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Monthly Questions and Answers 


I am considering installing a snowmelt system in the driveway of my existing home. Is this really economically practical? Signed, Tired of shoveling.


Dear Tired, my back hears you. The economics of a snowmelt are hard to calculate. A lot of it has to do with your location, frequency of snow fall and other variables. There are also different categories, as defined by ASHRAE as it pertains to Snowmelt operating costs. A lot of it has to do with consumer expectations. Can you live with a small accumulation, or is it critical that no snowfall be allowed to accumulate? Or can you live with a lot of accumulation and allow it to melt over a period of time. Depending upon your answer, the cost of installation and operation will vary. 

The advantages of a snowmelt system, if properly designed, installed and operated can bring a good return on the investment. For a homeowner, the convenience of having a slip free drive way are obvious. Elimination of the time spent snow shoveling is priceless, especially if you get hurt in the process. One potential benefit is that during the 9 months a year that the snowmelt system is not being used for melting snow, it is being bathed in free solar and ambient energy. If properly thought out, this “free” solar and ambient energy can be harvested and put to other uses, like domestic hot water preheat, hot tub and swimming pool heating and other  excellent uses. The following drawing is a conceptual drawing for being able to extract this free falling solar energy directly off of the slab, and place the energy in a DHW preheat storage tank. Obviously, if you intend to use it for other purposes, the system will have to be engineered and designed to your applications.

Commercially speaking the avoidance of slip fall hazards, employee loss due to injured backs shoveling snow and the associated wear and tear that snow melting chemicals have on interior and exterior finished floor goods in fairly substantial. Plus, if there is a continuous hot water load, it is entirely possible that the snowmelt system could produce more energy in a given year than it consumes in the process of melting snow. 

Residentially speaking, from a design stand point, it is very important that you use a good, qualified hydronic system designer with experience in this area. The energy loads on a snowmelt slab can be as much as 5 times greater per hour than a residential space heating load, and if the gas service coming to your home is not properly sized, operational issues can be expected. The designer must take all connected loads into consideration for the connected gas line, otherwise there may be issues with all connected loads during peak loading.

Another key feature to the economical operation of the snow melting system is the controls. We used to just use a simple on-off switch. After a few people inadvertently left theirs in the ON position for an extended period of time, and got a gas bill that was greater than their mortgage payment, we progressed to 12 hour twist timers. That worked, except you had to be there to turn in ON.

From there, we progressed to solid state controls that had the ability to sense slab temperature, outside air temperature and moisture falling on the slab. These controls greatly enhance the automatic operation of the snowmelt system and can reduce energy consumption by limiting the slab temperature during a MELT event to 40 degrees F.

As time marches on, the intelligence behind these controls has gotten even better. It is now possible, with an internet connection, for your system to anticipate a snowfall event, and turn itself from OFF to a preset IDLE condition, just below (i.e.30 degrees F) the melting point, and when moisture is detected, turn itself onto the MELT mode. This reduces reaction time and can allow for a significantly smaller physical plant needed to perform snowmelt.

Another important thing to remember is that when you melt the snow, you will end up with accumulating liquids, which if not properly disposed, can create a large ice hump that may make your or your neighbors driveway impassible. Think ahead and produce drainage to accommodate this situation.

Hopefully this answers your questions. If you need a good and competent designer installer, please visit the RPA web site and click on the Member/Product search buttons and see if you can find someone in your area to serve your needs. If that fails, feel free to contact mark.eatherton@radiantprofessionalsalliance.org and we will find someone to help you with your project.


 Is fluid maintenance a “requirement” on a good and true closed loop heating system?


 In the “Good Old” days, fluid maintenance was not considered a serious issue. All of the older cast iron boilers, and most all of the other metallic components in the system had enough physical girth to their construction that the known oxidation potentials in a truly tight, closed system was not considered an issue. In fact, the good ol’ boys always taught us that the black stinky fluids that came out of the drain valves on these wonderful heating systems was considered “Black Gold”. It was essentially the residual oxides associated with the oxidation (rusting out) of the thicker metallic components.

