The new house is superinsulated, 4,000 sqft of conditioned space, and a design heat load of 21,000 BTU/hr. The heat pump arrived today, a CM Tranquility 27 two-ton. The size is right for the heat load, but controlling it requires some thinking. I've been heating it this winter with a small woodstove, with output close to what the heat pump will deliver. I've noticed that the house does not cool down without heat or heat up fast with heat. The small difference between the stove output and the heat loss through the shell is quite small relative to the thermal mass of the house.

The house has three zones for the air distribution. The zone control is handled by an EWC Model UZC4. It has a fair amount of ability by itself to control staging of the heat pump and auxiliary heat, and it can work with various thermostats. I've been looking at thermostats that also could control the heat pump, with two stages of compressor and two of auxiliary (a small one for supplemental heat if required and a big 5 KW one for emergency heat). They also can control the Climadry coil operation on the heat pump, to get dehumidification without overcooling.

Control of heat pump staging is done by various methods. One is to upstage (too often via auxiliary, running up the electric bill) when the set point is so many degrees above actual, as when recovering from overnight setback. Another is to upstage after a certain number of minutes at one stage. None of these really seems appropriate for a superinsulated house with a "correctly sized" heat pump (meaning not grossly oversized). For one thing, it simply isn't appropriate to do overnight setback in such a house. Even without heat, the house typically won't get down anywhere near a usual setback temperature. Worse, that adequately sized heat pump could take multiple hours to regain that lost two degrees (two is what I've observed on bitter cold nights over this winter).

I'm looking for advice on control strategy and thermostat selection. I want the system to be patient and let the heat pump run long enough to do its job efficiently. I don't want a "usual" upstage strategy that will bump me into second stage or turn on auxiliary needlessly. I don't really care about how fast the house regains a single degree. Any thoughts?

Dick,

I have a Climatemaster ATP32U02 thermostat. It keeps my Tranquility 27 running nonstop at the setpoint or about 1 ˚ below it for days without upstaging. This is somewhat annoying me, because if I want to a get to the set point when the first stage heat output is just about the heat load, I have to change the setpoint! I choose the temp differential between stages. That model was replaced with ATP32U04. The new model can operate in temp differential mode like mine or in proportional integral mode. "The Proportional Integral (default) option will use a combination of
temperature differential and operating time to determine the appropriate
heating or cooling stages for operation. This algorithm may be configured
to allow the heating or cooling outputs to “de-stage” as the demand is being
satisfied, or to keep all activated heating or cooling stages energized until
the demand is fully satisfied, like the Differential algorithm does. " The quote is from: http://www.climatemaster.com/downloads/ATP32U03.04.pdf I am not familiar with your zone control.

If you figure out how to run Climadry in the first stage, without voiding warranty, please let me learn.

Regards,

Masoud

Posted By DickRussell on 21 Mar 2011 07:36 PM
 ... when recovering from overnight setback. ...

How much of a setback are we taking here? Generally Geothermal systems are most efficient when you just leave them at the same temperature all the time. While it is possible to see some modest savings with a small setback (with AUX heat disabled), larger setbacks are not recommended.  You shouldn't be using the same kind of setbacks, that you would do when you have a gas system, since the recovery is so much longer. Geothermal is a gradual heating method, unlike a gas system where there is an instant blast of hot air when the system kicks on. If you house is so well insulated, I would recommend just leaving it at one temperature all year round, and forget about setbacks all together.   

A 4000SF house with a 21kBtuh design heat load is likely best controlled by the very complex and sophisticated patent-pending SIAFI comfort algorithm:

(Set It And Forget It)

I don't think the same setpoint year-round is satisfactory for most folks...winter heating setpoint is typically 5+ degrees below most folks' summer cooling setpoint

If you can find a stat with configurable upstage temperature differential, grab it and set that parameter for 2*F.

I recently came across that feature on a very cheap builder-grade non-programmable stat...I just can't remember the brand / model now.

If the purpose of the set-back is to save money, then leave it alone and comfort yourself with the knowledge that you spent money to give a 4000sf home a 21kbtu heat loss.
If it is for comfort then who cares if it costs a few pennies to catch up.
If it is to win the "I pay less than you to heat my home" contest, then do the morning recovery with the pellet stove.
There might be a thermostat available that can do exactly what you want, but most work by some sort of timer or differential that isn't going to let a recovery take hours. If you consider that one in the thermostat business is supposed to deliver desired temp, it makes sense that it is going to be hard to prevent it from doing so.
j

I do not see anywhere in Dick's post that he expressed a desire for set-backs. In fact he commented above:

"For one thing, it simply isn't appropriate to do overnight setback in such a house. Even without heat, the house typically won't get down anywhere near a usual setback temperature."

Both my tstat and the new version have user adjustable differential temp (from 1˚ to 4˚) between stages. Humidity control differential is fixed at 2%, a narrow band, resulting in frequent compressor cycles in 2nd stage, for Climadry.

Regards,

Masoud

Masoud, you have it right. I won't be setting back at night, as there would be no point in doing so. For that matter, I don't even need a programmable thermostat, since nothing will change from day to day. Set and forget.

After spending a lot of time reviewing the EWC zone control and thermostat manuals, I have settled on the CM ATP32U04 thermostat, which was on the list from the distributor in the first place. I had a phone conversation with the EWC tech support on this to confirm some things. The CM tstat will give me flexibility to delay upstaging for up to two hours in [edit:] proportional mode [not differential mode], and it can turn on electric as third stage if it wants to. I doubt it would have to do that unless two hours isn't producing enough heat with just the compressor to keep me close to set point. Or I can use differential mode; if the differential is met so that it want to upstage then it probably needs to do so. We'll see. Anyway, the tstat lets me do a manual emergency mode, which I gather energizes W1 but not Y1 or Y2, which the zone control interprets as emergency mode. It then will lock out the compressor, turn on the first electric heat unless already on, and three minutes later will turn on the second strip. I'll just move wires so the small 1 KW strip in the duct is activated as third stage and the 5 KW installed in the cabinet as second level of electric. If things aren't going well after third stage is on, something is wrong with the heat pump and I can do a manual emergency mode through the tstat. That ought to work.

If the tables in the heat pump documentation are close to reality, I'll have about 20% excess capacity above design heat load. That will gradually fade as the heat exchanger gets fouled over time until it needs cleaning. Considering the long time it takes for the house to respond to changes in outside air temperature, with the thermal mass of the house damping things considerably, I may never even need third stage. Time will tell.

Masoud, I'll be following that Climadry frequent cycling issue this summer. In theory it ought to be a good feature for this particular house, but how well it does the job in practice remains to be seen.

" I don't really care about how fast the house regains a single degree. Any thoughts?"

"Regains" sounded like a set-back thing to me, but I do tend to get glazed over eyes in long posts.

My observation is that the heat pump in this house is "grossly oversized", so little 2nd stage should be required and almost no aux.
Instead of changing the stat, one could install toggle switches for 2nd stage and aux activating them only when needed.
j