Skip Navigation
Home Improvement @lemmy.world bbbbb @lemmy.world

Heat Pump switch-over temperature?

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?

2
2 comments

  • I’m on mobile and tbh don’t have time to dig into this too much, but I think you would do well to calculate the point of optimum efficiency for your specific scenario. I am in a similar scenario and hope to put together a spreadsheet that I could share if it doesn’t already exist elsewhere, but here is what I would do:

    1. Find out if your heat pump IOM specifies the minimum ambient temperature before electric auxiliary heat kicks in (sometimes called “em heat” or emergen heat). Electric heat will always be the most expensive source of heat, so you should use oil heat when temperatures drop below that.

    2. Look up the peak electric rate (in $/kW-hr) for your utility company. Use this to calculate the cost, per hour, to run your heat pump in all temperatures tabulated in your link. This will tell you how expensive it is to run your heat pump in a worst-case scenario.

    3. Calculate the cost to run your boiler, per hour. This is where you will have to do your own homework on efficiency of your boiler, rate of consumption, and cost of oil in your location. Hard to say if the boiler will run at full capacity or part load, but most are capable of running between 20-100% of nameplate capacity (5:1 turn-down). Summarize your findings into a coat to run your boiler per hour.

    4. When the answer for #2 exceeds the answer for #3, you’ve identified your switch point! Note that this relies on a number of assumptions, like that the heat pump is running full capacity.

    • 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.