It was considered a protective coating for the metallic components in the system.

These older cast iron components also had significantly wider water passage ways to accommodate any partial blocking of the water ways by these free oxides. This is not as common in todays hydronic heating systems. Today, our selection of appliances construction material vary from a much thinner cast iron, to a thin stainless steel, a slightly thicker aluminum alloy and even some thicker mild steel flue gas passage ways, and even tighter water ways. The manufacturers of these highly efficient appliance do call out (in most cases) a require working fluid pH in order to retain their manufacturers warranty. This gets overlooked many times and can result in early heat exchanger failure, especially when a newer tight water way boiler is connected to an older cast iron gravity circulation system. In certain popular European style water tube boilers with narrow passage ways, these oxides can and do accumulate in the lower sections of the heat exchanger, resulting in poor cooling of the heat exchanger surface, and subsequent overheating and eventual premature failure. On these older systems, it is strongly recommended that a system cleaner/conditioner be added to the system upon start up, and flushed out after a period of time, and corrosion inhibitors reintroduced into the working fluid. Additionally, these system will continue to lose their free oxides, so I’d recommend the use of a dirt removal/air separator to insure that none of these oxides ends up creating any additional long term issues.  

In addition to possible boiler blockage issues, there have been numerous documented cases of the free oxides blocking off the water passage ways of radiant floor heating systems using smaller (less than 1/2” in inside diameter) on lower floors where velocity is low. If caught early enough, these oxides can be force purged out of the system and flow restored. In worst case scenarios, it becomes necessary to temporarily install an ultrasonic pulsing pumping/filtration system to soften and remove these deposits in order to re-establish and maintain flow.

Although fluid pH control and fluid maintenance are not a current code requirement, they are a strongly suggested “Best Practice” and will be mentioned in the “Best Practices” manual being produced by members of the RPA. As a good business person, a contractor should look at fluid maintenance as a potential additional profit center for their business. Knowing about a problem before it becomes a major problem can save your consumers a lot of money, not just for maintenance, but also the cost of operation.


 Can I use non oxygen barrier tubing with todays high efficiency appliances?


As with any hydronic heating question, there is only one correct answer, and that answer is, “It depends!”... But that’s probably not the answer you were looking for.

Oxygen diffusion through the walls of plastic tubing are well documented. And it has been a point of contention for years among contractors in the field.

The first and most important part of this answer has to do with the appliance manufacturer. Some appliance manufacturers will not warrant their equipment if utilized with non oxygen barrier tubing, so buyer beware. As far as the code is concerned, they currently defer to the RPA’s good old installations standards guideline, produced by the previous committee’s and administration of the RPA.

There are three prescriptive methods to deal with non barrier tube in a system possibly containing ferrous components.

The first method has to do with the non use of ferrous components. All materials in contact with the working fluid can not have any ferrous components in contact with the working fluid. Although the cost of the tubing is inherently less due to the lack of an oxygen barrier, on certain jobs, the tubing savings can be consumed by the increased cost of non ferrous pumps, air separators, tubing and other components. I think it is important at this point to tell you that oxygen and oxidation affects more than just ferrous components. It has been my experience over the years that rubber components also suffer oxidation degradation over time with continuous exposure to highly oxygenated fluids. It’s not just the metal components you have to worry about, but also all of those gaskets and other rubber components typically found in a hydronic system.

The next prescriptive methods requires the use of an all non ferrous heat exchanger to isolate the non oxygen barrier portion of the system from the ferrous components of the system. Typically, a stainless steel brazed plate heat exchanger is used to isolate the OPEN oxygenated portion of the system from the CLOSED loop portion of the system. This method has the disadvantage of requiring additional pumps, air separators and other components. It also comes with a thermal efficiency penalty caused by the heat exchanger. A person can oversize the heat exchanger to help alleviate the thermal operating issues, but a person has to stop and ask themselves if the tubing savings justifies the additional expense to make it work trouble free.

