prog. therm. and heat pump questions

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Yeah, that's what I meant.

The other possibility is to stop the setback well before you get up.

You could obviously have 3 systems but thats not likely to be economic unless you are using close to free surplus systems.
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When it is sized to the load the problem is it would only shut off for an hour or two as the house cooled, then it would run steady until after the sun came up to catch up. No real point in setting back.
The only way set back works with a heat pump is if it is grossly oversized. All that accomplishes is set back without auxiliary heat. It would short cycle inefficiently except for when it was trying to recover from a set back.
In an environment with an ambient dewpoint above 60F there will be problems with a grossly oversized system in cooling mode.
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That is just plain wrong. The whole point of a set back is that less is pumped at the setback temp because the losses are lower. How much lower depends on how well insulated the house is.

Wrong again.

Not if you one of the systems sized so that doesnt happen.

Again, not if you have more than one system.
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27. Rod Speed Dec 16, 3:47 pm show options
Newsgroups: alt.home.repair, sci.engr.heat-vent-ac, misc.consumers.frugal-living
Date: Sat, 17 Dec 2005 07:47:00 +1100 Subject: Re: prog. therm. and heat pump questions Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse

The whole point of set back is to lower the temperature for a while.This would imply that the house cools down and the systems stops running during this cool down period. Then it would run less per hour due to the setback indoor temperature until such a time that it need to start warming up the house so that it was back up to temperature perhaps when the occupants awoke or perhaps returned home from work.
The problem is shortly after the temperature dropped to the setback temperature, the heat pump would end up running steady just to get back up to temperature.
Typically in the temperate states where air source heat pumps are used, they are sized with the cooling load in mind and use the heat strips. One sized for the full heat load will be grossly oversized for cooling resulting in summer time humidity control problems.

No not wrong just unsucessful in educating you.

So you are saying size one for the cooling load, one for the heating load and one for speedy recovery from set back then?
Three heat pumps now. Plus all the ductwork and backdraft dampers. Let's try to keep this practical and not go to hypothetical extremes to prove this is possible. Noah er I mean Nick is a bad influence on you.

Well like I said if you want to install three heat pumps, knock yourself out.
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To reduce the amount of energy consumed to keep it at the higher temp.

Correct.
Correct.
Wrong with the situation being discussed, with more than one system so it doesnt take a long time to get it up to the normal temp again.

And what was being discussed was having more than one system and doing that so it doesnt take a long time to get back to the normal temp and doesnt use the heat strips to do that.

Yep, completely wrong.

You cant even keep track of what was actually being discussed.

Yep, I mentioned 3 for a reason.

You dont have to duplicate/triplicate all of those.

Wasnt doing anything like that.

Nothing to do with Nick at all.

Two and three was what was being discussed.
Your claim that setback has no value is just plain wrong with THAT situation.
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If you installed two complete heat pumps to save on and speed recovery, the added cost of the second system would be more than the savings provided by not using any strip heat. If you installed a slightly oversized 2 stage heat pump, it would run in low stage all summer providing about 65% total cooling capacity. Then it would run in high stage (High Capacity) during heating season when it was below 45 degrees outside. You could possibly set back in this case. Note that installing a two stage heat pump usually adds about $2,000.00 to an average installation. The average life of a quality, properly installed air to air heat pump is 15-20 years nationally. If you would save $2,000.00 over that time span, then install the two stage system. If you install two complete systems, add about $6000.00 to $10,000.00 to the cost of the first system, assuming you had room for the extra ducts and registers and equipment. Tough way to save money. Like going around your elbow to get to your asshole. (Or is it the other way around:-)? ).
Strretch
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Yes, that's why I said it would only make sense if the extra was surplus etc.

You wouldnt need all the ducts and registered duplicated if you just used it for the faster return from the setback temp.

Yes, that's why I said it would only make sense if the extra was surplus etc.
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That makes sense. Then again heat strips have their place. It isn't hard to think of a situation where they would save money (assuming net.thought.police still allow thinking), eg in a house with lots of heat loss and little thermal mass and a wimpy heat pump, eg 1000 Btu/h-F and 1000 Btu/F and a (70-30)1000 = 40K Btu/h heat pump on a 30 F night. If the heat pump can only keep up with a 40 F temp diff, no setback is possible, so a 10 hour night requires 400K Btu of heat, or 200K Btu of electrical energy with a COP of 2. With heat strips, the house can be 50 F most of the night, with 100K Btu of heat pump electrical energy and 1000Btu/F(70-50) = 20K Btu of 8 AM strip energy, so the strips save 80K Btu, ie 23 kWH of electrical energy, and the low-cap heat pump is cheaper.

