# Question about water supply for tankless heater



## GrtLksPlbr (Aug 12, 2014)

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


----------



## mugdiller (Oct 21, 2015)

http://www.bing.com/images/search?q...f8f2198697d739b53d278ao0&mode=overlay&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


----------



## newyorkcity (Nov 25, 2010)

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.


----------



## Plumbus (Aug 4, 2008)

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 $$$.


----------



## dhal22 (Jul 30, 2010)

Plumbus said:


> 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!!!!


----------



## Cajunhiker (Dec 14, 2009)

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.


----------



## Redwood (Sep 8, 2008)

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...


----------



## rwh (Dec 17, 2014)

Redwood is right. I swapped out my 110volt baseboard heat for 220v. Twice as efficient


----------



## mugdiller (Oct 21, 2015)

newyorkcity said:


> 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


----------



## GrtLksPlbr (Aug 12, 2014)

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.


----------



## Phat Cat (Apr 1, 2009)

GrtLksPlbr said:


> 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.


Unless this builder is trying to be super GReen, the only other explanation is cheap. 

The smart arse response running through my mind is, don't worry about under sizing the tankless unit. Global warming will catch up eventually and make up the delta in water temp rise. Lol

Kudos to the OP for at least considering the possibility, that's what drives innovation.


----------



## joeplumber85 (Jul 16, 2012)

Put a cheap electric tank downstream of the tankless with a mixing valve and recirc it back to the tankless. It won't lower the delta t it will lower the gph which will have a similar effect but let the tankless run more efficiently. It will also act as a buffer tank to prevent "cold water sandwich".


----------



## johnmeto (Oct 13, 2015)

you can recover some warmth from channel water, on the other hand I would suggest simply knocking up a unit.


----------



## bct p&h (Jan 19, 2013)

So you're worried about bacteria growth in a tank running at lower temps but not in a tank that would sit and allowed to be brought up to ambient temperature? Sounds like basically the same problem to me.
Tell the builder to stop being cheap and pay for a properly sized unit.


----------



## GrtLksPlbr (Aug 12, 2014)

bct p&h said:


> So you're worried about bacteria growth in a tank running at lower temps but not in a tank that would sit and allowed to be brought up to ambient temperature? Sounds like basically the same problem to me.
> Tell the builder to stop being cheap and pay for a properly sized unit.


 I'm worried about both. I know that legionella can be an issue in warm water systems that aren't hot enough to kill the bacteria, but part of my question was if it was a concern in a ambient temp tank as well. It would seem so to me.


----------



## bct p&h (Jan 19, 2013)

GrtLksPlbr said:


> I'm worried about both. I know that legionella can be an issue in warm water systems that aren't hot enough to kill the bacteria, but part of my question was if it was a concern in a ambient temp tank as well. It would seem so to me.


This is from OSHA

Q. What water conditions are best for growth of the organism?

A. Warm, stagnant water provides ideal conditions for growth. At temperatures between 20°C-50°C (68°-122°F) the organism can multiply. Temperatures of 32°C-40°C (90°-105°F) are ideal for growth. Rust (iron), scale, and the presence of other microorganisms can also promote the growth of LDB.

That is just legionella. I'm sure there are a lot more bacteria that would love to live and breed in a nice room temperature storage tank.


----------



## Best Darn Sewer (Dec 23, 2012)

Am I not understanding something? Why not put in a Grundfos comfort system circ pump w/ bypass and set the timer to the most used times? Naviens have a dip switch for when a pump is in use. Or use the internal one that comes on the 240a. Won't that help by pre-heating the water before using it?


----------



## dhal22 (Jul 30, 2010)

Best Darn Sewer said:


> Am I not understanding something? Why not put in a Grundfos comfort system circ pump w/ bypass and set the timer to the most used times? Naviens have a dip switch for when a pump is in use. Or use the internal one that comes on the 240a. Won't that help by pre-heating the water before using it?


I don't think that will work. Are you talking about recirculating hot water back to the tankless while you're using hot water?


----------



## Best Darn Sewer (Dec 23, 2012)

dhal22 said:


> I don't think that will work. Are you talking about recirculating hot water back to the tankless while you're using hot water?


Sure. Why not? I'm asking seriously. Would the pump not help? I guess it wouldn't work well after a minute or two of use. It would seem like the tankless wouldn't have to work as hard at first but once the GPMs increased on the outlet side it would exceed the limits the pump. 

Hell, now that I think about it more I guess it's not such a great idea. Never mind.


----------



## mugdiller (Oct 21, 2015)

Best Darn Sewer said:


> Sure. Why not? I'm asking seriously. Would the pump not help? I guess it wouldn't work well after a minute or two of use. It would seem like the tankless wouldn't have to work as hard at first but once the GPMs increased on the outlet side it would exceed the limits the pump.
> 
> Hell, now that I think about it more I guess it's not such a great idea. Never mind.


That works to avoid a "cold sandwich", so your hot water is actually on demand. The pump has nothing to do with the demand the unit can handle. Each Unit is rated for a certain GPM that can be produced, in this plumbers case, his incoming temp is 50 degrees, so you actually have to adjust your GPM down, as most of those ratings are based on 70 degree water. if not sized properly the unit will still produce your 120 degree water, however flow rates will drop dramatically once you exceed the designed GPM on the unit, same thing happens if you crank it up to 140, great, but you just lost GPM as it is now working 20 degrees hotter. And this is why the customer who bought their tankless water heater at Menards doesn't like it, the 18 year old know it all has no idea what a BTU is


----------



## plumber joe (Oct 17, 2008)

Run 200' of cold 1" water pipe across the basement and back before introducing it into the heater, normal house temp is apx. 70 degrees, could give you a temperature rise of 15-20 degrees.


----------



## plumbdrum (Nov 30, 2013)

Run a dedicated cold line from water meter or well and then Instal a self regulated and rated heat wire and insulate the line going to tankless


----------



## newyorkcity (Nov 25, 2010)

*Wow*



plumbdrum said:


> Run a dedicated cold line from water meter or well and then Instal a self regulated and rated heat wire and insulate the line going to tankless


That is a great idea. Just be sure the heat trace cable's thermostat is activated at a high enough temperature that it is not off most of the time.
Also, be sure the heat trace manufacturer allows you to insulate over their cable. I think the off the shelf cable does not.


----------



## plumbdrum (Nov 30, 2013)

Maybe this companyThermon.com


----------



## GrtLksPlbr (Aug 12, 2014)

I've been away for a bit, but just caught up. The project is on hold while some cost-reducing design changes are done to the whole house. Thanks for the discussion and suggestions, though.


----------



## ItalStal (Nov 12, 2015)

I love the simple solution of adding a tank to a tankless setup. Its a novel idea. The new navien tankless units have a hidden (to the homeowner) tank with a recirc pump. About a litre of water is always heated to assist. 

I would say upgrade the unit, or add a second unit given space and service availability.


----------



## tater6061 (Feb 25, 2013)

http://www.overstock.com/Home-Garde...ermostatic-Mixing-Valve/10417583/product.html


I think I have a solution for the Delta T


----------



## jeffreyplumber (Dec 7, 2009)

Hope Project goes through at a decent price. If saving money is the plan just go with the small unit they will get less flow causing them to save even more money. I think any other way will increase cost. How a bout a tank heater? and ditch the tankless altogether? Ive done reasearch that says it takes 20 years to get a savings on tankless over tank heater. Im not sure on this but thats what my source said. 
Was this strictly a cost issue? I know many people are trying to be Green and will pay more for the feeling of doing right with the planet (not me) Is this the motivation for a tankless?


----------

