# Domestic hot water conversion



## hroark2112 (Apr 16, 2011)

I'm looking at a commercial project where they want to remove the existing boiler and replace it with tankless. There is a storage tank which will be staying. The storage tank has 4 connections, not counting the T&P. Currently the cold water comes in to the storage tank at the bottom and the hot water takes off the top. The boiler ties in 2 separate connections and utilizes a circulating pump & aquastat. There is also a building recirc pump that ties back in at the cold water inlet.

Here's my question. They want to replace the boiler with 2 tankless heaters. What extra do I need to do to make the system work best? I am planning on adding a check valve in the line as well as a wye strainer with blowdown valve. We plan to do a full cleaning on the storage tank while the system is down just to make sure we are starting out spotless. 

A few of the piping diagrams I've seen show an expansion tank on the loop. I've never put an expansion tank on the hot side. Input?

Thanks in advance!


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

Awful idea. Using a tankless to feed a tank takes the idea of the tankless away. I'm assuming it is a conventional boiler feeding the tank now. Try to talk them into either running just the two tankless or replacing the boiler with a high eff boiler like a htp elite ft.


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## Plumbus (Aug 4, 2008)

I agree with btc. Condensing boiler manufacturers units in a variety of btu's.


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## hroark2112 (Apr 16, 2011)

I understand....but this is what they want.


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## wyrickmech (Mar 16, 2013)

Check on condensing boilers. The last thing you want is a customer that is complaining about it not working right. Intellahot makes a unit that is H stamped you might look at them also.


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## Redwood (Sep 8, 2008)

hroark2112 said:


> I understand....but this is what they want.


Tell the customer, "I'm the phuckin plumber here."

Using an indirect water heater as a tankless storage tank is about as dumb as it gets...


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## wyrickmech (Mar 16, 2013)

Reds right delete the storage and size the tank less for the project. I miss read and thought you were talking about heating.


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## hroark2112 (Apr 16, 2011)

They don't want to go with a new boiler.

Here a drawing I found on this kind of installation. Thoughts?


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## Catlin987987 (Nov 12, 2010)

bct p&h said:


> Awful idea. Using a tankless to feed a tank takes the idea of the tankless away. I'm assuming it is a conventional boiler feeding the tank now. Try to talk them into either running just the two tankless or replacing the boiler with a high eff boiler like a htp elite ft.


I have seen it done many times, many of them work pretty good. The heat exchangers stay clean because there is a low temp rise, and they are still more efficient than an atmospheric boiler. The biggest thing you should worry about is the pump sizing, hopefully the tankless comes with an internal pump you can use.


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## ZL700 (Dec 8, 2009)

You wont find a tankless with an internal pump you can use because it will be controlled as a recirculation option and the tankless with have no way of accepting the storage tank call for heat


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