What cutout temp do I run my heatpump to?

[icenfire01]

This fall I had my local HVAC tech install THJF24S41S3 heat pump (Colman 14.5 SEER) which is backed up by my older Carrier 92% AFU propane furnace (1998 I believe). My installer set the changeover point to 40* F which I think is quite high. As of today propane is just under $1.50 (cheap!) and my local electrical rate is about $.07 kwh (I think). Today I just lowered the changeover temp to 30*, is that sufficient for my setup?

 

[Jackfre]

Every heat pump will have a performance curve. Find the performance curve and power input for your unit and it will simplify your choice. Also you could check the amp draw/run time/cycle time and get an idea as well. Overlay all that information on the comfort in the house and you can decide. Also, ask the contractor why he chose 40* to bring in second stage.

 

[icenfire01]

Quite frankly I think he chose 40* just because. This was only his second heat pump installation. He was actually hesitant to sell me this one, he just doesn't believe in them I think.

 

[icenfire01]

Where do you find a performance curve? I have looked up the specs on line but have not seen that.

 

[pseudorealityx]

Good luck getting legit performance info from a Coleman unit.

You should be able to get the COP at 17 degrees (low temp) and 47 degrees (high temp). If you've got that, you can at least get an idea of cost per btu. Obviously the gas furnace is easy to calculate.

 

[icenfire01]

Good luck getting legit performance info from a Coleman unit.

You should be able to get the COP at 17 degrees (low temp) and 47 degrees (high temp). If you've got that, you can at least get an idea of cost per btu. Obviously the gas furnace is easy to calculate.

Im not following you, what do you mean by COP?

 

[pseudorealityx]

Coefficient of performance, BTU per KW input.

As temperatures fall, heat pumps get less and less efficient. They provide less BTU per KW input. All heat pumps are rated at both 47 degrees and 17 degrees, and a COP will be provided at each of those data points. Most traditional heat pumps will be ~2.5 COP at 17 degrees.

If you know your BTU per KW, you can determine your BTU per $$. And once you do that, you can find out where the switchover point is.

 

[jvitez]

COP: coefficient of performance, the heat output vs the electricity input ratio.

An example would be a geothermal heat pump with a COP of 2.5. That means for every watt of electricity the heat pump uses, it puts 2.5 watts of heat into your house. Any type of pure electric heat would have a COP of 1.0, ie 1 watt in, 1 watt out, or 100% efficient.

AFU is fuel burning appliance's version of COP. Your furnace converts 92% of the energy contained in the NG burned, into heat. Eight percent goes up the flue. So you could also say your NG furnace has a COP of 0.92.

Hope that helps.

 

[icenfire01]

That's makes a lot of sense, thanks for the explanation.

 

[zmaxmotorsports]

Set it between 25-30 and see how it works,you can fine tune it from there.

 

[sms1974]

In northern ohio we generally set them at 30 with a propane furnace. It does depend on heat pump and furnace efficiencies.

If your getting propane for a buck and a half I'd prob shut it off around 35/40

 

[bzinsky]

coleman = luxaire = york

https://www.theacoutlet.com/docs/00402924THJF.pdf

 

[icenfire01]

Thanks for that I had been looking on line and gave not seen that chart.

 

[pseudorealityx]

So yeah, depending on your exact matching, COP at 17 degrees is ~2.5ish. Good enough to be useful for calculating this. A couple of degrees on either side of the peak is tiny... maybe a few cents per day difference.

 

[icenfire01]

So for every Watt of electricity in should give me 2.5 btu's. How do I know how many watts my heatpump is using?

 

[pseudorealityx]

Ran the numbers... given $1.50/gallon for propane, and $0.07KW-hr for electricity. This assumes a 3.5 COP at 47 degrees, and a 2.5 COP at 17 degrees.

With the heat pump, at 47 degrees, you're spending ~$0.58 per 100,000 BTU
With the heat pump, at 17 degrees, you're spending ~$0.82 per 100,000 BTU

With the propane, at any temperature, you're spending ~$1.78 per 100,000 BTU


Conclusion: The heat pump is always more effective, BUT loses capacity as the outside temperature lowers. Use the heat pump until it can no longer keep your space heated, and then switch. You'll have to figure that out by trial and error, and things like humidity and wind will change the switchover point there.

 

[bzinsky]

Ran the numbers... given $1.50/gallon for propane, and $0.07KW-hr for electricity. This assumes a 3.5 COP at 47 degrees, and a 2.5 COP at 17 degrees.

