Put a smart thermostat on a heat pump
7 January 2006
A. You’re right. You’ve made the correct conclusion about heat pump operation that unfortunately confounds many heat pump owners and keeps the controversy of heat pump efficiency versus comfort at a boil. Controls are an integral piece of the puzzle. By the way, one of the settings on the controls is marked Emergency. Setting it there will take the outside portion of the system completely off line and you will be heating by electric resistance heat only.
If heat pump controls are set in a normal fashion and someone comes along and pushes the temperature setting up more than two degrees, the auxiliary heat will come on to assist the heat pump to arrive at the desired temperature quickly. What that basica lly means is the heat pump part of your heating system, which is commonly described as an air conditioner working in reverse, needs help from an electric furnace device, which is like a big toaster grid of resistance heat strips inside of the air handler , to heat things up to the desired level in a reasonable time. The reason one wants to avoid relying upon the resistance heat to such a rapid, more than two degree temperature elevation is resistance heat is quite expensive to operate in energy usage in comparison to the cost of using the basic heat pump operation.
Most garden variety heating system programmable thermostats are just timers like an alarm clock. You set it at 65º Degrees from 10PM until 5:30 AM. When the alarm goes off at 5:30, it just jacks the setting up to wherever you've set it to go-- say 72º- - and the furnace cycles on and by the time your feet hit the floor at 6:30 the house is toasty warm.
You can-- and many have-- place such a programmable thermostat on your heat pump system and the perception of the operation of lowered night heat with a morning warm-up will be there but it actually costs you more in energy usage than if you'd left the h eat pump set at the daytime temperature all night long! That's because when the signal from the thermostat comes to start things up, it thinks it's going to a gas or oil furnace and goes right to the higher setting causing the auxiliary heat to come on and stay on until the desired temperature is achieved and stays there.
The key is the words "Heat Pump" in the programmable thermostat’s name. It must be specifically designed to elevate the temperature setting-- and to wait for it to achieve it-- one degree at a time. That means if you've turned the setting at bedtime to 65º, the heat pump programmable thermostat has to figure out how many hours early it has to start working on raising the temperature to get it to the morning setting.
It has to be the type to which you refer to do you any good. Other manufacturers make heat pump specific programmable thermostats but you have to be sure they’re compatible with your system. Unless you are very sophisticated in these matters, I don't su ggest that installing such controls is a do-it-yourself project. I've seen some poorly rigged control messes installed by blissfully ignorant homeowners who take great offense at the suggestion that their handiwork is counterproductive.
When I'm walking around a house with a new home owner and the house has a heat pump system I ask them if they have ever lived with a heat pump before. If they haven't-- or even if they have-- I recite a little speech about how heat pumps are designed to be set at a desired temperature and then left alone and that the little lever on the top or side of the thermostat is not a gas pedal to be stomped on when you want to change things fast. Jamming the setting up to 80º when you really want 72º won't make it get there any faster. It doesn't work that way. That goes for gas or oil heat, too.
If the temperature in the room with the thermostat does not achieve the set temperature within a reasonable period of time-- say a couple of hours-- then something is wrong. And it's not necessarily the heat pump that's the problem. I was in a new waterf ront home the other day with the wind blowing outside and the system just couldn't keep up-- even with the auxiliary heat cooking away. What was wrong was that the painter had removed all the weatherstripping from the three water facing double door sets to paint them and hadn't replaced it yet. A strong frigid breeze was blowing in around the edges and centers of all of those doors competing with and defeating the heating system, while the owners were yelling at the heat pump.