About a year ago we completed construction on a weekend home on Long Island NY that is heated and cooled by a open loop geothermal system.  The system works great but I have a question on the thermostat setting.

We leave the temperature at 55 F during the week in the winter and an hour before going out on Friday I turn the temperature to 70 to have the house warm when we arrive.  The problem is this turns on the auxiliary heating coils and defeats the efficiency of the system.

The question:  is it more efficient to leave the temperature at around 66 F from Sunday evening to Friday evening and move it up only a few degrees before arriving or is the way we are doing it more efficient?

Thanks,

--Vince

This is a tricky situation... tricky in that you may be saving by leaving it 55 for 5 days or so, rather than just a night set back.

The first idea that comes to mind for me is putting an outdoor sensor on the electric heat, and setting it to balance point temp. Then let this be the determining factor as to whether the electric heat gets to come on.

If this or some other control logic can't be easily set up to keep your electric heat from coming on in your situation, you would probably be better off leaving it at 66 like you say.

As a comparison/side note... we have a church building in the area that has 47 tons of EarthLinked DX. Typically, the LDS church sets the unoccupied temp. down to 62-65. In the two buildings with EarthLinked (each 47 tons retrofitted), they maintain 68 deg. unoccupied, and bump it to 69-70 throughout the building in occupied. By comparison, a similar building, at the same elevation, 15 miles away, spends an extra $9,500 per year in energy, even with the 62-65 setback.

The point is that geothermal is so inexpensive to maintain, that it's not really worth having to have the electric heat supplement a big temp. change from unoccupied to occupied.

Do you know the heat load loss of the home and the capacity of your heat pump(s)? Does the auxillary heat ever come on when you are out on the weekends?

There is a experiment going on right now on another thread to answer at long last the effectiveness of set backs over roughly an 8 hour period .
I would say when this experiement is completed it would be very interesting to you
I have absolutly no doubt that using setbacks for 5-6 days at a time is saving you money even with use of aux to pick the temp back up
some may disagree , I think Im pretty much solo on that one
Once we have some what of an idea on how much set backs (if any ) save over an 8 hour period it would be eisier to take a stab at how much you could save ( taking into account your high KWH rate )
Then with that rough estimate and the cost of how much someone like me would charge you to put in an outdoor temp sensor
and figure a ROI
It may only be costing you an extra 2 bucks a time that you jack the temp up depending on the size of the aux strip
If the controll cost 2-300 or more thats allot of weekends

Interesting question.

Other folks are in the process or researching daily setbacks, and they'll likely learn that savings are fairly minimal. A week is another story - it would pain me no end to have to leave a system turned up for 5 unoccupied days per week. That said, there is certainly the issue of a long aux run during recovery wiping out setback savings.

I wouldn't want to disable aux entirely, if for no other reason than its role in maintaining freeze protection in an unoccupied home, as well as needing it while there on very cold days.

I think I'd look for some way to partially disable Aux - an 'away' switch to keep it off unless needed for freeze protection - say via a temperature switch. One could program recovery to occur on geo only (give it plenty of lead time) and then upon arrival set aux for normal, that is running as needed to maintain setpoint during cold weather.

It occurs to me that your combination of open loop and un occupied periods make set-back and disabling aux heater an okay idea. A 7 day programmable thermostat would let you start raising temp 10 or 12 hours ahead of time, aux. could be turned on once you arrive to make up the difference. The usual consequence of not enough heat without electric coil is unlikely if 55* is all you desire while it is off. Many of the newer stat's can even learn how long average recovery takes and do some self adjustment.
Might want to check with installer before changing operation.
Good luck,
Joe

Vince,

Do you have a remote controlled thermostat "to move it up ... a few degrees before arriving"?

If you have a programmable stat, maybe you can set it up to start early on Friday for recovery from setback in several stages (events) in 2-3 degree increments, smaller than what it takes to trigger aux heat. Depending on your thermostat, perhaps you can keep back aux heat by adjusting time delay and/or temperature differential for electric heat.

Regards, Masoud

Good point , I assumed there was a smart t-stat installed allowing adjustment over the internet .

Yes, I am able to adjust the t-stat over the internet.  I can move it up a few degrees at a time but that is a drag.  There should be t-stats that move up in 2 degree intervals for these types of systems.

--Vince

No,  the only time it goes on is in the morning because I set the temp to 64 at night because I like to sleep when it is cool.  So when it comes back on the Aux goes on.

--Vince

The emergency heat is the aux coil.  If electricity goes out for a long period, well I have a problem.  My plan is to get a natural gas backup generator, but that will not happen for a while.

I am reluctant to turn off the aux, because it is my only backup in the event the system goes down.

I am surprised that they do not produce a thermostat that gradually goes up and down in 2 degree intervals, and easy thing with digital thermostats.

Thanks,

--Vince

You can program in 2 degree bumps with some or get one that "learns" recovery time. Of course this is all impacted by the weather. If you can access stat by internet, hook elec coil to emergency only (break aux. terminal connection) so that it can be remotely activated if your homes temp alarm notifies you that primary heat is down. A simple toggle switch would let you employ it as aux. while you are there.
J

Honeywell 9000 IAQ stat is the best! It has a lockout of your auxilary heat in 5* increments from 5* to 65*.

What is wrong with the idea to program the thermostat in steps, like going to 57 in the first hour, 59 in the next hour, and so on, so the aux heat never reaches the 3 degrees temp differential where it calls for the 3rd stage