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Duel Fuel cut over with defrost mode

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jrongo

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Discussion starter · #1 · (Edited by Moderator)
I am using this website to try and calc. my cut over outside temp for best $ savings:



I am using an electric rate of .186 per kw
I am using a COP of 3.17 (but not sure about defrost mode) @35 deg.
I am using an oil burner Eff of 80%
I am using a oil price of 2.499

I do I factor in defrost mode?

Thanks so much.

Cheers,

jrongo
 
#4 ·
yeah, often when I post something dumb the lurkers rush in to prove how smart they are

I am hoping we will both learn something
again it does depend on humidity ect so it isn't going to be a perfect calculation
 
Discussion starter · #5 ·
Yes - I am interested to hear feedback as well
no way the COP at 0 F is 1.82 if in defrost mode say 25% of the time...


Temp COP

-10 1.34
-5 1.59
0 1.82
5 2.03
10 2.24
15 2.43
17 2.5
20 2.61
25 2.77
30 2.94
35 3.17
40 3.36
45 3.55
47 3.63
50 3.76
55 3.94
60 4.1
65 4.25
 
#6 ·
jrongo said:
Yes - I am interested to hear feedback as well
no way the COP at 0 F is 1.82 if in defrost mode say 25% of the time...


Temp COP

-10 1.34
-5 1.59
0 1.82
5 2.03
10 2.24
15 2.43
17 2.5
20 2.61
25 2.77
30 2.94
35 3.17
40 3.36
45 3.55
47 3.63
50 3.76
55 3.94
60 4.1
65 4.25
Click to expand...
COP can certainly be 1.82. COP does not take defrost into account, but you will normally create more watts of heat than the watt input. Problem is you can not generate enough heat, so supplemental is needed.

paul
 
#8 ·
If you want to allow for defrost loss. then your switch over should be at a COP of 2.5, at 2.6 your ahead of the game. At 2.5, your about even, using the heat pump and allowing for defrost.

At A COP of 2.5, and the electric rate you posted. A heat pump would cost $21.80 for 1,000,000 BTUs of delivered heat.

Oil at 80% and $2.499 a gallon, would cost $22.31 for 1,000,000 BTUs of delivered heat.
 
#9 ·
I think you need to find out how many BTU's of heat the unit can produce at given temps. Your unit is still efficient down to single digits and below but it likely won't produce enough heat to keep your house warm.
 
#10 ·
Rabbler said:
I think you need to find out how many BTU's of heat the unit can produce at given temps. Your unit is still efficient down to single digits and below but it likely won't produce enough heat to keep your house warm.
Click to expand...

Don't confuse efficiency with saving money.

The heat pump will be more efficient at 10°F then the oil furnace is. But will cost more to use for the same amount of heat.
 
#11 ·
I don't think I am.
I was commenting more about comfort.
If the heat pump was running 24/7 at 10F outside temp, sure it might be more efficient than the oil furnace but if it can only keep the house at 50F it's certainly not an ideal setup.
Wouldn't you want to switch over to your aux heat just a bit below the point where the heat pump can't keep up with the heat loss of the building?

This assumes the HP is always cheaper to operate than the aux heat.
 
#12 ·
Rabbler said:
I don't think I am.
I was commenting more about comfort.
If the heat pump was running 24/7 at 10F outside temp, sure it might be more efficient than the oil furnace but if it can only keep the house at 50F it's certainly not an ideal setup.
Wouldn't you want to switch over to your aux heat just a bit below the point where the heat pump can't keep up with the heat loss of the building?

This assumes the HP is always cheaper to operate than the aux heat.
Click to expand...
Your reasoning is right.

Just, I think the thread is about cost, not comfort.

In the OP's case. The heat pump will probably cost more to use at 40 or 35° then the oil furnace. And at 40 to 35°, the heat pump will probably still be able to handle the heat loss of the house.
 
Discussion starter · #13 ·
beenthere said:
Your reasoning is right.

Just, I think the thread is about cost, not comfort.

In the OP's case. The heat pump will probably cost more to use at 40 or 35° then the oil furnace. And at 40 to 35°, the heat pump will probably still be able to handle the heat loss of the house.
Click to expand...

You are right, the question was only about cost. I have no back up heat strips in my duct - I go heat pump or oil after the cut over.... I am going to use 34 degrees and hope the thing the heat pump does not run 20 hours a day - I wish I could come up with a hard cacl'd number...

Best,

jrongo
 
#18 ·
At worst. You could find that it can't handle the heat loss at 25. In which case you set the switch over higher. But even in that case, it still won't have cost as much as oil for the heat that it gave you.

Got some dual fuels out. That the heat pump runs first(our electric is cheaper then your posted rate) no matter how cold it is outside. When the indoor temp drops. then it switches to gas.
 
#19 ·
beenthere said:
Got some dual fuels out. That the heat pump runs first(our electric is cheaper then your posted rate) no matter how cold it is outside. When the indoor temp drops. then it switches to gas.
Click to expand...
I just got my Lennox system installed with the HW IAQ thermostat. The thermostat does just that. There is a aux lockout and a HP lockout with an adjustable droop setting. Above aux lockout (I set at 30), it will always run HP. Below HP lockout (I set at 20), it will only run backup heat. Anything btwn the 2, it will run HP first and if the indoor temp falls more than the droop setting (currently set at 2 deg), then it will kick on the backup aux heat. Not cold enough yet in PA to put this to test but thats what the manual says. So, this takes the guesswork out as the thermostat does the thinking for you.
 
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