DIY Hydronic Floor Heating

[Exeric]

I think the biggest advantage to under subfloor pex (if you don't have extremely cold winters and you have a crawl space) is that it allows much better flexibility for installing different floor coverings. For instance, in my house I have 3/4" diagonal planks for the subfloor over which I'm installing 5/8" plywood as an underlayment.

A back of the envelope R value for 3/4" hardwood is about 0.9 or so. Assuming the plywood and planking are about the same that would come out to around R = 1.8 before any floor covering is laid. That is actually a good thing provided you back insulate it for an under subfloor pex installation job. It means that the difference between a linoleum or a thick carpet floor covering will have much less effect on the required heat output from the pex.

Taking the opposite case with the pex installed directly under the floor covering you will have a change in the R value between a linoleum and carpet installation where it goes from an R value of about .25 (1/4 plywood) and about R = 3 for heavy carpet. Like I say, these are back of the envelope estimates just to show the trend. There would be no easy way to accommodate those different temperature requirements between those extremely different R values that the heat must penetrate. That will definitely effect the value of it on resale of the house.

The other problem with above floor installations, except for pex in concrete, is that the pex radiates directly into the floor covering. Even with aluminum spreaders there will be significant cold spots where the spreaders don't exist.
On an underfloor pex installation on the other hand the heat will migrate in all directions through first the subfloor and then the underlayment. This will provide a lot more even heat to the floor covering.

I think maybe you have overstressed the importance of direct heat conductivity through short distances to the floor covering. If you have to do it that way because of limited access underneath, well fine. But if not then below subfloor installation has a lot going for it in milder climates.

[Vlad]

Originally Posted by Exeric

I think the biggest advantage to under subfloor pex (if you don't have extremely cold winters and you have a crawl space) is that it allows much better flexibility for installing different floor coverings. For instance, in my house I have 3/4" diagonal planks for the subfloor over which I'm installing 5/8" plywood as an underlayment.

A back of the envelope R value for 3/4" hardwood is about 0.9 or so. Assuming the plywood and planking are about the same that would come out to around R = 1.8 before any floor covering is laid. That is actually a good thing provided you back insulate it for an under subfloor pex installation job. It means that the difference between a linoleum or a thick carpet floor covering will have much less effect on the required heat output from the pex.

Taking the opposite case with the pex installed directly under the floor covering you will have a change in the R value between a linoleum and carpet installation where it goes from an R value of about .25 (1/4 plywood) and about R = 3 for heavy carpet. Like I say, these are back of the envelope estimates just to show the trend. There would be no easy way to accommodate those different temperature requirements between those extremely different R values that the heat must penetrate. That will definitely effect the value of it on resale of the house.

The other problem with above floor installations, except for pex in concrete, is that the pex radiates directly into the floor covering. Even with aluminum spreaders there will be significant cold spots where the spreaders don't exist.
On an underfloor pex installation on the other hand the heat will migrate in all directions through first the subfloor and then the underlayment. This will provide a lot more even heat to the floor covering.

I think maybe you have overstressed the importance of direct heat conductivity through short distances to the floor covering. If you have to do it that way because of limited access underneath, well fine. But if not then below subfloor installation has a lot going for it in milder climates.

I just want to remind where all discussion is coming from. Radiant hydroponic heating system is low or even very low temperature heating system. This makes possible using heat pump for heating water. Performance and COP of heat pump depends on temperature difference (outside/inside). The main goal to make water temperature as low as possible. This is why adding R value to your floor covering will lead to water temperature increase. My installation is hybrid above floor but I have extra layer of plywood above pex. In my installation I have 5/8 plywood and hardwood or tiles (about 20%) my water temperature was max @ 95 F.

I don't think it is a great idea to put tiles or carpet or any other flooring right on top of pex.

I am absolutely happy about performance of my heating floor system.

