[Bob Sanders]

It's -21c (-6f) right now and my heat pump is still working fine with no heat strips (except of course when in defrost).

I have set the heat pump cut off at -23c (-10f) even though the data says it will continue to deliver 15,000 btu at that temperature. I'm debating whether that's a good cut off point or I should continue to let it run past that and allow aux heat to come on and make up the difference.

At -10f it's still pumping out usable heat but the COP at that point is down to 1.67... still cheaper than resistive heat but not by a whole lot. The debate I'm having is whether or not I'm actually saving money at that point, or losing it in the long run with a shorter life span for making it work for longer hours.

 

[roughneck]

How big is your heat strip, and how is it staged on? What's building heat loss at -10?

 

[yuri]

I vote for losing it.

Those condensor fans unless ECM have sleeve bearings ( not balls ) and will wear out.

Also compressors are not built like they were 20-30 yrs ago and you will shorten it's life IMO. Constant vibration on the circuit board outside will shorten it's life.

In the REAL world you don't get somethin for nothin.:vs_no_no_no:

 

[WyrTwister]

Which shortens the life of a compressor . Start / stop or continual running ?

God bless
Wyr

 

[beenthere]

As long as its COP is above 1.3, its saving money on the heating bill. Below a COP of 1.3, the defrost cycle on a time and temp eat up most, if not all of the savings.

 

[yuri]

Which shortens the life of a compressor . Start / stop or continual running ?

God bless
Wyr

Both. Parts break eventually from vibration, metal fatigue. I have seen compressors break internal components like connecting rods or drive shafts. It is still a engine/motor/machine.

Excessive starting is hard for ACs especially in very hot climates as they will overheat.

 

[Bob Sanders]

How big is your heat strip, and how is it staged on? What's building heat loss at -10?

2 stage.
stage one 10Kw
stage two 5Kw

The heat pump is on stage 2 most of the time (continuous) at at about -8f so I'm guessing heat strips will be required to start subsidizing at around -10f.... although I'll probably start cycling with the 10kw strip

 

[Bob Sanders]

As long as its COP is above 1.3, its saving money on the heating bill. Below a COP of 1.3, the defrost cycle on a time and temp eat up most, if not all of the savings.

Unfortunately there is no data beyond -10f, but by the way it's dropping on the chart I'll probably hit a 1.3 cop at around -12f (-25c), so if I decide to keep it running then the cutoff I guess should be there.

I'm pretty impressed though. I didn't think I would make it this far down without a little subsidizing with heat strips!

 

[roughneck]

Is it holding or loosing space temp at -10 outdoor ambient?
If it makes you feel better it's 64* here right now. Heat hasn't run in almost a week. Air conditioner ran during Christmas lol.

 

[Bob Sanders]

Is it holding or loosing space temp at -10 outdoor ambient?

Actually I don't know that. We have only been below -10 for a few hours so far this year. Not really long enough to make any serious discoveries.... plus at the time I had the cut off set for -10 at which point the heat strips took over full time. I think I will drop the cut off to -12 just to see how it works.

 

[Bob Sanders]

If it makes you feel better it's 64* here right now. Heat hasn't run in almost a week. Air conditioner ran during Christmas lol.

LOL!
I think it was this time last year that we were OFFICIALLY colder than Mars so we're doing pretty good!

 

[supers05]

Why don't you actually measure your heat gain and cop. If have a thermometer, you do it. (Digital would be better)

1) turn off the compressor and let one of the elements run. Measure the temp rise over the AH. The 5kW might be better as it will allow a longer run time before cooking you out of there. If you run 2 speeds of your blower, then measure both speeds.

2) calculate actual air flow(s).
CFM = kW * 3.41/(1.08 * DeltaT)
(If you can get an actual amp draw and voltage during operation that'll be more accurate)

3) run the compressor without heat strips. Again measure heat rise. (IIRC you have 2 stage, you'll be in 2 stage at this point but it'll be cool to know the performance of 1st stage if you can manage)

4) Measure amp draw and voltage of outdoor AND indoor unit.
Power consumption (VA) = Volts * Amps), adding both outdoor and indoor units

Real power consumption is VA/PF, which the PF would be at the most 0.9 for a compressor, usually lower but it's unlikely that you'll be able to measure this, so use 0.9.

5) calculate BTU output
BTU = 1.08 * CFM * DeltaT

6) Calculate COP
= output power/input power
= BTU/ (3.41 * input power)

7) If you feel like a scientist by now, you can measure energy use during defrost and for how long your demand defrost is running and how long. Then you can calculate the average COP.


Cheers!

 

[Bob Sanders]

Actually that's a good idea. The IAQ thermostat already has the ability to measure delta t on each stage so the info should already be on the logs.

 

[user_12345a]

You must have very low heat loss, being able to heat with 20000 btu or less at -21c.

 

[Bob Sanders]

You must have very low heat loss, being able to heat with 20000 btu or less at -21c.

It's a bit surprising too because my house is 103 years old. I redo the siding and added 2" inches of foam at the same time years ago. I guess that did a better job than I thought it would.

 

[user_12345a]

maybe the strips are coming on a little and you don't know it or it's giving more heat than you think.

The 2" foam on the outside must do better than batts because of the closed cell nature of the material and no thermal bridging. I'm sure it also provides a really nice air barrier.

 

[Bob Sanders]

maybe the strips are coming on a little and you don't know it or it's giving more heat than you think.

No.
I've been watching for that very carefully. That would also show up in the bill, and it has been quite the opposite. The savings so far has been pretty substantial.

 

[WyrTwister]

One way to monitor each strip , is to use an amp meter to test each one . If it is indicating amperage , it is consuming electricity and producing heat .

This is even more positive than using a volt meter .

God bless
Wyr

God bless
Wyr

 

[Bob Sanders]

One way to monitor each strip , is to use an amp meter to test each one . If it is indicating amperage , it is consuming electricity and producing heat .

This is even more positive than using a volt meter .

God bless
Wyr

God bless
Wyr

The strips have already been checked for shorts to neutral and other things. They're in good condition and are only on when the thermostat directs them.

The IAQ thermostat keeps logs of when each stage comes on and for how long they run.

 

[user_12345a]

Also compressors are not built like they were 20-30 yrs ago and you will shorten it's life IMO. Constant vibration on the circuit board outside will shorten it's life.

I honestly thought that compressor is one of the few things that is still very well made, usually don't fail unless abused. The scrolls have no valves, supposed to last even longer than the old recips.