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Radiant Floor Heating/radiant floor heat system for small home


QUESTION: Hi Morgan,

I have been reading through many of your responses and have found it very educational.

I am remodeling a 680sf lake home in Wright Co, MN.  I have gutted the entire building and removed the wood floor.  I insulated the interior of the block foundation with R10 rigid foam and filled it in with approximately 2' sand.  I plan to insulate the perimeter and the entire 680sf with R10 rigid foam prior to pouring a concrete floor embedded with pex tubing.  I plan to insulate the 4" walls with R13 and the ceiling with R38.  540sf has 8' ceilings.  The other 140sf faces south towards the lake and has a vaulted ceiling with extra windows.  We will be using the home as a year round weekend getaway and plan to heat it to approximately 50 degrees during the winter.  It may be 20 years before we consider living there full time.  
I am hoping to use hydronic radiant heat as the sole primary heat source.  Natural gas and electric are available.  The water is from a well.  

I noticed in many of your responses you recommend a natural gas high efficiency tank type condensing water heater as a source for both domestic and space heating for smaller homes.  Is that what is referred to as a "indirect system" that separates the domestic and space heating?  If yes, is it possible to use propylene glycol in the space heating and would you recommend it for my weekend use?  

I also read your response to someone with a 640sf weekend getaway home in Ontario and you originally recommended the tank type condensing water heater but later in a 2nd response suggested the use of a dual purpose space heating/tankless water heater.  I had previously looked into one of the options you listed, the Quietside DPW, but was turned off by the recommended minimum 900sf and the possibility of short cycling due to the minimum 27,000 btu output.  I contacted the manufacturer and they suggesting a "Bypass, Diff., Man, Fix 3.6 Psi" accessory that connects to the manifold to eliminate this problem.  Are you familiar with such an accessory?  Do you think this unit would work for my application?

Thanks for your time and knowledge,


ANSWER: When designing radiant heating systems for small houses and cabins the real challenge is finding equipment small enough to meet the load and still operate with efficiency and comfort. As you suggest, over-sized equipment will make equipment cycle more often than it should leading to increased component failure, poor efficiency and less-than-optimal comfort.

To be comfortable in any space you have to maintain the design temperature as closely as possible keeping the heating source ON more than off.

I am currently unhappy with the lack of small condensing boilers and the combination or combi-water heaters or boilers leave much to be desired.

For the combi-boilers/water heaters (they are getting harder to distinguish and in many cases it is just a matter of labeling) the peak demand of filling a tub or running a shower typically requires a 100,000 Btu burner in cold climates with cold ground water. These combi-units can work very well if the potable water is soft and the fixture load is matched to the units output. Unfortunately the heating loads are going down while the energy it takes to generate on-demand shower water does not change. In you own case the heat load will likely be less than 20% of teh on-demand load of one shower head. So in the coldest weather your combi-boiler will be cycling on and off within seconds (short-cycling). This is akin to driving in stop and go traffic burning up equipment without much progress. It may be that your new combi may not even stay on long enough to satisfy the thermostat!

Before deciding on any heat source or radiation you must do a heat load to determine the size of the heat source and the radiation required to meet the demand on the coldest day. Big glass makes this job more demanding since window present the largest load in most new construction and retrofit remodels. It may be that you will have to supplement the floor heat with a wall panel in order to heat the space on the coldest day.

The tank-less water heater is usually not a good match since it is set up for the typically high (60+psi) city or well pressure and requires a separation or sub-assembly between potable and space heating water. Tank-less water heaters are almost always too big for space heating even when modulating to its lowest fire it will still be cycling.

When designing heating systems for cabins up north we use several strategies for keeping things from freezing up. First, a freeze alarm, since no heating system is 100% reliable in all conditions. Second, we suggest a sealed combustion fireplace with standing pilot or battery ignition. Finally, if we have to, we will specify a minimum level of propylene glycol to protect PEX in the slab.

Most modern condensing boilers will have frost protection mode but all require power to operate in any mode. No help there.

The smallest combi-boiler I am aware of still requires 100,000 Btu input. No help there.

Since the space is so small combustion air can only be practically had from outdoors so a sealed combustion unit is the answer. A sealed combustion water heater will store the water you need for bathing and have the added benefit of thermal mass presented by the stored water. This allows the use of small amounts of energy for small zones or loads without compromising comfort, economy or reliability. System life is extended and maintenance is reduced.

Most conventional water heaters are now deemed "suitable for space and potable water heating". Their suitability is the real question and can only be properly determined by doing the math and sorting out the various options.

If you send a drawing we can help.  

---------- FOLLOW-UP ----------


Will a 40gal direct vent (sealed combustion) natural gas water heater 0.59-0.62EF with side connections for space heating be suitable for my attached floor plan?  I seen models from AO Smith, State, and American on the internet.  I would prefer not to spend the money at this time on the weekend getaway for a power direct vent 0.67EF or condensing 0.95EF water heater unless it is the only option.  It doesn't seem necessary for a power direct vent when the water heater will be adjacent to the exterior wall.  Maybe install a more efficient replacement water heater in 15 years.  Is it recommended to utilize the one water heater with a heat exchanger off the side connections for space heating and the top connections for potable in a closed loop system?  I would then have a well pump w/pressure tank, water softner, and water heater.  Is this a more efficient setup than 2 separate water heaters?  Will this system be suitable for heating the cabin at 50deg during the winter and increasing the temperature for weekend use?  Would an electric boiler or in slab cable be a better option for this small load?  FYI - the north 21.25sf, east 28.5sf, south 71sf, and west 31.5sf of Milgard vinyl windows are 1/2"-19/32" GAP, Air Fill, LOE04, U 0.33-0.34, SHGC 0.26-0.31, and VT 0.48-0.58.

