ASK THE EXPERTS: Hydronic Heating

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This system from Warmboard uses aluminum cladding to distribute heat.
This system from Warmboard uses aluminum cladding to distribute heat.

I very much enjoyed John Siegenthaler’s article, “Renewable Hydronic Heating” (HP152), and wondered if he might comment on lightweight systems that use aluminum as a heat conductor as opposed to those that use concrete as a thermal mass. I was recently made aware of this type of system and the benefits to the overall flooring process when looking at building a home over a basement foundation. Details of the system can be found at warmboard.com. I hope to hear another opinion on this.

Rick Wimberly • via email

I’m glad you enjoyed the article. There are several radiant panel systems that use aluminum plates or claddings as “wicks” to pull heat away from the tubes and spread it across the panel surface. The product you mentioned—Warmboard—is designed as a complete subfloor/radiant panel system. It is about 1.125 inches thick and, as such, can serve as a structural subfloor and replace conventional plywood or OSB subfloors.

The key to success with any of these products is to keep the water temperature required as low as possible. As I suggest in the article, design the distribution system so that it can supply the maximum required heating load without exceeding a supply water temperature of 120ºF—even lower if possible. The lower the water temperature, the higher the efficiency of heat sources such as solar collectors, heat pumps, and condensing boilers.

Warmboard uses 12-inch tube spacing; other aluminum plate systems use 8-inch tubing spacing. They should not be covered with more than R-1 from flooring materials, and should have at least R-19 underside insulation. This type of system would provide about 19 Btu/hr./ft.2 of output, when operated with a supply temperature of 120°F.

I would also highly recommend the use of PEX-AL-PEX tubing (versus standard PEX) for any system involving aluminum heat dissipation plates or claddings. PEX-AL-PEX with aluminum is less likely to make any expansion noise as the system warms up.

John Siegenthaler • Appropriate Designs

Comments (6)

Tom M's picture

Asic, I understand where you're coming from, it's just that a tank can only hold so much water and if you use that tank to heat a cold room the heat can dissipate quickly, unless you have one of those passive homes with a small swimming pool full of water that is heated up over the entire year thus a large amount of tempered water to use.
As far as heat in a basement slab, unless the hole is insulated first, there is a lot of heat loss to the ground. In my opinion a basement, if it is truly underground, should not be heated and should not have to be heated. The whole purpose of a basement, especially if in the northern part of the earth, is to create a buffer zone between the living space and the earth and it should stay a constant comfortable temperature year round. A heated basement has all that surface area to loose heat, kind of like a heated swimming pool, 75% of the surface area is the earth which is normally cooler than the air temperature but most people concentrate their focus on the losses coming from the top of the pool.
Actually I like the idea of using a sunroom of sorts with a large slab as the main storage of SHW using radiant. That way there you can heat the slab to whatever temperature is available. Then when that room gets real hot, hot air can be pumped out of it into other parts of the home. All you need to do is zone off different rooms with your radiant heat so that way there each room can call for hot water when needed and when they are satisfied the hot water is sent to the sunspace to get that as hot as possible to store heat for when needed.

asicengineer's picture

Tom, you have the traditional view on concrete thermal mass radiant heat and my comment was to provide the counterpoint...that there are multiple advantages of the lightweight non-concrete system and it has worked out well for me and might work out well for others.

I get that a 70 degree mass won't radiate into a 70 degree room. That's part of the problem. To store useful heat, you have to store it well above room temp, so pound for pound, a tank of water at 120 degrees F stores a lot more heat than a concrete floor at 75 or 80 degrees because you can slowly drop it down from 120 to 80 or so and provide good warmth to a room. You can't pump a concrete floor up to 120 degrees for storage because you would overheat the room and potentially ruin the flooring. A concrete floor, no matter how massive, doesn't hold a lot of energy if you are bounded by about 80 degrees before it is too hot and 70 before it is too cold. Given a choice, I'd rather a thin, light radiant floor at the perfect temperature and with separate solar storage tank than a thick heavy radiant concrete floor at never quite the right comfort level.