The third prescriptive method is to maintain corrosion inhibitors in the system fluids to keep the pH in control and away from the range that typically cause fast degradation of ferrous metal components. They key to this method is annual maintenance. All chemical treatments must be non toxic, because if they are considered toxic, the authority having jurisdiction will require the installation of a reduced pressure principle backflow preventer, along with double walled heat exchangers for the domestic hot water heat generators in the system. Non toxic corrosion inhibitors are readily available, and it is my suggestion that a bright and flashy notice be attached to the primary heat source letting future service technicians know that there are special chemicals in the fluid that should not be drained into the sanitary sewer, and that they must be considered non toxic, and that they will require regular annual testing, maintenance and replacement.

If non toxic antifreeze is used in the hydronic system, it may qualify as a good corrosion inhibitor, but again, buyer beware. There are some non toxic glycols that can be purchased without the required corrosion inhibitors. The use of oxygen scavengers would appear to make sense in this case, but in reality, is probably a waste of time and money due to the continuous readily available stream of oxygen making its way through the walls of the tubing and into the working fluid. If the pH of the system fluid is not tightly controlled and is allowed to become acidic, serious damage can result due to the accelerated corrosion that will take place with this less than desirable working fluid. It is not a set it and forget  it proposition. It should be checked, tested and adjusted annually if necessary. You should also check with the heating appliance manufacturer because certain non ferrous boilers (think aluminum alloy boilers) require a completely different set of operating parameters (pH).

Fluid maintenance is even a good idea on tightly closed hydronic heating systems. Oxygen, like many other fluids that mother nature controls, despises any imbalances in content. If there is more oxygen on the outside of the pipe than on the inside of the pipe, she wants the oxygen content on both sides of the pipe to be the same, so she lets oxygen permeate through rubber flange gaskets, expansion tank diaphragms, packing glands on valves, and packing glands on some larger pumps. This continuous supply of oxygen is always looking for something to oxidize, and iron based components are the perfect luncheon companion for this omnipresent gas. Systems that are kept in the continuous circulation mode with a micro bubble resorber style of air removal system do have less oxygen content than non continuous circulation systems, but as soon as the circulators shut off, mother nature kicks in and balances the system oxygen content out.


I have a couple of questions:

  1. What size PEX is most commonly used in concrete reinforced slabs?
  2. What size wire mesh is most commonly used in reinforced slabs?
  3. Is there any correlation between the size of mesh and the size of the PEX being applied in reinforced slabs?


The answer to question number one depends upon the application. If it is a residential or light commercial radiant floor-heating panel, then 1/2-inch is typically used. On larger commercial spaces, 5/8-inch and 3/4-inch tube are used. If it is snow-melting application, then 5/8-inch and 3/4-inch are the norms.

The use of mesh for tying tube is done by numerous different methods. Using the cold rolled steel mesh that one can get from Home Depot and other building outlets is difficult because it has a tendency to arch up due to the way it was rolled initially. This is referred to as “memory.” It is difficult, if not impossible to work with, especially for tying to tubing. It also only comes in 4-foot wide rolls.

A preferable method is to use 8x20-foot flat sheets of 10 Gauge x 10 Gauge flat welded wire mesh, 6-inches on center. These are usually available from concrete form suppliers who also provide reinforcement materials. This mesh lays flat and stays flat. It is usually tied together using wire ties, or alternately, nylon ties. The metal wire ties are much more robust under the unwatchful eyes of the concrete placement/finishing contractors. When tying at 9-inches on center, it becomes somewhat difficult on every other pass because the tubing has a tendency to slide down to the next grid, especially near the return bends. This might require the addition of nylon “stays” made with nylon ties to avoid this movement, or I have also placed twice as many sheets so I get a 3-inch grid, which is much more conducive to tying tubes at 9-inches on center. This is done on steep grade applications to keep the tube from creeping during the pouring process.