As far as installing 1/2" pex tubing is it ok to attach directly to the rigid foam at the bottom of the 4" concrete slab or should it be elevated to the middle of the slab.  Is it recommended to loop around walls and cupboards or is it ok to be under them if at the bottom of the slab?

Thanks for any information you can provide,


ANSWER: Though popular, and readily available, the QuietSide boiler is rarely a good match for low-load (small) radiant floors e.g. cabins or basement and is certainly not for DIY. Often times when a boiler manufacturer has trouble with its initial presentation in a given market, they decide to short-cut traditional distribution and go directly to the consumer via internet wholesalers or big box stores neither one known for their technical support. I do not think of a buffer tank as a design strategy, rather a re-design tool used when others have made serious design mistakes.

Though we do specify anti-freeze for some of our more remote cabin designs it is not ideal and not generally compatible with low-mass heat sources such as the QuietSide. Great care must be taken to provide the proper mix and brand of anti-freeze and one must remember to de-rate the output of the boiler and up-size the circulator to accommodate the inefficiency imposed the the system by the inclusion of anti-freeze. If you have a serious risk of frost damage an alternative heat source and/or a freeze alarm is our preference.

When specifying boilers for clients out of our immediate service area, we always survey local contractors, and listed distributors to assess the level of service for the heat source we are interested in. Often times the "perfect" boiler for a given radiant floor is not available or adequately supported in the market we intend to place it. Any boiler or water heater is only as good as the people that design the system, install it according to specification and maintain it according to the manufacturer's recommendations. In, that, order.

In small homes, maintaining acceptable air quality is more difficult than in larger homes due to the small volume of air and the lack of infiltration. To achieve or maintain good air quality three things must be considered. First, the quality of construction; the tighter the envelope, the worse the quality of air. Second, the sources of pollution; people, animals, cooking, clothes washing and gas appliances. Finally, the ventilation; all sources of pollution must be exhausted and a controlled source of fresh air provided in the most efficient manner possible without compromising creature comfort.

Naturally, a properly designed ventilation system is must; we use energy recovery ventilators in our designs. But if you plan to use gas to heat space and domestic hot water in a small space you must have a source of combustion air. With an atmospheric (gravity, naturally aspirated) water heater or boiler a free-air source (4-6" pipe open to the outside) must be piped to the mechanical room to provide oxygen and maintain a minimum chimney draft. This means you must have a mechanical room separated from the living spaced by an air-tight door for comfort and efficiency.

This is why we use a direct-vent, sealed-combustion water heater for many of our designs. Commonly piped in PVC these appliances draw fresh air from outdoors and vent the byproducts of combustion back out again without the detrimental pollution of SOx and NOx, which has gone down the drain with the condensate.

If you would rather go low-tech, a properly designed mechanical room can be built and atmospheric water heater used of dual-purpose duty. In this case a single-wall Bradford White CombiCor style unit may be in order as it is quiet, will not require a buffer tank and has a built-in heat exchanger to separate potable DHW from space heating water anti-freeze. We often design the laundry and an extra bath into our mechanical room designs. But again, this takes some engineering.

Unfortunately, this is not the place of design work, as there are too many questions that cannot be answered without a proper heat load.

If you care to contact us directly we would be glad to help you design a radiant floor system for your lake cabin.

---------- FOLLOW-UP ----------


I downloaded Radiant Works heat loss and design software from WattsRadiant website and calculated a heat loss of 22000Btuh.  I like the idea of using a dual purpose direct vent sealed combustion water heater piped with pvc, a heat exchanger to separate the potable and space heating, not using glycol, and installing a sealed combustion gas stove with standing pilot for backup as you suggested.  Unfortunately I am not prepared to pay for the ultimate American Water Heaters Polaris unit for my weekend getaway. What are your thoughts on using the American Water Heaters Power Direct Vent PVG62-50T60-NV unit with side located space heating connections?  I found it at Lowes on clearance and out the door for $600.  It has a 0.67EF and qualifies for a $150 rebate from Xcel Energy.  Thats a final price of $450.  It would be several years before I would see the savings from a $2600+ Polaris unit.  The biggest downside I see from the unit I am looking at is that I will experience extra standby loss with a 50gal water heater in a 1 bathroom home.  

Your thoughts on using the PVG62-50T60-NV unit for the near future until we retire and live at this home and upgrade to a Polaris type unit would be greatly appreciated.  I am assuming it can be plumbed exactly the same, making any future upgrade easier.  



Badger Boiler Versa Hydro
Badger Boiler Versa Hy  
We design hydronic radiant floor heating systems everyday, but not via open forums in general or this venue in particular. In order to be accurate a Manual 'J' heat load must be performed by an experienced designer and the various options considered from an experienced perspective.  

If you would like us to design a radiant floor or combi, space and water heating, system for your particular home, please send all the pertinent information directly to:

Thank you.  

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Morgan M. Audetat


Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........


30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

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