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Old 01-09-2017, 05:21 PM   #1
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Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


I recently purchased a large house with two 5 ton Trane heat pumps. They appear to be working properly but run constantly (it was a similar story in the summer). Has anyone done a comparison of efficiency in temperatures below approximately 28-30 degrees?

Here are my current settings in my thermostats:
Stage 1 heat (seems to be full blower and 'low' stage compressor) at 0.5 degrees below set temperature.
Stage 2 heat (seems to be full blower and 'high' stage compressor) at 1.5 degrees below set temperature.
Stage 3 heat (seems to be full blower, 'high' stage compressor, and 15kW auxiliary electric heat strips) at 2.5 degrees below set temperature.
I do not see any settings for compressor lockout in my thermostats.

The last few days it has been very cold (daytime highs in the mid 20s and lows in the mid teens) which results in both heat pumps running nearly 24x7. The systems seem to be able to maintain the temperatures within 2 degrees of the thermostat settings (set at 68, maintains at 66-67), so they run in stage 2 heat.

I'm wondering at what point is it more efficient to run in stage 3 (auxiliary heat strips) instead of stage 2? Even though stage 3 uses much more energy, won't it run less frequently? Or does it use so much more than it is not even a valid consideration if stage 2 can maintain temperature within 2 degrees?

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Old 01-09-2017, 05:38 PM   #2
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


The fan speed should be lower on first stage than second.

Don't worry about run time.

a 2-stage heatpump will run a lot, that's the whole point of having 2-stages.


Properly operating heatpumps will always be more efficient than heat strips even at 15f and below, it's just that the capacity isn't there. Compressor amp draw drops as it get colder, but not proportionally to capacity.

Leave the systems alone and allow the electric backup to come on only when necessary. the outdoor unit should continue to run and the electric backup cycles to supplement.

Run time may be shorter with electric elements, but 15kw is a hell of a lot of electricity. The 5 ton heatpump may be drawing 3 to 5 kw depending on stage and operating conditions.

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Old 01-09-2017, 08:29 PM   #3
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


I look at this as a layman. We have one 5-ton, single stage 15 SEER HP in our 2,600 sq. ft. two-story built in 2007. With outdoor temps in the twenties I don't flench at maintaining 72*f during winter months and the unit cycles off usually hourly. Even in the teens we heat solely with the HP (though I am cautious about implementing setbacks at this point). The past few days with lows hovering at 5*, yeah, the HP could only maintain 68.x - 69.0 with a set point of 70 (I rolled it back a couple degrees during that cold period) and I had to engage the electric strips a couple times (which I loathe).

So If I moved from my experience just outlined and moved into Captain's home I would be inclined to want my system checked. But then again that's why I'm a layman and others of you are pros.

Last edited by bfletcher7; 01-09-2017 at 08:31 PM. Reason: Added clarification to my HP
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Old 01-09-2017, 09:15 PM   #4
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


not all of us are pros.

You're getting pretty good performance out of that machine, though i'm betting it's really oversized for a/c and you have to crank it down to pull the humidity out.

If the op's units are 2 stage and running at 3 tons each on low, i think long cycles are to be anticipated.

There are capacity tests using a thermometer and calculator (if the backup heat kw is known - airflow can be estimated, from there can calculate heat output), but i'm betting most techs won't do them. They'll throw gauges on, look at the pressures (which don't tell the tech jack on their own), if lucky check the indoor split and call it good.

without performance calculations there's no way to be sure.
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Old 01-09-2017, 10:24 PM   #5
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


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Originally Posted by user_12345a View Post
not all of us are pros.

You're getting pretty good performance out of that machine, though i'm betting it's really oversized for a/c and you have to crank it down to pull the humidity out.

If the op's units are 2 stage and running at 3 tons each on low, i think long cycles are to be anticipated.

