Tankless oil-boiler water heater vs. other options?

Discussion in 'Tankless Water Heater Forum' started by Bratan, Apr 11, 2014.

  1. Bratan

    Bratan Member

    Messages:
    36
    Location:
    New York
    I just moved into a house that has oil boiler (Crown TWZ150 series) that also has integrated tankless water heater. I know very little about boilers/heaters so my clueless question to the experts is this an efficient setup?
    Will adding a hybrid (electric) heat pump water heater improve efficiency and save me some money? :)
    House is in New York (Warwick), 2200 sq ft (3 heat zones) and there's plenty of space in the boiler room (which is also pretty warm when boiler operates).
  2. Dana

    Dana In the trades

    Messages:
    2,969
    Location:
    01609
    A heat pump water heater completely separate from the oil boiler will be a cheaper way to heat hot water, and would allow you to run the boiler at a lower and more efficient temperature. With most tankless coils you need to keep the boiler at 160F or higher to get reasonable hot water performance out of it, but if you're only using it for space heating you can set the low-limit down to 140F, which reduces the idling losses of the boiler by quite a bit.

    The heat pump water heater will reduce the boiler room by a couple of degrees as well, which reduces the heat loss out of the house, and during the summer months it will be dehumdifying a bit.

    Since money is finite I'd be remiss in pointing out that you might get better bang/buck out of installing a high efficiency mini-split heat pump and heating a large space heating zone with it. It's ~2x the , but would be 4x the oil savings, and will typically pay for itself in less than 3 heating seasons at recent years' oil pricing. It's about half the cost of heating that zone with oil with an 85% efficiency boiler.

    Also, if it's a basement boiler room and it isn't insulated the greater fraction of that idling loss of the boiler is lost through the ~R1 of the exposed above grade foundation + R2 band joist, not to mention the huge air leak between the foundation and foundation sill (a leak larger than all window & door leakage combined in most homes that have not been air-sealed with blower-door guidance.)

    The TWZ150 has 179,000BTU/hr of output which is a RIDICULOUSLY oversized unit for heating a 2200 square foot house. Tighened up a bit and with storm windows or clear-glass double panes the heat load of that place probably under 50,000BTU/hr with an uninsulated foundation, and would be be under 40,000 BTU/hr. Oversizing was often done to improve domestic hot water performance, but 3-5x oversizing for the space heating loads takes a real toll on the performance, especially when the place is cut up into three zones, probably NONE of which has enough heat emitter to deliver the whole burner output into the space, which means it probably short-cycles with burn lengths of 2-3 minutes tops. Turn up the thermostat on your smallest zone (smallest being the zone with the least amount of radiator or baseboard), turn the other zone stat down, then time the burns and report back. A short-cycling 85% AFUE boiler 4x oversized for the design condition heat load will run at about 70% as-used AFUE, with a large fraction of the difference being dumped as standby overheating the boiler room, and up the flue during ignition cycles. There are retrofit controllers that can tame that a bit, but let's first see if it's really necessary- if you have a lot of high volume high mass radiator it might be doing OK, but if it's all fin-tube baseboard I'd be surprised.

    This boiler is cold-start tolerant but you need to set up the controls for cold-starting, which would be the right thing to do once you've divorced the domestic hot water heating function from the boiler. The manual (linked to above) discusses how that is done, and you may have to replace part of the controls to achieve that end. (I didn't read the thing cover to cover nor will I, but YOU probably should.)
  3. Dana

    Dana In the trades

    Messages:
    2,969
    Location:
    01609
    A heat pump water heater completely separate from the oil boiler will be a cheaper way to heat hot water, and would allow you to run the boiler at a lower and more efficient temperature. With most tankless coils you need to keep the boiler at 160F or higher to get reasonable hot water performance out of it, but if you're only using it for space heating you can set the low-limit down to 140F, which reduces the idling losses of the boiler by quite a bit.

    The heat pump water heater will reduce the boiler room by a couple of degrees as well, which reduces the heat loss out of the house, and during the summer months it will be dehumdifying a bit.

    Since money is finite I'd be remiss in pointing out that you might get better bang/buck out of installing a high efficiency mini-split heat pump and heating a large space heating zone with it. It's ~2x the , but would be 4x the oil savings, and will typically pay for itself in less than 3 heating seasons at recent years' oil pricing. It's about half the cost of heating that zone with oil with an 85% efficiency boiler.

    Also, if it's a basement boiler room and it isn't insulated the greater fraction of that idling loss of the boiler is lost through the ~R1 of the exposed above grade foundation + R2 band joist, not to mention the huge air leak between the foundation and foundation sill (a leak larger than all window & door leakage combined in most homes that have not been air-sealed with blower-door guidance.)

