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Duel Fuel cut over with defrost mode

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3.1K views 18 replies 6 participants last post by  klingon  
J
#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
 
J
#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
 
B
#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.
 
R
#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.
 
B
#12 ·
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.
 
B
#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.
 
K
#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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