Does it pay to turn down the heat at night when you have a radiant-floor heating system? David Meiland and a heating contractor are having trouble seeing eye to eye on that question.
Meiland says the house they’re discussing is a slab-on-grade with R-10 insulation below and PEX tubing cast into the slab. In another part of the house, the floor is framed, with PEX installed below in tandem with aluminum plates that help distribute the heat. The boiler is a 30-kW electric model with an outdoor reset.
In a Q&A post at GreenBuildingAdvisor, Meiland writes, “The heating contractor advised the homeowner that setting the night-time thermostat temperature too low would cause the system to work very hard in the morning to get the slab back up to temperature, and therefore waste energy.
“My opinion is that lowering the setpoint at night is a good thing, as the system will put fewer BTUs into the house, saving energy. When the system ramps up in the morning, it will reheat the slab without any loss of efficiency. Fewer total BTUs will be put into the house.”
Meiland thinks the heating contractor has it wrong, confusing a period of high demand in the morning with low efficiency.
That’s the focus of this month’s Q&A Spotlight.
High-mass floors respond slowly
While there is a mix of flooring types in the house, several readers pointed out that the high mass of a concrete slab will not change temperatures very quickly — and that has a bearing on the practicality of turning down the heat at night.
Cramer Silkworth, for example, writes that if the house is well insulated, its occupants may not experience an overnight drop in temperature even with the thermostat turned down. “The slab’s mass combined with a well-insulated envelope may carry it through the night with only a small drop in temperature,” he says.
When morning rolls around, he adds, it will take some time before the slab can be brought back to temperature. “The system won’t ‘work hard’ to bring the slab back up to temperature (it may be at full load, but the heat’s not being lost any faster than it normally would),” Silkworth says, “but it will take some time, such that you’d need to set the system to come on a couple hours before you want the morning temperature reached.”
The relatively slow response to change also can lead to uncomfortable temperatures both in the morning and later in the day, Keith Gustafson says.
“If the setback is too deep and the concrete loses significant temperature, it will take hours to reheat, and the residents may be uncomfortable,” Gustafson writes. “Radiant just does not recover like baseboard or hot air.”
“In the morning,” he adds, “your thermostat turns back on but the physics of thermal mass say you need to put ‘x’ BTUs into the slab before it will rise in temp, and thus allow the room to rise. If you put enough BTUs into it quickly enough to ramp room temperature up, then the mass will cause it to overshoot — and how does having the room at 78 degrees 3 hours after you go to work save energy?”
Dan Kolbert makes the same point. Not only does it take a long time for the temperature to come back, but it puts a strain on the heating system. “One client went away on vacation, turned the heat down to 50,” Kolbert writes. “It took almost a week to get back to temperature, and she had very little hot water while she waited.”
Would better thermostats help?
Writing from the San Francisco Bay area, Monty Worthington describes the trouble he’s had in regulating temperatures in his new house with radiant-floor heating and very basic thermostats. After some tinkering, he’s learned that a 3-degree setback overnight and in the middle of the day seems to work. But he has to program the heat to kick on at 5 a.m. in the bedroom in order to be comfortable two hours later when he gets up.
Meiland says he had the same experience while living in that area in a house with a staple-up radiant floor system. “When the temperature swings were in the 30-degree range, it was necessary to start gaining or losing heat 3 to 4 hours in advance,” Meiland says. “This was a quick-responding system: 7/8-in. fir plank subfloor, 5/16-in. hardwood over, well-insulated underneath.”
AJ Builder suggests that a forced-air heating system has faster response times than radiant-floor heat, but Worthington wonders whether a more sophisticated thermostat would help.
Silkworth had earlier suggested that some thermostats are capable or learning how long it takes the heating system to reach the set point and automatically compensating for it.
Kevin Dickson writes, “The newer Honeywell thermostats can ‘learn’ the temperature response of your house and compensate to prevent overshoot in any house.”
The heating system may be overkill
GBA senior editor Martin Holladay adds another wrinkle to the conversation: in a tight, well-insulated house, you don’t need much of a heating system when the design temperature is a relatively balmy 30 degrees, as is the case with Worthington’s house.
“What you need is a good airtight envelope with high levels of insulation and excellent windows,” Holladay says. “If you have those features, you won’t need much heat at 30°F — certainly not a high-mass radiant floor with a condensing gas water heater. Your internal loads — the refrigerator, lights, and TV — will provide much of your home’s heat. A few ductless minisplit units are all you will need for cool nights.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
First, a bit of a rant on radiant floor heating. While there is no denying the thermal comfort of a radiant floor heating system, the following claims are often made but are just not true:
- Radiant floor heating is more efficient than other heat distribution systems.
- Radiant floor heats objects and surfaces, not the air (this is often cited to support #1 above).
Even at peak delivery, radiant floor heating systems deliver a maximum 60% of their heat by way of radiation. The two factors driving radiant heat transfer are surface area and surface temperature differences. At floor temperatures in the low 90s (F), net radiant transfer, particularly with people (our surface temperatures are in the low to mid 90s), is minimal. And the thermal comfort of the warm floor is more one of [no-glossary]conduction[/no-glossary] to your feet than radiation.
And as a slow-acting, high thermal mass system, any initial air temperature from setback is steadily eroded until at equilibrium, the air temperature profile of a radiant floor heating system is pretty much the same as any heating distribution system and more dependent on the performance of the building envelope.
Just because someone decided to name this system “radiant floor heat” does not mean that the laws of thermodynamics and rules of heat transfer are suspended by terminology.
It’s not that radiant floor heating cannot work well; it’s just that it is a very expensive way to provide space conditioning — and one that only provides heating, not cooling, at that.
With any high-mass heat distribution system, setback can be a real balancing act between thermal comfort and energy efficiency. Sure, setback saves BTUs, but if the main advantage of radiant-floor heating is thermal comfort, the prolonged thermal lag of a high mass heating system in the morning or when you return home in the evening is not likely to be very welcomed.
And depending on solar gain, particularly in the morning, overshoot can be a big problem with nighttime setback of high-mass heat distribution systems. Certainly, setbacks cannot be as deep as with faster-acting heat distribution systems.
We have ramp-up, multi-input setback thermostats for heat pumps, ones that use outdoor temperature readings and patterns to minimize strip heat use. I don’t know if similar thermostats are available for radiant-floor heat, or if the heat pump thermostats can be used with radiant-floor heat, but they are not inexpensive and can be “fooled” by dramatic weather changes.
The long and the short of it: Specify or use radiant-floor heating for its steady delivery of thermal comfort, not because it is more efficient or well-suited to energy savings associated with setbacks.