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Community and Q&A

Too small a boiler

user-6623302 | Posted in Mechanicals on

Can a boiler which just meets your heat loss be too small to use an indirect water heater? Indirect WH seem to rely on significant amounts of boiler to do the job. If you are at design outside temperature, do you have to make a choice between hot showers or warm house. Is there some point were a standalone water heater is necessary and what is the effect on overall energy use. Oil is my fuel of choice if that matters, no gas.

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  1. user-2890856 | | #1

    Maintaining a jacket temp to insure good DHW performance year round is a fool's errand . Lots of folks say having one appliance is best , in this case whether fuel be NG , LP , oil , they'd be wrong . Use a stand alone , well insulated tank type water heater if using cast iron boiler .

  2. GBA Editor
    Martin Holladay | | #2

    In general, I agree with Richard on this one -- an indirect water heater connected to a boiler uses more energy than alternative methods during the summer.

    The brief answer to your question is: Yes, it's possible for a boiler to be too small to provide both domestic hot water and a home's space heat during design conditions (roughly speaking, that means during the coldest day of the year).

  3. user-6623302 | | #3

    Thanks for the info. I currently use an oil fired DWT which also provides space heating in season. I know this is not the best in the warm seasons but I hate to go electric but II guess that is going to be the best choice.

  4. Expert Member
    Dana Dorsett | | #4

    Even the smallest oil boilers tend to be more 2x oversized for the space heating loads of typical houses, and often don't hit their AFUE numbers (which are based on 1.7x oversizing factor) in situ. Even if sized EXACTLY for the 99% outside design heat load (not likely), a zone controller that gives priority to the water heater will not leave you cold, either in the shower or in the house, unless you're taking a dozen back to back showers during the very coldest hours of the year. The recovery time of the indirect is VERY short when taking the full output of the boiler, too short for the house to drop even 1F in temperature during the recovery period.

    The effects of oversizing and the off-season water efficiency of oil fired boilers at different oversize factors and control strategies can be found here:

    You'll find that with heat purging controls on the boiler (see System #3) the efficiency hit in hot-water-only situations or at even 3x oversizing isn't nearly as bad as with dumb aquastat controls. (See Tables 2 & 3 to cut to the chase, but it's worth reading the whole thing to understand what's going on.) That type of boiler control was not standard equipment on most oil boilers 10+ years ago when those laboratory tests were performed, but are now becoming the norm. Retrofit heat purging economizer controls are available and (usually) fairly easy to install as a DIY. (Boiler #3 was a Energy Kinetics System 2000, for the record.) Even without an indirect water heater heat purge controls are well "worth it", with a very high IRR on the investment.

    A typical electric tank water heater is 4500 watts, or ~15,000 BTU/hr. If your oil boiler has more than 15,000 BTU/hr in excess of the space heating load, you'll get better domestic hot water capacity out of the indirect even at design condition. A typical standalone gas burner delivers 30-32,000 BTU/hr to the water. The smallest oil fired burners deliver about twice that of a gas burner, 4x that of an electric tank.

    If not going with an indirec water heater, heat pump water heater located in the boiler room can "harvest" the boiler's standby losses and some of the distribution losses, and lower the heat loss of the boiler room by lowering it's temperature a degree or three. It would also provide summertime dehumidification for that space.

  5. Expert Member
    Dana Dorsett | | #5

    OK I've been pondering it for awhile- what means " ... an oil fired DWT..." in 'merican?

  6. GBA Editor
    Martin Holladay | | #6

    It's a Domestic Wa Ter heater.

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