Are programable thermostats helpful with geothermal systems?

I live in west central MN (half way between the Twin Cities & Fargo) and will be building a new SIPs home this spring.  I'm looking at geothermal vs. air source heat pump for my HVAC.  The plan is forced air for the main floor & radiant heat for the basement.  I'm was thinking about trying NEST programable thermostats.  However, one of the HVAC contractors I talked to mentioned that with geothermal, you don't want to have the thermostats changing temps throughout the day because this will make the system less efficient.  Is this true, or would a programable thermostat provide additional cost savings similar to when paired with a traditional furnace?

In a heating dominant climate like MN, is it worthwhile to install a geothermal system with SIP construction?  Or would it make more sense to use the money saved by not installing geothermal on other areas of the house & rely on the SIPs for energy efficiency?  We're planning to live in this house for at least 15-20 yrs, so that helps the return on investment factor.

Any insights would be appreciated!

Thanks!

Joel

It depends

For 3 yrs I have not used the programable feature of my stat with GSHP.

Deep set backs can possibly bring on electric resistence (AUX) heat trying to get back up to set temp. I would think that is the same with air source or ground source HP's.

ChrisJ

Even for those of us who don't use setbacks or "7-day programming," it sure would be
a treat to find a thermostat that isn't totally brain-dead in the aux heat department.

I've tried three spendy t-stats, so far, and their internal [pardon the expression] "logic"
seems to be based entirely on simple-minded algorithms incapable of noticing that
the heat pump is doing fine on its own, and the temperature is moving briskly toward
the desired setting.

Instead, they go into panic mode and call for aux heat the moment they detect a small
"droop" -- even if it was caused by a manual change-of-setting, just 30 seconds ago.

I have also considered trying a 2nd generation NEST, but unfortunately, their web site
is saturated with dumbed-down marketing fluffy-talk and devoid of useful information...

...in accordance with HVAC industry standards,

Looby

Joel, a properly executed SIP house will be very slow to lose heat, even in a MN winter. A properly sized GSHP or ASHP system won't be able to flood the house with heat quite the way a traditional furnace would, as the latter typically is grossly oversized for the job. Accordingly, a heat pump installation likely will take a lot longer to recover from a nightly setback when it's really cold outside. Many thermostats are set up to upstage, eventually to electric strip supplemental heat, if the set point has not been reached within a certain length of time or the temp is not recovering fast enough. The thermostat also might be programmed to go to second stage if the actual temp is say two degrees below setpoint, and to electric strip auxilliary if the difference is some increment (eg 1 degree) below that. So you can see where the contractors you spoke with talked about reduced efficiency if you change setpoints often. Engaging straight resistance heating (COP=1) always degrades overall COP and is something to make sure doesn't happen without good reason. Thermostat setup is important for any heat pump system.

There are apparently some thermostats now that address the staging issue by allowing the heat pump to bring up the temperature more gradually, without upstaging if the unit can do it.

Then, too, consider that the benefit of programmed setbacks is the reduced heat loss resulting from setback times time spent at the reduced temperature. A very well insulated house loses heat so slowly that the cumulative reduction in heat loss is minimized. This is an argument for "set and forget" in a very well insulated house. Given where you are, make that house tight and very, very well insulated as first priority. Then almost any heating system will cost relatively little to operate. A heat pump can be a very good long term strategy for new construction, as the system will be small.

One of our Prestige thermostats selects stages based on the degrees I enter (Version 2.0), the other (Version 1.0) has 'intelligence' to manage staging.

I rarely have Aux heat enabled on our thermostats. It's easy to go into the Prestige installer setup and 'tell' the thermostat that there is no Aux heat. Before doing this, make sure this is OK with your installer.  You may not have Emergency Heat if the heat pump locks out.

Daily setbacks do not provide much savings, if at all, with efficient houses and GSHPs. Plus you scarify your comfort.

1) Why don't you go radiant all the way? You are investing money to create those hybrid houses, where you need a w-w heatpump and air handlers anyway, and then you take a knee at the 5 yard line. Air handlers need relatively high water temps (105F or higher) to put out any heat, but you can design the radiant supply temp for 85F or less. Efficiency gain between 85F and 105F = 30% for the life of the system


2) ASHP in central Minnesota? How is this going to work?


So you wonder about setback, but your whole approach here has inefficient design features, impacting the system efficiency much more.

Efficiency gain between 85F and 105F = 30% for the life of the system

The % savings in using radiant varies widely depending on outdoor temperature (edit: if you use outdoor reset as you should) and with source temperature.

Setback certainly saves money, but only to the extent that the house (or office) actually cools down. As houses get better insulated, they cool down more slowly. If you want to do setback, consider a larger buffer tank + hydronic radiant to store up enough excess capacity to heat back up quickly (without aux heat).

The savings of the operation at a certain supply temperature are independent of the outside temperature. A buffer tank is always needed in an hydronic application unless you use only zones which have high enough flow rate to satisfy the needed flow rate of the heatpump.

1)  I'm not sold on using gypcrete radiant heat for the main floor & 2nd story.  Not much expertise with local contractors.  Plus I will still need an air handler & ducts for A/C.

2)  ASHP are being used more & more frequently in MN.  2 of my neighbors have them currently.  I don't know any specifics on their systems, but I know they have been very happy.  They both use dual fuel systems for supplemental heat in the winter.

As far as thermostats go, it sounds like the general consensus is programable/learning thermostats aren't that important with very tight homes like SIPs.

1) Why would you embed it into gypcrete, it would just increase the response time of the radiant system. A top of the floor application would do much better. Unless you are pouring the floors anyway.
2) I don't see the benefit of building a dual fuel system, significantly increase the upfront costs without giving you the operational costs benefit.

If you do radiant half hearted, it is better to not do it at all. Get good forced air system which can handle your whole house and enjoy it. Don't invest into all the radiant setup and only do the basement floor with it.

"2) ASHP are being used more & more frequently in MN. 2 of my neighbors have them currently. I don't know any specifics on their systems, but I know they have been very happy. They both use dual fuel systems for supplemental heat in the winter."

The information thus far offered suggests you have not determined the specifics of much for your new home. It also suggests that in and of themselves SIPs automatically create a high efficiency envelope......and you ask about setting back a thermostat.

Before I wrote "it depends" which would normally spark the question "on what?"

2 different contributors here have demonstrated that setting back their geo saved money. There are reasons it may not be true with your home.