Standard Pex for Heating

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Old 11-15-16, 09:52 PM
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Standard Pex for Heating

I'm curious about using standard (non-oxygen barrier) pex for small heating loops. I've had small instances, like one or two bathroom floors which I'd like to heat using the potable hot water source. For anything more than that of course I'd switch to a real boiler system.

Such an "open system" is described here: The Open System | | DIY Radiant Floor Heating | Radiant Floor Company

I know the arguments against such a system. My question here is primarily about pex. Does running an open system like this with standard (non barrier) pex pose a risk to the pex, water heater, or other system parts? If so how is it different from any other pex usage?

Most barrier pex is not rated for potable water. Of course you'd have to use pumps & parts rated for potable water for the system. I'd use a stainless pump.

There isn't much difference between the open system described in the link above and running a recirculation pump on a standard potable water system. You're just adding some heat transfer plates and running the pump a lot more often.

Thoughts?
 
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Old 11-16-16, 06:10 AM
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You can use standard PEX for such an application. The oxygen barrier is used in closed systems to minimize corrosion of cast iron boiler parts, pumps, etc, by minimizing the amount of oxygen in the system. In an open system like you describe, there are no cast iron components and in any case, fresh oxygen is constantly being supplied by the "new" water flowing into the system.
 
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Old 11-16-16, 10:41 AM
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Are you able to quantify in any way the amount of oxygen from various sources? We figure there's oxygen in the water coming in from the street...how much? How much oxygen is getting in through the pex pipes as water circulates?

Will the open system kill the anode rod in my water heater or destroy the tank faster? What about the short galvanized pipes coming off of the water heater tank before it converts to pex? Most of the house is plumbed in pex and copper but there are a few short bits of galvanized running to fixtures where I didn't replace the pipes because they were in good shape and I didn't want to rip open walls.
 
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Old 11-16-16, 11:46 AM
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Tap water varies quite a bit. Dissolved oxygen can range from 0 to 18 mg/liter. Somewhere in the middle is probably average, but it depends on source, mineral content, temperature and other minor factors.

I don't have any information on how much oxygen diffuses through standard Pex, but the oxygen barrier Pex must not have diffusion greater than .1 mg per liter of water in the pipe per day.

Lets say for the sake of argument that non barrier Pex diffuses 100 time more oxygen than barrier Pex, or 10 mg/liter per day. 50 feet of nominal 1/2 pipe has about 2 liters of water inside. So in a day, that would add 20 mg of dissolved oxygen to the water. That would barely move the meter on a water heater full of water.

You can see why it is a problem in a closed system with perhaps hundreds of feet of pipe. It builds up over time and it is multiplied by the long length.

In your case, with a small loop and fresh water all the time, I don't think it's going to make a significant difference.

Sure dissolved oxygen will reduce the life of your tank and iron components, I just don't think the additional oxygen from a small PEX loop will be a significant part compared to O2 in normal water.
 
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Old 11-16-16, 05:49 PM
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Thanks so much for the clear analysis. Now I'm not so worried about the couple non barrier heat loops I've installed.

By the way, the only reason I would not use barrier pex is because it seems almost all of it is not rated for potable water. The only type I found that was rated for potable water and had a barrier was very expensive.
 
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Old 11-16-16, 08:20 PM
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I guess that brings me to a second question:
What are the actual dangers of using oxygen barrier pex in a potable water system? Are they not rated for potable water simply because the manufacturer sees no reason to jump through whatever hoops are needed for the rating?
 
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Old 11-17-16, 06:36 AM
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I did find mention that meeting the NSF rules for potable water requires extra cleaning steps (of the tubing itself) during manufacturing. I guess that would add cost that manufacturers don't see as necessary given the primary end use.

Couldn't find any details on what it is they are removing via the cleaning, but I guess I wouldn't want to take a chance.
 
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Old 11-17-16, 12:54 PM
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I have one bathroom floor installation where the home will eventually have a real gas boiler but it might be a year. I used oxygen barrier pex and wish I could temporarily run the floor from the potable water heater. I've never done a heat exchanger but it seems like you'd be in for at least $200 in parts for a bare bones setup. I'm actually thinking about using a point of use electric water heater just to run that one bathroom floor for the winter.

I wonder if I could make a super simple heater exchanger soldering two 10' copper pipes side by side. The bathroom floor probably has a max output of 2,000btu.
 
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Old 11-17-16, 01:13 PM
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There are some plate type water to water exchangers on ebay for a little over $100. No idea if they would work for your application....
 
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Old 11-17-16, 09:29 PM
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I've been look at those plate exchangers but having trouble finding info on pump sizing. An exchanger plus stainless pump would cost more than a small electric water heater. Though, I could probably retask the pump at some point.
 
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