Why not a single-speed compressor with low and high airflows?
Nick
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Nick,
If you check the data, or measure it yourself, Strips cost 2 to 3.5 times as much per BTU as the heat pump. So you set back by shutting off the cheap heat (Heat Pump Compressor), and recover by using the expensive strip heat. Setback increases the electric bill if the heat strips come on during recovery. In addition to warming the air during recovery, you also have to warm the entire thermal mass of the house and all it's contents. THAT is ia big part of the added power consumption.
Here in Myrtle Beach, SC, a lot of homeowners are relocated yankees (like me), used to setback. They setback in the winter and their electric bills go up. They call me to check their system, I explain proper operation, they stop using setback in the winter and their electric bill goes back down. Happens every winter, many times.
Many heat pump setback thermostats lock out strips during the beginning of setback recovery. On those, recovery takes a LONG time. After a number of hours, the strips are allowed back on, recovery occurs and electric bill goes up.
Note that recovery without strips only works in mild weather, so setback efficiency depends on your climate. It does not work well in my climate.
By the way, my wife, like many women, has to make several potty trips to pee every night. If I installed another setback thermostat and her tush gets cold on a regular basis every night, I will get the cold shoulder when it comes to sex. That alone is enough reason not to use a setback thermostat.
Stretch
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I have a 12 year old thermostat (Chronotherm III) that has the adaptive recovery feature as you suggested. When the system comes on (when coming out of a setback), it will try to reach the set point without bringing on the auxilary heat strips. It will only bring them on if it "looks" like it's not going to meet the set temp at the time set. Seems to work pretty well, but if it's down in the 'teens, forget it. That's when the gas logs kick in anyway.....;-]
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Nope, not with a very lossy house that loses more heat at the higher temp over the setback time than that 2/3.5 times.

Thats always been a furphy.

Depends entirely on the thermal mass of the house.

Doesnt mean that there arent situations where setback works. You dont get called out to those situations.

Still doesnt mean that there arent situations where setback works, particularly with deliberately oversized systems that dont need to use strips to come back off setback.

Bet it does with a bigger system.

The obvious approach there is a heated toilet seat, stupid.
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Stretch wrote:

That's logical, since the whole purpose of a heat pump is to be more efficient than resistive heating.

Also logical, but aren't there programmable thermostats that are smart enough to deal with this? I have a cheapo programmable thermostat which is supposed to be able to intelligently determine how much time it takes to recover. A well-designed programmable thermostat for a heat pump would be very conservative about this and would err on the side of recovering way too early (hours early) rather than having to kick in the resistive heating.
I don't know that such thermostats exist, but it seems like it would not be all that hard to make one that is smart enough to avoid having to turn on the resistive heating due to setback. Machine learning techniques are pretty good these days, and the thermostat has good information available to it (the average rate the temperature drops when the system isn't running gives it an idea of the load, and the duty cycle gives it an idea of its capacity relative to the load). The worst problem seems like it would be changing weather (where it's 30F colder one night than the previous), but even that can be solved if the system realizes the temperature is dropping a lot faster over the course of the night than it was the previous night and knows not to expect last night's data to be a good indicator.
- Logan
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Or just use the inside and outside temps and the measured performance of the system at that outside temp to know when it should come off the setback to have the inside temp back to normal temp at the time specified. Not a shred of rocket science required at all.
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And if the occupants of the house are going to be out for the day, is it really necessary to get back to normal temp for the short time in which they are getting up, showering etc and heading out ? It may make more sense to do that stuff at the setback temp and let the system bring the temp back up to the normal temp in the late afternoon before they are due to return instead, and be able to use the higher outside temp to get back to normal temp without using the strips at all.
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Stretch wrote:

