The cool nights are upon us here in central PA and it won't be long until I'll get to use my new Tranquility 27 for heating the house. My question involves setting back the heat during the day and at night. When I had propane heat I always set the heat back to conserve energy and save my bank account. I've heard that I probably shouldn't set back the heat on my geothermal system because it may cause the auxiliary heat system to kick on which could eat up the savings of setting back the daytime and night time temperature.

So, should I set back the thermostat or should set it and forget it? If I should set it back, how far can I drop the temperature without the auxiliary kicking on when the timer kicks in to recover mode?

This really depends on the features of your thermostat as this is what controls the auxiliary heat.  I have disabled my auxiliary  heat and now set back my thermostat.

I probably should talk to Joe about this. It seems at night if you can make it kick down so it doesn't turn on the auxillary (night being colder and thus reducing your margin between full geo and geo+auxillary) you could get a pretty good savings.

The problem with a large set back, summer or winter, is the signficiant heating / cooling of the structure and its contents that occurs, in addition to obviously the inside air temp.

I experimented this summer with a number of setbacks, ranging from 0 to 10° (F) of setback.

I found that there's a point within this range that optimizes costs.  For me it's about 4° of setback.

It was pretty interesting to see how much the structure/contents heat up with large setbacks, and how long it took to then remove the heat from the structure/contents.  And removal of the moisture requires more cooling too.

The ultimate experiment was when we traveled for 2 days.  We set the tstats to 90°, thinking that we'd hardly run the HVAC during this time period (Dallas location).  This was indeed true.

But, it took a good couple of days to recover.  And the HVAC KWH spent recovering was more than the KWH saved.  It was a good experiment to go out to the extreme to see that there's an 'economical set back point.'  I now use 4° as my 'economical set back point.'

I would think that this 'economical set back point' would vary as a function of geographic region.  I.e., in a northern climate, and assuming aux heat is disabled (as Dean notes, above), it might take a long time to recover from a 4° set back.

To experiment to find the 'economical set back point' would probably require some way to track HVAC KWH consumption.

Best regards,

Bill

Any setback should reduce heat gained or lost by a structure and thus reduce system load and cost to operate. The only way I can see that "the HVAC KWH spent recovering was more than the KWH saved" would be if HVAC system efficiency (EER or COP drop) is much lower during recovery. Aside from the obvious need to avoid heat strip operation during recovery in heating mode I wouldn't expect that to happen. If anything, longer cycle times during recovery should slightly improve efficiency.

Then I got to thinking about how a closed loop system operates: Excessive cycle on times during recovery could push a closed loop (Entering water temps) out beyond its design temperature. That would cause system efficiency to decrease, possibly substantially. Whether that loss of efficiency exceeds savings from reduced load during setback would have to be calculated on a case-by-case basis.

Bill - is this the phenomenon you encountered during your experimentation with setbacks?

My thermostat is a Honeywell Vision Pro (TH832U I think).

• Dewayne-
  How many degress do you set your thermostat back?
•  Can I aslo disable the auxillary with this thermostat and then kick it on if needed?

I was originally thinking of a 4-5 degree set back. I guess I can start with that and I find if the auxillary heat is kicking on either check to see if its affecting my energy bill or eliminate the setback.

This was my experience when I set the thermostat for the geothermal on vacation mode for 3 days.  I think I set it for 85.  When I returned from the trip and turned off the vacation mode, it was the next morning before it felt comfortable.

I followed up with my HVAC guy about the proper temperature setting during the day when I am at work and he didn't think I should set the thermostat above 78 during the summer in North Texas for the same reasons Bill cited above.  This is probably not accurate but my impression is that a house that does a good job of keeping out the heat in the summer must make it a little harder to remove it.

Regarding Bills comment about tracking HVAC kwh consumption, I have been using The Energy Detective with the "Footprints" software interface and am pretty happy with it.  The receiving unit itself holds 13 months of electrical usage stated in kwh and dollars.  The software interface has some quick and easy graphs that can show usage by the hour and I can download the data into Excel.  For ~$200, I see it as a pretty good value.  Their website is www.theenergydetective.com. 

I set mine back from 70 to 65 in the winter and 75 to 80 in the summer.  I disconnected the "W" wire in my thermostat as this is what turns on the auxiliary heat.

