Radiant heat - lesson learned

[HoosierBuddy]

Hey guys,

I just wanted to pass on a lesson I learned in doing my radiant heating of my garage/bonus room/breezeway project I built in 2006.

Ready?

What I found out was that installing PEX heating coils underneath a subfloor, and then insulating under the PEX to force the heat up into the area above is not very effective.

My garage slab has pex in it with 2" of HD polystyrene under it. It works great.

My bonus room above the garage is heated by a 2nd zone pump via pex loops under the subfloor. The heat there is only effective if I crank the boiler up to about 160-degrees.

Our breezeway is heated with the same sort of arangement under the subfloor. The heating there has not been effective even if the temperature on the boiler is cranked.

So...what I had to do was add a ventless gas heater to the bonus room to help heat the area during the coldest part of winter.

On the third zone, the breezeway, I added 2 small modular heaters (basically coils for the zone's water to flow through with thermostatically controlled fans that turn on when the coils get hot) to supplement the heat AND went back in this summer and installed the specially designed aluminum plates that staple to the subfloor around my pex, adding about 50% more pex tubing in the process. The goal being to get my water to transfer more heat to the subfloor before returning to the boiler. After I put the 6" of fiberglass batting back under those loops I also added a layer of FBF insulation along the bottom of the floor joists to try to protect (as best I could) against thermal bridging to the crawlspace.

Coming into the fall, I've turned my boiler temperature back to 150-degrees and am hoping for the best. During an early cold spell, it seemed the aluminum plates helped a lot, but it's still early.

I do know that just runing the pex in the joist space under the subfloor and clipping it up to the subfloor every couple of feet doesn't work worth a damn even with 6" of fiberglass batting under it. You just can't get enough heat transfer from the water to the floor. The water comes back to the boiler and it's still hot. The thermostats in zone 2 and 3 are calling for heat, but the boiler isn't even firing, because the water is still hot. Intolerable.

SO...If I had it to do over...and I DON'T...I would have gone with one of the systems that allows you to install PEX above the subfloor in both my bonus room and my breezeway. The bonus room could have easily been equipped with that special grooved subfloor that the PEX snaps into. The bonus room has laminate flooring except for the ceramic tile in the bathroom The breezeway could have been (somewhat easily) covered with litecrete above the subfloor and my ceramic tile could have gone over that.

So...my advice is don't do what I did and IF YOU DO...at least spring for those aluminum plates. They do seem to help some.

Phil

 

[72Tunaboat]

I have to disagree with you on a few things. Granted, Doing a retrofit under the sub floor isn't as efficient as having the tube buried in the slab. When you run tube in the joist bays, you Must use a foil backed insulation(foil side towards the tube) if you want to see any results. I used a product called Reflectix. You have to leave some air space between that and the tubing. Also, you need to staple it a bunch to the joists and box in the ends so that is a sealed airspace.

What happens then is that the tubing heats that sealed airspace and the foil directs that heat up through the floor. You still need to insulate under the Reflectix with some batting under the Reflectix(I used R21).

The water temp required for this type of radiant is much greater than what you run in your slab. You would need a mixing valve to temper the water down for your slab.

There's a few ways to do these type of installs, but they do work if done correctly. I have it in my house and it works great.

Remember this is just my .02 and valued accordingly.

 

[redsky49]

With all due respect Hoosierbuddy, it sounds like you have more of a heat transfer problem than an insulation problem.

If this was my system to correct I would need the following data:
Leaving water temperature
Return water temperature
Flow rate (gpm)
Indoor/outdoor temperatures

As a rule, if there is no temperature drop in the loop, you have mis-sized some aspect of the design - flow rate, loop length, heating load, etc.

If the metal pans have increased the radiating area and this lowers the return water temperature, I think that you have confirmation that heat transfer is the issue. Are you able to accurately monitor and/or record water temperatures?

As always, offered only as opinion.