    The TWZ150 has 179,000BTU/hr of output which is a RIDICULOUSLY oversized unit for heating a 2200 square foot house. Tighened up a bit and with storm windows or clear-glass double panes the heat load of that place probably under 50,000BTU/hr with an uninsulated foundation, and would be be under 40,000 BTU/hr. Oversizing was often done to improve domestic hot water performance, but 3-5x oversizing for the space heating loads takes a real toll on the performance, especially when the place is cut up into three zones, probably NONE of which has enough heat emitter to deliver the whole burner output into the space, which means it probably short-cycles with burn lengths of 2-3 minutes tops. Turn up the thermostat on your smallest zone (smallest being the zone with the least amount of radiator or baseboard), turn the other zone stat down, then time the burns and report back. A short-cycling 85% AFUE boiler 4x oversized for the design condition heat load will run at about 70% as-used AFUE, with a large fraction of the difference being dumped as standby overheating the boiler room, and up the flue during ignition cycles. There are retrofit controllers that can tame that a bit, but let's first see if it's really necessary- if you have a lot of high volume high mass radiator it might be doing OK, but if it's all fin-tube baseboard I'd be surprised.

    This boiler is cold-start tolerant but you need to set up the controls for cold-starting, which would be the right thing to do once you've divorced the domestic hot water heating function from the boiler. The manual (linked to above) discusses how that is done, and you may have to replace part of the controls to achieve that end. (I didn't read the thing cover to cover nor will I, but YOU probably should.)
  4. Bratan

    Bratan Member

    Messages:
    36
    Location:
    New York
    Thanks Dana! That's a lot of great information I need to digest! I will try to time how long burns take (but yeah I noticed it comes in short bursts).
    BTW are you sure about 50,000 BTU/h? I live in pretty cold climate so I think it's something like 50 BTU/h * sq ft, which comes up around 110,000 BTU/h which of course is still less than 179,000 (I don't know why previous owner installed such a monster). Also it's an open floor-plan house with high cathedral ceilings so I think some energy loss might be from that :(
    It sounds tho that I definitely should go with heat pump for hot water. I've been doing some more research and someone mentioned that it takes approximately 1 gallon of oil per day to heat up just hot water. If that's true, then ouch > $1200 per year! Plain electric water heater will probably more efficient than what I have...
  5. Dana

    Dana In the trades

    Messages:
    2,969
    Location:
    01609
    Most houses in FAIRBANKS AK have heat loads smaller than 50BTU/ft of conditioned space. I'm pretty sure the heat load of a 2200' house in Warwick NY won't be over 50K unless there is unsual amounts of low hanging fruit on the insulation & air sealing upgrades.

    Warwick NY is in US climate zone 5, and has a 99% outside design temp of about +10F, maybe a coupla degrees cooler than that.

    http://weatherspark.com/#!dashboard;q=warwick, ny

    Realistic heat loads on 2x4 construction are typically under 20BTU/ft in US climate zone 5, around 15 BTU if it's pretty tight and has foundation insulation, not too many huge window area, etc.

    For example, my 1920s vintage 2x4 framed ~2400' house in central MA in a location 3-4F colder than Warwick in winter, where the 99% design temp is +5F. It has a head load in the ~35K range @ +5F, after some air-sealing and insulating the 1500' of semi-conditioned basement (not counted in the 2400' house size.) Before I started tightening it up it was running a bit north of 45K. I still have the original single pane 6/6 double hungs and it has 1980s vintage clear glass storms- this isn't some superinsulated house, it's nowhere near current code-min. If you have glass in the windows, and doors that actually shut, there's no way you are at 50BTU/ft x 2200' = 110KBTU/hr. even if it doesn't have insulation in the walls.

    Tankless coils run at about 50% efficiency best case in summer, and you'd be lucky if your hot water heating is going any better than 75% in winter at your oversizing factor. But assuming you use 63 gallons/day of 130F water (about the volume & temp of the DOE EF testing), and your incoming water averages 50F. Thats 8.34lbs/gallon x 63 gallons x (130F-50F)= 42,034 BTU /day for hot water heating. A gallon of oil has about 138,000BTU/gallon, but assuming your annualized average is about 70% efficiency(optimistic, to be sure) that delivers only 96,600 BTU/gallon into the hot water. So it's really more like half a gallon/day average for heating hot water, not a gallon, but it's still pretty expensive at recent years' oil pricing.
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