Perhaps.

I don't disagree that theoretically heat gained should equal heat loss.  But, my observation, after the 2 day period of the house empty with tstats set to 90°, was that my cooling KBTU per day (heat extracted from the house) didn't return to a normal pattern until after 2 days.  And on an emperical basis, it looked like it required more KWH to recover than what I saved.

It was obvious, though, that if the house were to stay sitting at 90° for 3 or more days, then there's a cost savings.  But it looked at best 'a wash' at 2 days.

Influencing all of this I believe are many factors.  The outside temp wasn't the same each day.  And since my house is not very air tight wind pattern/intensity each day will make a difference.  And my closed vertical water loop isn't sized to handle a large load on a continuous basis, so I'm sure EWT went up higher than normal.  And I was trying to cool the house back down during hot parts of the day.  And I'm sure other factors I haven't thought of.

All in all, I've settled in on 4° setbacks, no aux heat (don't even have it installed), and needing more than just a couple of days to justify setting back all the way to 90° in cooling mode.

Best regards,

Bill

I use two tools.  One has already been discussed here in an earlier thread, the Web Energy Logger (WEL) (see http://www.welserver.com/ ), that put's the information automatically out to the Web (my data is at http://welserver.com/WEL0043/ ).  It's a more expensive option that the excellent suggestion offered above by 183eej, at about $800 (less in an 'a la cart' form), including the power toroid current sensors and the pulse-based KWH monitor.

The tool I use is simply an ammeter hooked around a wire.  Pretty easy to get an immediate reading, and to move it around to different wires.  Very convenient for example at a circuit breaker box.

Best regards,

Bill

As soon as we can predict the weather, we can predict savings from set-back. This question has come up more oftn lately with, folks who never expected to buy geo. most recently a nice couple who we cost up at 64 degrees as they would shoot for 68 during the day and 60 at night.
I submit three things.
1) If it's more than a few degrees of recovery you probably aren't saving much.
2) If you activate the auxiliary heat it probably costs more.
3) If you aren't comfortable 1 and 2 don't matter.
We know geo is less expensive to operate, so be comfortable; but wisely so. If you enjoy cooler temps at night, set your thermostat to recover 1 degree an hour to avoid activating the auxiliary coil-or turn off the breaker to the aux heater in the milder months.
J

Comfort is indeed the issue. Why do we forget that? We all hover around this efficiency argument/issue. If you've already spent the $ to install the geo, you can now afford to be comfortable. You're not worried about spending thousands this winter to heat the home.

A wonderful option/accessory/capability of geo is its ability to be on "auto-changeover" mode. Set the temp. at what's comfortable and LEAVE IT ALONE! You're talking about possibly saving a couple of pennies per night. Why not just be comfortable? And if you think you like to sleep with it cooler, just set it 2 or 3 degrees lower, then, as Joe says, set the t-stat to do 1 deg. per hour to avoid the electric heat. However, you'd most likely find that leaving it the same temp. will still be comfortable while sleeping. Maybe you won't need such heavy blankets now, too.

I agree with you Clark.  With a heating and cooling bill that is averaging $40/month, any percentage cost improvement I could squeeze out of a thermostat set back could be measured with a micrometer.  And the "set it and forget it" feature was a nice surprise I didn't know I was getting.  Is that a feature that is unique to geo?

My thought on it was at night when the outside temperature goes up, is the most likely time that the back up heat will kick on to make up any difference. If dialing it back enough degrees kept the geothermal from kick on, it might save a bit. I'm also wondering if (possibly specific to my region) it would make sense to switch from DTE's heating rate in the winter to interruptible space conditioning in the summer -http://my.dteenergy.com/otherInformation/residentialRates.html if it's even possible. My biggest concern would be if the geothermal systems can be safely interrupted that way - I don't want to bust 20K+ equipment to save $100-200 a year at best. It does look like being on time of day useage for the system it would make sense for both cost and comfort to cool down to 66-68 the A/C in the summer at night and then let it reach back to equilibrium in the mornings at 72 or so when peak useage kicks in.

The negative of a lower interruptible rate during cooling season is that you are likely to get interrupted during the hottest part of the day when everyone else has the AC on full bore.

