The system would just have to be adjusted to take full advantage of peak and off peak hours. Since I don't have this type of service, I would assume that peak pricing would be during the day, between 9am and 5pm. Recovery times would have to be restricted to hours before or after that time. So if you have the system turn down at 10pm and back up at 6am, that would give it 3 hours to recover before peak. The system would then turn down again at 9am during peak and turn back up after 5pm. These limitations could be easily worked around now that you know that you are indeed saving $. 

One thing not covered in this study was recovery time, I guess it would depend on the system and how big of a setback you used. As for my own house, a 4 or 5 degree setback takes less than an hour to recover without AUX heat. 
   

Great point Masoud. Folks on a time of day meter might benefit from higher evening temps and lower daytime.
Bill and Dewayne are logging KW for us though, so oddly dialing up or down at night could save money depending on electric provider.
J

I'll add another footnote that says that the conclusions are based on KWH consumption, and cost could vary depending on pricing schemes.  (In my case, KWH and cost are the same as I pay the say KWH rate regardless of time of day.)

Best regards,

Bill

Posted By TechGromit on 02/10/2009

... One thing not covered in this study was recovery time, I guess it would depend on the system and how big of a setback you used. As for my own house, a 4 or 5 degree setback takes less than an hour to recover without AUX heat. 
   

This is a good point.  The experiment isn't valid if the recovery requires a long recovery time (good portion of the day) and/or a lot of Aux heat.  How many degrees you can go for a setback is probably related to amount of capacity one has before significant run time is required for recovery and/or significant aux heat is required.

Or if you have an exceptionally tight home where solar gain, cooking, and electric lighting can substantially affect the amount of daily KBTU required to heat the structure.

Best regards,

Bill

So I finally have some results.

I collected my HDD's and KWH's from 9 pm until 9 am since my house has huge solar gains on some days and none on other days.

I ran for 7 days with an 8° set back and then 7 days with no set back.

HDD_D	KWH	KWH/HDD
-20.51	18.5	-0.90
-21.91	17.95	-0.82
-20.52	16.36	-0.80
-20.1	15.97	-0.79		Days with set back
-14.66	13.78	-0.94
-12.39	8.76	-0.71
-15.78	13.81	-0.88
	average	-0.83
HDD_D	KWH	KWH/HDD
-16.33	16.44	-1.01
-15.88	10.11	-0.64
-18.42	20.16	-1.09
-19.37	19.26	-0.99		Days with out set back
-20.14	22.67	-1.13
-24.24	25.57	-1.05
-18.45	20.34	-1.10
	average	-1.00

My aux heat was shut off.

The heat pump had recovered from the night set back by 9 am each day.

While not the best method,  it does give us an idea of how set backs  affect cost.

At least in my situation....your mileage may vary.

Dwayne, this is very helpful.

My observation of your results is that they are similar to mine:

1. Running with a tstat setback scheme certainly did not result in greater KWH consumption.  I.e., the KWHs needed to heat the structure back up after a period of setback did not offset the KWH savings from the setback.

2. And indeed, it may be possible to even conclude that there's a cost savings associated with a tstat setback-recovery scheme (avg. index of 0.83 versus 1.00).

So, it looks likes it's reasonable to tell people that like cooler temps at night, via using a setback-recovery tstat scheme, to go ahead, as it won't cost them more KWH.  And it may well save some KWH, although our limited experiment duration periods allow for a lot of influencing factors.

I think a key footnote to this conclusion is that both you and I have plenty of capacity - i.e., we're not capacity limited.  And so our water loop EWTs have time to recover each day, etc.  I have reservation in making the same conclusion (i.e., offering the same advise) to someone who is limited in heating capacity - someone who requires almost all day to recover from a night time setback.  I'm not convinced that these individuals would not be adversely affected.  But that's another experiment.

Thanks for contributing - this has been a very interesting and useful subject.  My counsel, to those who like it cooler to sleep by, will be to go ahead, it won't penalize your KWH consumption, as long as you have adequate capacity.

Best regards,

Bill

Bill,

Well said.....I would agree.

I would guess that where someone is using aux. already, this would not be a good idea (of course I guessed before that it wouldn't be a good idea anywhere....).
I'm surprised by the results, with or without disclaimers....and appreciate the time you both took to try to answer the question.
Perhaps another wives tale is laid to rest here ;)
J

Joe, agreed.

I'd guess the experiments' results don't extend to situations where a lot of aux heating is required, where essentially you're mixing a lot of COP of 1 (aux) with a much higher COP (heat pump).  My guess, without facts, is that aux heating would need to be less than 5% of the total run time for experiment results to hold up.

In my case I rarely get out of 1st stage (haven't at all this month so far) - in Dwayne's case, I suspect he's shifting to 2nd stage at some point during his multiple-hour recovery period in the morning.  But neither of us use any aux heat, or struggle to heat our homes on the coldest of days.

I was surprised to see Dwayne's results pretty similar to mine, as his home is much more energy efficient than mine.  I think this underscores that capacity is a significant factor.

I think you can clearly advise to your customers that a setback-recovery tstat scheme will not cost KWH, and may even save KWH, for those customers that have a well designed (on the water/DX and air sides) and properly operating ground source (water-to-air or DX) heat pump.  I wouldn't think any of this applies to radiant heating, by the way.

Best regards,

Bill

I would be very interested in an AC analysis
specificly
1 - does the same hold true for ac mode


Joe I would agree with the statement 100%, as long as timers and temp diff controll aux heat , the application of this info would be very limited

How well would a set back work for cooling in summer with DTE's rates? From 10 am to 7 pm it's ~12.5 cent/ kWh and the rest of the time 6.5 cents / kWh? Would it make sense to throw the house from 72 to 68 around 7pm, especially if the system has a huge amount of spare cooling capacity? Any thoughts on that? Even if it was only break even, I'm interested in the concept because of comfort. I'm told people fall asleep most comfortable when the air is at 68.

at night the system could run twice as long for the same money
Now if you are comparing it to 72 all the time It would cost more ( not much with those rates )
If compared to 68 all the time it would save
Set back usualy refers to using the system less when you need it less
What you seem to be talking about is using it more
If you make beleive you used to keep it at 68 all the time and now you set back to 72 during the day you will save money hand over fist

Look into the chilli pad - I was refereed to a few times here and looks cool
basicly heat or cool just your mattress pad

Seneca,
This goes with what Masoud mentioned earlier, with less expensive rates at night, dial down makes less sense in the winter nights than the summer. Can we talk Mrs. R into 62* on those summer nights?
The beauty of all of this is, we're splitting hairs over the last 40 bucks while our propane neighbors are dialed to a balmy winter 58*.
J

How much cooling do we need at night in Michigan? For Dexter average low temp in July is 63 F. If humidity permits, I will set thermostat back to off at night, and open a couple of screened windows.

Regards, Masoud