In answer to question number three, there isn’t usually any correlation between tube size and the reinforcement steel to which it is being tied.

In some cases (commercial grade slabs) the contractors are given a “grid” of steel reinforcement rebar to which to tie. All of these methods should be held up on “chairs” to ensure that the mesh and tube end up being supported mid-slab. An installer can also used “adobe” bricks to accomplish the same feat. Ideally, the tubing should end up within 2-inches of the emitting surface, but in my 36 years I have NEVER seen it end up that way. Big-footed concrete guys can send it to the bottom of the slab in a heartbeat.

The tubing is then tied to the metal supports using nylon ties, or steel ties depending upon the contractor’s preference. If you do decide to use nylon ties, make certain to cut off the loose end tails or you may come back to a concrete porcupine with the nylon ties sticking through the finished cement. I have also used a battery powered rebar tying tool that was fantastic. It cuts the labor associated with tying tube by almost half! The initial expense is somewhat large, but so is our field labor. If you decide to go this route, you will need to adjust your field labor downward to compensate for the significant labor reductions, and will also need to compensate your material costs upward slightly to reflect the cost of the pre-rolled wire ties used with the rebar tying tool.

In most residential cases, the engineering specifications do not call out the required use of reinforcement mesh, so it ends up being an added strength feature to help resist settling and associated cracking.

Although I am not aware of any documented failures of PEX tubing tied on metal with metal ties, I have personally witnessed some tubing deformation where it was tied with steel twist ties too tight to steel mesh. Many hydronic heating contractors exclude the materials and labor required to place this mesh grid for tying from their installation contracts. This, in my opinion, is a big mistake. The concrete contractor does not understand why he is placing the mesh, and does not take the hydronic contractor’s wishes, wants and needs into consideration. If the general contractor demands that his lesser expensive crew be allowed to provide and place the mesh, make certain that you provide the specifications and that it is installed in a manner that is conducive to your installation manners and methodology. Otherwise, it is going to cost you extra labor in order to do your job. Besides, why miss out on the opportunity to get additional labor and material dollars?

Lastly, there are a bunch of extruded polystyrene insulation panels with knobs on them that are coming out to displace the need for mesh in non-critical load bearing situations. It provides a good place for locking the tubing in and provides the insulation necessary to control the flow of heat.


I recently read an article on the Internet that claimed that radiant panel heating systems can cause Legionnaires Disease. I have a radiant heating system in my home, have elderly parents living with me and want to insure that they are not exposed to this deadly disease. Can radiant heating systems cause Legionnaires Disease?


Yes and no. In the case of a radiant floor heating system that is truly a fully closed self-contained loop, no. Read on please.

First, let’s look at where the organisms responsible for this disease come from and what causes amplification. The bacteria responsible for this disease are omnipresent in the soil. If it’s in the soil, it’s also in the water. A Centers for Disease Control study a few years ago determined that 90 percent of the participants in the study had been exposed to the bacteria, most probably due to their drinking water. The normal drinking of water is not how it becomes deadly, but this test basically confirmed its omnipresence. So, it is established that the bacteria is everywhere. In fact, there are cases of farmers and others who regularly are exposed to dust in suspension, inhaling this dust and contracting the disease. We are all exposed to it from our potable water supplies, whether they are treated or not. The bacteria have a certain temperature range of tolerability, and in certain temperature regimes can actually reproduce quite well, thereby increasing the levels of exposure. Their ideal temperature for proliferation is between 77 and 108 degrees F. As water temperature increases, the bacteria’s concentration decreases due to scalding death. At sustained temperatures above 140 degrees F, they cannot survive. Hence, the reason the Europeans maintain their DHW storage tanks at 140 degrees F and mix down using an anti-scald mixing valve to a lower, safer temperature of operation. But we are talking about space heating systems here.

