Hi folks, I got a heat pump installed at my home and I'm looking to better understand when/how it switches over to backup fuel in the winter.
My unit is the LENNOX ELITE EL18XPV SERIES, which I found heating specs on here
It is installed in conjunction with a heating oil fired boiler and an aqua coil as the backup heat source.
When it was installed, I asked, and the installer indicated that the heat pump will switch over to the boiler at about 37F.
What I don't fully understand though, is that this unit was billed as a variable speed/inverter, so it seems it should be able to run at lower temperatures than 37. The website above seems to indicate it should produce 31,800 BTU/h at 17F, so can I set the "switch-over" temperature of outside to be lower to use less oil?
Ah, ok, good point. In my install, I did not opt to have them wire and install electric auxiliary heating strips for my unit, and the boiler & aqua coil is I believe being controlled by a control system that makes the unit think the boiler is a heating strip. 17 F is about the coldest it got in my climate last winter, so I guess that's a good starting point.
So it seems my Heat pump alone would run at 4.2 kW for 31,800 Btu. at 17F. My blended electric rate is about $.12/kwH, so about $.5/hr to run.
For my house, I believe the load calculation would be that I need about 48,000 Btu to effectively heat. Our boiler runs at 0.75 gal/hr max, and heating oil was $4.69/gallon last winter, so the boiler would still need to run at ~20-25% of capacity(assuming it's a linear curve) at 17F to reach that btu number based on 138,500btu/gal x 0.75gal/hr x 0.8(not so sure on efficiency) x 20% capacity = 16,620 btu, which I believe at 0.75gal/hr x 0.2 = 0.15gal/hr would be ~$0.72/hr. So that would give me a total cost of ~$1.2/hr for 48,420 btu.
Without a heat pump, I guess I would solely be burning oil, so that would need to be about 57-60% load, so 138,500btu/gal x 0.75gal/hr x 0.8 x .6=49,860btu, which I believe at 0.75gal/hr x 0.6 = 0.45gal/hr would be ~$2.1/hr.
It seems like based on my electric rates and the values from that heat pump, it should offset roughly half of my heating oil usage at the 4.2 kW electric consumption assuming the boiler has some sort of ability to modulate oil in take.