The short answer seems to be that setting back will either save energy or it won't.
I keep mentioning, and everyone seems to continue excluding in their calculations, the fact that even with resistance heat engaged the heat pump will also be running. The net COP during recovery will be the weighted average of the heat pump COP and 1:1, and thus greater than 1:1 when the heat pump COP is greater than 1:1. The recovery COP will depend upon the ratio of heat pump and resistance heat capacities. This is why I suggested trimming the backup resistance heat to a minimal number of elements, staging the remainder in with either t-stat programming, time delay, or return air temp. Regardless of overnight setback there will be times when the system has been set back, such as vacation, and quick recovery will be desired. During these times the remainder of the resistance heat will be called upon as needed, but will never come in during normal operation.
Now here's something that nobody has yet addressed: If it is sufficiently cold in the house and outside, as in after the unit has been turned off for a long period in sub freezing weather, or perhaps was down due to some failure, then the heat pump alone may never *ever* recover at all. In fact the temperature could actually drop during the recovery attempt. Don't doubt it for an instant, as a lifelong tech I have encountered this more than once.
This is a situation in which the COP of the heat pump is a misleading number. Though it may be greater than 1:1, no energy is saved over the heat strips because the house simply never warms up, and all of the runtime is effectively purely wasted energy and thus wasted dollars. The capacity is simply not enough to overcome the current level of heat loss. Thus when it is exceptionally cold in the house the heat strips will save energy during recovery. While this doesn't typically apply to normal overnight setback, it does however illustrate that the difference in COP of two heat sources doesn't equate directly to the difference in energy used and/or money saved. IOW, though the energy savings may be intact on paper, they are meaningless because you are getting nothing ot show for the money spent.
During recovery without backup heat, the heat pump has to start recovery sooner, so that the added runtime of this method of recovery can indeed be greater than the cost of strip heat during recovery, infinitely greater if the heat pump's capacity has dropped so that it just equals the heat load in which case there is no recovery, period, just hours and hours of runtime with nothing to show. But even under normal circumstances we won't instantly switch to greater savings without backup heat, there is a curve involved, it's a continuum, and there must be some specific point along that curve in which the energy use is exactly the same with either method of recovery. That point depends entirely upon the system installed [it's configuration and sizing], the house in which it is installed[R values, infiltration, and thermal mass], the outdoor temp, and the indoor setback temp. There are even more variables to consider than these, but these are the major ones.
You should be able to figure out from the argument above and those made in the thread by all of you, how each of these variables affect the balance, i.e. which direction of any changes in these will tip the balance in favor of backup without resistance heat or backup with resistance heat.
In summary, the energy savings gained or lost depend upon very many factors and no blanket statement should even be attempted. Every system would have to be tested in the field in order to determine its energy saving potential during various ambient/indoor temp combinations, and/or off or set-back periods. IOW it's too complicated an issue for anyone here to claim to have made the correct statements about savings unless its those who said "it depends" :) Easiest way is to check your light bill before and after you changed your setback habits. Even if you show a savings for the one month out of two tested, this will still be meaningless unless you are sure that the month with the lower bill was as cold as or colder on average than the other and you used exactly as much additional electricity or less on other appliances, all of which contribute to the total heat load.
hvacrmedic
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Rod Speed wrote:

That is sure economically viable.
To get a heat pump to work with setback, you are oversizing the system. Either a single oversized unit, or multiple units. It takes a rediculous situation to make setback work on a heat pump.

Yes multiple systems, now as high as three, just to prove a point that you can setback a heat pump.

Sure, buy 3 heatpumps to prove set back. Spend extra money on the ductwork involved for three systems, including back draft dampers, lol what a crock

Well I believe you were speaking first of two systems, then realized you needed three, so as not to be short cylcing in the heating mode, and in the summer having too much sensible cooling that the stat would be satisfied in 5 minutes run time.

Three systems sure, that is practical,

No you could have three systems share a common supply and return, just get it to work there einstein.

No not at all just install three heat pumps. Or two heat pumps and a central AC. Hey maybe you could get a two stage one and a central AC. SUre is a lot of tap dancing to prove that you could viably set back a heat pump.

Setback has a value, just not with heat pumps.
A fossil fuel system sized right on the money for the heat load in a high thermal mass home may not be the best system to be setting back either.
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The economic viability wasnt being discussed.

Nothing rediculous about using a surplus system to make setback work in a situation where it isnt economic to increase the insulation substantially and its quite lossy.
It can be a viable approach, particularly if the normal temps are only set say in the late afternoon prior to when the house will be occupied for long when not in bed.

Its just one way to have a viable setback with a heat pump.
The other obvious approach is to have the setback temp right thru till say 3pm because the occupants arent likely to need the normal temp in the hour or so when everyone is running around having showers etc before heading out of the house in the morning. Then even just a properly sized single system should be able to come back off setback quickly enough to not use the strips with a properly designed controller.
Even if it starts coming off setback at say 1pm because it takes 4 hours to get back to normal temp with those outside temps, its still going go save power over no setback.
And it may well be possible to design the entire system so the outside part of the system uses passive solar too if its only going to come off setback after midday.

