We heat and cool our home geothermally (water to air system.)
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 building's phase. The lower extremes may force the auxiliary heating to start up (maybe electric?). It might be cheaper to keep things at a low wick rather than replacing lost heat with expensive heat.
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...
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 warm continuously.
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 temperatures.
The fellow you are talking to may not be able to tell you! One point to consider is you can "stress" the ground by pulling a bunch of heat out of it at one time. If you leave an area colder, then bump it up
5-10 degrees the heat pump will run longer than normal and pull an abnormal amount of heat from the ground. Once the ground gets too cold, the equipment does not as efficiently also. Same with air conditioning, but then you are putting heat into the ground.Generally with heat pumps they are slower to heat up the home than say gas or electric, so the equipment runs longer to get to the occupied temps. I am with your heating contractor, leave the temperature constant, unless you have an area of the home you can close off and leave cold for many days. I would not bother to set back the temperature once or twice a day like you might with electric or gas heat. If you want to save money, wear a sweater, and turn the temps down a couple degrees. Greg
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.
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 do.
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.
Your explanation makes sense to me (and is essentially what I offered to someone else in this thread) but...
Though, indeed, the efficiency of the system decreases as the temperature of the well goes down, could that loss of efficiency compensate for the significant savings we would have if we were to drop the temp of our house by, say, 10 degrees for 8 or 10 hours each day?
(Though there may be parallels to industrial applications, I'll stick with home heating for my example.)
For a given outside and inside temperature, the house loses a constant amount of heat per hour, and that amount must be replaced if we are to keep the internal temperature constant.
If the internal temperature of the house is allowed to drop, two things happen. First, there is the direct energy savings because it takes fewer BTUs to keep the house at the lower temp; but perhaps less obviously, the rate of heat loss to the outside environment is decreased. (Because the greater the temperature differential, the more rapid the rate of equalization.)
So, for the eight hours or so that the interior temperature was lowered, there are savings for two reasons: We are providing less heat to the house, and we are losing less per hour of what heat we do supply.
When we decide to go back to the original interior temperature, at every stage (prior to reaching that temp) the hourly rate of heat loss is something less than it would be when we reach the desired internal temperature.
Now, of course, heating up the house those 10 degrees will take a bushel of BTUs, but (unless I am way off here) that would have to be fewer than those saved.
I well understand that the efficiency of the system goes down as the well cools, but it seems to me that the diminished efficiency, though regrettable, is more than balanced by the savings at the lower temperatures.
With all of this, I may be completely out to lunch, but I'd love to understand where I am going astray.
I have a closed loop slinky coils in three trenches as deep as the backhoe could dig. Covered back with the clay that came out of the trenches. I think it is a 3 ton unit conditioning 2700 sf, a little east of Atlanta Ga. No booster heat. Hot water heater option. When it is extremely cold, ie 10F or extremely hot 100F the unit runs a lot. It makes lots of hot water when the delta T is enough for the unit to run more than a few minutes. I think the thermostat is at 74 in the winter and 78 in the summer. My wife my cycles the thermostat a degree when she is too cold or too hot. No 68F in the winter that I grew up with. Warm blooded woman I married.
I have no idea if setback works as she is awake when I sleep. Rolling the thermostat when you have people living different shifts does not work so swell.
The neighbor down the road has a couple of geothermal units that they zoned for the main part of the house they lived in and another zone and unit for the extra bedrooms. They claimed power bills less than my house even though their house was much bigger. Makes me mad enough to finish insulating the concrete walls in my conditioned basement.
I have not thought about how dry the earth might be and how that might affect the efficiency of the unit. Not a lot of rain over the summer. I do know I have added dirt to the trenches once in 10 years. I need to add a few more inches to some of the trenches close to the house as they have settled. Settled is good suggesting better heat transfer, maybe.
If you can get the same night to happen back to back read your power meter. My house seems to do about 1000 kwh a month or about 38-45 kwh per day average in the coldest of winter days. You could read the meter before you go to bed on a normal night and read it at 8 am. Next night assuming same wind conditions, cloud cover and night temps do the setback and rollup. You might get your answer. Or put an hour/minute meter on the air handler and get similar results. The hour/minute meter might be more accurate. You could also attach thermometers to the two water lines to see the delta T and what if any measurable influence the setback does to the well temps.
I don't think you will see significant savings setting back the temps. Also consider the time it takes to recover after a 10 degree setback. It is very possible that equipment may run longer to recover the temps than it would to just maintain a "normal" occupied temp, more possible when you consider the stress it may put on the well. Again, I would set the temps and leave them at one set point, unless the home is not occupied for days. Can I ask where you may be located? Greg
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.)
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...
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