I have a heat pump with aux heat for which I bought a programmable thermostat. The RTH7400D to be exact
What is the recommendation on setting the temp difference between wake and sleep times so that I am not going to make the system work to hard and not get the energy savings that I bought the prog. thermostat for in the first place?
The programmable-stat isn't going to magically save you any money vs a non-programmable stat. It has one purpose: To control temperature, just like any other t-stat. A thermidistat OTOH also controls RH, within limits providing constant comfort at constant temp, something the regular thermostat won't do. IOW, a constant temp setting won't provide uniform comfort on a regular stat, either programmable or non-programmable. IOW, you may find yourself frequently overriding the schedule on your programmable stat, thus defeating its true purpose, which BTW isn't what you claimed it was above.
The energy savings enter in only in the fact that you are setting back the temperature for periods of the day, something that a programmable stat isn't a requirement for. Basically any time the unit is off you are saving. It isn't the stat that will save you money, it does nothing to the system by its mere connection to the system to improve its inherent efficiency, it is only the lower than comfort temp settings scheduled in winter, and higher in summer that save you the money. Thus for the greatest savings just turn the system off when you aren't home, and set back at night to whatever freeze level in winter or sweating level that you care to withstand in the name of savings.
Recovery time between sleep and wake settings isn't an issue as far as energy cost is concerned, but it should be to be taken into account when adjusting the schedule times.
BTW, in reference to my above comment: People don't buy programmable stats to save money, they buy them to save them from the 2-second hassle of manually setting back every night, and/or for improved comfort via RH control when available. Comfort control OTOH typically reduces overall economy, FWIW. Thus, unless you aren't home, then the only way that you are going to save money with *any* stat is to live uncomfortably in your own house. I'm almost positive that this is the reason that 1 out of every 100 programmable stats that I encounter in the field are running in HOLD mode with no schedules set. HTH.
hvacrmedic
"Recovery time between sleep and wake settings isn't an issue as far as
energy cost is concerned, but it should be to be taken into account when adjusting the schedule times"
It sure sounds like it would be a big issue on a heat pump system with electric aux heat. I think his concern was if he set the setback delta too big, the aux electric heat could kick in.
Make that 99 out of 100 :)
I should also add, while I'm at it, that some stats do offer programmable options for backup heat, and may provide some savings over a standard stat in which backup heat will always be energize during recovery from setback. In multiple stage systems some programmables will also offer net savings over the non-programmable by running the system longer in the low stage. Sorry for these technical omissions.
hvacrmedic
The same across the board.... Or, no more than 2F degrees difference between the two.
The most common question asked is : What temp saves me the most money? The answer is, as low as you can stand it.
With programmable stats, any real savings you think you had, you kill when the aux heat comes on...if needed for recovery.
Any decent digital stat maintains a 1F differential, so that alone saves you a ton over a mercury stat. I said decent...the one you have is a marginal Honeywell....bottom end of the scale, particularly if you got it at Lowes or Home Depot.
I immediately addressed this in the follow-up post that I made, but yes, you are correct. This is however something that scheduling alone, what he asked about, won't cure. That option, if available, will be in the setup menu which is typically difficult for the average homeowner to make sense of. They could easily change some settings that don't need to be changed. If he is interested in optimizing recovery via heat strip lockout, then he should check his literature or contact the manufacturer.
hvacrmedic
Conventional Heat Pumps get an average COP of 2 to 1. Thus if the overnight (set back) runtime is reduced by an amount of time that is twice the recovery time with heat strips enabled), then energy is saved over not setting back. Keep in mind that the COP during recover will be greater than 1 to 1 because the heat pump will be running along with the heat strips.
At low ambients, where the COP of the heat pump is barely, if at all, Greater than 1 to 1, then it is impossible to imaging that heat strip assisted recovery will overcome the savings of setting back. OTOH, at higher ambients, it hardly matters much--the savings, or loss, whatever the case may be, only amounts to pennies per day.
Unless I see some studies that supersede this, then I say setting back overnight will save money. I also suggest that the system be set up to run minimal backup heat, not at the stat, but at the unit. Rheem used to install a klixon in the air handler that locked out a strip or two until the air temp dropped below 65ºF. Not a bad thing. Staging the strips is also a good idea. This can be done in conjunction with a 3 stage heat stat, or by installing a long time delay on a portion of the strip heat. There are plenty of options, but none of them are going to save
*substantial* amounts on the energy bill. If energy efficiency is that great of an obstacle then either shut it off, or get a higher efficiency system. I really don't think that piddling with t-stat settings is worth the time unless you've done the calcs that prove that any savings from doing so are going to be noticeable. It sometimes amounts to spending a dollar to save a dime. Figuring in the cost of the programmable stat vs a non-programmable, payoff might take 10 years, it might take one, and it may never pay for itself, depending upon system design.hvacrmedic
Depends on the climate, no? Might be 3:1.
Thus? :-)
Nick
Switch off the strip heaters if any and turn off the heat at night and set the thermostat timer ahead to make the house comfy when you get up.
There's no magic: reducing the indoor-outdoor temp diff at night makes less heat flow. That's Newton's (1642-1727) Law of Cooling.
Or opening all the house windows in wintertime.
How can he be wrong about his true purpose? :-)
Nonsense. He can be perfectly comfy sleeping under a quilt in a 40 F bedroom.
Nick
From that angle one could simply turn the unit off, t-stat no longer a factor :)
hvacrmedic
Sure. Swiss mountain folk believe it's healthy to climb under quilts with bedroom windows wide open in freezing weather.