Its just ONE way to make setback viable with a heat pump.

You dont need all that stuff for the system thats just ensuring that you can come off setback without needing to use the strips.

Couldnt bullshit its way out of a wet paper bag...

Wrong. I JUST said that three can be useful in SOME situations.
You dont even need two if you come off setback after midday, just an adequately sized system that doesnt need to use the strips on most days in winter. And if passive solar is used to help with coming back off setback after midday, thats going to make setback viable even with a heat pump. And wont necessarily cost much at all hardware wise.

Thats an entirely separate issue to what was being discussed, the silly pig ignorant claim that setback can never be viable with heat pumps.

Doesnt have to be 3, or even 2.

Perfectly possible if you dont have them all running at once, gomer.

Just one way to do setback with a heat pump.

Wrong when stated as absolutely as that. Most obviously if the setback lasts until after midday and passive solar is used instead of the strips for coming off setback.

Having fun thrashing that straw man are you ?
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Rod Speed wrote:

No that's the problem. You want to argue the fact that you can install three heat pumps to prove that you can set one back fine, I am just pointing out to you that it is a dumb idea.

I believe i mentioned I mentioned if you were trying to set it back during the day while occupants are at work, then have it recvover while the sun is still shining.

Yes I agree, if some one wants to have a viable scheme for setback then spend the money on three heating systems. Dumb idea but you have proved a point.

Occupants gone during the day, recover before the sun sets.

Without the multiple heat pump scheme, you will save energy just the slow recovery will be cool inside. Not maintaining a comfort point. So keep the home cooler save energy at the cost of comfort. Set it to 65 all the time if you want.

Well so far you have come up with tree heat pumps to prove a point. Going to extremes, rediculous.

You cannot bull shit me and say you have ever had a redundant system share common ductwork. You obviously never have. At least not with direct drive equipment typical of a residential application these days. Maybe you got lucky on some belt driven stuff. Rookie.

Well so far. without wasting power on heat strips I have seen it proposed that a system with triple the heat capacity be installed or you install three heat pumps to prove a point.
Don't bring a knife to a gun fight next time. At least Nick can misapply numbers he crunches you seem to have nothing.

Like I said you obviously have never designed a working system with multiple equipment sharing common ductwork. If you had a succesful project of this nature then I suspect that some contractor saved your ass. You probaly threw up a lot of rhetoric, tried pointing fingers at everyone else but in the end the owner paid extra for your lack of experience.

I did not realize you were Socrates the Scarecrow. Next time I will simply light a match.
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Nope.
Lying, again. I JUST rubbed your nose in the FACT that your pig ignorant claim that setback isnt viable with heatpumps is just plain wrong.

No it isnt, most obviously if the extra heat pump is surplus. That can indeed make set back viable and save real money.

No you didnt.

That aint the only way to have a viable setback with a heat pump.

What I just said.

All you have to do is come off the setback well before it need to be back to comfort point. That may well be very viable if you only want the comfort point in the evening after the occupants have returned.

No cost in comfort if you come off the setback with plenty of time for the slower recovery to the time you need the comfort.

Makes a hell of a lot more sense to use a setback, and come off that well before the house needs to be back to the comfort temp, when the heat pump is working more efficiently with that return to comfort temp happening after midday etc.

Lying, again. It was just ONE approach to get setback viable with a heat pump.

Couldnt bullshit its way out of a wet paper bag.

Couldnt bullshit its way out of a wet paper bag...

Couldnt bullshit its way out of a wet paper bag...

Couldnt bullshit its way out of a wet paper bag...

I never said that they had to all be running at once, you pathetic excuse for a bullshit artist.

Couldnt bullshit its way out of a wet paper bag...

Couldnt bullshit its way out of a wet paper bag...
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Well Ron you are a smidge slower than your psuedonym suggests. You have never had redundant systems share common duct work else you would have a clue as to what I was saying.
I hired a guy from ARC Industries once, his nickname was Ronnie the Rocket. If I told him precisely what to do, he would do a good job.
So far, heat pumps can be successfully set back with rediculous situations. Go back and read carefully, sound out the big words.
You also have never had redundant systems share common ductwork nefore. Search 'back draft dampers' and think about forward curved direct drive fans spinning backwards.
Like I said Nick will spew forth some misapplied numbers and the odd time he is on the money. Unfortunately , you do not have a clue.
Rod Speed wrote:

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