Years ago, when radiant floors were first gaining traction, an idea was proposed to simplify hydronic radiant heating systems by allowing the use of a single contiguous potable fluid (DHW) as the medium for washing and bathing, as well as the circulating medium to radiant panel heating systems. In concept, this sounded like a great idea. It would reduce the cost (no extra heat exchanger, pump, expansion tanks, etc.) and require less labor to install. In reality, the industry was inadvertently creating Legionella amplifiers. As this water lays in the system during non-heating periods, it becomes water of a “questionable character.” These types of systems have been allowed for many years under both American and Canadian plumbing/mechanical codes. The code officials addressed it by requiring the use of a timer to operate a circulator to keep water from becoming fouled, and in the case of systems using only zone pumps for circulation, it made sense. However, these zoned pump systems are a rarity, and most systems use individual zone valves with a single pump. There is no way of guaranteeing that all of the zone valves will be opened when the pump is running in its attempt to avoid stagnated water in the heating circuits. Then, there is the issue of dumping heat into a space zone during the cooling season.

Another effort by system manufacturers was to pipe the system such that with each draw of hot water, the incoming cold water “flushed” the water out of the zones, into the DHW heating system, and then flowed to the points of use. Again, this might work on a system that has perfectly balanced circuits and no zone valves, but once again these ideal conditions are rarely seen in the real world. In reality, people with these types of systems were being exposed to the uncontrolled production of condensation in their radiant panels, and the issues associated with that. Many of them were abandoned and re-piped to avoid running too cold water through their radiant floor/wall/ceiling systems.

The only guaranteed way of avoiding unnecessary exposure to this potentially deadly disease from your space heating system is to have an isolation heat exchanger that separates the potable DHW from the non-potable space heating water, and pipe it in such a manner that the incoming potable water on a hot water draw flushes out the heat exchanger to avoid stagnation potentials. In the case of water treated with chlorine, it was thought that the bacteria can’t survive exposure to the oxidation of chlorine, hence the reason for requiring recirculation and flushing. It has been found that in order for our conventional chemical sanitation methods to be effective, the required concentration of chlorine would have to be approximately 10,000 times higher than the minimum currently required by law, and exposing ourselves, and our piping systems, to concentrations this high is extremely detrimental.

The Legionella blooms and multiplies significantly. Exposure to this elevated bacteria count when a person’s immune system is depressed (something as simple as a head/chest cold), through the inhalation of these deadly bacteria causes bacterial pneumonia. Per the CDC, it is the most commonly misdiagnosed disease in the world. Legionella is not the only bacteria that can withstand our normal methods of chemical sanitation. Pontiac Fever and others have similar growth characteristics to Legionella and present just as many health risks.

For more information on this subject, visit the Center for Disease Control’s website at www.cdc.gov/ and enter the term Legionellosis in the search engine. Or, simply type the key search word Legionellosis into any search engine.

Prevention by design is the most effective means of avoiding exposure of this potentially fatal disease. The RPA has proposed changes to the new Uniform Solar Energy and Hydronics Code (USEHC), slated for delivery in 2015, that will attempt to address the issues associated with open radiant heating systems.


The hardwood flooring system in my new recently constructed home had some issues with shrinkage this winter during the heating season. When I contacted the hardwood flooring contractor, he told me that the water temperature being sent to this particular hardwood floor should never exceed 85°F. He told me that the only time they had issues of shrinkage with hardwood floors is in situations where it is placed over the top of a radiant floor heating system. When I approached my mechanical contractor and asked him to turn the water temperature going to this floor down to 85°, he said that there was no way that it would be able to keep my dining, living room and kitchen areas comfortable when it got cold outside. I asked him to go ahead and turn it down anyway, which he did, and when it got below 30°F outside, as he predicted those areas are extremely uncomfortably cool.

Another problem that appears to be related to this is that when it got extremely cold outside, the bedrooms in the area directly below these hardwood floors got uncomfortably warm.

Can you give me some guidance and direction?

