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# Too good to be true?

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Member Posts: 103
edited November 2017
Looking for unlimited shower hot water or as close as I can get.

Keep 30 gal of indirect 80 degree hot water on standby 24 hours a day vs 50 gal at 140. When hot water is required the 80 degree water then goes to an electric tank less whole house heater providing from 5 to 9 gpm anywhere in the house depending on which heater is used.

Is this do able? Does the math work?

Will the added cost of using electric be offset by the savings in 80 degree vs 140 or will physics prevail and make all things equal?
www.afinalsalute.com

• Member Posts: 684
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Bacteria
• Member Posts: 103
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What about using a heat exchanger off the boiler to bump the incoming water temp up to 80 degrees before getting to the electric tank less, then no water will be on standby? Should be no problem for a dedicated 141K BTUs to bump the incoming water up say 40 degrees in the winter and less than 30 in the summer.

Yes, No, Maybe?
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• Member Posts: 23,287
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Whatever you come up with, don't store water at temperatures between about 70 to 80 and 120. Please...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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You MUST store above 120 as has been said, 140 is optimal. Why not store 50 gal at 140 and mix down to 80 for the electric ODWH? Need to know the boiler size, pump size, and recovery for the IDWH to tell, but My gut feeling would be that mixing down to from 140, would use less than the continuous recovery rate of the IDWH, then you electric ODWH would pick up the slack, and you have continuous hot water up to 9 GPM as long as the electric ODWH is at least 53 Kw (assuming a 40 degree delta T for 80 to 120 deg at 9 gpm). 53 KW will require a HUGE circuit though.
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"53 KW will require a HUGE circuit though" -- this, of course, is the real problem with on demand water heaters. If we were to assume that you have a standard 240 volt single phase system, you are looking at about 220 amps. Good luck...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 103
edited November 2017
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Why are we hashing old theory? Changed my question after "Bacteria"

Now I'm changing to this theory.

Instead of using a 36kw 150 amp unit at 105 degrees, I could indirect at 140, mix down to 80 and only heat my master shower with smaller tankless. This would only require around 11KW and a 50 amp breaker getting me 3 gpm at 105 degrees.

Another idea, just forget about the tankless all together providing the dedicated 141 BTUs from my boiler to the 50 gal indirect is enough to keep me in hot water till I'm done.
www.afinalsalute.com
• Member Posts: 684
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With the boiler from your other thread and with a mixing valve you will probably be able to recover at the same rate as you use with the correct sized inderect from what I have read.

But you haven't posted the heat loss figure. It may be oversized. I've seen a couple posts where you could do this with an 80k boiler
• Member Posts: 350
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cnmdesign said:

Another idea, just forget about the tankless all together providing the dedicated 141 BTUs from my boiler to the 50 gal indirect is enough to keep me in hot water till I'm done.

Bing, bing, bing!

• Member Posts: 103
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Ended up getting a 50 gal Burnham stone lined indirect.
Planning on running it off of one of my four zones.