Plumb two HPWH in Series or Parallel


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Old 03-18-14, 10:33 AM
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Plumb two HPWH in Series or Parallel

First, let me explain my situation. I have a 4100 Sq Ft Single Story house, with copper pipes buried in the slab. 80 gal Elec water heater with Grundfos Recirc Pump. Because of the length of the recirc system, and the heat loss in the slab, I have to keep the recirc going from 6am to 6pm every day. Otherwise, it takes 15 minutes to get hot water to the kitchen ( last stop on the HW loop). Cost for Hot water is $150/mo

Bought an external HPWH last year, thinking it would help , and it did. However, it can't keep up with the heat loss,and the HW temp continues to fall during the day. By the end of the day, water is down to 90 degrees or less. It get's cool enough during the winter that the upper element of the Electric Heater is kicking on several times/day!

Did a lot of calculations, from several different directions, and they all came up with the BTU/hr demand of approx 10,000 BTU/hr. The HPWH has a 6200 BTU/hr capacity, so not enough.

I've looked at alternatives ( point of use water heaters, solar,etc) but came up with the solution to add a second HPWH. This would give me 12,000 BTU's/Hr capacity, and should be able to keep the water hot throughout the day. I found a good deal on a second one, but wanted to discuss whether this is a good idea, and whether to add the HPWH in series with each other, or run them in parallel.

Thanks for your thoughts
 
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Old 03-18-14, 02:29 PM
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I would seriously be looking at ways to re-route (using PEX) the hot water piping out of the slab. You might be able to find a less expensive means of heating the water but you will ALWAYS have the heat loss into the slab and earth. Getting the piping out of the slab and you can properly insulate the piping. Added plus is that piping in a slab will eventually leak and need to be re-routed anyway.
 
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Old 03-20-14, 10:00 AM
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re-piping

I had considered re-piping, but the cost estimates were staggering ( $12-$20K), vs a few hundred for a HPWH. Payback would be in decades, rather than months. I'm old enough that I would never see the payback on the re-pipe. Willing to risk needing one in the next decade- if it happens- so be it.

Now, back to the question, Serial or Parallel installation? Serial would be much easier for me, and since both would be running simultaneously , not much issue of one wearing out before the other. Parallel involves some more serious piping considerations, including flow back valves and also the volume of water through the auxiliary piping. Concerned primarily with speed of recovery, as the available volume at 80 gal's should be good enough.
 
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Old 03-20-14, 12:35 PM
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Update

Talked to the HPWH company today. They said no way to ire them together, unless I replaced the one I have with a different model. Pursuing other options now.'

Thanks.
 
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Old 04-05-14, 06:17 AM
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I think that point of use would be the best way to go. No, I don't necessarily mean an "instant" or tankless heater. With a 4100 sq.ft. house you should be able to find some space to put a full size (30 to 40 gallon) water heater. You might even be able to have the new kitchen water heater also feed a nearby bathroom with a new hot water pipe through the attic.

If you already spent (or plan to spend) money on hardware (the water heater) you might as well put it near the point of use to cut down on energy lost to the slab in a long pipe run.

If you do go with a second water heater next to the first and keep the recirculating loop, it would not be a waste of energy during the heating season since the recirculating loop behaves like a forced hot water space heating system.

Two tank type water heaters next to each other can be plumbed in your choice of series or parallel regardless of make and model.

The performance difference is not great. I think that parallel gives more hot water at first during a peak demand since the second to kick on (doesn't matter which kicks on first) will kick on sooner thus you have both heaters in action sooner. With series connection the second heater won't kick on until the first has "run out" of hot water and cooler water has reached the second. If the usage period is long enough to get both heaters going, series and parallel behave the same thereafter. For parallel connection, adjust the cold inlet valve for the heater that kicks on first so they both kick on a roughly the same time during heavy usage.

With low demand it doesn't matter whether the heaters are in series or parallel. While both heaters may still be on at the same time, each won't be running for as much time.
 

Last edited by AllanJ; 04-05-14 at 07:10 AM.
 

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