By, Ryan Carda 
and Doug Carruthers
Many a thrifty homeowner has reasoned that it will require less energy (and therefore cost less) to adjust their thermostat settings when they are out of the house or sleeping. It makes sense that when they aren't in the home, there is no need to keep the temperature at the perfect 70°F so in the winter, they set it back to 60°F.
In fact in 1995, the EPA established technical specifications for programmable thermostats that included guidelines for this practice, known as 'thermostat setback'; as part of the Energy Star program because...
"The average household spends more that $2,200 a year on energy bills- nearly half of which goes to heating and cooling. Homeowners can save about $180 a year by properly setting their programmable thermostats and maintaining those settings."
| Setting | Time | Set Point Temp (Heat) |
|---|---|---|
| Wake | 6:00 am | <70°F |
| Day | 8:00 am | Setback at least 8°F |
| Evening | 6:00 pm | <70°F |
| Sleep | 10:00 pm | Setback at least 8°F |
And they are exactly right... when you follow their recommended schedule (shown right) and you are using a conventional fossil fuel system, thermostat setback is an effective means of saving energy and money.
But, for geothermal systems, the conventional wisdom does not hold.
Why It Works for Conventional Heating Systems
A conventional heating system, like a natural gas furnace, is typically oversized to meet the needs of the conditioned space. A fact we discussed in more detail in the Three Most Common Mistakes Made by Geothermal Designers. The truth is that from a cost perspective, a furnace rated at 60,000 Btu/hr and one rated at 30,000 Btu/hr are only a couple hundred dollars apart.
It should also be noted that, unlike with heat pumps; there isn't a significant decrease in the operating efficiency of a conventional system when they are oversized. Ultimately, it really doesn't make that big of a difference to the initial price or operating cost to install a MUCH larger system than needed when a conventional system is used.
Which is why thermostat setback is so effective with fossil fuel burning equipment. The oversized equipment with all of its extra capacity is able to quickly heat up the air, the floor, the walls, furniture and everything else in the space to recover from the recommended 8°F setback when the homeowner returns. Plus, with the higher relative supply temperatures provided by fossil fuel systems, the homeowner can stand over a floor vent like a campfire if they are really cold.
Why It Does Not Work for Geothermal Systems
Geothermal systems are specifically designed to meet the needs of the conditioned space so they are not able to quickly recover from setbacks. To meet the required demand and recover from a setback period, the geothermal system will most likely need a supplemental heat source like an electric resistance heater or a back-up fossil fuel furnace. When the setback period ends, these supplemental systems are called into action and, all of the pennies saved from using geothermal, are eaten up by the cost to operate the secondary system.
The key to maximizing the operating cost savings of a geothermal heat pump is to allow it to do as much work as possible without help from secondary systems. Thermostat setback forces the geothermal system to fall back on supplemental systems more often and so increases the average cost of operation for the entire heating and cooling system.
Why Not Oversize the Geo System.
If geothermal systems were designed to be as oversized as conventional systems, no one would ever want to install them. The initial cost would be much higher than necessary because for every Btu/hr of capacity your equipment is rated to handle, there is a connected amount of loopfield that must be installed. Which means oversized systems, require oversized loopfields and a couple of extra 200ft bores don't come cheap. Which is the primary reason that geothermal systems are designed to meet the space needs much more closely than conventional systems.
Also, oversized geothermal systems aren't as efficient as properly sized systems because of the nature of how heat pumps work. Heat pumps of any sort operate more efficiently if they are able to run for longer stretches of time and achieve a nice consistent set of operating conditions. An oversized heat pump will complete its job too quickly and will suffer from what is known as short cycling, which doesn't just decrease system efficiency it also decreases system life.
From a comfort standpoint, if the homeowner uses a thermostat setback strategy with a geothermal system, they will not be pleased. For the reasons already mentioned, the geothermal system will take longer to bring the house up to temperature. The less apparent issue for many homeowners is that geothermal systems have lower supply temperatures so camping out over vents doesn't provide the instant gratification of a fossil fuel furnace or even resistance strip heat.
Basically, oversizing a geothermal system is bad no matter how you look at it.
Leave It Alone
At this point you can come to one of two conclusions, either geothermal is a bad choice because you can't use thermostat setback or (and this is the right one) thermostat setback is a bad choice for homes using geothermal. Thermostat setback with geothermal is unnecessary and detrimental. The are a few big things to remember:
- Thermostat setback is a work around; a home hack if you will, that came about in an effort to make a conventional system cheaper and more fuel efficient.
- Geothermal is already more efficient than any conventional system and the estimated $180 saved annually with conventional systems using thermostat setback is only a fraction of the operating cost savings the homeowner sees every year because of their geo system.
- Using thermostat setback with geothermal WILL cost more money than maintaining a constant temperature.
The long and the short of this whole discussion is that a home using geothermal should have its thermostat set to always maintain the desired temperature for occupancy... and then the buttons should be removed. The best part about geo is that if done properly, the homeowner could forget that they even have a thermostat because they are just that comfortable.
Realistically, no one will ever leave their thermostat alone entirely. Adjustments need to be made for parties, mothers in-law, a kid with the flu and life in general but, as a general rule, if geothermal is installed in a home, the best advice for the thermostat is to... Set It and Forget It!