I did it in suburban Philly years ago - no cooling from 1-5 PM on weekdays. It worked during heat waves so long as I remembered to set the stat real low before leaving for work - house 'stored' coolness. The rate break was substantial and I was single at the time.

Wiring it into an interruptible panel wouldn't be hard - what the utility does NOT want you to be able to do is switch back and forth as it negates the effect of shaving their peak.

My device already has to be on it's own panel. They have a time of day rate around here for separately metered devices that heat the home like geothermal (or an electrical heater, which apparently would be cheaper to run than a 92% efficient propane heater at current rates). They give the device about 5.46 cents / KwH plus .67 cents per billing day between oct. to Nov. . Versus the 10.5 / KwH they charge for a normal home (first 17kwh per day, 11.9 cents per KWH over 17). The rate, however, goes up to 10.685 / KWH plus .67 per billing day in the summer (5.25 cents less during off peak).
My understanding is the interrupt is supposed to be at most 15 minutes per hour at most. Not sure if I'd want to do it for a couple reasons. 1. My wife is stay at home. 2. If the cycling does any damage to the equipment (saving a couple hundred a cooling season while breaking some very expensive equipment, not worth it).

On the auto-changeover, most any heating/cooling system COULD be set up that way. Any heat pump should be set up that way, especially geothermal.

We have both time of use and utility interrupt. I talked them in to interrupting my signal wires from my T-Stat to the geo. So the geo unit just sees its call for heating or cooling shut off. The funny thing is I have it set so far out it would never run "on peak" or during a demand so its fine with me if they want to shut it off.

AFAIK, disconnecting power to an HVAC system for 15 minutes every hour should do no harm as long as the makes and breaks are quick and clean. Much of the time the compressor cycles on and off several times per hour anyway, particularly in the case of a zoned system.

A 75% duty cycle (off 15 minutes every hour) is an interesting concept. System wide, the utility gets to cut its peak load by up to 25% of the connected interrupted load - likely very useful for them to shave peaks and avoid buying excessive really expensive peak power from either distant or high cost generators. This justifies a substantially lower kwh price.

This situation offers a challenge and opportunity for a competent HVAC contractor. In order to meet building load he or she should effectively derate the selected unit by 15-25%. One should assume that the contractual maximum interruption interval will occur on design (or hotter) days since that is when the utility's grid is under the most stress, so the HVAC system will be off 25% of the time during very hot afternoons.

Steps I would take to allow my customer to both avail themselves of a substantially lower interruptible kwh rate and stay fairly comfortable during interruptions:

1) drop the indoor design temp a degree or two to accommodate slightly lower thermostat settings during hours of 15 minute interruptions - this allows the customer to let the mass of the house itself to store a bit of extra cooling to tide over the 15 minute off cycle. I don't favor generic indoor design temps - the customer should be carefully questioned as to thermostat setting preferences since a few degrees difference in indoor design temp substantially affects cooling load calculation. Beware that customers with a stated preference for lower thermostat settings at night may be reacting to an existing system oversized or poorly ducted so as to provide poor dehumidification.

2) Slightly oversize the system to accommodate the down time While it is normally an anathema to oversize a system, the challenge and potential economy of an interruptible rate suggest certain limited compromises. Ensure ductwork is sized for the slightly oversized equipment.

3) Educate the customer as to inability to precisely match thermostat setpoint during periods of interruption.

4) Advocate strongly for a 2 stage system to minimize effect of oversizing (better dehu at part load)

5) Consider inclusion of a separate minisplit unit not on the interruptible panel in the largest room (or wife's stay-at-home office, whatever the case may be) to keep comfortable during peak interruption days.

This all may seem to be quite a bother but the economies of an interruptible rate are considerable and this sort of thing is the wave of the future as energy costs rise, supplies tighten. Price signals will increasingly be sent to consumers in ther form of demand, time-of-day, and interruptible rates.

To throw in a little local experience, our local utilities have been disingenuous at best regarding interruptible service. While they promise no more interruption than 15 min/hr and no more than 3x's/day, we have seen 15 on and 15 off for hours, running in a hot spell ,so often that (after several not enough a/c calls) we'll only install interuptable service if H/O agrees to install an extra ton of cooling to speed recovery of short cycled system.
J