 

[HoosierBuddy]

Heat transfer is the problem. That was my whole point. I wish I would have installed the tubing in lite-crete to enhance heat transfer from the pex to my ceramic tile.

The modulating tankless boiler has a digital readout for the water temperature. The boiler short cycles. It throttles down to 25,000 BTU input (it's lowest setting) and the return water temperature quickly rises above the "off" setting. Meanwhile, out in the breezeway the thermostat is showing 67-degree air with a 71-degree set point. The zone pump runs and runs and runs and finally the boiler kicks back on for another minute.

It's definitely a heat transfer issue. If I had my pex in litecrete under the tile...instead of loosely tacked in the joist space...I wouldn't have this problem.

That's my point.

Raising the water temperature helps, but the higher the water temperature the less the efficiency of the boiler. An efficient system would be characterized by a low water temperature being able to raise the room air temperature to the thermostat's set point.

The aluminum plates snap around the pex and are stapled solidly to the subfloor. In theory, they should allow for conductive heat transfer to the floor, which (again in theory) should be much more effective than convective and radiant heating alone. Obviously if more heat can be pulled out of the pex the system will be much more effective.

My point though is I view the aluminum plates as being sort of "mickey mouse" compared to lite-crete or (maybe) the special subfloor that gets the pex out of the joistspace and up into the room that's being heated.

Also...I should point out that I only installed the plates this summer in the breezeway, where I had access from the crawlspace. To install them in the bonus room I would have to take the ceiling down in the garage. Possible, but it's 13' in the air and there are a lot of lights that would have to come down first. The ceiling is seamed steel attached with screws...so it would be easier than drywall...but still one royal PIA. IF the aluminum plates make a world of difference in zone 3, my breezeway, I might bite-the-bullet and do it for the bonus room (zone 2). We'll see.

Phil

 

[chevelle67]

you can easily figure out if any of the "fixes" you are doing is actually helping by calculating the btu`s being used. formula = 500 x Delta T of water (supply - Return) x gpm = BTUH.

 

[72Tunaboat]

Regardless, you are not going to be able to get enough heat through floor without raising the water temp, and the slab cannot take that much heat coming through it. You will need 2 different water temps for the slab and the under floor stuff.

 

[HoosierBuddy]

Regardless, you are not going to be able to get enough heat through floor without raising the water temp, and the slab cannot take that much heat coming through it. You will need 2 different water temps for the slab and the under floor stuff.

What's the slab going to do? Explode?

I've been heating it going on 3-years now. Thanks for offering your opinion.

Phil

 

[72Tunaboat]

It will crack. Under normal conditions your water temp going through the slab should not exceed about 110 degrees.

and you are correct, it is only my opinion.

Good luck

 

[Charles (in GA)]

I only install and service hydronic heat for a living. You homeowners and do-it-yourself guys probably got it figured out without any help, so why bother posting if you want to argue.

It would have helped and been beneficial had you posted this information in your first post in this thread.

Charles

 

[HoosierBuddy]

It will crack. Under normal conditions your water temp going through the slab should not exceed about 110 degrees.

and you are correct, it is only my opinion.

Good luck

The guy that installed mine does it for a living too. And he's a heck of a nice guy. That doesn't mean I like how it worked out.

Oh...you thought I installed mine myself?

No. I'm just trying to make it work in an acceptable manner.

The thing about concrete slabs is, they all crack eventually. That's my opinion. However, mine doesn't have a crack in it anywhere yet...and that was with the water temperature at 160 for the past 2 winters. That zone has the actual boiler in it though, shares a wall with the house, and we keep the t-stat about 55-degrees...so it hardly ever actually sees flow. Maybe that's why what I'm seeing doesn't match up with what you're saying.

Phil

 

[Dragster Racer]

So, will a driveway crack if the sun heats it above that temp? I'm not sure I can see how that would always be the case.

 

[sneezer41]

the reason for a lower temp in a slab is at 150 degree water it may become uncomfortable under foot, not an issue in the garage