I've got concrete floor radiant heat in the basement which I regret because all winter long I know it is losing heat to the earth. The installer claimed not much because eventually it will warm all the earth near the house and be finished, but I'm not sure about that math. Seems suspicious to me but I haven't done the calculations. Either way I'm sure it is a whole lot more than a nice insulated 500 gallon tank of water would be losing plus any heat lost from a storage tank in my utilities room would just go toward heating the house.

As far as wood flooring, I have had a lot of success with engineered wood laminates that float (aren't nail or glued) on the floor. I'm on year 4 of success with outdoor temps going from 100 for a summer max (rarely) down to 0 for a winter low (commonly). I don't have AC so the floor does load up with summer humidity, but so far, so good. Many wood laminate floors are spec'ed to allow installation on radiant heat. Like you say, wood isn't a great conductor. It conducts heat about 1/5 as well as concrete (engineeringtoolbox dot com). But on the other hand, there is likely to be several times greater thickness of concrete than wood so the overall heat transmission isn't vastly different.

Fred Golden's picture

I was reading a book called "Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian (Sep 15, 2006)" and would tend to agree with James Kachadorian that you can in fact store thousands of Btu's of heat at near 70F in a concrete floor. I think he mentions 30 Btu's per cubic foot of concrete, per degree of temperature rise. While 64 pounds of water is much more heat per cubic foot than concrete, both will store heat at 70F.

Your room temperature might vary more with the solar heated concrete floor. And using two set point thermostats, with one higher setting for when the solar heated tank is warm and the other as a minimum setpoint for a cooler night is helpful to sort of "Overwarm" the room when you have plenty of solar production to warm the home for those upcoming cooler days.

Also cost comes into effect. It is hard to beat the cost of concrete block and low cost PEX tubing as a heat exchanger material. Mr Kachadorian own home is heated with passive solar and the heat is stored in 12" concrete blocks laid on their sides, forming air channels from the south to north ends of the house. This slowly circulates the 65 to 75F air inside the home to warm the blocks cooling the air, or slowly warm the air cooling the slab back to 66F. By allowing his home to stay between 66F in the mornings and 78F after a nice sunny afternoon, he can collect well over 100,000 Btu's of heat daily, and store most of that in the ventilated floor system.

Certainly a active solar system with 200 gallons of storage would help prevent the need to run the backup wood stove, but he does not feel the need for such a system when his house rarely needs to run the heater - even on a 20F sunny day.

Local code now requires at least R-10 insulation under a heated slab floor. Heat transfer into the ground is indeed very slowly, and super insulating between the 50F ground and heated areas is not nearly as important as insulating between 105F outside temp and 75F inside temp, or 0F outside and 65F inside temp. You can have well over 30F temperature difference between the air outside VS only a few degrees temperature difference between ground and inside air temp. You lose significantly more heat to warm air leaving the structure than heat lost through the walls of a well insulated home.

Tom M's picture

asic, thermal storage is just that, storage. if you are an engineer, you should understand that a mass at a temperature say 70 degrees will not radiate heat to a room that is more than 70 degrees. the whole idea of storage is that you store a mass of heat at a certain temperature so when room temperature goes below the storage temperture it radiates heeat into the room. Using wood on top of a radiant floor is crazy. Wood is an insulator, though it will make its way through over time. If you want instant heat, forget solar and go conventional. If you want to save and use solar, gain what you can, when you can, store it so it will provide when solar is not avaliable. Concrete or mass storage bypasses or eliminates tank storage......

asicengineer's picture

I installed warmboard in my addition a few years ago and have been very happy. It does behave differently than thermal mass type radiant and I find the difference to be a good one. Basically, without the thermal mass, the response time is much faster, meaning an hour or two, not a day or two. For solar this means you'll want to put a thermal storage tank someplace, but the upside is much easier control of the heat of the room with just a regular thermostat. No need to have crazy outside thermometers and weather forecasting to predict when you might need more heat pumped into a cement floor. Also no need to have hard tile like with concrete installation. I installed a nice wood floor and because the temps are low, there is no warping or cracking. So, personal opinion, skipping the thermal mass concrete makes for a much easier and nicer system.

Tom M's picture

Aluminum fins draw the heat from the tubing, concrete stores the heat so there shouldn't be an either or when comparing the two. A system that incorporates both should be considered for maximum heat distribution and storage.

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