It sounds like your thermostat turns the auxiliary heat strips whenever you have more than degrees between the ambient and set temperatures. You should install a thermostat with an outside temperature sensor so you can lock out the back up heating. The Honeywell Vision Pro is one example, but there are others. I would hope your ground source heat pump would be able to keep your house warm down to zero degrees.

Your load calculation came out to 3 tons, but the builder decided you should have a 4 ton unit. What was the cooling load calculation? In a air source heat pump, the size is determined by the cooling load. I assume the guide lines are the same for geothermal.

It seems your geothermal heat pump is using a lot of energy considering you have an average well insulated house and you keep the temperature at 66 degrees. When was the last time you had the system serviced?

That electricity usage is the whole house, including hot water (though the desuperheater helps in the summer). I thought system was sized for heating load in colder climates like ours? We have the system serviced once a year, in August. Change the filters every 3 months.

We went with the contractor who bid the smallest unit since we had a 4 ton, a 4.5 ton, and 1 contractor called for a 2 and a 3 ton, 1 heat pump for each zone. Cooling load came out to 27169 Btuh after accounting for latent cooling and equipment loads. We keep it 75 during the summer, when it's muggy we'll set it to 72 at night.

We do have outdoor sensor, Honeywell IAQ t-stats. Since we don't do much of a setback we don't see the Aux Heat kick in, except when it was below zero a couple of weeks ago. But with a 15-yr old washer/dryer (and a teenager) I think we use a lot of hot water.

The system needs to be sized for cooling otherwise it will short cycle in the summer and have poor humidity control.

"Since we don't do much of a setback we don't see the Aux Heat"

The amount of your set back should not determine whether the auxiliary heat is activated or not.if you are above the lock out temperature.

What outdoor temperature do you lock out the auxiliary heat?

There was no lockout set, I just set it to 50 degrees about 10 days ago. Left no lockout on HP, b/c with COP that high I assumed that it will always be more efficient to run the HP than the Aux. But 50*F may be too high a lockout for Aux Heat - how low should I set that?

I'm assuming our electricity usage doubles from May to Feb due to increased cost of heating water, not only is groundwater getting colder but DSH isn't doing much of anything to preheat it by Feb. Fan and compressor running more then too, even if Aux Heat doesn't kick in (we also see rise in dog days of summer, but not as much since it's just fan and compressor not hot water then)?

I suggest setting the lockout at 30 degrees and see if you feel it is comfortable. If the supply feels warm and the house can recover from the set back reasonably fast, then you can consider going lower. A lock out of 50 degrees is effectively no lock out. I am surprised your installer did not set this up for you.

I have no experience with geothermal, but I would be disappointed if you could not take the lock out down into the 20s. Some owners of the Carrier Infinity Greenspeed have reported very low lock out temperatures and it is an air sourced heat pump. I don't why a geothermal heat pump can't give the same performance.

I still don't see how to do Man J calcs with air temp ("DB") instead of EWT, LWT. Installer took the HP lockout default of 40*F (!) off and left it no lockout, but also set no lockout for Aux Heat (I think no lockout on that may be a default setting on Honeywell IAQ?).

I don't remember if system was calling for Aux Heat often, and what outside temp was, when we were trying to use 3 degree setback. But since we changed to 1 degree setback, the only time I've seen Aux Heat called for was when trying to come up that 1 degree while it was -8*F outside.

So do you think my hypothesis about fan & condenser running more in deep winter (avg temp below freezing), plus trying to heat colder water in Feb, contributes more to kWh usage during winter than the Aux Heat does? I don't mind turning lockout down to 20 instead of 30 - heck, we leave the breakers off when nighttime lows aren't below freezing yet. Not quite sure I want to risk having them off when lows are single digits (or negative) though...

Climatemaster and Honeywell don't give any guidance on lockouts.

I don't think there is much difference in heating water February versus. At least not enough that would be noticeable.

Your auxiliary heat strips have been turning on unnecessarily. Having no lock defeats the purpose of having a geothermal heat pump. You were minimizing your set back just to avoid having the heat strips turn on. Your installer has made several mistakes in my opinion.

The thermostat manufacturers can't give guidance to setting lock out temperatures. They have no idea where you live, the insulation quality of your house, and how big is you system. A default of 40 degrees is a conservative. It is still better than no lock out.

When do you think the Aux Heat was turning on, with only 1* setback? I haven't seen it come on. But we do hear a lot more air blowing through ducts when it's cold out (runs more frequently, not sure about longer).

I did put Aux Heat lockout to 25* tonight, but can turn off breakers for real test. Problem is lows in single digits expected midweek, I'm not sure I want to leave breakers off then, can do it the next couple of days when lows only going down to about 20* though.

Oh, and the 40* default lockout temp was to lockout the HP, not the Aux Heat! Glad the installer changed THAT default setting, our bills would have been through the roof with HP not running any time outdoor temp dropped below 40!

This post was edited by ajsmama on Sat, Jan 18, 14 at 21:53

Just had meter read on the 17th, usage went from 60.12 kWh/day last month (avg temp 33.1) to 69.29 kWh/day this month (12/18 - 1/17) when avg temp was 30.6 - but that included nights below zero as well as days in the mid-high 40's the past couple of weeks.

Since I just set the lockout to 25*F a day after the meter reading, we can see what effect that has - though the lows (heck, even the highs!) are predicted to be below that starting tomorrow, so there may actually be no effect (if the cold weather keeps up as usual at the end of Jan through mid Feb reading, I expect the lockout will make no difference and usage will climb into mid-70's kWh/day).

I'll report back on next month's meter reading.

Even with lows at 0 & single digits (haven't seen negative numbers again, though they were predicted), I haven't seen the Aux Heat running. Air blowing through vents (on first floor) is 80-81 degrees - I haven't seen it get hotter than that even checking multiple times per day whenever I hear the fan blowing.

Since the lockout for Aux Heat has been set to 25, I figured it would be kicking in, but since it's not, I assume that isn't the reason for the higher electric bills when average temp drops below freezing (not quite 7 winters in the house yet). Will report back on meter reading mid-Feb but as reported above, it did go up from mid-Dec to mid-Jan.

Usually about 75-77 kWh/day used when temps are 27-31 degrees, the one time we had 82 kWh/day was an estimated bill, average temp 22.5, though the meter reading the following month wasn't much lower since the temp then was 27.8.