This is our first heating season with our units.  With it being very cold and windy outside, when our system runs in the 2nd compressor stage, and the temperature gets close to satisfying the thermostat, the system "down-stages" to first stage.  After a period of time second stage engages again.  I assume this is because 1st stage just isn't able to finish satisfying the thermostat.

If this is the case, is it better to:

a) let the system continue running, and switching between 1st and 2nd stages continuously,

OR

b) should I set the DIP switch on the unit to finish in 2nd stage.  This would possibly satisfy the thermostat and shut off the unit off until the next thermostat call for heat.

Which is better for the system, and which is better for the closed horizontal loop in terms of preserving ground heat for later in the winter? And why?

Thanks!

P.S.  If option b:

For our split unit, when the thermostat down-stages, I believe (based on observations) our thermostat would change blower speed to medium even though the compressor stays in 2nd stage.  Would the system running in second stage and having a medium fan speed be a problem, or would the supply air just be warmer?

For our all-in-one unit I assume (no observations) the blower speed would stay on high if the unit stays in second stage despite a 1st stage (downstage) call from the thermostat.


As a side note, I would switch the "finish in second stage" off in cooling mode for better dehumidification.

We always set them to finish on second stage...less wear on the system components and compressor.

Bergy

Whenever we get near design temperature which is 2 degrees for Indianapolis, I kick on the wood fireplace. We have 20KW strip heaters, but I keep those turned off, for obvious reasons. Side note, we live in a hardwood forest.

Thanks Bergy. I changed the dip switch. The all-in-one unit did keep the blower on high after the downstage, and the geothermal system magically turned off after only 1 or 2 minutes after the (attempted) downstage by the thermostat. I forgot to time how long it was before the next on cycle though, (maybe tomorrow.)

heimdm, we too have an alternative aux heat source. My plan is to run our propane fireplace when 2nd stage runs nearly constantly (before electric aux would think about turning on). For us, propane (purchased in the summer) is about half the price of electric aux. I still hope it gets warmer outside soon though.

Any other thoughts/comments?

Though I normally defer to guys like Bergy with much more boots on the ground experience than I, in this case I disagree:

Worst wear on system components is during starts. Long run times do little harm. For analogies consider that lightbulbs fail upon being switched on and most folks prefer to buy a used car that has been driven mostly on highways.

Longer runtimes distribute heat more evenly.

System is typically more efficient in lower stage.

System runs more quietly in lower stage - lower blower speed and power

I don't see a major difference in loop field operation, but I think it would be slightly more efficient in continuous rather than intermittent operation, again using an automotive analogy - driving at a steady 60 mph is more efficient than varying between 45 and 75

So, is it correct to say that a long (possibly non-stop) run with the system cycling between stages isn't any worse for a closed horizontal loop than getting the thermostat satisfied by keeping the system in 2nd stage?

With it being so cold and windy here (temperatures are unusually cold for this area), my primary concern is preserving ground heat for use later this winter. Our EWT was 39.2f a few days ago (3,000 feet of 3/4" HDPE for 5 total tons in SW VA). The printout from the installer show 32 degrees for a minimum loop temperature (but the loop is protected to 20 degrees). Am I worrying unnecessarily?

Yes, probably.

Whether you take the heat from the ground via 20 hours at low stage vs 15 hours at high stage will make little long term difference in loop temp since either way you are extracting about the same amount of heat from the ground.

Oddly enough, you take slightly MORE heat from the ground per unit heat delivered to the home in low stage since higher COP at low stage means less total heat comes through the utility meter, but the difference is small.

Geome,

What's the brand and model of your heat pump?

Bergy

Your loop size is about the same as we use in mid MI. No worries there.
The most efficient system uses the bare minimum to get the job done (the most efficient anything for that matter).
If your weather has been flirting with design lows as ours has, your unit should be running most of the time.
You are set up in 3 stages most people are eager to disable the 3rd yet you wish to disable the 1st from doing it's job. Do you see the contradiction in that?
Let it run.....
Good luck,
Joe

I have a Climate Master Tranquility 4 ton system, just recently my installer replaced the thermostat to the newest model at my request.

