In-joist radiant heat

 
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hotblast1357
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Post by hotblast1357 » Mon. Jan. 24, 2022 7:25 pm

Hey everyone, looking for opinions.. after this heating season I will be installing in-joist radiant heat in the upstairs floor joist cavity, before I finish the downstairs, right now the downstairs is unfinished and wide open, I will be removing the oil furnace and ductwork, house is 30x38, 16” o.c. Joist, I’m thinking 4 equal sized zones, that will be living room, kitchen, bedroom/bathroom, bedroom/bathroom, is it easier to buy 4 rolls of 300’ of pex for each zone, or buy a 1000’ roll and one of those pex un-spoolers? I can get a 1000’ roll cheaper.. and would have some left over for future use.. I am using heat transfer plates, a 4 port manifold which will be in the center of house under stair well, I have a constant circulator now that feeds the manifold in the basement that has dhw, basement radiators on it, and I’m thinking I can just put a circulator with flow check valve on spare port and then have that feed the radiant manifold.. am I best to just use the existing thermostat for the hot air unit and have it control that 2nd circulator?

Thank you in advance!

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Rob R.
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Post by Rob R. » Sat. Jan. 29, 2022 2:18 pm

I could go either way on the roll length. What size pex are you using? Normally with 1/2” you try to keep each loop 300’ or less. A 300’ roll would be handy in that regard.

I would also rethink your zones. It is nice to keep the bathrooms warm and the bedrooms cool. You say 4 zones but only mention 1 thermostat…do you just mean 4 loops? Is all of the flooring material the same?

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 6:31 pm

Goin with 1/2”

Yes 4 loops, 1 zone.

Each zone will be around 310-340’ including the runs to and from the manifold.

All flooring upstairs is linoleum on top of 3/4” plywood.

I want the whole house just one temp, it is now, no need to make one room different than the other…

Obviously I’ll need to throw a mixing valve in there somewhere also..

 
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Post by Rob R. » Sat. Jan. 29, 2022 6:33 pm

I would consider 5/8” tubing. You can go up to 400’ with that.

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 6:53 pm

Can you point me towards the aluminum panels for 5/8?

That’s the other argument I see a lot… do you need the aluminum panels? My grandfather next door to me has in joist radiant heat, with a older rubber type hose, just simply stapled to the bottom of the floor, with nothing under it, open joist, no insulation or anything, and it heats his house just fine…

 
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Post by Rob R. » Sat. Jan. 29, 2022 7:11 pm

Some of the plates will work with either size.

You can get by without the plates if you run a high enough water temperature. I know some folks that did a staple up system without the plates and even with 180 degree water in it the room gets cool in weather like this.

I have not checked your math - are you planning one run of pex per joist cavity or two?

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 7:41 pm

2 runs of pex per joist, joist are 16” on center, house is 30x38..


 
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Post by Lightning » Sat. Jan. 29, 2022 7:47 pm

So what can you expect the BTU heating to be per foot of pex?

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 8:50 pm

Lightning wrote:
Sat. Jan. 29, 2022 7:47 pm
So what can you expect the BTU heating to be per foot of pex?
Enough lol I don’t know

 
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Post by CoalisCoolxWarm » Sat. Jan. 29, 2022 9:34 pm

When looking at the infloor heating, I read the metal transfer plates and PEX-AL-PEX were the better way to spread the heat for the flooring.

Not exactly the same thing but throwing that out there. Are you planning to use PEX-AL-PEX ?

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 9:37 pm

Nope no pex Al pex here just pex.

 
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Post by LTStorm07 » Sat. Jan. 29, 2022 10:35 pm

I can tell you that my coworker just purchased a house with radiant floor heat that was added as a later thought after tearing out hot water baseboards. His floors do have the 1/2 barrier pex and plates, double passes, and believe it or not, it struggles in temps (0 degrees) like this. It has full fiberglass insulation sealing it into the joists and all. He had a HVAC company come check it out and they told him even with plates and all, there simply isn't enough radiation for his floor plan + ceilings. Apparently the previous owners had to crank the temp way up to make up for it. They were running 160 degree water unmixed. The HVAC company told him without the plates it would had really struggled. The homeowners put the system in themselves. He's looking at ultra low temp mini splits to help things out now since he can't keep the floor running over 150 degrees safely. His floors are all tile and hardwoods.

 
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Post by hotblast1357 » Sat. Jan. 29, 2022 10:48 pm

I could see where tile and hardwood would hinder the transfer of heat… I only have a sheet of plywood and roled linoleum so it should transfer heat quite well… of course it is a worry of mine that it won’t heat properly…. But I guess there’s no way of knowing till you do it.

I see you mentioned “+ceilings” so I’m assuming you mean he has vaulted or tall ceilings?

 
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Post by LTStorm07 » Sun. Jan. 30, 2022 12:21 am

hotblast1357 wrote:
Sat. Jan. 29, 2022 10:48 pm
I see you mentioned “+ceilings” so I’m assuming you mean he has vaulted or tall ceilings?
Yep, he has vaulted ceilings with a fairly open floor concept and even with fans pushing it back down it doesn't help.

 
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Post by Rob R. » Sun. Jan. 30, 2022 7:49 am

LTStorm07 wrote:
Sun. Jan. 30, 2022 12:21 am
Yep, he has vaulted ceilings with a fairly open floor concept and even with fans pushing it back down it doesn't help.
A heat loss calculation of that home would likely reveal that the heat loss is too high to be offset by radiant floors alone. A 2 stage thermostat and some radiators would have been a simple solution.

As for the temperature of the water, that is just a means to an end. You do not want the floor to be warmer than 80 degrees or so, or it will be uncomfortable.


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