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Marathon and a heat pump water heater

user-471779 | Posted in General Questions on

Planning on Installing a heat pump water heater 80 gal rheem. and dumping it into two marathon water heaters… 50 gallon each. One of the marathons would be right next to the heat pump the other 80 feet away. Big long house…. The one 80 feet away would feed 1 1/2 baths, kitchen sink, dishwasher, washing machine, and potentially outdoor shower…

I know there will be about a gallon and a half sitting in that 3/4 line.. Essentially I am preheating the water going into the marathon…

Here is the question.
So if I feed the Marathon that is 80 feet away from the heat pump with a well insulated 3/4 line…. Do you think I would be using more energy overall with this method than to just feed cold line to that heater and let it do all the work….

I know there will be about a gallon and a half sitting in that 3/4 line.. Essentially I am preheating the water going into the marathon…

your thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    The most obvious way to reduce your equipment cost would be to buy just one Marathon, not two. Why do you need to install a Marathon heater directly beside the heat-pump water heater?

  2. vensonata | | #2

    I am in the same situation, 80 feet to two bathrooms with showers and sinks. My thoughts follow similar to yours however.... The heat pump water heater is noisy so it must be isolated in the mechanical room. But waiting for hot water becomes an exercise in patience, doesn't it? So a point source electric only heater is necessary. On demand uses too much peak power and requires rewiring so that is a write off. The question comes down to the size of the tank being fed at a distance by the heat pump tank. I suspect that even a 3-5 gallons storage tank is adequate. The long line empties into the small hot tank and the line attains full heat since there is only 1.5 gallons in the line. The hot water from the heat pump merely passes through the small point source heater then. So it is not necessary to have a large expensive remote heater at all. The little tank does the same job better.

  3. Reid Baldwin | | #3

    It depends upon how much of your hot water demand is in short draws. Suppose that your HPWH uses 1/3 as much electricity as your Marathon to heat a given amount of water. For simplicity, assume your cold water supply is near the same temperature as your interior air. For a draw of less than 1 1/2 gallon that is not close in time to another draw, your setup uses 1/3 more electricity because both water heaters have to heat the water. For a long draws (but less than 80 gallons), your setup approaches 2/3 less electricity.

  4. charlie_sullivan | | #4

    It depends on the usage cycle. If were dominated by handwashing and short bouts of dishwashing most of the heat that the heat pump puts in would get lost from the 3/4" line, so you'd be better off feeding that remote marathon with cold water directly. On the other hand, if it's mostly longer draws, the 1.5 gallons in the line is a small fraction of the total water drawn each use, and you'd be better off feeding it from the heat pump water heater.

    You might be on the edge where it's not clear--a really good dishwasher might only use three gallons per wash. If I did the math right, heating 3 gallons with the heat pump plus 1.5 (that was sitting in the pipe) with the marathon uses the same energy as heating 3 gallons with the marathon. But that's assuming the ambient temperature in the house is the same as the incoming water temperature. If you are in a cold climate, that's not true, so that makes your scheme seem more attractive.

    I doubt the remote tank needs to be 50 gallons. Dumping 1.5 gallons of room temperature water into a 20 gallon tank sitting at 130 F only drops it to 125.5 F, even without active heating during that flow. If that seems problematic, a thermostatic valve on the output or on the shower control should take care of it. You'd then need less space and have lower standby losses.

    I am guessing that you are using the extra tank next to the heat pump because you have the heat pump set to never use resistance heat, and you want the marathon to recover faster than the heat pump's tank recovers using just the heat pump. Whether that saves energy compared to just using one of the heat pump water heaters combined heat-pump/resistance heater modes depends on usage cycles, and of course on the control strategy the HPWH uses to decide when to use resistance heat. So it's hard to say whether it's a good idea.

    Getting weather data to evaluate options for envelopes and heating systems is a lot easier than getting data on hot water usage cycles. And the variation from family to family within a region is a lot more for hot water usage. So it's hard to predict the impact of different schemes with much confidence, even with detailed modeling.

  5. user-471779 | | #5

    I am definitely getting into the weeds here for sure... Is there any way to determine what kind of standby loss is in the 3/4" insulated pipe filled with 1.5 gallons? because that is really the only downside right? I imagine if an hour later the sink is run again... could that water still might be 90 degrees or so? which will take much less energy to heat in the marathon?

    Or do I just recirculate the other manifold on the far side of the house.. Or tankless electric for just the sinks and dishwasher.. and let the showers and washer wait for the hot water 45 seconds supplied with 1/2 from the utility...

  6. Expert Member
    Dana Dorsett | | #6

    This image was originally on a website, and is the result of in-situ measurements of temperatures in a potable hot water system at different pipe insulation R values:

    Clearly with R2 you have maybe 15-20 minutes before the hot water is too tepid to be useful for many applications, and at R4 that might stretch to 30 minutes before it drops below 90F.

    Current code min is R3.

    No matter what, after a full HOUR in a ~60F basement the residual temp of the water isn't a very useful for most hot water purposes, and nowhere near 90F.

    The article may be archived somewhere in cyber space, and now defunct website is up for sale, want it? :-)

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