With the heat pump, at 47 degrees, you're spending ~$0.58 per 100,000 BTU
With the heat pump, at 17 degrees, you're spending ~$0.82 per 100,000 BTU

With the propane, at any temperature, you're spending ~$1.78 per 100,000 BTU


Conclusion: The heat pump is always more effective, BUT loses capacity as the outside temperature lowers. Use the heat pump until it can no longer keep your space heated, and then switch. You'll have to figure that out by trial and error, and things like humidity and wind will change the switchover point there.

I was about to say wow propane is expensive (nobody around here uses propane for heat) But that electricity price is really low.

Electricity in my region, from several providers, is about 16-20 cents per kwh, that includes supply and distribution. Supply hovers around 9-13 cents and distribution is about 7.

 

[icenfire01]

I thought $1.50 was really cheap considering last year it jumped to over $4.00 a gal. And the price jump is THE reason I installed a heat pump!

 

[icenfire01]

Ran the numbers... given $1.50/gallon for propane, and $0.07KW-hr for electricity. This assumes a 3.5 COP at 47 degrees, and a 2.5 COP at 17 degrees.

With the heat pump, at 47 degrees, you're spending ~$0.58 per 100,000 BTU
With the heat pump, at 17 degrees, you're spending ~$0.82 per 100,000 BTU

With the propane, at any temperature, you're spending ~$1.78 per 100,000 BTU


Conclusion: The heat pump is always more effective, BUT loses capacity as the outside temperature lowers. Use the heat pump until it can no longer keep your space heated, and then switch. You'll have to figure that out by trial and error, and things like humidity and wind will change the switchover point there.

This is really helpful thank you for the info. Can I ask how you came up with the calculation? I would like to run the number for when propane changes price and I have to switch to space heaters again... [emoji35]

 

[bzinsky]

So for every Watt of electricity in should give me 2.5 btu's. How do I know how many watts my heatpump is using?

2.5 COP means its 2.5 times efficient as resistant heat, such as a space heater.

Resistance heat is 3.41 btu's for every watt.

So at 1500 watts, a space heater is putting out 5,115 btu's. The brand of space heater is irrelevant. All resistance heat puts out the same amount of heat per watt. Even an incandescent light bulb, or a TV, or computer monitor. All that energy it consumes turns into heat, and it's at 3.41 btu's per watt.

A heat pump with a COP of 2.5, at 1500 watts of consumption, is 12,787 btu's.

I didn't realize a until about a couple years ago it was possible to heat a place using only electricity and be more efficient than resistant heat. I was thinking, if all electric heat is the same, how is it possible to be more efficient. I had a serious "ah-ha" moment when I figured that one out. The difference is the heat pumps absorb energy from one place and transfer it to another, hence the term "air-source" and "ground-source" heat pump.

 

[yeldogt]

Heat pumps are more efficient then resistance heat way down below 0 ........with expensive electric they are still more efficient then propane into the singles. Since HP's are also sized for cooling in the summer (smaller capacity need) .. at some point the HP is not going to be able to give you enough heat .. even though the heat is it giving you is cost effective.

Another problem with most heat pumps -- the gas furnace can not run with the HP -- so you can't let the HP run and use the furnace to make up the difference .. this would be great with propane. You can with heat strips and hydronic coils.

Most HVAC guys just pick a standard temp that they know will work ..40. With all the advances in HP technology things are really changing. Todays variable speed compressors are allowing for large capacity units to be installed that will act like smaller unit in the summer to cool and dehumidify and still have the high capacity in the winter when the temps drop.

You are just going to have to play around a bit and test out where the system starts to fall behind. HP's are designed to run .. they don't like setbacks ... so when it is running all day and can't keep up as the sun goes down -- you have found the lockout temp.

 

[pseudorealityx]

This is really helpful thank you for the info. Can I ask how you came up with the calculation? I would like to run the number for when propane changes price and I have to switch to space heaters again... [emoji35]

At this point, you can just multiply what I gave you by a multiplier for the new price...


If it goes to $2/gallon of propane, then it would:

$1.78 * ($2/$1.5) = new price is $2.37 per 100,000 BTU.

 

[zmaxmotorsports]

coleman = luxaire = york

https://www.theacoutlet.com/docs/00402924THJF.pdf

Coleman/evcon has been a manufacturer for other peoples stuff,not just york for years.