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Domestic hot water heat recovery: Two systems or one?

Edward Cambridge | Posted in Mechanicals on

We have a large footprint house with multiple bathrooms. The Power-Pipe product seems to be an excellent way of recovering shower waste heat. The question is how to deal with multiple bathrooms that are more than 20 feet apart in a 3 story house. A central drain would be an easy otion and allow the use of one power pipe BUT the hot water woudl need to traverse a great deal of PVC pipe before getting to the power-pipe. It would also be complicated to plu,b to feed from the power pipe to to the mulitple showers drawing from it (in order to plumb the cold side of each). In this configuration a Weissman or BUderus system woudl be used as it could also provide some space heating for the radiators.

Another option would be 2 power-pipes, each feeding a separate how water system. This is a renovation of a large ~6500 sqft home and so the idea of 2 DHW systems is not crazy. That woudl allows the natural gas system to be the main system for the house with (perhaps?) a simple electric tank provisioning 2 bathrooms that are distant from it.

Given the cost of electricity, the likelyhoold that PV may well become power of choice, the low cost of electric tanks but hampered the inefficiency of a plain boiler – what does the math work out to be?
Really struggling with this one….

Help?

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Replies

  1. D Dorsett | | #1

    The lateral drain distance in PVC drain doesn't matter a whole lot. Since the surface area contact between the water and PVC is small, very little of the heat is transferred to the PVC. When the drain is vertical the water clings and spreads along the inside of the drain resulting in 5-10x the surface contact (which is why gravity film drainwater heat exchangers need to be mounted vertically.)

    It's pretty common for the drainwater heat exchanger to be configured to feed the entire cold water distribution system in the house rather than a direct home-run to the shower & hot water heater. The output temperature of a typical 40-60% efficiency heat exchanger is at room temperature, or slightly above. In the event that another cold water tap is being used, the higher flow on the cold water side of the heat exhcanger results in a lower output temp than when just the shower is running. In practice this is rarely a perceptible problem.

    But every house has different plumibng constraints.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Edward,
    There are lots of moving pieces of this puzzle. Dana Dorsett has provided you useful information -- don't worry too much about horizontal (or nearly horizontal) PVC pipe.

    On the question of "One water heater or two?", you might want to read these two articles:

    Domestic Hot Water: No Perfect Solution

    Hot-water circulation loops

  3. Edward Cambridge | | #3

    Martin,
    Thanks for the additional information. I had read the article on circulation loops and plan that approach.

    One question that is wrapped up in my earlier post is "at what point does the benefit of gas for water heat get outweighed by using a separate electric resistance boiler"
    Knowing that this house (at some point) will have PV, I have it in my mind that a traditional electric boiler is not so wildly inefficient in an application such as this one (if a power pipe is used)

    Anyone ever looked at that comparison (gas vs electric when a power pipe is used)

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Edward,
    When you use the word "boiler" instead of "water heater," I assume that you are talking about space heat as well as domestic hot water.

    Every designer has to make their own judgment on this one. When it comes to space heating, I would always advocate ductless minisplits over an electric-resistance boiler. (The minisplits will use 1/3 as much electricity.)

    When it comes to domestic hot water, the usual advice (for an all-electric house) is to install a heat-pump water heater. But if the house has enough room for a big PV array, and the local utility offers decent terms on their net-metering contract, it's possible to make a good argument in favor of an electric-resistance water heater.

  5. Edward Cambridge | | #5

    Thanks Martin.
    The plan is to use an indirect Gas to provide hydronic (radiator) heating as well as supply hot water needs.
    The second water heater would be to serve a distant set of bedrooms (leaning against a second system at this point)

    The house will ALSO have a ducted minisplit system (Fujitsu given better performance in-duct)
    The challenge has been getting support to do an ONLY heat pump system. Fujitsu's top rated installer in the Boston area would not do the install. They consider the multi-head systems to be very problematic to troubleshoot and were very reluctant to move forward. They put me in touch with the regional fujitsu rep at Sweeney Rogers Geraghty who frankly was nice but not that helpful.

    So, we're coming out with a largely minisplit heated/cooled house with radiators as supplemental/backup. This is a large old victorian (9k sqft) and although it will be superinsulated (4" exterior foram and densepack) it's still BIG. Alexander Bell is going to be diong a heat-loss by room and so we will have a better handle on what's required but this is our current thinking.

  6. User avatar
    Dana Dorsett | | #6

    The term "boiler" can refer to a potable water heating appliance in the UK or some of the Commonwealth countries, sometimes referred to as a "cylinder" as well. But in North America "boiler" refers exclusively to space heating equipment. A tank of water heated with a space heating boiler is called an "indirect hot water heater".

    So, terms like "indirect gas" get to be a bit confusing.

    Unless you share the costs of natural gas & electricity, there is no way to determine whether/when there is a benefit to one or the other. In eastern Massachusetts the retail price of natural gas is about $1.25/therm or $12.50/MMBTU in recent years' average, whereas electricity is about 18-22 cents/kwh, depending on your supplier.

    https://www.eia.gov/dnav/ng/hist/n3010ma3m.htm

    https://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a

    At 3214 BTU/kwh that's 293 kwh/MMBTU, or $53-64 /MMBT 4-5x the cost of natural gas, if used at the same efficiency.

    In an EF3.0 heat pump water heater than drops to about $20/MMBTU.

    For a 0.67 EF gas hot water heater that rises to about $18.28 /MMBTU.

    For an HSPF 12 right-sized ductless heating it's 83 kwh/MMBTU of heat delivered, which costs $15-18/MMBTU, which is not very different in cost than heating with a standard hot water heater & panel radiators.

  7. Edward Cambridge | | #7

    Dana, you have exposed my British roots and without hearing my accent!

    Your mathematical solution answered the question concisely. It makes no sense to use electricity to boil water.

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