[Exeric]

Originally Posted by Vlad

I just want to remind where all discussion is coming from. Radiant hydroponic heating system is low or even very low temperature heating system. This makes possible using heat pump for heating water. Performance and COP of heat pump depends on temperature difference (outside/inside). The main goal to make water temperature as low as possible. This is why adding R value to your floor covering will lead to water temperature increase. My installation is hybrid above floor but I have extra layer of plywood above pex. In my installation I have 5/8 plywood and hardwood or tiles (about 20%) my water temperature was max @ 95 F.

I don't think it is a great idea to put tiles or carpet or any other flooring right on top of pex.

I am absolutely happy about performance of my heating floor system.

I agree with everything you said. The only place I differ is regarding my particular house and a lot of houses similar to mine, one's with an existing diagonal planking subfloor. The only other choice for me is pex on top of underlayment, which I agree isn't too good.

But it really would not make it difficult to have low water temperatures in my system. Just as Mr. Bean says, run pex closer together. You get more heat with the same temperature. Plus if one is really anal about it, on my system with with two wood layers before floor covering you just might be able engineer one system that accepts the two insulation values of say R=2.7 (3*0.9 wood floor layers) and R=4.3 ((2*0.9) + 2.5 a single carpet layer with 2 wood layers.) That is a difference ratio of insulation values of about 1.6. That should easily be doable in one carefully engineered system for a floor covering change by a future owner. If one originally installs 3/4 hardwood floor covering then engineer the system so it has the reserve to heat 1.6 times the current total insulation value if carpet is installed later. Options are always good.

[AC_Hacker]

Originally Posted by Exeric

...on my system with with two wood layers before floor covering you just might be able engineer one system that accepts the two insulation values of say R=2.7 (3*0.9 wood floor layers) and R=4.3 ((2*0.9) + 2.5 a single carpet layer with 2 wood layers.) That is a difference ratio of insulation values of about 1.6...

Difference ratio of 1.6?

Are you inventing a new kind of math here?

If you have just gone from R-2.7 to R-4.3, you have just increased the thermal resistance of your floor covering by 59.3%... and you still want carpet and carpet padding on top of that?

Above is a temperature chart for a product called QuickTrax, which I do not think is anything special, but it does illustrate what we are talking about.

The diagonal lines are there to help you calculate how much you will have to raise your feed water temperature to compensate for the insulating effects of various floor coverings. And if you install your PEX below the subfloor, the subfloor becomes part of the chain of insulation.

Floor covering makes a very big difference in the temperature of the feed water required to supply a given number of BTUs per hour.

When you increase the insulating effect of floor coverings, your boiler is going to have to work harder to overcome that thermal resistance... and your wallet is going to have to work harder to feed your boiler.

And this is exactly why we are so interested in high-efficiency, low temperature heating.

You should check out the link to Vlad's DIY radiant floor install, he did a great, innovative job. It must be said too, that he also did a highly effective job of building a very tight, well insulated house that holds in the heat that his floors produce. The value of reduced infiltration and superior insulation can't be overstated.

(NOTE TO VLAD: I still think you need to insulate your crawlspace.)

Best,

-AC

[Exeric]

Hi AC,
The point I was trying to address was that efficiency is not always the be all and end all. This is especially true in more temperate climates. If one is using a renewable resource such as a solar collector or collector(s) to heat the water then it may be less of an issue. Your mileage may vary.

For instance, if you go for maximum efficiency with a heat source going almost directly into an example hardwood floor covering, then the pex will be seeing an R~0.9 . If the person decides to change to carpet later it will go up R~2.5 minimum, probably more. I don't see how that system will be able to handle that 180% increase. In other words, the change in a less efficient system compared to this more efficient heat transfer represents a 60% vs 180% increase in heat requirements with a change to carpet from hardwood. It is just good engineering to have a robust but simple simple that allows one to change to carpet at a later date without reengineering the system. It will come at the expense of maximum efficiency. This may be impossible in a very cold climate but in other circumstances it may not, especially using a renewable resource to heat the water. It also avoids the hot spots and installation problems of pex next to the floor coverings.

Also, if you look closely in my previous comment the R = 4.3 included the carpet on top of two layers of wood.

