Very basic in-floor / radiant / hydronic heating questions

[NothingNew]

Hi, folks,

I have few basic questions that I hope the cumulative experience of this forum can answer:

1) What is the optimal slab thickness to support in-floor heating?
2) What is the optimal height to place the heating tubes within this slab?
3) What is the optimal amount of foam insulation to place under and on the slides of that slab?

Thanks!

Brian

 

[PoorUB]

No clue what optimal is, but 90% of the floors I did had 4 inches or more depending on the use. Floor heat didn't come into the equation for floor thickness. Tube placement? We always laid it right at the bottom, most of the time stapled to the foam. Foam thickness? We always did 2".

 

[haveissues]

Just throwing it out there that some research suggests the radiant is more effective in the center of the slab.

 

[yeldogt]

A basic slab is 4" thick and the recommended under slab is 2" of foam w/ a VB.

Can be a case made for trying to get the PEX higher in the slab. There is a "radiant" aspect to a slab floor ... so depending on the overall slab temp and PEX spacing you can feel the tubing in some applications. Have never really noticed it except in an industrial setting A loading dock where the 3/4 tubing was widely spaced comes to mind --- they pumped higher temp water through the PEX. It was all open so this was to keep things from freezing and to provide some comfort. This is not going to happen when it's inside and an insulated garage.

I normally tie the PEX to mesh and don't really worry about where it ends up -- Another thing is if you are going to build any walls I'm always worried about hitting a tube when the wall plate gets attached.

In my opinion is always better to just add a bit more PEX ... PEX is cheap and it's better to not max out the loop length and spacing to save a few pennies. Often it's one more loop. W/ 1000 ft of reg PEX not much over $300 -- it's not going to add much and the whole system works better. Get the pipe in and order the correct spaced manifold after. Some of the high end PEX like Viega comes in 1200 lengths.

You can use Pex - AL as well -- I find it easier to use

 

[finn]

One thing to consider is that going from a 4” slab to a 6” slab doubles the thermal mass, which slows the system response. It will take longer to heat up and slower to cool down, which can be a problem when the outdoor temperature swings widely, particularly in the shoulder season.

Placing the tubes in the middle of the slab is tougher than just stapling it to the foam. One of my floors has developed cracks along a tube run, probably because the tubing floated too close to the surface during the pour. A piece of the concrete has actually spalled out in one area, exposing about four inches of Pex. I don’t like it and don’t have an easy fix, but it hasn’t caused more than an aesthetic problem so far, though. The floor is probably twenty years old.

 

[haveissues]

One thing to consider is that going from a 4” slab to a 6” slab doubles the thermal mass, which slows the system response. It will take longer to heat up and slower to cool down, which can be a problem when the outdoor temperature swings widely, particularly in the shoulder season.

Placing the tubes in the middle of the slab is tougher than just stapling it to the foam. One of my floors has developed cracks along a tube run, probably because the tubing floated too close to the surface during the pour. A piece of the concrete has actually spalled out in one area, exposing about four inches of Pex. I don’t like it and don’t have an easy fix, but it hasn’t caused more than an aesthetic problem so far, though. The floor is probably twenty years old.

when I did mine I put it under the rebar so it would stay in the middle.

 

[NothingNew]

Thanks, everyone.

I got to speak with my plumber today. He matched most of your recommendations
- 4" slab
- at least 2" of insulation
- place the Pex in the middle of the slab with wire mesh

One new item that he mentioned has me going with a separate hot water heater now. I had intended to do a mixed / open contact system with my existing potable water, but the potential of Legionaire's if the water isn't flushed often enough has me thinking differently now, as we (typically) travel for some time each year.

 

[PoorUB]

If you have insulation under the slab I fail to understand that it makes any difference where in the slab the tubing runs. The intent is to heat the slab and in turn the space. Where is the heat going other than the slab?

 

[NothingNew]

There have been a few studies on placement within the slab, but this is the one most cited:

Plumbing & Mechanical Engineer | Plumbing & Mechanical

Comprehensive source for engineers and designers: Plumbing, piping, hydronic, fire protection, and solar thermal systems.

www.pmengineer.com

That said, I'm going with my plumber's advice.

 

[finn]

You definitely don’t want to commingle your slab water with your potable water, got reasons you understand.

Do look into combi boilers that have a separate coil for dhw. My hvac guy ended up upgrading my new shop boiler to a combi for mo charge. Using one device saves on floor space, which you will always need more of.

 

[yeldogt]

If you have insulation under the slab I fail to understand that it makes any difference where in the slab the tubing runs. The intent is to heat the slab and in turn the space. Where is the heat going other than the slab?

I agree ... especially with constant circulation where you are using as low as possible

 

[kj_mustang]

This is what I did on mine. Tie the pex to the bottom of the wire mesh and place the mesh on stands helps hold it at a good location off the foam and far enough down to not be hit by anchors when attaching walls. Use a heat exchanger for the floor loops to keep those separate from the DHW.

 

[mcbane]

I put my tubing at the bottom of slab because I wanted to be able to safely put in a concrete anchor without concern for hitting the hydronic tubing. When I researched this, most of the hydronic heat experts were recommending that you use as thin a slab as possible and get the tube as high in the slab as possible so that you reduce the system time constant (time lag from when boiler fires up till slab surface is warm). I imagine that the reduced time constant can be important in a house where a room might be in the sun part of the day and the system is trying to rapidly respond to that sort of change. Can be uncomfortable if your slab is at 85 degrees in the morning just as the sun starts coming in the window.

In my shop the hydronics are only intended to prevent freezing and make the place usable in the winter, which for me is around 50-55 degrees. That means a slab temp in the mid to low 60s so it isnt possible to overheat the place on a warm spring day. So I have no concerns about time constant. And if the system is run at a more or less steady state, I agree with the above commenters that with an insulated slab and sufficient density of tubing it shouldn't matter where the tubing is placed.

In any case, my tubing and concrete are done so no going back.