Nick
Thus, yes, at some higher COP and/or warmer climate the trend will reverse. That's why I said in summary that economy depends upon the system :)
hvacrmedic
I don't quite see a "thus" there. Would you further explain your logic?
Would you agree that night setbacks always save energy if the heat strips are switched off, even though that may require longer reheating after the setback?
Nick
I'm sorry, that won't be possible. I can however add more conclusions at will. It's quite easy once you get the hang of it :)
I'm not in the mood to drag in formulas at this point in time, having worked all weekend, till dark hours. IOW, I'm too damned tired :)
You've probably worked through this already, so if you would, could you please provide some math to analyze the set-back savings of various COP systems with and without resistance heat enabled?
Without strip heat enabled it *seems* obvious that the monetary savings of setting back would be greater with the lower COP system. IOW, with a COP of 1, you have effectively the economy of straight strip heat, in which case setting back definitely helps the pocket book. OTOH I'd be very grateful if you could prove otherwise, as my wife is a setting-back fanatic and I'm tired of waking up to a cold pisser. I suppose I could shorten it a bit, but hanging it in the water easier than aiming when you haven't had your coffee yet.
hvacrmedic
My friend Rich says he can easily supply multiple inconsistent conclusions.
Perhaps later, when you have rested.
Well, we might agree that night setbacks always save energy with heat strips switched off, even though that requires longer reheating after the setback, but a longer setback with a faster recovery using strips might save more, if the house can stay cooler longer. This wouldn't apply to houses with zero or infinite thermal mass, but it might with a finite mass.
In scenario 1, keep a house with G = 200 Btu/h-F of thermal conductance at
70 F for 8 hours on a 30 F night with 8h(70-30)200 = 64K Btu from a heat pump with a 2:1 COP, using 64K/2/3412 = 9.38 kWh of electrical energy.In scenario 2, the house has C = 10K Btu/F of thermal capacitance and an RC time constant C/G = 50 hours, so it cools from 70 F at midnight to 30+(70-30)e^(-8/50) = 64.1 at 8 AM. The house returns instantly to
70 F at 8 AM, with powerful strip heaters that consume (70-64.1)10K = 59K Btu, or 17.3 kWh, almost twice scenario 1.In scenario 3, the house has a wimpy 10K Btu/h heat pump with a 30+10K/200 = 80 F Thevenin equivalent temp and no strip heaters. It cools to a minimum temp T (F) in t hours and reheats in 8-t hours, so T = 30+(70-30)e^(-t/50) and 70 = 80+(T-80)e^(-(8-t)/50) and t = 1.7 h and T = 68.7 F. Reheating for
6.3 hours takes 6.3x10K/2/3412 = 9.23 kWh, with a small setback savings.More typically, with C = 5K and G = 500 and RC = 10 hours and a 60K Btu/h heat pump with a COP of 3 and a 30+60K/500 = 150 F equivalent temp, we would use 8h(70-30)500/3/3412 = 15.63 kWh in scenario 1. In scenario 2, the house would cool to 30+(70-30)e^(-8/10) = 48.0, and the heaters would consume (70-48)5K/3412 = 32.24 kWh at 8 AM. T = 30+(70-30)e^(-t/10) and
70 = 150+(T-150)e^(-(8-t)/10), so t = 6.0 h and T = 52.0 in scenario 3. Reheating takes 2.0x60K/3/3412 = 11.9 kWh, for a big setback savings.Some people say the COP of a heat pump can be less than one, but that seems hard to believe. OTOH, they do wear out.
You might enjoy
Nick
I wonder why you end up cross posting this stuff. No one is going to subscribe to all the groups you belong to find the original post.
You must have a compelling need to drag this crap over to the 'HVAC Criminals' because you are a closet mental masochist.
snipped-for-privacy@ece.villanova.edu wrote:
Scenario 1, setting back the temperature of a dog house or other pathetically small structure, irrelevant.
Typical, LMAO
What is the time constant as that 60K heat pump warms up? Would it ever reach steady state and a COP of 3? Well maybe when it was running after the setback period ended.
60,000 steady state heat output, 20,000 load each hour hmm, wonder how many times that will cycle on and off drawing LRA? Well if a dog year is 7 years, a Nick year must be 3 years.Maybe assume it takes 15 minutes to stabalize and work backwards from there for your time constant.
my bad, the load is 20 the system puts out 60, 3 times the load, therefore it exceeds the load by 200%.
A valiant effort, but several variables are missing. Still it's probably an ok ballpark analysis for comparative purposes. I wouldn't value the actual numbers beyond their ability to provide inequalities (More than/less than). They are too ambiguous. Still I don't see yet where we are in disagreement. Maybe you weren't trying to disagree. I dunno.
Sure it can, else there wouldn't be an economic balance point. As the ambient drops the suction pressure drops too, mass flow is reduced, thus he capacity is reduced as ambient temp drops. Runtimes are also longer and take place with reduced volumetric efficiency, thus COP drops as well. At what point would this curve level off? Wouldn't it continue to fall right on through the 1:1 mark? At 0ºK (absolute zero) the COP would theoretically be 0:1. Assuming a COP of 2:1 at 40ºF thus we can thus interpolate a COP of 1:1 at an ambient temp somewhere between absolute zero and 40ºF.
Thanx, very good site. The rim deflector is a must have item. :)
hvacrmedic
Abby Normal wrote
Completely trivial to find it using groups.google
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