There are capacity tests using a thermometer and calculator (if the backup heat kw is known - airflow can be estimated, from there can calculate heat output), but i'm betting most techs won't do them. They'll throw gauges on, look at the pressures (which don't tell the tech jack on their own), if lucky check the indoor split and call it good.

without performance calculations there's no way to be sure.
That's not exactly true. You can compare the pressures and amp draw and the indoor/outdoor temps to the manufacturers charts and get a rough idea. (+/- with lineset changes) They do have charts to match the specific outdoor unit to indoor coil. It's not laboratory accurate, but it won't be miles off either.

You are right about some techs rushing through things though. There aren't many that would check the charts, or even amp draw on a habitual basis.....



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Old 01-09-2017, 11:07 PM   #6
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


I like the simplicity of diy testing with a thermometer, especially with electric backup where u can calculate the airflow.

then only if a problem is suspected is it necessary to spend money on a tech.
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Old 01-10-2017, 12:22 AM   #7
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Quote:
Originally Posted by user_12345a View Post
I like the simplicity of diy testing with a thermometer, especially with electric backup where u can calculate the airflow.

then only if a problem is suspected is it necessary to spend money on a tech.
That's why i offer my help here. I might learn something in the process....

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Old 01-10-2017, 01:58 AM   #8
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Everything else said above is fairly accurate. The compressor is on average +3 x more efficient then the electric elements. (upto 5x in warm weather) Each system is a bit different though, so you do have to look at the specs of the unit and, like user mentioned, and test if it's actually living up to its potential. I'd expect a economic break even point well below 20*f though. (capacity is a different story. Too many variables to guess at that one.)

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Old 01-10-2017, 04:47 AM   #9
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Quote:
Originally Posted by CaptainHook View Post
I recently purchased a large house with two 5 ton Trane heat pumps. They appear to be working properly but run constantly (it was a similar story in the summer). Has anyone done a comparison of efficiency in temperatures below approximately 28-30 degrees?

Here are my current settings in my thermostats:
Stage 1 heat (seems to be full blower and 'low' stage compressor) at 0.5 degrees below set temperature.
Stage 2 heat (seems to be full blower and 'high' stage compressor) at 1.5 degrees below set temperature.
Stage 3 heat (seems to be full blower, 'high' stage compressor, and 15kW auxiliary electric heat strips) at 2.5 degrees below set temperature.
I do not see any settings for compressor lockout in my thermostats.

I'd change that to just 2 for the aux heat.

The last few days it has been very cold (daytime highs in the mid 20s and lows in the mid teens) which results in both heat pumps running nearly 24x7. The systems seem to be able to maintain the temperatures within 2 degrees of the thermostat settings (set at 68, maintains at 66-67), so they run in stage 2 heat.

I'm wondering at what point is it more efficient to run in stage 3 (auxiliary heat strips) instead of stage 2? Even though stage 3 uses much more energy, won't it run less frequently? Or does it use so much more than it is not even a valid consideration if stage 2 can maintain temperature within 2 degrees?
Somewhere around -10F. Or, if you know the COP of your heat pumps, when the COP drops below 1.3

When the COP drops below 1.3, time and temp defrost makes it about even with electric resistance heat, as far as energy used.

If they have on demand defrost, your good to about a COP of 1.1 to 1.2, as they will go into defrost very little at temps below 20F

Quote:
Originally Posted by user_12345a View Post

You're getting pretty good performance out of that machine, though i'm betting it's really oversized for a/c and you have to crank it down to pull the humidity out.
Being 2 stage units, he shouldn't have trouble with humidity control if the blowers are set up properly.
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Old 01-10-2017, 05:57 AM   #10
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Quote:
Originally Posted by CaptainHook View Post

I'm wondering at what point is it more efficient to run in stage 3 (auxiliary heat strips) instead of stage 2? Even though stage 3 uses much more energy, won't it run less frequently? Or does it use so much more than it is not even a valid consideration if stage 2 can maintain temperature within 2 degrees?
Have a look at the Trane data.
My heat pump comes with a data sheet stating the COP at different temperatures. As BT says, it's worth it to fly alone with the heat pump right down to a COP of 1.3 or so. Only use your heat strips as supplemental to that point.

