prog. therm. and heat pump questions

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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?
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Russell Coleman wrote:

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
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"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.
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snipped-for-privacy@optonline.net wrote:

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
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RP wrote:

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
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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
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Nick Pine wrote:

From that angle one could simply turn the unit off, t-stat no longer a factor :)
hvacrmedic
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Sure. Swiss mountain folk believe it's healthy to climb under quilts with bedroom windows wide open in freezing weather.
Nick
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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.
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CBHVAC wrote:

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 65F. 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

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Depends on the climate, no? Might be 3:1.

Thus? :-)
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

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

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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
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snipped-for-privacy@ece.villanova.edu wrote:

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

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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
http://www.homeclick.com/showpage.asp?itemid=202929 , or
http://www.cleanbutt.com/PRODUCT.ASP , with optional wireless remote control.
Nick
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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 <-- "but they only scoff"
Your old conductance, now typically at 500 lol. Anyways you are saying 70 indoors and 30F ambient I believe so it would mean a heat loss of 40x500= 20,000 Btu/hr with your mind set.
But of course you will have a heatpump with a 60,000 Btu/hr heat output on this home that exceeds the load by a mere 300%.
So a unit capable of about 60,000 Btu/hr of heat output based on a 30F temperature source will most likely have a cooling capcity of at least 6 tons.
You always neglect solar gains on homes IN YOUR GOOFY COOLING SCHEMES and use your temperature based 'Conductance' so carrying your mindest to a cooling environment when it is an AVERAGE (LMAO) 88F outside, they only need 500x(88-70)=9,000 btu/hr for sensible cooling , that 6 ton just out to cool it. You don't have to worry about a latent load in this scenario Noah, because your system would be so grossly oversized its is going to be 80%RH as a minimum.<-- "works in a zoo"
Or if you would use a real cooling ambient with your scheme and 'conductance' then you need sensible cooling of 500x(95-70),500 Btu/hr. And you wonder why I give you grief when I say that you neglect the effect of the sun on cooling. <-- "they just scoff"
Maybe this unit with a would be available in single phase, if it was some kind of 5 ton system on steroids that moved 2600 CFM.
In reality, I would doubt a home that actually required a 20,000 Btu/hr heating system in a 30F climate would need much more than 3 or 3.5 tons of total cooling in a 95F climate.
Just another attempt to cross post drivel that does not work in the real world. So put in a heating unit three times the required size to prove that you can set back a heat pump. Pathetic.
Maybe look at applying a Biot Number or two when you are looking at heat released, then heat required to warm something back up. <-- " subliminal hint"
You may also want to consider the outdoor coil defrosting in your calculations as well.
In a cold environment (sub zero) with a high thermal mass, a system sized to match the heat load may shut off for a couple hours at night when setback, then would be running steady until after the sun rose to catch up. This is based on something real, not a fictious scheme where the equipment is 3 times the capacity of the design heating load.
With heat pumps and or high thermal mass buildings, set back is not always a good scheme. If you set it back during the day, such as when occupants go to work, think about having the space back up to temperature before the sun sets.
If you shut off the strip heaters, sure it is possible that a properly sized heating system could eventually catch up to the required temperature but the problem is the sun will have risen and the occupants have left for work already before it does.
Then think with some passive solar, the house would not cool off all that much while the sun was shining. Run time is down quite a bit, when the sun is shining. From experience with R2000 homes, the systems can be off for hours while the sun is shining in cold weather.
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Abby Normal wrote:

my bad, the load is 20 the system puts out 60, 3 times the load, therefore it exceeds the load by 200%.
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Completely trivial to find it using groups.google

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Abby Normal wrote:

Hey Ronnie, get er up to temp before the sun sets.
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Pity that was clearly about a setback during the day, not the overnight actually being discussed.
Pity you also pig ignorantly claimed

Cant even manage a consistent line in pig ignorant bullshit.
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