Just heard back from my HVAC guy...he said I don't need an outdoor sensor and aux lockout for a geothermal system. Thoughts??

We always do!

My non-expert opinion:

There is a difference between "need" and whether it is a good idea for you based on your desired variations in house temperature. If you keep the temperature in your house at a constant temperature and infrequently (only a couple of times per week) vary it by more than 2 degrees at a time, then you may not "need" it.

On the other hand, if you regularly (daily) use set backs in excess of 2 degrees, I think the aux lockout could save you money by not having the heat strips come on when the geothermal system could otherwise recover in a reasonable time frame. If you use set backs in excess of 2 degrees when the outdoor temperature is below 40 degrees, I would get a different thermostat with a much lower aux lockout temperature. The saving may pay for a different thermostat.

As an alternative, you could try reducing your setbacks to no more than 2 degrees when the outdoor temperature drops below 40.  2 degrees is an estimate.  Your aux may still come on depending on your thermostat program and how long it takes to recover.

You need to keep in mind Joe's caution. No point in having it save you some money at first, but later have it come back to bite you later in the winter.

Experts - what would you suggest that homeowners with a closed loop (horizontal or vertical) set their lockout temperature to (x degrees over the system design temperature) so as to not have loop problems, as Joe described, in the winter?  This may not be an easy question to answer, but please provide some guidance if possible.

Lockouts on stat are generally in 5* increments. I set thermostats such as the IAQ at next to balance point plus 5.
So if bin report suggests your balance poit is 17* I would set the stat for 25*. I am comfortable that this setting will not greatly impact deep winter performance.
The other important issue at hand is not to inadvertantly void a warranty (by doing things installer doesn't sign off on) just to save a couple killowatts.
Also do not forget that running a heavy compressor for an extra 20 minutes at 3-4 COP is no less expensive than 5 minutes of auxiliary coil at 1 COP.
joe

Joe

I am sorry to hear that you've had so much trouble with time delay relays. I  have not had this experience.

Timed delays are prevalent in the systems already for  compressors, staging aux electric, and thermostats etc. Can you describe the troubles you have had?

We do use setbacks of about 10 degrees at night and during the day when we're not home. I don't know how soon the thermo is turning on the compressor to heat the house back up 10 degrees in time for 4:00 PM. Clearly, if it has to turn on at noon and run for 4 hours, perhaps it would be better to just have aux come on at 3:45 PM.
How would I know?
Isn't the point of the smart thermostat (VisionPro, which I have) that it will adjust the time when it calls for heat in order to avoid aux heat if possible?
If I discovered that the system was indeed coming on hours and hours in advance, even with just compressors, what could I do except lessen the setback?

Yes - you do not want aux involved in setback recovery.

Joe

I am sorry to hear that you've had so much trouble with time delay relays. I have not had this experience.

Timed delays are prevalent in the systems already for compressors, staging aux electric, and thermostats etc. Can you describe the troubles you have had?

-Of course virtually all geo units have some sort of time delay feature in the PC boards and many thermostats as well. I was speaking (in regards to the comments of this thread) about after market, add on time delays. I have had many of these "chatter" or not close reliably, which has caused harm on conventional airconditioners to contactor or compressor (as there is no redundant short cycle protection) as well as not close at all.
If you haven't encountered this then I guess I need to know which TDR you are using.

We do use setbacks of about 10 degrees at night and during the day when we're not home. I don't know how soon the thermo is turning on the compressor to heat the house back up 10 degrees in time for 4:00 PM. Clearly, if it has to turn on at noon and run for 4 hours, perhaps it would be better to just have aux come on at 3:45 PM.

-.....It would be better not to dial it down so far.
I would never encourage a 10 degree set back on a geo system or a conventional air conditioner. These are tightly designed systems and if sized correctly would labor to catch up or not restore temp at all with out auxiliary on a cold day or hot day in cooling dominated climates.
We need to change your mindset on how to best save energy with a geo system.
That sort of usage in my mind would void any sort of operating cost or performance responsibility of the installer.

