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Radiant heat vs. split system heat pump

Alex Ivkovic | Posted in Energy Efficiency and Durability on

I have a 24 x 24 foot family room with a 16 foot cathedral ceiling. It’s built over a garage. It has radiant heat running off an oil fired boiler (the rest of the house has radiant baseboard. The room has a split system heat pump that is used to provide AC. During the week the room only needs to be heated for a about 5 or 6 hours in the evening. Does it make more sense to keep the radiant at a lower temperature, say 65 and run the split system in the evening or run the radiant at a higher temperature. It typically takes several hours for the radiant to come up to temperature

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Alex,
    If I understand you correctly, you are describing a room with two heating systems. One system burns oil, and the other is electric.

    To answer you question, we need to know what your goals are. Do you want to reduce your energy costs? If that's your aim, we need to know how much you pay for your fuel oil, and how much you pay for each kilowatt-hour of electricity.

    Perhaps you have another question -- one connected to comfort. If that's your question, the answer will be more complicated.

  2. D Dorsett | | #2

    Describe what you mean by "...radiant baseboard..." in greater detail. (The term means different things to different people.)

  3. Alex Ivkovic | | #3

    Martin- the heat pump is a Daiken LV Series HSPF is 11. Current electrical cost is .21 per KWH
    The radiant heat runs off a 25 year old boiler that has 4 zones. 1 for the family room radiant in question, it has a mixing valve to take the temperature down to 130. 2 zones are for the baseboard heat in the remainder of the house and the 4th is a hot water loop to an external hot water tank. Current oil cost is 2.00 per gallon. I originally thought of this question in terms of energy cost bu the comfort question is also interesting.

    By Baseboard heat I mean your standard slant fin available at Home Depot style baseboard

  4. GBA Editor
    Martin Holladay | | #4

    Alex,
    Your oil heat (assuming 80% efficiency) is costing you $1.76 per 100,000 BTU.

    Your electric heat (assuming a COP of 2.5) is costing you $2.46 per 100,000 BTU.

    In your case, the oil heat is 28% cheaper than electric heat. Use the boiler -- especially since it's already in use to heat the rest of your house.

  5. Alex Ivkovic | | #5

    That makes sense, thank you!

  6. Expert Member
    Dana Dorsett | | #6

    Fin tube baseboard isn't radiant heat- they are convectors. Cast iron baseboard (BaseRay, et al) has a significant radiated component, as do short thin profile convecting panel radiators (Runtal, etc.)

    During 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. See figure 5:

    http://www.nrel.gov/docs/fy11osti/52175.pdf

    Assuming 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.

    An 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):

    http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/images/Figure5_lg.gif

    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. 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. 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.

    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. See Table 3:

    https://www.bnl.gov/isd/documents/41399.pdf

    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. 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.

    If the boiler IS short-cycling on zone calls (which it might be, if you have so little baseboard that it takes hours to bring the space up to temp with the boiler) it won't even make 70%. You can buy some (but not all) efficiency back with a retrofit heat purge controller (eg Intellicon 3250HW+) which can be a DIY job for ~$200 or less for those with electrician skills.

    If you look at the boiler's nameplate for it's input BTU and DOE output BTU, then measure up the length of the baseboards on each zone (the length of the sheet metal, not the connecting plumbing, and not just the length of the finned section under the tin) it's possible to assess the impact of the oversizing factor for the zone radiation. You could also put a datalogger on the burner motor for few days and see what the minimum & average burn lengths are, and the average duty cycle, etc.

    If you have some mid to late winter oil fill-ups with the exact fill up dates, it's also possible to assess your whole-house heat load using the boiler as the measuring instrument, which would be a good way to determine the total oversizing factor. See:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

    If the boiler is tuned up regularly it may have been re-jetted to a lower firing rate at some point in the past 25 years, which typically lowers it's raw combustion efficiency slightly but typically improves it's duty cycle and net operating efficiency for a better "as used" AFUE. Competent burner techs will usually tag the boiler with the model number of the nozzle used, and the measured steady state combustion efficiency. If you have that information, that too would be useful.

    Almost no residential oil boilers in the US are hitting anywhere near their tested AFUE numbers unless it has better than average jacket insulation, a buffer tank, and heat purging boiler controls. But just how far below depends on the system particulars.

  7. Expert Member
    Dana Dorsett | | #7

    The above was perhaps unduly complex, but the short answer is that the oil boiler is probably hitting well under 80% efficiency, and the Daikin will deliver quite a bit higher COP than Martin's estimated 2.5 if you let it run night & day rather than turning it on when you use the space in the evening, and off when you go to bed.

    The particulars affect the real numbers (see prior post) but my guess is that except during the coldest weather running the heat pump will be less expensive than the oil-burner most of the time, and dramatically less expensive when it's above 40F outside.

    I'm happy to get into the particulars, and there are probably cost effective ways of improving the efficiency of the fossil burner (well worth doing) but probably not to the point that it would actually be cheaper than running the heat pump.

    If the mini-split meets it's numbers the cost would be $1.92 per 100,000 BTU. (During the shoulder seasons it'll beat it's numbers.)

    If the boiler is only running at 75% as-used efficiency (it's probably less, due to oversizing and short-cycling- see prior post) the fuel cost is $1.88, but it has the additional overhead of the pumping, burner blower, & control power, making it a wash, at best.

    Bottom line, it's ALWAYS going to be better to run the heat pump during milder weather, but the crossover temperature at which it becomes cheaper to run the boiler (if ever) depends, and needs more analysis.

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