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Community and Q&A

Add on demand to boiler system

Bill Skaff | Posted in Mechanicals on

We have a baseboard hot water boiler that is also used to heat our DHW(built in 1988). Seems we have larger than average gas bills even in summer. I have noticed that the boiler comes on even after 1 shower.
I am not sure if the temp set points are too low, boiler needs inspection, or to eliminate the side arm from boiler to DHW( Then add an on demand water heater). Bill over $100 mth seem very high for 2 people.

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  1. GBA Editor
    Martin Holladay | | #1

    You guessed correctly. Using a boiler to produce domestic hot water during the summer is inefficient.

    There are many variables: older high-mass boilers are worse than newer low-mass boilers. Domestic hot water systems based on a tankless coil are much worse than systems that have an indirect tank.

    The bottom line is that lots of options (including, in many cases, a heat-pump water heater) are cheaper to operate. However, buying a new water heater is expensive. So your decision on what to do about your inefficient set-up will be based on how long you plan to live in the house, and whether or not you have any extra money in a savings account.

  2. GBA Editor
    Martin Holladay | | #2

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

  3. Bill Skaff | | #3

    Thanks for the info. I'll let you know what we decide.

  4. Expert Member
    Dana Dorsett | | #4

    Heating water with an embedded tankless coil can be grossly inefficient in summer, but with a cold-start boiler and retrofit heat purge controls with an indirect hot water heater (operated as the priority zone) it can reduced to being worse than a standalone fossil-burner tank type water heater.

    See the summertime efficiency of tankless-coil types vs. system #3 (an oil boiler with heat purge controls and an indirect) in Table 2 this document:

    A worn out ~73% combustion efficiency boiler with an embedded coil for heating water lsuch as system # 8 delivers about 37% efficiency for summertime water heating- literally half it's steady state efficiency.

    An 87% combustion efficiency boiler with heat purge controls and an indirect delivers 75% efficiency heating water in summer, or about 85% of it's steady-state efficiency.

    A 30 year old gas burner is typically delivering about 78-80% steady state efficiency, but if oversized and kept at a high temp to support domestic hot water used it's AFUE is in the dumpster. But gas burners can be cold-fired without significant damage, so if set up for cold firing and retrofit heat purging retrofit controls it should be able to muster about 0.85 x 80% = 68% effiency during the summertime, which is comparable to an Energy Star standalone tank.

    But the additional load and heat purging controls will also help the as-used AFUE during the heating season as well, reducing the standby losses, and the total number of ignition cycles. The net annual savings is better than just turning off the boiler for 3-4 months, and heating with a tankless water heater, but the installed cost is comparable. The greater the oversizing factor of the boiler, the larger the savings will be.

    To assess whether retrofit boiler controls and an indirect would work...

    What boiler manufacturer & model /make?

    What are the nameplate input BTU and D.O.E output BTU?

    How big is the existing side-arm heater?

    To assess the oversizing factor of the boiler, run a fuel-use based load calculation:

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