Joe

What's a guy supposed to do who likes 68 during the day, but below 60 at night for sleeping...SIGH. Open a window?

Within reason, yes. Close the bedroom door, open a window an inch or so and sleep happy.

A smart thermostat, sometimes called adaptive recovery, might minimize aux during the recovery, but the step change inherent in an 8-10 degree setback might overcome that

Try lighter bed clothes, fewer blankets, turn the ceiling (or other) fan on, etc. Before geothermal, we set back 10 degrees. Now that we have geothermal, our setback is 3 degrees (with an aux lockout). Much easier to get out of bed at night (if you have to).

So, with the money I save on not setting back the thermo so much, I can pay to run a ceiling fan. Seems goofy to me. What I need to find out is how much electricity is used to bring the house back up during adaptive recovery. Seems like Geo has been around for decades but the data still isn't conclusive. Does anyone know of any websites that have posted the different electricity uses to compare adaptive recovery on geo with maintaining temperature?

Again your mindset is still on a furnace oversized by 30-50%. The only way it is cheaper and possible to do huge temperature swings is with a grossly oversized system.
If it is a comfort issue for you then pay to run the auxilliary coil. So what if it costs you 50 cents/day, your still going to spend less than you would on a gas furnace. Again auxilliary coil of like btu with a heat pump that runs for 15 minutes uses the same amount of electricity as a heat pump running for an hour at 4 c.o.p. Let 'er rip and catch up quickly. Do not run compressor for 4 hours straight to "save money".
In terms of fuel savings though you propose to do the furnace equivalent of idling the car all night and then flooring it. Adaptive recovery or no you are going to run at full steam (2nd or 3rd stage) for hours in cold weather.
Joe

A ceiling fan set on low speed uses well below 50 Watts in most cases - a pittance compared with heat pump or aux. Operating one that way might give you same perceived room temperature as dropping whole house to 60.

An instant internet answer to your question is unlikely to be either accurate or easy to find owing to huge variances among houses, loops, climates, soils, etc.

I'm with Joe on this. A 10 degree setback is huge, reasonable only for a multiday absence.

I like the car analogy so much that I'll attempt to refine it further: Compare a week of geo operation to a round trip cross country - you could drive 12 hours per day at 60 mph taking short breaks for gas and food. Another approach would be to drive 90+ for fewer hours per day and take long breaks. 90 mph is needed to 'recover' from long breaks, but you can easily guess which method will save lots of gas money.

Our energy star rated 52" ceiling fan uses:
Low - 9.1 watts - 1633 CFM
Medium - 28.7 watts - 3629 CFM
High - 73.7 watts - 6423 CFM

I agree on the mindset comment, and I totally know where you're coming from since I have had a gas furnace/AC all my life until installing our geothermal system earlier this year. Geothermal is a different animal. You may be able to save what you expect and still be much more comfortable at the same time. I gradually decreased our cooling temperature this past summer by 3 degrees. It was much more comfortable in here and the electric bill didn't dramatically increase like I expected.

Compared to aux usage recovering from a 10 degree setbacks, you may spend less and have the temperature more comfortable by not setting back nearly as much.

I've been reading around the net, and come to several conclusions...

1. There is still A LOT of confusion about setbacks and recovery.

Many people try to point out that if the setback is more than a couple of degrees, then aux heat will be involved. This might have been true for older thermostats, but not with the Adaptive Intelligent Recovery. The AIR feature turns the heat on early (for me, stage 1 and 2 are both compressor, stage 3 is aux heat) in order to bring the house up to temperature. It doesn't matter if the setback was 10 degrees, because stages 1 and 2 will be used a few hours ahead of time to bring up the temperature. Aux only will come on during that time if the house isn't warming up as fast as the AIR thought it would, based on previous cycles. Aux would also come on if I mess with the programming and MANUALLY up the temperature more than 2 degrees.