A Frustrated homeowner.


Dear Frustrated,
Without being able to actually put “eyes” on your system it is virtually impossible for me to guarantee performance. However, your sentiments echo those of many consumers for which I’ve had to work for over the years and resolve these types of situations.

The first and most important critical detail is the requirement of placing insulation in the floor joist bays directly below the radiant floor heating system. If this is not done at the time of construction you will experience under heating on the upper floor and overheating on the lower level directly below the radiant floor. The myth that seems to have evolved is that “heat rises”. While this is true of some fluids (Water and or air) radiant energy travels omni-directionally, including downward, through the path of least resistance. Unfortunately, in many situations when the consumer is faced with the need to tighten their budget in the midst of their construction project the general contractor mistakenly decides to remove the insulation from the heated floor because he is used to placing the insulation in an outside wall or ceiling that is exposed to extremely cold weather on the other side of the wall. This is where the “heat rises” mistake is made, and the decision to remove the insulation is performed. The insulation that is placed below this radiant floor heating system is a must if you are going to control the direction of the flow of the radiant heat. If in fact this insulation package was removed in the process of construction, and the ceiling is already finished, it is possible that you can have some cellulose insulation blown up into the joist bays to give you the necessary R value required to control this directional flow of heat. Be very cautious when working around can lights in the ceiling of the basement, or any other heat generating equipment that could create an unsafe condition.

The second violated principle appears to be the lack of acclamation of the wood products that were applied to your floor. It is extremely critical that this wood be allowed to acclimate to the normal background relative humidity of the inside of the dwelling prior to being placed on the floor. This acclamation should occur after all moisture producing finished products have been applied and allowed to completely dry. This  includes but is not limited to, cementitious materials like stucco, drywall compounds, latex paint and anything else that has to dry out and give up its residual moisture. Unfortunately, the critical path method does not have any allowances for  the time necessary to allow these products to acclimate. Consequently, they are applied prior to becoming “normalized” and when the radiant heating system is turned on it does drive the excess moisture out of the wood unevenly, causing it to warp, shrink and crack. Your hardwood flooring contractors comments are not 100% true. I have seen hardwood floors in houses that didn’t have radiant floors that also experienced the same shrinkage issue.

 In addition to making sure that the wood is a stable as possible prior to placement, it is necessary to maintain a stable humidity environment inside the dwelling after the installation, otherwise the wood will swell and shrink at will as it absorbs and releases moisture. I have experienced this issue in my own house which unfortunately does not contain a humidification system. Doors get sticky in the winter due to internal moisture, and shrink during the Summer (dry climate).

The third violated principle pertains to the operating fluid temperatures of the fluid that is being circulated below the hardwood flooring system. Many years ago, the RPA provided some information to the National Hardwood Flooring Counsel, and in that statement they recommended a maximum surface temperature of 85°F. This is a fairly common recommendation with radiant floor heating systems regardless of the final finish  and is really more of a human physiology limitation as opposed to a mechanical limitation. If a human is in contact with a surface that is greater than 85°F their body thinks that their core is going to overheat and it causes their body to begin cooling itself through a process known as evapo- transpiration also known as sweating. Somehow, this maximum surface operating temperature mistakenly got converted into a maximum fluid operating temperature by the hardwood flooring contractor.

In reality, a hardwood floor can and will achieve surface temperatures far in excess of 85°F during the winter months in situations where it is placed in front of a south facing window. Again, if the wood is properly acclimated, and a decent and reasonable humidity level (30 to 50% rH) is maintained within the dwelling the wood will remain extremely stable for many years. I have personally witnessed hardwood floor surface temperatures in the range of 140° where it is being exposed to direct sunlight, and it didn’t even have a radiant floor heating system below it.
The required floor fluid operating temperature is something that a competent installer can calculate, but I would caution you that if you have placed any significant throw rugs over the top of a large area of the radiant floor,  that the fluid operating temperature will probably have to be raised slightly higher than what it would normally be. My suggestion would be to start out low and go up slow.

