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Old 10-22-2015, 02:02 PM   #1
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Default Question about water supply for tankless heater

I'm looking for some advice.

I have a builder who has spec'd a tankless water heater for a new build and has asked me to come up with a way to raise the temp of the water going to the tankless in an effort to reduce the delta T for the purpose of being able to produce more GPM without going to a larger tankless water heater.

Our typical ground water temps are 48-50 F, so I usually calculate a need for a 70F heat rise to deliver 120F water, which is max per our state code.

The only way I can think of to bring up the water temp supply to the tankless without using a tank style water heater to partially heat it is to use a simple uninsulated tank so that the water can gradually reach ambient temp, at times of zero consumption of course. Under demand, the pre-"warmed" water supply will give out eventually. This just doesn't seem like a very good approach.

Using a standard water heater to pre-heat the water would be more effective, but I worry about the possibility of bacterial growth at the moderate temps the tank would be operating at. Not a good idea, either.

Another issue with this idea is that while reducing the delta T will help the tankless unit produce more GPM at a set temp, it's not really a huge jump in output, maybe getting me 7+ GPM.

Well, that's it in a nutshell. Any constructive ideas?

Thanks, Scott
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Old 10-22-2015, 05:39 PM   #2
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http://www.bing.com/images/search?q=...verlay&first=1

you can reclaim some heat from drain water, however I would recommend just bumping up a unit, or tying 2 together, I deal with the same water temps here. whats the peak GPM you are looking at
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Old 10-22-2015, 05:58 PM   #3
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I think you answered your own questions in your post.
I downsize previously oversized space heating boilers regularly to serve the customer better.
Looking for a way to lower the delta T to avoid purchasing a larger tankless water heater is a different scenario and not a risk I would feel comfortable with.
I am not sure if the issue with not getting a larger unit is money, but it reads as if it is. If so:
I have started telling GC's that either they or the owner has to cough up the dough to do things right, instead of asking someone else to be the fall guy if things don't work out as planned. The same homeowners who cry that they are broke seem to find the money for the nicest granite and appliances.
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Old 10-22-2015, 08:18 PM   #4
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I'd tell your GC that I'm not a water alchemist. Changing the ΔT requires energy. The source of that energy boils down to $$$.
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Old 10-22-2015, 09:39 PM   #5
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Quote:
Originally Posted by Plumbus View Post
I'd tell your GC that I'm not a water alchemist. Changing the ΔT requires energy. The source of that energy boils down to $$$.
Beautiful!!!!
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Old 10-22-2015, 09:49 PM   #6
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The simple solution is to add a second tankless to help with gpm demand if you increase the temp to 140 degrees, and to use quality units that have the recirculating pump built into them.
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Old 10-22-2015, 10:01 PM   #7
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Just tell him to find a way to get a btu to do more...

1 BTU=1 DEGREE FAHRENHEIT INCREASE IN ONE POUND OF WATER.

Surprise us all with his brilliance...



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Old 10-23-2015, 12:26 AM   #8
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Redwood is right. I swapped out my 110volt baseboard heat for 220v. Twice as efficient
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Old 10-23-2015, 01:50 AM   #9
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Quote:
Originally Posted by newyorkcity View Post
I think you answered your own questions in your post.
I downsize previously oversized space heating boilers regularly to serve the customer better.
Looking for a way to lower the delta T to avoid purchasing a larger tankless water heater is a different scenario and not a risk I would feel comfortable with.
I am not sure if the issue with not getting a larger unit is money, but it reads as if it is. If so:
I have started telling GC's that either they or the owner has to cough up the dough to do things right, instead of asking someone else to be the fall guy if things don't work out as planned. The same homeowners who cry that they are broke seem to find the money for the nicest granite and appliances.
You put it in better words than I did, you're dealing with a 90-94% efficient application, adding a tank is not an option
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Old 10-23-2015, 09:20 AM   #10
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Thanks for the responses, guys.

This builder is one of those Mr. Wonderful types that is often clueless but believes that technology has an answer for everything.

I was pretty certain that there was no free lunch in this situation and your responses have confirmed that he's asking the impossible.
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