When I purchased my geothermal heat pump almost four years ago, the installer convinced me to "pay a little more" for a programmable thermostat. They explained that it would be more efficient. I could turn it back a few degrees while I was asleep and have it automatically came back up before we get up. Shortly after it was installed, I had to contact the local Climatemaster distributor about some issues I was having. The distributors rep told me not to use the programmable fuction. He said that a geothermal unit is designed to hold a specific temperature and not to change. Who was telling the truth? Where can I go to find specific testing data?

What issues were you having?

I set back 5 degrees every night, but I make sure our auxiliary heat doesn't engage during recovery.  Using auxiliary heat to recover may diminish (or possibly exceed) the cost savings from setting back.  Be aware, recovery times can take a long time.

Cost savings (if any) will vary depending on various things.  Maybe Doc has some data to share.  Personally, we are more comfortable setting back, so we set back.

It depends on when you setback and how much.

The most efficient systems are designed or sized to maintain the temperature at design temperature, even to use some supplement heat when it is very cold outside. Then of course response time is none existing. We advise against any setback or at least not more than 1-2 degrees. People follow their old habits, but usually from the second year they convert and they set it and forget it, and enjoy the comfort of their systems.
I don't even discuss those things with the customers. They get an advanced 3 stage programable thermostat standard, usually the honeywell 8300. Sometime I put the ecobee in if they would benefit from remote controlling. Again, I simply put them in if I think it is a good idea. Things change a bit with more proprietary controls, e.g. 7 series Waterfurnace etc.

Due to incremental capacity cost, a GSHP is (or ought to be) sized to match the design heat loss closely, with little or no excess capacity, so that the unit does the job all or nearly all the time, going to auxiliary heat only in extreme cold. While this minimizes cost, what is seen as a downside to folks accustomed to rapid temperature recovery provided by a grossly oversized heating system is the longer recovery times of a GSHP in very cold weather. Many (most?) units are two-staged, so that the unit can operate most of the heating season in first stage, at a higher COP. Output in first stage may be only 65-75% of maximum; the CM TTV026 unit is about 75%, according to their tables (I have one). For peak heating demand, the unit must upstage to maximum output, and if that still is not sufficient, the auxiliary (electric heat strips) also come on.

The control issue is when to decide that the unit should upstage. This is not a function of the GSHP unit itself. It responds to input signals on terminals Y1, Y2, and W in a multistage unit. No signal on Y1 or Y2 while W is energized is used for "emergency heat" mode, when the unit has failed and the compressor should not run. Making decisions on when to upstage is the function of the thermostat or, for a zoned system, the zone board, or sometimes both; the two must be set up to work together properly.

The control scheme used for making decisions, usually selectable, is to upstage either after a certain amount of time without satisfying the thermostat (or too low a rate of recovery) or when the measured temperature is a certain increment below setpoint. For example, the thermostat could be set to turn on the unit in first stage when the temperature is one degree below setpoint, upstage to second stage at two degrees below setpoint, and to turn on auxiliary strips at three degrees below. If night setback is used, and the temperature drops say 5-8 degrees by morning, then an upstage likely will occur with the first scheme sooner than needed (impatient thermostat) and right away (both second stage and auxiliary) with the second scheme. This is why some GSHP owners disable the electric strips, leaving them for truly emergency use by manual intervention. This also is why some do not use setback programming at all. In the case of a very well insulated and tight home, overnight temperature drop may be only a couple of degrees much of the winter. Since savings is a matter of how much the inside-outside difference is reduced and how long it spends reduced, that cumulative small fraction reduction times very little time reduced times a heat loss that is very low in the first place renders setback not worth the effort - set and forget.

Recognizing the inherent problem with using control schemes designed for oversized heating systems on slow-recovery (properly sized) GSHP units, some thermostats (I believe I read about this recently) have different options for more gradual recovery, to enable longer recovery time without upstaging needlessly. Perhaps others can provide detail.

I know that there are some thermostats such as 1f97-1277 or equivalent that have an energy mode that when set, will gradually ramp the temperature. ie if you set back 3 degrees at night and then warm the house back up 3 degress by 7am, it will start hours ahead ramping 1 degree at a time until it gets to the final temp at 7am.
It has definitely been proven that using 2nd stage is less efficient so ramping 3 degrees all at once will make you bills go up .
What they don't tell you when they say to set it to one temperature and leave it alone is that there is the comfort factor.
People want it cooler when they are sleeping.
I used these in almost every install and they work great.

I tell homeowners to never set back more than 3 degrees at night or when they leave for work during the day. I also tell them that anytime it gets below 32 degrees(our design temperature is 24) that they should use no setback because of the long recovery times and the possibility of auxillar heat coming on.

With the efficiency of a geothermal system today(up to 5.3COP rated) there are little savings from setbacks, but chances are the programmable thermostat also has other features that a non programmable would not.

We have also had several problems with the ClimateMaster thermostats. I think they are made by Honeywell?Had several G signals fail this year and a couple O signals...its a real shame. I am looking for another one I like. I use HBX for Radiant and Venstar for our W-W hydronical air coils...

We have also had several problems with the ClimateMaster thermostats. I think they are made by Honeywell?Had several G signals fail this year and a couple O signals...its a real shame. I am looking for another one I like. I use HBX for Radiant and Venstar for our W-W hydronical air coils...

One other draw back to night time set backs in my area is folks get off peak electric rates at night. Dialing down at night and recovering during higher electric rates is the worst possible scenario for operating cost.
Dick Russel and Sky describe best that geos are not designed to recover huge temperature swings. If you lower temperature for nightime comfort, then understand it may cost you more.

We like it cooler upstairs at night also. The Honeywell 8000 has 4 time slots for temp settings. Downstairs, zone 1, it's kept at 70 all the time, dropping to 68 at night. It's set to do 68 at 10 pm, 69 at 6 am, and 70 at 8 am, and 70 at noon. This way the temp never bumps up more than a degree at a time. Upstairs, we like it a little cooler at night so it's 67 at 10 pm, 68 at 5:30 am, 69 at 7:30 am, and 70 by 9 am. We have an infant son, and he naps at all hours, so maintain temp upstairs is necessary. We've found on the coldest days so far, the system runs stage 1 (it's a 4 ton 2-stage plus aux geo for about 2200 sq feet total) 90% of the time, and kicks in stage 2 occasionally during the temp changes. In the past, I used to do 5 degree setbacks, and aux would engage every morning. No longer!

I believe the Honeywell has a smart recovery feature that seeks to prevent use of aux strips via ramping, as has been noted, but Dick, Sky and Joe have it right, IMO.

If I had low night rates, I'd set it for 75 and sleep atop the sheets, and then coast through the high rate window as far as I could.

This thread should be a sticky.