The hardwood floor shrinkage can be addressed by a competent hardwood floor servicing company, the lack of insulation can be handled by a competent insulation contractor, and the fluid operating temperatures can be adjusted by a competent radiant heating contractor. Once all of this work is done, you can enjoy your radiant comfort.


I see so many backflow preventers on boilers plugged due to dripping. What is your take on this? Should I plug them with an iron pipe size plug or replace?


Generally speaking, backflow preventers leak because they were doing their job at one point in time, and a piece of scale or rust got caught beneath the washer seat causing them to drip continuously. My presumption is that that you are talking about a spring-loaded double check valve with an intermediate atmospheric vent.

This typically happens because there is a drop in the incoming water pressure going to the appliance that the device is protecting. Under ideal conditions, the valve should reset and not seep water from the relief port. Too often, that "ideal" happens in pristine laboratory conditions, and not the real world.

The correct thing to do, is to replace the bad leaking backflow preventer, and at the time of replacement install a single seated spring-loaded check valve upstream of the backflow preventer. This will keep the backflow prevention device from seeing any droops or drops in incoming water pressure. It should keep the backflow preventer from weeping due to reduced pressure on the inlet side. A single seated spring-loaded check valve in and of itself is not considered adequate protection for residential applications.

In most cases, in a residential setting, the pressure reducing device that is downstream of the backflow preventer has an integral single seated check valve that should keep the backflow prevention device from seeing any increases in water pressure on the downstream side of the back flow preventer as the system water is being heated. In the off chance that the pressure reducing valve does not have a single seated check valve (older valves), I recommend that you install one in the course of service and replacement.

Although the days of people using dangerously toxic chemicals for providing water treatment for these closed loop heating systems are in the past, there may be enough residuals in the water to potentially cause someone to get ill. Many older gravity conversion systems had a Honeywell #1 Heat Generator device installed, and it contained a considerable amount of mercury. This mercury was in constant contact with the circulating fluids causing the water to become water of "questionable character". To say nothing of all of the oxides that are in the system. It is just not worth taking a chance. We must protect the health of the consumers. Plugging the relief vent is not an acceptable practice because it renders the backflow prevention device useless.

In certain areas of the country, it is required that a reduced zone principal backflow preventer be installed, even for residential applications, and it is required that the backflow preventer be tested by a certified backflow prevention tester annually. Consider yourself and your residential customers lucky in that you are not required to have RZP types of backflow preventers in your residential settings. It typically costs the consumer a minimum  $100 to $150 to have these devices tested on an annual basis. Most construction codes require the use of an RZP type of backflow preventer on commercial boilers in commercial settings.


We are looking at a job where the only place to put a boiler is in the attic.  We've got the blessing of the building department but we're unsure how to deal with the condensate and Pressure Relief Valve.  There are vent stacks available but are they legal to use?  Can you provide some guidance?


Although you may have permission from the AHJ, it is important that you follow the appliance manufacturer's recommendations as it pertains to accepted and allowable locations for the installation of their product. One boiler manufacturer that I am familiar with explicitly disapproves the installation of their product in an attic. It is assumed that the attic will be an "unconditioned space", meaning that it will not be operated or maintained in a freeze free environment.  Even with heat tape trace protection, there is no guarantee that the heat tape will not fail when it is extremely cold outside, thereby causing the condensate disposal system to freeze and break. Subsequently causing significant water damage. If you can overcome that particular hurdle, then the other things you need to take into consideration is the treatment and disposal of the condensate. The condensate must be neutralized so that it will not dissolve the piping system into which it is being placed. The local code will dictate the allowable and recommended method of condensate disposal as it pertains to the existing plumbing and venting system. Based on my many years of experience as a plumber, it is acceptable to "wet vent" one fixture unit into a vent stack of a given bathroom group. A condensing boiler will generate approximately 1 gallon per hour of condensate per 100,000 btu per hour of energy consumed.  There are also some new code approved fittings that will fit beneath a lavatory sink to allow condensate to be properly disposed. The  local authority having jurisdiction would be a better person to ask this question because their interpretation of this portion of the code is subject to the field authorities final approval.

