Side Arm Heat Exchanger used on Water Heater for Radiant Floor Heat

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Old 02-14-15, 06:09 PM
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Side Arm Heat Exchanger used on Water Heater for Radiant Floor Heat

I would like to use a $85.00 "side arm heat exchanger" to extract heat from a gas water heater that has 3/4" side taps. I need to provide a small radiant floor heat load of 14,000 BTU in a concrete basement floor.
The reason I would like to do it this way, is so I could eliminate the $200.00 bronze pump on the HWH side of the heat exchanger.
I think I would pump the radiant floor heat (80 degree Max return water) to the bottom of the heat exchanger end port. That's the port of the heat exchanger with the most friction and could be pushed threw with the pump.
This way the water in the HWH side of the heat exchanger would be cool enough to drop down and allow the hot water to be drawn into the top which would be the last place the radiant water could get heated to I hope about 100 Deg. The side ports of the heat exchanger are 1" sweat. Overall length is 17 1/4" long. Also the water heater is big enough to satisfy the loads. I possibly would set the water temp. to 140 degrees Max.
My concern is would the water be cold enough to drop down the heat exchanger. At times when the water heater sees domestic use the temperature could be 50 degrees at the bottom of the water heater.
Will it work?
 
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Old 02-14-15, 06:27 PM
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Will it work?
I have strong doubt.

I believe you'll be pumping the floor heating side WAY faster than the thermosiphon on the other side can transfer heat into the water.

Also, you will have very little temperature difference between the floor water and the water heater water... remember that the closer in temp, the less heat transfer. Google "heat transfer Driving Force" and you should be able to find more about that.

How much is that heat exchanger?
 
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Old 02-14-15, 06:56 PM
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$85.00 plus shipping for the copper heat exchanger.
All I would need for supply temp is 100 degs max. If I were running the tank at 140 Deg I think it could do it. Worse case I would plumb in the bronze pump. I'm tempted to use the water in the tank and a bronze pump, theres no other metal that could rust.
Chicago doesn't like that.
I did this with a plate heat exchanger with a pump on each side and had to slow down the water on the tank side of the heat exchanger to a trickle to get 100 deg. supply water going into the slab.
I found a small 10 plate on e-bay for $50 back then.
 
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Old 02-14-15, 07:38 PM
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I think that's a cheap price for that! I have some 2" surplus copper around here and have thought about manufacturing a few and selling them on ebay. With the fittings I would have to buy (I don't have a "T-drill") it comes damn close to that price. I can't afford to build them that cheap!

If the city don't like pumping the domestic water through the floor, I would think they would have a problem with using a water heater that wasn't ASME rated (bearing the "H" logo) for space heating. Is yours 'accepted'? H stamped?
 
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Old 02-15-15, 03:22 PM
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I am not sure the potable water from the water heater is going through the floor. Unless I am not sure what I am looking at this is a heat exchanger. at first glance I thought it was a low loss header. Flow could be an issue as Trooper stated. Potable water would be a no-no into radiant floor.
 
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Old 02-15-15, 03:39 PM
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It's a sidearm heat exchanger...

The options were either to use a HX or to pipe direct...

I wouldn't want to pipe direct either... who wants nasty old stagnant water that sits in the pipes all summer long coming out the taps? not me!
 
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Old 02-15-15, 04:52 PM
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Correct, potable water laying in pex tubing getting cold. Hmmm. cold water, plastic pipe only missing a bit of legionella for a major problem. Bring it back to the water heater, maybe not hot enough to kill the bacteria. Now it is airborne and you get sick.

I always get a bit concerned when someone says sidearm heater. I have heard so many things sidearm heaters even indirect water heaters.
 
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Old 02-23-15, 09:40 AM
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Thanks for the help. I have since found a web sight that has a calculator for heat exchangers. The flow on the side of the water heater at 140 degrees would have to be about a 1/3 of the radient side at 100 degree supply and 80 degree return. I pretty sure your right about low flow on the water heater side. What would need to happen is the water in the heat exchanger would have to drop down into the lower part of the tank with gravity so that the hot water at the top could enter the heat exchanger, the moment the heat goes into the heat exchanger its rising also. So the to temps would fight each other causing very little flow.
I can't find it now, but I read in some towns its OK to use domestic water from water heaters as long as the water is circulated through out the year for 5 mins. or so on a timer once a day. I'll try to find it.
 
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