Last heating season I learned that short ground source heat pumps (GSHP) run times produce almost no hot water. I adjusted the thermostats so that they would only call for heat once per hour. While this helped some, by making each cycle a little longer, we – never – got truly “hot” water.
A neighbor of ours relies almost entirely on their GSHP to supply hot water, and it works, so why not in our house? As we enter into another heating season, and we get less free hot water from our solar hot water system, I am struggling to think of a way to make greater use of the very efficient desuperheaters in both of our GSHP units.
After some quick experimentation, I – may – have found an answer. I had previously disabled the setback feature on our thermostats, preferring instead to keep the inside temperature at a “steady state” around the clock. My previous experimentation with setbacks lead to some equipment lockouts, but I am going to try once again.
Now, I will allow the inside temperature to drop to 65 degrees F while we sleep, and have the house warmed up to 68-69F just before we get up in the morning. Last night, I allowed the setback for the upstairs while keeping the downstairs set as it had been. Since the outside temperature dropped into the mid 30s last night, the inside temperature upstairs did in fact drop to 65F, and the thermostat made sure it was 68F by 5:30AM. This morning, we had 80 gallows of 90F water. Interesting. Especially given the relatively mild outside temperature, and the not particularly cool inside temperature. We had higher temperature water than usual.
Tonight I will allow the entire house to cool to 65F over night (weather permitting) and have it warmed to 68F upstairs and 69F downstairs by 5:30AM. With both units running longer than they “normally” do to increase the inside temperature 3-4 degrees F, I have hopes we might actually have 80 gallons of usable (115-120F) hot water. If we do, I might just change my opinion of the desuperheaters. I’ll let you know how it goes.
Ed
Why not lower thermostat even lower at night?
Why not turn your thermostats down even further at night and during the day when no one is home (I'm assuming no one is usually home during the day)? My thermostat drops to 50F at night, comes up to 68F in the morning, drops back to 50F during the day when no one is home, and up to 65F in the evening. Weekends days are different of course. As long as you have enough blankets on your bed you won't even notice except for those hopefully rare occasions you get up in the middle of the night. If your house is as tight as it sounds it is, it shouldn't even cool down all that much Plus it should recover pretty quick in the mornings. Plus if I understand correctly, this scheme would generate more hot water for you.
Well...
So, why not le the interior temperature drop to 50F at night? Well, I may be able to try that as the days get colder. However, my first thought is that it will draw too much heat from the standing column wells in a short period of time.
By having the heat pumps run once per hour when needed during waking hours, there is time for the wells to recover (heat naturally enters the well water from the ground). When a lot of heat is extracted from the wells over a much shorter period of time, the water temperature drops. As the well (incoming water) temperature drops so does system efficency. Since we do not employ an "emergency bleed," we risk the systems locking out due to low water temperature (risk of freezing).
Basically, if I do try to lower the nighttime temperature further, I think it will need to be done slowly to see what happens.
At this point, I cannot say with certainty that the lower nighttime temperature gives me more hot water, but it looks like this may be true. I just have not grabbed some decent measurements.
As for lowering the thermostat during the day... I work from home. The thought of wearing a jacket in the house while working at my desk is "chilling". Plus, when the sun is shining, the house usually warms to 72F during winter days. :)
I will try lowering the thermostat even more at night when the nights are consistently colder and we have a chance of dropping to that temperature. Right now, the lowest we've seen is one night in the low 30s, and I don't think the inside temperature even got down to 65F.
Ed
Mr. Green Dreams
Quick Update - Hot Water
The night time setbacks are working wonderfully. We are waking up to 50 gallons of water in the 110-115 degree range.
Here is a small warning you anyone with a GSHP that might want to try this... you need to disable your emergency / 3rd stage or your electric bill will go throught the roof. When the system works to heat the house in the morning, it will call for emergency heat (electric resistance) unless you take steps to avoid this. Since ours is turned off (permanently?) via separate circuit breaker it can't come on.
On cold, but sunny days, I think we are making more hot water than during the summer. Interesting. We get 50 gallons from the GSHP, and the solar hot water system does the rest. Nice.
Now if I could just get my kids to close the doors behind them. ;)
Mr. Green Dreams