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Rob Dumont

posted Nov 1, 2010, 5:03 PM by ASHRAE Saskatoon President
What is your opinion about radiant floor heating for houses?

There are positives and negatives:
On the positive side, most people seem to like that type of heating, which almost always consists of pipes carrying a warm liquid placed in the floors. To distribute the heat from the pipes to the floor surface, a concrete layer is poured around the tubes; alternatively, heat fin plates, often of aluminum, are attached to the pipes. Various floor coverings such as ceramic tile or thin wood floors have been used as the top surface. Although not ideal, radiant floors can be covered with wall to wall carpets. (A higher water temperature will be needed to drive heat through the insulation that the carpet provides.)
Here are some of the advantages:
1. The radiant floors generally have a warmer floor surface temperature, which makes the floor more comfortable in the heating season. However, in a very low energy house not much space heating is required, and the floor will generally not be very warm.
2. There is an absence of noise from creaking radiators or convectors, and there are no drafts from forced air ducts.
3. There is no smell of burnt dust from higher temperature heating surfaces, and special cleaning of the floor is not needed.
4. Compared with conventional forced air systems with conventional fan motors, the electric power to drive the liquid pump is often only about 1/5 that of the power to distribute an equivalent amount of heat by forced air.
5. With some boilers or water heaters, the same heating source can be used for both space heating and domestic water heating.
6. The water temperatures in the radiant systems can be low, and with the right kind of boiler the low water temperatures can result in higher efficiencies. (The lower water temperatures can even allow the boilers or water heaters to condense–a good thing provided the boiler or water heater is designed to handle the condensation.)
Many homes in Europe use radiant floors. I have heard that in Germany radiant floors are the system of choice for new homes.

On the negative side, there are a number of challenges with radiant systems:
1. Generally there is a substantial cost premium to incorporate radiant floor heating into wood frame construction. The concrete topping, aluminum plates, heat distribution pipes and manifolds used for the floors make for a more expensive system than a forced air system. Concrete and aluminum are also relatively high embodied energy materials compared with wood. In a multi-storey house, you generally also need some insulation beneath the radiant floor systems for good temperature control, and that insulation adds to the cost.
2, The extra weight of the concrete topping can add costs to the structural system for the house. A 38 mm (1.5 inch) thick layer of concrete around the floor piping adds a dead load of about 15 to 20 pounds per square foot. This is significant relative to the live floor loads of about 30 to 40 pounds per square foot for typical residential floors.
3. There is no ventilation air distribution with the radiant heating systems; a separate ventilation system will be needed to ensure good indoor air quality. Forced air systems do not have that disadvantage, as the ducts can carry ventilation air as well as air for heating (and cooling.)
4. A well-insulated and house does not need a lot of space heating; as a consequence the floor surface temperature will not be very warm and the "warm floor" effect will be small. For instance, to provide 3 kW (10,200 BTU/hr) of heat to a well-insulated house with 100 square metre (1076 square foot)floor area with an indoor air temperature of 22 C, the floor surface temperature needs to be only 3.6 C [6.5 F] higher than 22C. This elevated temperature would only be needed on the coldest day of the yar.
5. Inexpensive condensing boilers with a long track record of durability and longevity are not yet available. A number of builders have used residential water heaters, but the efficiency of typical water heaters is not very high compared with condensing equipment. I have heard of severe condensation occurring in some water heaters that are used for space heating with radiant systems. The problem is that the return water temperature from the floor system is cold enough that continuous condensation occurs in the flue gas chimney of the water heater. The condensation drips down and splashes on the gas burners, creating carbon monoxide and potential air quality problems. As with any house using combustion equipment, I would recommend installing a carbon monoxide sensor. Some water heaters are certified for use as both space and water heaters, and these are a better choice.
6. Night setback of temperature in the house is more complicated with the concrete topped radiant floors, as the thermal mass of the concrete delays the temperature fall in the house following setback, and prolongs the amount of time for the house temperature to recover.
7. Temperature controls for radiant floor heating systems tend to be more complicated; sometimes the systems use an outdoor air reset temperature to vary the fluid temperature in the pipes according to outdoor conditions. Untrained repair people can have problems diagnosing such a system.
8. Radiant heated concrete floors don’t work all that well with direct gain passive solar heating systems. On a cold night the radiant floor will have to be warm to deliver heat to the house. When the sun comes out and starts delivering passive solar heat, the house thermostat will call for the pump to stop delivering heat to the radiant floor. Even though the heat flow is cut off, there is a lot of stored heat in the concrete floor, and that heat combined with the passive solar heat contribution can overheat the space. I noticed this phenomenon in a newly built commercial building here in Saskatoon with a radiant floor and a large passive solar aperture. In February there was so much heat available from both the radiant floor and the passive solar contribution that the staff had to open an outside door, because the air conditioner was not set up to run when the outdoor temperature was cold.
9. It is generally more difficult to incorporate air conditioning or cooling with a radiant floor system. Some work has gone on with passing chilled water through the piping systems, but there are questions about the efficacy of that approach. A possible problem is that the floor surface temperature can go below the dew point temperature and mold will occur under carpets, furniture, cardboard boxes, etc. I would proceed with extreme caution with chilled floors, particularly in any location subject to high humidity.
10 Radiant floor systems can be very expensive to repair. One example I have heard of is an older radiant floor system in a church near Vancouver, B.C. To save money, iron pipes were used in the radiant heating system under the extensive ceramic tile floors. Several of the iron pipes have started leaking after about 20 years, and a major cost will have to be incurred to replace the entire floor system.
11 Some types of plastic piping used in radiant floor systems have had problems. At least two plastic piping systems have had major class action lawsuits associated with leaks.

For the house that my wife and I designed, I looked into radiant floor heating, but decided against it. There was a substantial cost premium, and that cost along with the fact that we wanted hardwood floors which are not very compatible with leaking pipes made us choose a forced air system.

In general I prefer to see dollars spent on improving the building envelope through better windows, thermal insulation, and air sealing rather than on heating systems. All of these expenditures on conservation will also improve thermal comfort.

However, for those who have the money and inclination, radiant floor systems can be a satisfactory heating system. They have the best cost-effectiveness when used with concrete slab-on-grade construction in buildings that don’t need air conditioning. Just don’t skimp on the underfloor insulation and be very careful about solid concrete thermal bridges to the outdoors. The property of concrete that makes it a relatively good conductor of heat from the radiant floor piping also makes it a very good source of perimeter heat loss from concrete slabs.