DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > Using Tankless water heater for floor heat?

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Old 09-05-2013, 08:56 PM  
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Default Using Tankless water heater for floor heat?

I have run into a strange one. Long story short the neighbor has offered to pay me to finish a job someone else started and bailed out on. It amounts to finishing out a bathroom and office in a rural machine shed. Most things look decently done and no big deal, except for floor heat which, according to the owner, was to be run through a tankless water heater. Has anyone familiar with a way to do this? The domestic water is already plumbed to the water heater and to various outlets for a shower, clothes washer supply, sink and scrub sink. The PEX tubing is sticking out of the floor near the mounted tankless heater. I am trying to figure out if there is a way to install a circulating pump and use the tankless heater without some kind of heat exchanger and not polute the domestic water. I am open to ideas and suggestions as this is a new one on me.

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Old 09-05-2013, 10:38 PM  
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Old 09-06-2013, 06:29 AM  
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I don’t see how you can do this using the same unit for domestic hot water and heat both. There could be a heat exchanger out there for this I don’t know.

We did the heating of my nephews house using a 40 gallon gas hot water tank and it worked out great. He has a second tank for his domestic needs. We ran his as a non-pressure system with an open stand pipe. We also added a drain to the end of the runs and a tee connection to supply make up water and to purge the lines of air by circulating water under house pressure thru the system on start up. There is very little lost over a years’ time thru evaporation (maybe a quart) all he has to do is turn on the fill valve until water comes out the stand pipe.

In our case the water comes back to the tank only 10 degrees less than it went out and the closed loop is very efficient and the minerals and such in the water are only what is in the water to fill it the first time and nothing builds up in the tank like normally happens in a hot water tank. In your case I’m not sure how you would separate the water returning or how the tankless unit would handle the hotter in coming water.
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Old 09-06-2013, 06:40 AM  
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Maybe that's why he bailed?
You can't mix Potable & non-Potable water together so this heater would be just for floor heating in a closed loop.
So you have to literally replace the function of the boiler with the tankless unit in a setup as bud has described.
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Old 09-06-2013, 07:45 AM  
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Thanks guys. This confirms what I thought. No way to mix the domestic water and the heating system together without cross contamination. I also did a little more digging and found that this heater is not rated for floor heating, so I will just have to look at another alternative for heat. At least I am doing this hourly and the owner is buying supplies so no sweat for me.

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