My heat pump still allows for 15,000 btu at -10F with a cop of 1.67 so I push it right to about -12F before I lock it out.

Trane SHOULD have this data somewhere for you to get a hold of.

I have a two stage and personally speaking, I have found through a bit of bill comparing that the whole thing is more efficient with low stage bypassed on the heat pump. The COP is better in high stage heat pump (not heat strips) than low stage. As a result I don't run low stage at all in the Winter. I have the heat strips set up to act as supplemental heat (set to keep temp within 1 degree) starting at around -2, but I find that the heat pump handles its own until about -5 before the heat strips start becoming active.

Last edited by Bob Sanders; 01-10-2017 at 06:13 AM.
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Old 01-10-2017, 10:43 AM   #11
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Thanks for the replies. This morning it was approximately 18 degrees and I noticed that the upstairs heat pump did turn off temporarily as it reached the thermostat setpoint of 68 degrees. I have outdoor units 4TWX6060B1000A and 4TWX6060A1000A but am not able to locate any published COP. This is the only information I was able to find:

"A.R.I. STANDARD 210/240 RATING CONDITIONS — (A) Cooling 80F. D.B., 67F. W.B. air entering indoor coil, 95F. D.B. air
entering outdoor coil. (B) High Temperature Heating 47F. D.B., 43F. W.B. air entering outdoor coil, 70F. D.B. air entering indoor
coil. (C) Low Temperature Heating 17F. D.B., 15F. W.B. air entering outdoor coil, 70F. D.B. air entering indoor coil. (D) Rated
indoor airflow for heating is the same as for cooling."

I am unclear on what "D.B." and "W.B." mean.
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Old 01-10-2017, 11:13 AM   #12
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Quote:
Originally Posted by CaptainHook View Post
Thanks for the replies. This morning it was approximately 18 degrees and I noticed that the upstairs heat pump did turn off temporarily as it reached the thermostat setpoint of 68 degrees. I have outdoor units 4TWX6060B1000A and 4TWX6060A1000A but am not able to locate any published COP. This is the only information I was able to find:

"A.R.I. STANDARD 210/240 RATING CONDITIONS (A) Cooling 80F. D.B., 67F. W.B. air entering indoor coil, 95F. D.B. air
entering outdoor coil. (B) High Temperature Heating 47F. D.B., 43F. W.B. air entering outdoor coil, 70F. D.B. air entering indoor
coil. (C) Low Temperature Heating 17F. D.B., 15F. W.B. air entering outdoor coil, 70F. D.B. air entering indoor coil. (D) Rated
indoor airflow for heating is the same as for cooling."

I am unclear on what "D.B." and "W.B." mean.
DRY BULB
WET BULB

One of the reasons I went with the heat pump I did (goodman 4 ton) is because all the data was readily available and pretty detailed. I don't know trane so it may not be that way.

At any rate the goodman 5 ton 2 stage DSZC160601 which should be somewhat comparable to yours, states:
Low Stage: btu output 11,500 at -10 with a cop of 1.21
High stage: btu output 16,800 at -10 with a cop of 1.47.
(note the high stage has a better efficiency)

Provided the machines are somewhat comparable then you should be good till about -11 or so before you should lock out and shift strictly to strips. Of course you should expect long running times at temps that low.
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Old 01-10-2017, 11:15 AM   #13
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


I'll look it up on the portal later if you can't find it online.

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Old 01-10-2017, 11:21 AM   #14
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


Thanks, I looked pretty hard. The system was installed in 2005 so that might be why it's hard to find online.
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Old 01-10-2017, 12:26 PM   #15
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Re: Heat pumps, large house, and efficiency-Stage 1 vs. 2 vs. 3


-10f is a bit of a strech, it may save a little bit but the heat output is probably so low, it's not worth the wear.

traditional air source heatpumps aren't suitable for frigid weather.

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