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Community and Q&A

Domestic Hot Water Heating Options

user-1147664 | Posted in Energy Efficiency and Durability on

I’m looking for some advice on the most efficient, cost effective way to heat DHW for my project currently being built. The project is in zone 6 and includes two residences, a main residence and an “in-law” apartment separated by an attached garage. Both homes will be spaced heated by the same pellet boiler located in the basement of the main residence. I’m thinking that an indirect fired hot water heater might be the most efficient atleast for the main residence but I’m not sure about the feasibility of doing this for the in-law apartment. Also, I’m concerned about having such poor efficiency heating water with a furnace in the summer time, albeit our heating season is 7-8 months a year so maybe this isn’t as big of a concern.

Is it worth having two heaters for the primary residence: an indirect fired for the winter and switch over to an electric or heat pump heater in the summer months? Or is the upfront cost of two heaters not worth the gain in efficiency? Are their concerns with heating the inlaw apartment with an indirect hot water heater as well? Otherwise I’ll likely purchase a heat pump water heater for the inlaw residence. By the way, the in-law apartment will also have a basement to locate the water heater.

Thanks in advance for your advice!

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  1. GBA Editor
    Martin Holladay | | #1

    As you may know, your question about the "most efficient" water heater and the "most cost-effective" water heater are different. When it comes to water heating, I think that efficiency doesn't matter much; equipment cost matters much more.

    To answer your question, the most efficient water heater is either a heat-pump water heater (with an efficiency of about 200%) or a ground-source heat pump with a desuperheater (efficiency hard to measure, but probably at least 300%).

    There are too many variables here to come up with a simple answer to your question. For example, how much do you pay per kWh of electricity? Will the in-law apartment be occupied full-time? How much hot water per day will that apartment use?

    If your electric rates are low, the in-law apartment may just need a small electric-resistance water heater, located close to the bathroom or kitchen.

    For the main house, you have lots of options. For example, you could set up a large electric-resistance Marathon water heater near your boiler, and install a flat-plate heat exchanger with a loop to your boiler. Ideally, this loop would have its own dedicated circulator, and be controlled by an aquastat in the water heater tank. During the winter, you could turn off the electric-resistance elements; during the summer, you could turn the electric elements on and let your boiler get cold.

    For more information, you may want to see All About Water Heaters.

  2. user-1147664 | | #2

    Thanks Martin, your response was helpful. 200% efficiency is certainly compelling. However, I do like your idea of combining an electric resistance heater with a loop into my boiler. Our electric rates are just shy of $0.16kwh. Knowing my electric rates does your opinion change? It seems I'll need to calculate the payback of purchasing a heatpump heater versus your idea to combine electric and indirect.

  3. GBA Editor
    Martin Holladay | | #3

    A heat-pump water only makes sense if you have somewhere to put it. If you put it indoors in a cold climate, it will rob space heat from your building.

    More information here: Heat-Pump Water Heaters Come of Age</a.


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