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Old 09-01-2014, 07:40 PM   #1
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How to add radiant hydronic floor heat to current hydronic boiler?


Hi all,

I have a crown hydronic boiler for my baseboard heat. I am remodeling kitchen and will be removing the baseboard to allow for more cabinets. Can cut the baseboard out and connect pex to the same loop where it was, making the run for the heated kitchen floor then just reconnect into where the other end of baseboard would be?

The floor will be tiled.

Let me know if you need anymore info to better answer. Thanks for reading!

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Old 09-01-2014, 08:27 PM   #2
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How to add radiant hydronic floor heat to current hydronic boiler?


That method will tend to make the floor to hot to walk on.

Would be better to make it its own loop, and use a tempering valve to limit the water temp.

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Old 09-01-2014, 09:17 PM   #3
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How to add radiant hydronic floor heat to current hydronic boiler?


Probably should have given more detail. The pex will be run in the joist bays under the subfloor in crawl space. I plan on using some type of heat transfer plate like ultra fin. So the heat would have to travel through subfloor, backer board, thinset and tile. Does that Still sound too hot or not hot enough to heat room?

If it sounds ok, then my concern is dropping from 3/4 baseboard copper to 1/2 pex then going back to 3/4 copper. Would that cause any issue with remainder of loop?
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Old 09-02-2014, 04:07 AM   #4
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How to add radiant hydronic floor heat to current hydronic boiler?


It would still be too hot.
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Old 09-03-2014, 07:45 PM   #5
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How to add radiant hydronic floor heat to current hydronic boiler?


Ayuh,.... Baseboard runs 180 water, radient floor heatin' runs 140/ 150 water,...

You'll need to make it, it's own zone, 'n temper the water,....
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Old 09-03-2014, 08:03 PM   #6
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How to add radiant hydronic floor heat to current hydronic boiler?


Not sure what a tempering valve as beenthere said but I think if you tee in a mixing valve from the return at the output, that might work.
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Old 09-03-2014, 10:23 PM   #7
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How to add radiant hydronic floor heat to current hydronic boiler?


There's nothing wrong with using 180 deg water for radiant floor heating. There are some challenges but nothing show stopping.

You'll need to use less pipe under the same amount of floor area, and you'll need to add something (aluminum plate?) to dissipate that thermal energy to the same surface area.

The idea is that 50 feet of pipe running at 140 degrees is going to transfer about the same amount of energy as say 30 feet of the same pipe running at 180 degrees (that was just a guess by the way so don't use that for your math). The problem you must overcome is to disperse the energy from that 30 feet into the same amount of area as the 50 foot pipe.

If your boiler system is closed loop, a tempering valve is a bad idea.

Also, I doubt trying to use the return line to the boiler would work either.. it might, depending on how its been sized but something tells me the return water will still be too hot. Boilers don't like thermal shocking so they are normally set up to keep the return water within 20degF or so of the discharge side.
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Old 09-03-2014, 11:54 PM   #8
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How to add radiant hydronic floor heat to current hydronic boiler?


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There's nothing wrong with using 180 deg water for radiant floor heating. There are some challenges but nothing show stopping.

You'll need to use less pipe under the same amount of floor area, and you'll need to add something (aluminum plate?) to dissipate that thermal energy to the same surface area.

The idea is that 50 feet of pipe running at 140 degrees is going to transfer about the same amount of energy as say 30 feet of the same pipe running at 180 degrees (that was just a guess by the way so don't use that for your math). The problem you must overcome is to disperse the energy from that 30 feet into the same amount of area as the 50 foot pipe.

If your boiler system is closed loop, a tempering valve is a bad idea.

Also, I doubt trying to use the return line to the boiler would work either.. it might, depending on how its been sized but something tells me the return water will still be too hot. Boilers don't like thermal shocking so they are normally set up to keep the return water within 20degF or so of the discharge side.

The aluminum plate that I am hoping to use is called ultra fin, but I'm sure there are other competitors. From what I read, the pex is several inches below the sub floor which creates a convection source of heat spread out by the aluminum plates. Less pipe means less money, so I'm ok with that.

Main concern is going from the 3/4 copper to 1/2 pex. Does that introduce any issues as far as pressure (psi) or it's just the same as domestic hot water 3/4 to 1/2, a non issue?
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Old 09-04-2014, 03:56 AM   #9
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How to add radiant hydronic floor heat to current hydronic boiler?


If you pipe your pex loop in series like the baseboard was. You will have very poor heat in the other rooms.

Pipe it up as its own loop, and use a tempering/mixing valve. They are commonly used on closed loop systems where a radiant loop is added.
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Old 09-04-2014, 10:36 AM   #10
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Pipe it up as its own loop, and use a tempering/mixing valve. They are commonly used on closed loop systems where a radiant loop is added.
Tempered with what???
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Old 09-04-2014, 03:52 PM   #11
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How to add radiant hydronic floor heat to current hydronic boiler?


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Tempered with what???
You use the return water to cool the incoming flow. If the water is too hot you can end up with a myriad of problems, depending on the set up. If it is a true radiant floor, there is no way you want 140+ degree water flowing through it. That, to me, is the poster child of uncomfortable living space and disappointment
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Old 09-04-2014, 04:19 PM   #12
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Tempered with what???
A Canucker said, with the return water from the loop. The circulator is piped after the mixing/tempering valve, as the mix gets to set temp. The valve recircs more of the loop water into the mix. Its been done like that since before I got into the trade back in the 70s.
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Old 09-04-2014, 05:00 PM   #13
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How to add radiant hydronic floor heat to current hydronic boiler?


All of the boilers I've seen have the water returning to them at only about 20 degree's less than the discharge water.
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Old 09-04-2014, 05:10 PM   #14
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How to add radiant hydronic floor heat to current hydronic boiler?


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All of the boilers I've seen have the water returning to them at only about 20 degree's less than the discharge water.
I think it would depend some on the thermal mass of what you are heating and the temp outside.
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Old 09-04-2014, 05:47 PM   #15
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I think it would depend some on the thermal mass of what you are heating and the temp outside.
While there is certainly truth to that, I'm not so sure.

Most boilers do not like to be thermally shocked.. Cold water entering a hot boiler jacket causes rapid contraction of the jacket and that can cause problems.

While most of my experience with boilers is done with larger (>1M btu/hr) systems, everything I design will generally result in the return water being within 20 to 30 degrees of the discharge.

I'm certainly no expert in residential heating, that's for sure.. but basic common sense says that if the return water is cold enough to make it an effective tempering supply, then what does that say about the characteristics of the last heating load it past through? See my point?

I'm not saying the other guy is wrong, I'm just looking for an explanation on where this relatively cold tempering water is coming from..

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