With more information on the system and heat load, we'd be able to interpolate a bit, once we know where you are on the regression curves in Appendix 1 and Appendix 2. Since you have an indirect hot water heater zone, system is probably closest to Unit #2, but the boiler's raw efficiency may be closer to Unit #1. Most oil boilers 25 years ago ran 83-85% raw combustion efficiency, but only 70-75% efficiency when oversized by 3x (which is typical), and that's even assuming there is enough baseboard on each zone that it doesn't short cycle on zone calls. Only if it's oversized to the point of short-cycling on/off only a few minutes at time 5+ times per hour it will take a bit of an efficiency hit would it be more efficient to leave it off during the day and run long cycle as it brings the room up to temp. When used intermittently the efficiency is lower during the temperature recovery ramp, and even if you're only using the room 5-6 hours/day in the evenings it usually uses less power if you keep the room at temperature, due to the much higher part load efficiency, and due to the higher efficiency at the higher daytime temperatures. Mini-splits can both under & outperform that depending on it's sizing factor relative to the 99% condition heating load, and the 99% outside design temperature. See figure 5:Īssuming a COP of 4 during the shoulder seasons that would reduce your operating cost on the mini-split to $2.46 x 2.5/4 = $1.53 / 100,000 BTU.Īn HSPF of 11 means if sized correctly it should deliver 11,000 BTU per kwh, when installed in a zone IV location, which would be a seasonal average COP of 3.2 (not 2.5):
Cast iron baseboard (BaseRay, et al) has a significant radiated component, as do short thin profile convecting panel radiators (Runtal, etc.)ĭuring the shoulder seasons (40F outdoors and higher) most mini-splits will operate at a COP of about 4 running at mid-speed or low speed. Fin tube baseboard isn't radiant heat- they are convectors.
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