Thermostat for use with Heat Pump

[skipatroller]

I am looking for comments/suggestions/recommendations for a thermostat (programmable ?) for use with a heat pump system.

We built our house in Northern Idaho in 1992 and have a Lennox (2 stage ?) heat pump (heat pump is first stage and electric backup is second stage/emergency heat?) with a Lennox thermostat. Thermostat has 2 vials of mercury on bimetalic springs -- when first vial tips the heat pump is switched on. If the temperature drops to point of the second vial tipping then the Electric resistance heat is kicked in. (We went with a air to air heat pump as natural gas about 2 miles away, and ground source/geothermal heat pumps had a 20 year breakeven point compared to an air source heat pump. We "super-insulated" the house and we also supplement with a woodstove in the Family Room/Rec. Room and wood on the property)

Several years ago I thought about getting a programmable thermostat and went to the HVAC contractor that put in our system and they told me that programmable thermostats were a bad idea for heatpumps -- that you should leave it set at a constant temp.

This year our son is off to college (empty nesters now and empty pockets too with 2 kids in college). During the week, no one is home from 7AM in the morning until my wife or I get home about 5PM. I know that they have some pretty sofisticated thermostats now that can slowly ramp the temp so that only the heat pump is used (no electric back-up is kicked in)

I am thinking that using a programmable thermostat and letting it warm up the house during the afternoon (when the outside air is the warmest so the heat pump efficency is way up) would be a good move.

Like I said, I am looking for comments, ideas, suggestions and recommendations. Anyone have a programmable thermostat on a heat pump?

Thanks

[AllanJ]

What you are probably looking for is a thermostat system that treats the first stage (heat pump itself) as a separate heating system from the second stage (resistance backup/emergency heat).

So you would have different programmable thermostats for the first stage and for the second stage to achieve the different temperature goals and profiles to be accomplished with each stage.

For example you could arrange things so that during some hours the first stage only kicks on at a much lower room temperature than the second stage does.

I do not understand why programmable thermostats (temperature setback during certain hours) are inherently undesirable for a heat pump system. Except thre is probably some outdoor temperature below which the first stage should not be operating at all due to lower efficiency, you would need additional thermostats to cut out the heat pump. Then you might have yet another thermostat that would go directly to stage 2 if the outdoor temperature is too cold for stage 1.

Remember that each thermostatic element, no matter how it is programmed, controls a switch with just two positions: on, and off. Time and temperature must be used to determine when and how long the on position is sustained.

[suemarkp]

That is the main reason -- your aux heat, if electric resistance, is much more expensive than the heat pump heat. If you want to use setbacks, you need to get a thermostat that has some intelligence between the stages and even better also has some programmable temperature lockouts (but this requires an outside temperature sensor). Also, a properly sized HVAC system takes a while to bring the temperature up. If you've set back 10 degrees, you're not going to get that back in a few hours witrh most heat pump systems.

Most heat pump thermostats are kind of stupid and look for a difference of 3 degrees or more to jump to the aux heat stage. The Carrier Infinity (or Bryant Evolution) thermostat is great and gives you a lot of control. But it only works with infinity/Evolution systems. Even with this great system, I still only set back 2 degrees at night and when I'm gone.

I know there are generic thermostats that will do what you want, but you need to make sure you have enough wires from the T stat to the air handler. How many do you have? If you go with a simple programmable thermostat, you'll need to limit the setback temps to 2 degrees difference or less.

I know I've seen the topic discussed here, so you may want to try a search:
http://hvac-talk.com/vbb/forumdisplay.php?forumid=1

They aren't DIY friendly though, so be careful with what you ask and hope you can find it was already asked and answered.