Heat Pump set back
I've got a new Lennox xp15 heatpump.
Aux heat is propane gas furnace.
Seattle area, avg winter temps 30-45 degrees
3000 sq ft house
set to 35 degrees and lower the aux propane furnace will turn on.
I turned off the "switch to aux heat if indoor temperature has not been reached in 1 hour"
My temperature needs vary, so right now I don't use the programmable thermostat, I just leave it on Hold, and rise and lower as needed.
At 10pm every night, I turn my heat pump thermostat down to 64 degrees.
3 days/week, my set back hold is 64 degrees from 10pm-3pm the following day. (yes, 17 hours set back to 64)
Lets say it's 37 degrees outside.
At 3pm, I'll bump up the thermostat to 70 degrees (from 64). Heat pump will turn on with warm air coming out of the registers.
Comfort wise, I don't mind it taking 3+ hours for the heat pump to bring my house up to 70 degrees. I can feel warm air slowly heating my home
My question is economical and longevity.
1. Is it bad for the Heat Pump to work for 3+ hours to bring the house temperature up 6 degrees?
2. Propane is about $2.25/gallon now, is it more economical to run the propane furnace for 1 hour, to bring up the temperature quickly, then switch to the heat pump to maintain the heat? (on average, I know you probably need more info for an accurate answer)
At 2.25 a gallon for propane.
As long as your electric is less then 20 cents a KWH, the heat pump should always be cheaper with your outdoor temps.
Thats presuming that the HP's COP is 2.5 or greater at 30 outdoor temp.
Thank you beenthere and dac122.
I pulled my last electric bill to check KWH cost. Total bill / kwh = .094
I'm not quite sure how to calculate my COP easily
I have a 9 year old carrier weathermaker 58mvp propane furnace. I believe the AFUE is 92
Last months propane bill came to $2.19 / gallon
dac122, if you can easily figure out where the breaking point of cost efficiency for HP vs propane furnace, that would be greatly appreciated. If you want you can use .10/kwh and $2.25/ gallon for simpler calculation purposes.
You don't calculate the COP data. Contact Trane for data similar to this Goodman 14SEER 3 Ton.
Gas furnace with propane at $2.19 per gallon, 92% efficient furnace:
(1,000,000 / 91,600 BTU per gallon) x 2.19 / .92
Heat pump with electricity at 9.4 cents per KWH and COP = 3.11 at 35F:
(1,000,000 / 3413 BTU per KWH) x .094 / 3.11
You can substitute your data and calculate for all points on the COP curve, but this example should be in the ballpark and show that you can run that HP much lower than 35F for added savings. Obviously, defrost will eat into that savings. At some temp your HP is unlikely to satisfy your full heating needs, but you still be generating lower cost BTUs for part of your heat.
Thank you dac122.
My heatpump is a 4 ton 16.7 SEER and 8.7 HSPF, so perhaps my cost is even less than the above figures?
I reprogrammed the thermostat setback (0350) down to 25 degrees. Increments of 5 only. We're expecting overnight lows of 28 degrees, so I'll see if the house can maintain heat. I'll keep the house at 66 degrees overnight and bump it up to 70 degrees in the morning.
I love the heat pump so far. My only disappointment is that it is louder than I expected. The brochure and salesman says it is the quietest available but I can hear it, every time it cycles on, off, then goes into defrost mode, off, then cycles on again. Currently it's 32 degrees outside and the inside is an extra toasty 72.
Given your BTU cost disparity between propane and KWH, and your HSPF rating, I think you'll want to run that HP down to a much lower number if you want the greatest economic benefit. At some temp it will not be able to supply all your heating needs, but it will still be providing lower cost BTUs for some of your heating needs. If you don't like the cooler temps of the air then we are talking about a comfort balance point.
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