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Community and Q&A

Installing an Indirect Water Heater

michaeldrehl | Posted in General Questions on

I keep getting conflicting information about installing an indirect water heater with my current hot water boiler. I live in Northern NJ. I run the boiler six months a year and shut it off for the rest of the six months. The boiler is properly sized for my home.
I was thinking of replacing my conventional hot water heater with an indirect tank running of the current boiler. It would make sense for the winter time but I’m getting conflicting information for the summer time.
Here’s Martin’s take in one of the forums.
Too small a boiler – GreenBuildingAdvisor

Any advice would be helpful.

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  1. paul_wiedefeld | | #1

    What's the point exactly of the indirect? It's not the most efficient option and won't be cheap.

  2. michaeldrehl | | #2

    I needed to replace my heating system and hot water heater.

    1. paul_wiedefeld | | #3

      No real reason to tie them together IMO. An indirect will be less efficient than tankless and some tank options, assuming you have natural gas/propane. Oil has a lower efficiency ceiling which makes even less sense. A heat pump water heater placed next to a boiler could be a thermal purge of sorts during the winter.

      It may or may not last longer, but will cost more. Maybe it'll make sense if you stay in the house for a few decades, but I doubt any buyer cares about an indirect. Unless there's huge simultaneous hot water needs, I don't see the point.

      What fuel are you using?

  3. michaeldrehl | | #4


    1. paul_wiedefeld | | #5

      What's the conflicting info you're receiving?

  4. michaeldrehl | | #6

    That indirect water heaters are a good choice over conventional water heaters.

    1. paul_wiedefeld | | #7

      It's a "depends" situation. It's not that significant either way in terms of cost or pollution.

  5. nexp | | #8

    I switched to an indirect with a mod/con boiler from a standard atmospheric gas heater as part of a complete heating plant replacement. Summer (non-space heating) gas usage is less than half. Is it as efficient as a tankless heater? Probably not. But there’s a benefit of only having to vent one appliance and only have to maintain one burner. If you’re already replacing a gas boiler, I would lean towards an indirect. The new boiler likely has built in controls for it, and with a stainless steel tank it will last a very long time. I doubt it would cost more than a tankless once installation is included.

  6. woobagoobaa | | #9

    I'm in the same situation as Cody and made the same decision.

    One gas appliance for heat and DHW means one unit to be serviced annually. Don't forget to factor that expense into your math.

  7. nexp | | #10

    I would also point out that summertime efficiency is probably considerably better with a mod/con boiler than a cast iron boiler - not because of the mod/con part (when in DHW mode, the boiler will neither be modulating or condensing), but because the significantly lower thermal mass of the boiler means less waste heat.

  8. Expert Member
    Akos | | #11

    The best set up indirect connected to a modcon is a couple of points more efficient than a quality non-condensing power vented tank. The probably of having getting a good install and proper configuration is pretty low in most cases, so efficiency would most likely never be that high. The cost of parts and install is at least 2x of a power vent, so you are spending a lot of money for not much benefit.

    An indirect only makes financial sense if you have a thermal solar or wood/pallet boiler.

    There is also no easy way to decarbonize an indirect water heater setup down the road as most air to water heat pumps don't produce hot enough water for DHW.

    With a gas fired boiler in the basement, chances are there is a lot of piping and heat loss. The best setup is a heat pump water heater, cheap to install, no boiler connection or controls required and can always connect it to a PV setup down the road. If you really want to use some gas to heat water in the winter time, the simples is to connect a sidearm to the tank. This way if there is enough run time, most of the hot water would be produced by the boiler.

    If you must stay with gas, a quality power vented tank or tankless water heater is your best bet.

    I'm not sure what maintaince some of the previous commenters refer to but all gas tanks I've owned over the years, were installed and never looked at again until about 10 to 15 years down the road when they started leaking. Tankless do need to be descaled, so they are definitely more maintenance.

    As for indirect efficiency, I have two similar rentals, one with an indirect on a modcon, the other with a tankless condensing water heater. With similar water use in both places, the one with the tankless uses about 1/2 the gas in the summer.

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