Using a programmable thermostat with a heat pump
user-1088294
| Posted in General Questions on
We live in NC and have a heat pump for our heating/cooling. I can’t seem to find a clear answer on whether or not using my programmable thermostat will save energy. If I set the temperature back at night during the winter, the emergency heat comes on in the morning when the temp is ramping back up to it’s normal setting. This really makes the meter spin! Are there programmable thermostats made specifically for heat pumps that would avoid this problem?
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It depends a lot on the particular heat pump you have.
If you heat pump is oversized and 1-2 stages rather than a continuously variable/modulating version (you'll know if this is you), using a programmable set back will save in energy and wear & tear...
...unless it has resistance electric elements to back up the heat pump in heating mode. Which sounds like YOUR situation.
If by "emergency heat" you mean it has electric elements that kick in on long recovery ramps, you would be much better off with a "set & forget" strategy. Even when it's 25F outside the heat pump is at least twice as efficient as the electric elements. With all else being equal you might save 5-8% on the overall heating energy use with a set-back strategy, but if even 1/4 of the recovery ramp heating is from the resistance heaters, you'll be using more energy going that route.
If it's possible to turn off the resistance elements (or put it on a switch, for use during actual emergencies, such as when the heat pump portion needs repair) a set back strategy can save energy use, but you will have longer recovery ramps. Using a "smart" thermostat that "learns" your system, the length of the recovery ramp doesn't much matter- the thermostat figures out when to start the recovery ramp based on recent system behavior during the same part of the day. The smart-phone lovers seem partial to the Nest smart-thermostat (https://nest.com/ ), but there are other smart-thermostats out there- some wi-fi capable, others not. None can control the resistance elements directly, but many heat pumps can be modified to work without the internal resistance heating.
With any ducted-air heating system sealing every duct seam and register boot with duct mastic, and caulking between the boot and sub-floor/wall-board/ceiling gypsum is a good place to increase system efficiency, so long as the ducts are correctly sized for the coils and air handlers. (A duct undersized for the air conditioning can end up with frost-bound coils if the ducts leak. The right solution in those cases is to size both the ducts and HVAC units correctly, but that can be an expensive proposition as a retrofit where it was screwed up on day-1). In heating mode you don't have that issue, so go ahead and seal up the ducts now- if it causes cooling season issues there are better and more efficient work-arounds than simply letting the ducts leak randomly.
A lot of better thermostats have a setting to tell the thermostat that it is a heatpump - which help with control.
Also with the electric heat you could take it off the HP control - and add some standalone control very easy. Just need to wire the control signal through a low limit type control switch aquastat or freezestat. (Dynacon FS-33 for example) Depending on the circuitry a fan proving airflow switch might also be recommended (Cleveland Controls AFS-222 or similar)
I believe that Dana provided some great advice.
We operated our HP just as Dana described with the emergency resistance elements disabled and with an adaptive thermostat.
I suppose if you just disabled the elec heater; you could re-enable it anytime you are going to be out of town for a long period of time for emergency backup heat.
I'd like to clarify some misinformation here. Maybe I'm just somehow misreading Dana's post (he usually makes good points) when he wrote "None can control the resistance elements directly". Actually, all thermostats compatible with heat pumps control the resistance heating elements directly (there is a terminal and wire that controls them)..
If the thermostat isn't very smart, then setback can be a bad idea for heat pumps for the reasons already pointed out. But there are a variety of approaches that smarter thermostats use to recover without using the resistance heat and so energy savings can be captured. Some thermostats also include a control to shut off the resistance elements entirely above a certain outdoor temperature (using either an outdoor temperature sensor or internet weather service) and separate "strip heat cut-out" controls are available to perform this function for units without the feature. This control can save energy by avoiding the use of resistance elements caused by small changes in set point during milder weather (many thermostats will turn on the resistance heat any time the set point is 2F greater than the current temperature).
Thanks for the clarification Michael!
I don't get to deal with old-school heat pumps very often in my neck of the woods. Back in the mid '80s, in the Pacific Northwest was the last time & dealt with disabling the resistance elements on one of those, and the resistance heaters were definitely NOT under the direct control of the available thermostats of the time. I s'pose in the intervening 25+ years things might have changed... just a little bit? :-) Mea culpa!
Hi Rob, I live in Central NC and have found that the best approach here is to set back one degree for every two hours you're away in the daytime and set back 1-2 degrees at most at bedtime. That way, recovery in the warmer late afternoon/evening shouldn't kick in the resistance heaters, unlike in the early morning when it is coldest.