Using Radiant Floor Heating Systems for Efficient Residential Heating
From ASHRAE Journal Newsletter, June 9, 2020
Reducing heating loads in housing with variable solar gains can be achieved through increasing the south-facing glazing area. However, a risk for overheating exists after a certain glazing-to-floor area ratio threshold is met. Radiant floor heating systems can help mitigate overheating and save energy consumption.
A recent Science and Technology for the Built Environment article discusses several radiant floor designs and their effects on thermal comfort and heating energy requirements in a high-efficiency house in a cold climate.
Authors Sébastien Brideau, Associate Member ASHRAE; Ian Beausoleil-Morrison, Ph.D., Member ASHRAE; and Michael Kummert, Ph.D., Member ASHRAE, discuss the research.
1. What is the significance of this research?
In most climates, one of the ways to reduce heating loads in housing is to increase the south-facing glazing area. However, after reaching a certain south-facing glazing-to-floor area ratio threshold, risks of overheating exist.
Radiant floor heating, due to the thermal mass it often contains, is sometimes thought of as a means to smooth temperature swings. However, if careful design of the floor geometry and controls are not implemented, radiant floors can worsen the overheating problem and use more energy.
This paper discusses various combinations of radiant floor designs and controls and their effects on thermal comfort and heating energy in a high-efficiency home. High south-facing glazing and typical south-facing glazing areas are evaluated. Embedded tubes in thin slabs and tube-and-plate floors are evaluated.
2. Why is it important to explore this topic now?
As codes change, highly insulated windows become available and people want more energy efficient houses, increasing south-facing glazing might become part of the toolbox for designers wanting to reduce heating demand. An unintended consequence of using this approach is potential overheating, even during the coldest periods of the year.
The controls and design of radiant floors is relatively well understood for zones with low solar gains (or other heat gains), but little research was available on cases with high solar gains.
3. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?
The main takeaways from this research were the following:
- For “typical” south-facing window areas, all studied floor geometries were found to provide adequate comfort. However, lighter floor constructions, such as an above-floor tube-and-plate system (a low mass type of radiant floor), were found to provide the lowest energy requirements (up to 8% less energy than higher mass floors). This counters the perception that radiant floors must necessarily be associated with high-mass construction.
- For a “high” south-facing area (about twice the normally recommended maximum to avoid overheating), higher mass floors tend to help mitigate overheating, thus providing better comfort to the occupants (up to 7% improvement in comfort metric compared with lower mass floors). This is especially true if the tubes are located near the surface of the floor. A nighttime setback can also be beneficial with that slab.
- In almost all cases, the best controller was an outdoor temperature reset controller with indoor feedback. This type of controller is not common on the residential market but allowed for less overheating and lower energy demand.
4. How can this research further the industry's knowledge on this topic?
This research shows the importance of paying greater attention to the interactions between the radiant floor, controls and the rest of the building. The optimal radiant floor design for a house with a low glazing area may not be the same as the one for a high glazing area.
Additionally, this research encourages industry to pay greater attention to control strategies, which can have a large impact on system performance. We have shown that improving controls does not necessarily need to involve complex predictive controls, but tuning of basic controllers, or slight improvements of current popular controllers.
5. Were there any surprises or unforeseen challenges for you when preparing this research?
A surprise was that the difference in heating energy requirements between controller/floor geometry combinations can be as high as 25%. This means choosing the right floor type and control combination is very important to reduce heating energy requirements.
Another interesting fact was that nighttime setback slightly decreased the amount of overheating (and increased overall comfort), while also reducing heating energy demand. This shows that a well-controlled radiant floor can be used with nighttime setback.
Lastly, the fact that the temperature reset without indoor feedback did not perform particularly well was surprising to us. Adding the indoor temperature feedback element to this controller vastly improved the controller.