2. There is a lot of variance over the amount of setback vs the time of the setback, probably due to "everyone's house is different", but the overall picture seems to be no more than 8 degrees over 8 hours. So 60 at night by 10 PM and 68 by 6 AM is ok, but not 58 at 10 PM and 68 at 6 AM (10 degrees).

3. I see the point of the car analogy but driving at 90 isn't the same as having aux heat on. The AIR thermostat should absolutely prevent that from happening at all costs. Would it be better to have a system cycling on and off throughout the day to maintain 68, and cause wear and tear on the system, or just have it sit off for 6 hours, let the house cool, then turn on (no aux yet, just compressor) one for continuous run the hour or two needed to get the temp back up?

4. According to one website in Canada, the use of setbacks is highly NOT RECOMMENDED by their geothermal installers. I wonder if that is because our neighbor is *much* colder, or whether they aren't taking AIR thermostats into consideration.

5. I know that every situation is different, but I'm still very surprised there is so little real life data out there. I guess the expense and hassle of getting the logging equipment and posting the results is at issue.

If I was to collect data, what would I do?? Would this (below) make sense?
1. Buy/install some sort of amp/watt sensing device (TED, ie)
2. Alternate days...Let thermo do its AIR thing....the next day, set the thermo to 'hold' a temperature....next day, back to AIR. I wonder if this would mess up the AIR's learning curve, though.
3. Graph the results/compare the results to see if maintaining throughout the day uses the same/more/less energy than setting the temperature back.

6. (one more point of inconsistency) Outdoor sensors. My HVAC guy said I don't need one. He is the professional I trusted to install the system. That being said, reviews are mixed. Some people say the outdoor sensor with aux lockout is an absolute MUST at preventing aux heat coming on unless it's really cold..but again, the adaptive thermostat should be doing that already, shouldn't it? Some people say it's not necessary with smart thermostats, because the thermostat determines how much of the system needs to come on for heat (2 compressor stages, 1 resistance) based on previous "learning" and minute by minute measuring of how fast the house is heating (delta, probably?). The *only* two uses for an outdoor sensor with aux lockout seems to be:
1. if the AIR program, for some reason, can't get the house to temp on time, it could call for aux heat to make up the last bit of difference, and the aux lockout could prevent that if the temp was a certain level. Result? House would not get to temp on-time, but would eventually get there assuming everything else was normal.
2. if someone goes and messes with the program on the thermostat and turns it up more than 2 degrees. Normally, aux would come on at that point since we are "out of the learned program", but an outdoor sensor could prevent that if the outside was above a certain temperature.

If thermostats don't behave the way I described in this post and the previous, someone needs to build a better one! There is no reason that aux heat should/would come on if the thermostat is behaving intelligently and the heating conditions of the house haven't significantly changed (sudden blizzard, doors left open, etc.) even if the temperature during the day and night goes up and down several degrees.

Stuart,

I did an experiment like this on my system last year. I measured outdoor temps in order to have Heating Degree Days. I did one week of setbacks at night and one week with out setbacks at night. I compared the cost per HDD. The conclusion was that the set back did save a little money.

The results are posted somewhere on this forum.

The are some things to consider though. My system ( not designed by me ) is over sized. It will recover from the setback in about 2-3 hours. I have the aux heat disabled.

If a system is sized to cover 95% of the load, and has been set back at night on a design day or below, it might never recover until the outside temps warm up.

If you want to set back your temp at night, by all means go for it. Your point about longer run times is well taken. Just be aware that unless your system is over sized, on the coldest days of the year, your house might not ever get back up to temp.

You can read all about the test and results here

Geodean, mad propz to you for the forum reference and your work (and others) on really trying to produce concrete data! I read with great interest and will be visiting your welserver to see how things are going. I kinda wish I had more 'before' data to help me understand what this system is costing to run, but we only moved in 6 months ago now, so I have no electrical use data to figure out the basic cost, payback, etc. Regardless, the data seems to indicate that is definitely some savings with sensible setbacks and a smart thermostat.
Thank you so much!!