This is way OT...(again), but:
We heat and cool our home geothermally (water to air
We would, of course, like to decrease our costs further if
we can, and so have explored the benefits of setting our
thermostat lower at those times when the house (or parts of
it) are not occupied.
The folks who designed the heating system say that with
these systems, it is best to leave the set temp unchanged.
Of course, I have asked "why", but when I do, it seems that
smoke starts to come out of the phone. In essence, they say
that it is "best" but seem unable to say why.
Might any of you know what would be best in this regard ,
and particularly whether the issue of thermostat setback is
actually any different for geothermal systems?
Cycling a building in and out of extremes will bring into play the
The lower extremes may force the auxiliary heating to start up (maybe
It might be cheaper to keep things at a low wick rather than replacing
lost heat with expensive heat.
Just a hunch.
The need for the aux heat should be minimal at most for an
adequately-sized geothermal system. Some t-stats may demand it if the
temperature differential from setpoint gets too large, though, so it
should be ensured the rampup doesn't force that.
In general, the same rules apply -- a setback at night, for example,
will result in a lower average temperature so the effect is still there.
As a side note, had a ground-loop geothermal system in TN and liked it a
bunch. Am considering it for a replacement here...
Your reasoning is similar to mine...
We do not have any auxiliary source of heat: Our (9 ton
rated) heat pumps are more than sufficient to do the deed
even at 20 below.
For the life of me, I can't understand why the folks who
design the system say it is best (that is, less costly) to
keep the temp constant.
One possibility that I have thought of:
The cooler the water in the well, the lower the efficiency
of (and thus, the higher the costs of running) the system.
Suppose that each night, we allow the temp of the house to
drop, say, 10 degrees F.
Then, in the morning, lots of energy would have to be
extracted from the well in order to rapidly bring the house
up those ten degrees.
That would (obviously) cool the well, thus decreasing the
efficiency of the system, until the house warmed up. As a
result, the costs per BTU would go up during that period of
(relatively rapidly) re-heating the house.
Assuming that reasoning to be correct, the issue boils down
to whether the cost of that loss of efficiency is greater or
less than the savings to be had with the lower overnight
Thanks for any further thoughts,
All depends on the capacity of the well and how the loop is configured.
Unless the well is stagnant and of marginal size and the exchanger is
closed loop, I would expect that to be a minimal problem. If the well
weren't a well but a closed tank, maybe, but that's unlikely to be a
realistic model. Would have to know more to do a real
calculation/estimate, but I think it's not likely such a big issue.
I have been told by one installer here that owing to our very dry
climate there's an issue w/ ground loops and heat transfer. I've not
yet delved into it in sufficient detail to decide whether I think that's
hokum or not -- this guy hasn't yet actually installed a system, he's
just going on what somebody else has told him.
Would be interested in the capacity of the well, amount of exchange
tubing, etc., to support the system you have as a comparison. They
wanted to punch two or three holes here for deep ground loop, but at
$1500/ea, that gets terribly pricy quickly. Would have to have a second
well to go that route, but I'd think it could be only one although it
would not suit me to have it be a once-through in an arid area.
There is no auxiliary heat...
So, whatever heat energy lost by the house is replaced by
the heat energy extracted (at some cost of electricity) from
the well water.
My reasoning was that keeping the house warm when empty
would have greater cost than keeping it cool when empty
(that part seems obvious) and heating it up to comfort would
take less energy than that which would be lost were it kept
What am I missing?
You might want to verify that...
Our WaterFurnace heat pump maintains temperature very well when it
drops to below zero here in central Ohio. But if you raise the
thermostat by more than two degrees at a time, the auxiliary heat
coils in the "furnace" do kick on, and the electric meter spins like
crazy until the house temp is back within two degrees of the set temp.
In our case, the "auxiliary" heat is for quick temperature changes.
This is different than our old house with an air-to-air heat pump,
where the auxiliary heat came into play whenever the outside air was
too cold for the pump to generate sufficient heat. Fairly often, in
other words, since air-to-air pumps lose efficiency as the temp drops.
To use a setback thermostat, we would need one that raised the
temperature only two degrees at a time and/or would need to disconnect
the internal resistance coils. We are satisfied with a fixed
temperature of 69, and heating bills that are a third the size of
friends who have gas heat and homes that a half the size of ours.
Other than repeating it, and assuring you that I know what I
am talking about in this regard, there is little more I can
When we installed the system, we discussed this issue at
length with our contractor, the designer is the system, and
the system's installation folks.
Based upon their input, we opted for a design with capacity
sufficient to eliminate the necessity for any auxiliary
system. In fact, there is a box in the air handler that
would allow for the installation of such a resistance heat
supply, but it is empty.
All the best,
The thing to do is to add an exterior thermocouple to the aux heat
control so the aux heat doesn't come on unless exterior temperature is
at some preset temp. This can eliminate the mostly gratuitous usage.
What we did for the Water Furnace unit we had. (Unfortunately, I had
completely forgotten doing so and when we had moved and the new buyer's
inspection showed up the elements didn't work, I was gone and we ended
up w/ a service call to re-enable them to close the sale. :( ).
Anyway, there was also a setting on the thermostat that overrode the
"high" heat setting that could be used as well. Seems like that
thermostat was an option over the base one that came w/ the unit,
however. It had a setback option built in this worked with iirc,
whereas the other was a simple setpoint t-stat. This is quite a while
back now, memory's getting dim on precise detail.
I agree the units are well worth the initial extra installation cost,
particularly if don't have relatively cheap gas available...
It keeps coming back to the building's phase. It doesn't cycle at the
same speed your internal temperature does.
As you turn down the thermostat, the building's stored heat is given
up, some to the interior, but some to the exterior (loss).
To reheat the building's interior, the heat is not only heating the
interior, but also the building's mass.
So, when you turn down the thermostat, you need to later replace the
heat you lose. Once the building is up to temperature, you just
overcome the building's heat loss...like Lew's Ball.
I was going to try to equate this with the reason why when you
increase the waterflow through your car's radiator by taking out the
thermostat, your engine will overheat. The water HAS to spend time in
the rad to be able to give up its heat. So the thermostat slows down
the waterflow. Conventional thinking would suggest that by increasing
the waterflow, it should cool better. (There are a few caveats in
there too, so everybody keep their shirts on.)
But the time at lower setpoint more than compensates for the
differential loss. It's well-established in general that a setback
lowers overall heating costs in general. It would take unusual
circumstances for that to not be so.
Hi again, OP here...
That is just what I am trying to sort out:
For reasons that I have not been able to understand, the
Geothermal folks say that for their systems, it does not
work that way (and they seem to be consistent on this.)
All the best,
Might you know of some reasons that the general principle of
savings through setbacks would not apply to my geothermal
As I have said before here, I certainly do not (know enough
to) disagree, but I have no understanding of why that should
I don't think it is, in general, any different unless one of the
conditions outlined previously were to be true for a given installation.
I've not looked recently, but when we were looking into it initially,
Oklahoma State was the leading research university on geothermal. I'm
sure there are many others with useful information that a google would
uncover as well. The other place that had a wealth of geothermal
information online back then was also TVA (tva.gov). I would also
expect the EIA and DOE energy conservation web sites to be potentially
useful as well. You might try Water Furnace directly rather than the
local distributor/installer to see what they say -- I found them to be
quite knowledgeable with their evaluation/sizing software packages. It
wouldn't surprise me at all but what they could actually make that a
part of their analysis for your particular system -- of course, being as
it is already installed, they might not want to run another gratis...
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