Don't forget to provide freeze protection of the circulated space heating fluid as well (propylene glycol for example) because bad things happen when you least expect it and don't want or need it. The largest annual insurance company payouts for residential settings are claims due to damage caused by water. Don't become an insurance statistic.


What is the best way to use a DHW heater as a heat source for doing radiant heating?


In order to guarantee that there is no water of "questionable" character allowed to be created within the system, there must be a flat plate heat exchanger isolating the potable water from the space heating side (non potable or water which can become water of a questionable character).

In addition to the flat plate isolation heat exchanger, it is important to pipe the interface between the heat exchanger and the potable water system on draw such that the heat exchangers potable water circuit is flushed with each draw of DHW.

It is also critically important that during a pump forced heat extraction/transfer, that the location, type (spring check) and placement of check valves be located such to avoid any potential inadvertent back flow and subsequent mixing and or dilution on the DHW side of the system. The DHW heater must be approved as a space heating appliance before it can be used in these applications. Not that some critical components (T&P relief valves for example)  have intentionally been left off of the drawing for drawing clarification. Always check with your Authority Having Jurisdiction prior to performing one of these installations to make certain that it is in complete compliance with the intent of the code.


I need help with regard to soil conditioning below a slab in a roll in freezer with a 15°F set point. Would you know what kind of btu per sq/ft to drive for a “heatloss for below the slab”?


Do the math as follows:
Freezer set point = 15°F, Soil temperature to be maintained at 40°F =  delta T of 25°F, and 6"XPS insulation with a R value of 30.

Using the formula A/R*Delta T, A = 1, R = 30, delta T = 25, thence .0333 (1 divided by 30) times delta T (25) = .0333 X 25, for .83 btu/sq foot per hour.

The other formula commonly used as a checking tool would be U * A * DT,

1 divided by R value = U value. Hence, 1/30 .0333 = U value.

U (.0333) times A (1) times delta T = 25 would equal .83 btu/sq. ft. per hour. Check mate :-)

I would run this by a responsible tubing manufacturers engineering department to alleviate any potential future liability. If the frost control system fails, the formation of frost balls and slab heaving can be disastrous. If using waste heat recovery as has been done in the past, I would incorporate some means of audible/visible alarm to avoid issues before they become REAL issues. Don't forget to take into consideration the wicking potential of a shallow water table.


I am a homeowner with radiant ceiling panels which are hung just below the ceiling.  We have blown-in fiberglass insulation above the ceiling about R-18.  We are thinking about adding additional insulation.  Our question is:
1) given the 'radiant' loss of heat from the panel, would a radiant barrier insulation make sense in the attic above the blown-in fiberglass?  The radiant barrier would be about 8 inches above the panel. Our other option is adding more blown-in insulation. 
2) would a radiant barrier at that distance to cause the panel to be too hot?
3) with radiant heat in general, does a properly installed radiant barrier make more sense as an insulator than a large convection barrier?

P.S.  I am not asking you to worry about ventilation issues; we would probably use a perforated radiant barrier for that reason, though.


Generally speaking, a reflective radiant barrier has numerous critical requirements in order to be effective and remain effective. It should be 99.9% pure aluminum, should be as smooth as possible, and must have no dust or anything on its reflective surfaces. Also, ideally, it should have approximately 1" air gaps on either side of its reflective surfaces.
The general recommendation, as it pertains to insulation behind a radiant ceiling, is that whatever the code requires for a minimum R value for the climate, add 50% to the R value. If this is not feasible, based on available space, put as much insulation in as possible. Alternatively, you could use a foam product (check with code authorities regarding the application) which has a higher R value per inch than other insulating materials.