My old thermostat would work on differential, the new one works on a combination of differential and time. My thermostat will down stage close to set point.

I find that with this new thermostat it keeps the temperature much closer to the set point, and the house has been more comfortable.

With the cold weather we have had recently the system pretty much runs continuously except when we have a fire in the wood stove.

EWT temps have dropped from about 48 degrees in the fall to 35 at the moment, I think they might go up a little after the cold snap has passed. Night time temps have been 4-12 degrees for the past week, have not seen the thermostat engage AUX. heat yet.

If I set my thermostat to differential, and let it satisfy the set point in second stage, it will turn on again in 5 minutes. So I'd say leaving it run would be more efficient.

John

Ours has been running 2nd stage most of the time when it gets in the teens.  It only shuts down or runs the first stage when the sun is out on the really cold days.  Our system is new last fall so it is our first season.  The Aux heat has been on at least 4 different days for a period of time.  We plan on installing a gas stove as sup./emergency heat.  Can't say I have caught it turning of from second stage as it has been running most of the time.

Since the modulating scroll shifts "on the fly" I see no harm done to the compressor going between 1st and second and 2nd stage.
On the older 2 speed compressors that had to stop and restart between stages excessive cycling can shorten the life of the compressor and lower COP.

Thanks for all the great responses so far. I'm beginning to understand that I should let the unit run.

Bergy, the system is a WF Envision.

The loop is combined for both units, a 3 ton all-in-one for the first floor, and a 2 ton split for the second floor. There are three trenches 250' long with 4 pipes in each trench at 5 and 3 foot depths. If anything, one or both units may be slightly over sized due to rounding. Our installer used 600' per ton instead of the 500' per ton in our other quotes. Fortunately, it was also the lowest quote from a great contractor in the area. I know that this is not a typical situation where more pipe usually equals higher cost.

Joe, yes. Guess I still have a furnace mentality, thinking that the geothermal unit should shut off at some point rather than keep running by constantly downstaging, then upstaging. I expected the downstage when the thermostat was close to set point, but I didn't expect that 1st stage wouldn't satisfy the thermostat and upstage again. I was also thinking that constantly running is "bad" for the ground loop. So for this time of year, do you believe 39f loop temps are ok for us?

Geome,

If the system has a scroll compressor it does not need to finish on second. If you have a bristol twin single, it SHOULD be set to finish on second.

We no longer install Water Furnace so I'm not up to date...The "finish on second" feature was added to help the bristol from shutting down too much. They would shut down between staging and this lead to some premature compressor failures. I wonder why Water Furnace would keep that dip switch setting with the scroll's?

Sorry for the misunderstanding.

Bergy

Bergy, I always appreciate your input.

I would reverse my opinion this in case of a system that has to stop the compressor to effect a stage change. Finish on 2nd would save many stop / start cycles in that case.

Ditto the others where multiple compressors are employed.
No I don't think a 39* EWT is a problem, but I do think you misunderstand how little impact you can have on that. Size of the loop, temperature, sunshine, snow and design all impact it. The only thing you can do to impact it at this point is use more auxiliary heat or dramatically reduce your set point.
j

Thanks Joe. I appreciate the feedback on the loop temp. We turned back the thermostat 2 degrees (from 68 to 66) and used our propane fireplace as a supplemental heat source for 3 days. What you say about impact makes sense and even the measures we took may not have helped in a significant way.

Our performance summary sheet lists 32f degrees as the minimum loop temp. The lowest average loop temperature on our performance bin data sheets is 35f. The loop temp is currently 38.4f.

What happens at 32 degrees? Does the system stop working or become less efficient?

If a person waits until their loop temp is too low, and they have a good amount of winter left, they are stuck with aux. So, at what loop temp (let's use the given figures for argument sake) should a person consider taking measures like drastically reducing set point or using aux more?

geome

My loop EWT is Just below 35F, as far as I know they will operate fine down to 20f.

Last winter mine got down to 30F, so if yours is 39F now you shouldn't even reach 32F.

John