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Just moved into new house (actually 20 years old, just new to us). It has fuel-oil baseboard heat with six zones.

Richard Byrd | Posted in Mechanicals on

The system works well, but now that hot weather is here the furnace runs a lot to just heat the Hot Water Heater. That seems very inefficient to me. Running a 100,000 BTU/Hr. system just to heat water. Would not a well insulated electric HWH be more efficient and then the furnace could be totally off for 8 months a year?

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Replies

  1. User avatar
    Peter Engle | | #1

    Better yet, consider a heat pump water heater. It will cool and dehumidify the room it's in as well as using less electricity to provide hot water. Your actual payback, of course, will vary depending on your electric rates, hot water usage, climate and many other factors.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Richard,
    You're right. You usually don't want to run a 100,000 BTU/h oil burner to make domestic hot water during the summer -- although if you have a big, well-insulated storage tank -- an indirect tank -- it might not be as inefficient as you think.

    For more information on this topic, see Domestic Hot Water: No Perfect Solution.

  3. User avatar
    Dana Dorsett | | #3

    First, terminology: If it's hot water pumped into baseboard it's a boiler, not a furnace. In the HVAC world furnace implies ducted hot air.

    A 100,000 BTU/hr burner is about 3-4x oversized for the space heating loads of most 2x6 framed homes smaller than 3500 square feet. It takes about 200' of baseboard to emit at a rate 100,000 BTU/hr at an entering water temp of 180F out of the boiler. If it's cut up into six zones it's also probably short cycling on zone calls, since you CLEARLY don't have a large fraction of 200' of baseboard PER ZONE, and you might not even have 200' of baseboard for the whole house.

    It's worth measuring it all up zone by zone to figure this out, but since it's approaching end of life it's time to have the replacement figure out in advance, and whether there are cost effective ways of improving system efficiency.

    The boiler is probably ridiculously oversized solely to be deliver sorta-reasonable hot water performance, but even that requires a fairly high idling temperature, which increases the standby losses.

    If you have some wintertime oil fill-ups and amounts it's possible to estimate the whole-house space heating load using a fuel-use heat load calculation, as explained here:

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

    It's likely that a combination of adding an indirect water heater (operated as a "priority zone") and a retrofit heat-purging boiler control would be your best bet in the near term, eventually replacing the beast with something more appropriately sized for your space heating load. A heat purging control would allow you to drop the low limit to 140F (even low on some boiler models) and would maximally use the availble thermal mass in the system to limit the total number of burn cycles, lengthening the cycles. Without knowing all the details of your system I would expect a ~15% reduction in overall fuel use with this approach.

  4. Jonathan Blaney | | #4

    How about adding a properly sized buffer tank to the heating system?

  5. User avatar
    Dana Dorsett | | #5

    Buffer tanks aren't cheap, and the size needed for the late-middle-aged 100KBTU burner is larger than what would be needed for a more appropriately sized boiler that will (possibly sooner than later) replace it.

    Whether a buffer tank would be the best investment here requires more analysis, including the room by room heat loads relative to the room by room, zone by zone radiation output. In some cases a "reverse indirect" water heater as the central buffer can be an appropriate solution.

  6. User avatar
    Dana Dorsett | | #6

    Regarding the as-used efficiency of oil boilers for both space heat and domestic hot when oversized for the space heating load, see this document:

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

    Cutting to the chase, take a look at Table 2 and Table 3.

    In Table 2, the only system tested that has decent summertime water heating efficiency is system #3, which is a steel boiler + indirect using heat purge controls. While ~75% efficiency is ~11% less than the steady-state thermal efficiency, it's WAY ahead of the other systems. System #1 , the cast iron boiler with tankless coil is probably most similar to what you have, and it's summertime water heating efficiency is less than half it's steady state efficiency. Retrofitting a heat purging controller and lowering the idling temp would make it perform more like system #3.

    In Table 3 that same system #3 with the heat purge controller takes only about a 1.5% hit in average thermal efficiency even at a 3x oversize factor, whereas systems with dumber controls all lose 8-12% in annual efficiency.

    Twenty years ago only Energy Kinetics was shipping boilers with heat purge control as standard equipment. In the decade since that study was performed nearly all oil boilers now either come with a heat purging controls as standard equipment, or have that as a factory-installed option. Unless your boiler is an Energy Kinetics System 2000, or has already been retrofitted with a heat purging control, it's probably taking a 10% hit in thermal efficiency simply due to the oversize factor and high idling temperature needed for the domestic hot water, and another 5-10% related to short cycling on zone calls in an 6 zone system.

    This is why installing a heat purging retrofit economizer control is going to have a very short payback period, probably less than one heating season if installed as a DIY, usually less than 3 heating seasons if installed by a (non-gouging) pro.

    The phrase in the original post "...then the furnace could be totally off for 8 months a year..." implies a cooling dominated climate(?). In a cooling dominated climate it's probably worth looking at heat pump water heater, or standard electric tank and during the boiler off when it isn't needed.

    If the heating season is only 4 months long and it's a warmer climate the oversize factor could be north of 4x. In that case the payback on a DIY heat purge controller might take as long as two heating seasons, maybe five if installed by a pro (still "worth it" ) when using a ludicrously oversized boiler ONLY for space heating on a micro-zoned system, not for hot water too.

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