Best,
Exeric

[AC_Hacker]

Originally Posted by Exeric

...If one is using a renewable resource such as a solar collector or collector(s) to heat the water then it may be less of an issue...

Actually, it is more of an issue rather than less of an issue.

Fossil fuel is very energy dense, and has high exergy, in other words it is capable of easily producing high temperatures. It was created around 100 million years ago. It is stored ancient sunlight. We, as a planet have already used up half of our supply.

If we did have a limitless supply of this stuff, and if it was very cheap, then no problems... build a pretty hardwood floor, staple up your PEX under the subfloor, forget about aluminum spreader plated, install wall to wall 1/2 inch foam carpet padding, install luxurious, thick wall to wall pile carpet, and settle back in your comfortable chair and know that all is good with the world. Just keep turning up the boiler until your feet feel toasty.

But unless you lived in the an area of extremely mild winters, don't think that you would be able to heat your house with low exergy renewables, it would be a mistake. Renewables won't deliver the higher temperatures that would allow the heat to fight its way through all of the thermal barriers. With solar and with heat pumps you don't have the option to keep turning up the temperatures until the thermal barriers are overwhelmed.

Think you can turning up the sun? Good luck to you.

My point is that the design principles that will make heating with renewables possible, will make heating with fossil fuels very economical.

But obviously, not everyone is interested in economy... not yet.

-AC

[Vlad]

I think first of all you have to determine your heat source.

If the source is high temperature boiler or DHWT or wood burning boiler higher temperature in the underfloor loop doesn't matter. The temperature range will be just higher, so higher supply higher return. The most important to keep your efficiency will be to keep as much heat inside your house and not outside like crawlspace (I will insulate my as soon as I have extra time and $$)

But if you want to use HP or solar or even condensing boiler you have to fight every degree. The lower the better. This is just because this systems work the best and the most efficient when temperature is low. I spaced pex @ 12" and have no problem at all. I do agree spacing @ 6" would allow to lower my temperature from max 95 to max 92 or so.

I think many people just focus on floor heating and forget about air supply. Air heating/ cooling must be a part of your HVAC system. This heating season I will try to run 2 systems together.

On other hand space pex @6" is a real pain. You are limited 300' with 1/2 pex. If your room is larger then 10x12 (this is a size of my den) you have to put 2 loops in parallel in one room. Balancing is going be a real pain.

[AC_Hacker]

I came across this interesting comment regarding radiant floor designs HERE.

So far as I can tell, the guy who did the post lives in Worcester, Mass, and has an average Heating Degree Days calculated at 7510, assuming a base temp of 68F.

He gives some info on his house construction, which is useful. He also really understands the efficacy of spreader plates, and also the superior performance of extruded plates over stamped ones. He didn't spell it out, but it sounds like he has staple up with stamped plates, and is switching to extruded plates, because he wants to keep his boiler running cooler, so that it is functioning in condensing mode.

If he had built his radiant floor above his subfloor, like Vlad did, he would probably not need to go to extruded plates.

Interesting too, his comment on radiant barriers not being as useful when running cooler floors.

You will not achieve 85F floor temps through 3/4 " subfloor and 120-130F AWT in suspended tubing or staple-up PEX, but you might get better than 75F with staple-up Onix (which I don't recommend, BTW- there's a long and mostly bad history of EPDM tubing in radiant systems.) For any staple-up you'd need considerable hotter water than that to hit 85F floor temps.

But you don't NEED 85F floor temps either (and 85F floors are rather uncomfortable in the first place!) Start with a Manual-J heat loss calc for that room, and the number of square feet of available floor space that will have tubing underneath it. You get about 2BTU per square foot for every degree the floor is above room air temp. If your design condition heat loss in the room is say, 1500BTU/hr and you have 250 square feet of radiant floor, the max floor temp you'd need a floor temp of about (1500/250)/2= 6F above room temp, so at 70F room the floor would need to be 76F at peak temps, which is achievable with ~125F water using better-quality sheet-metal plates and 8" o.c. tubing, but not as a staple-up or suspended tubing without much higher water temps. If you need anything more than that you need to go with extrusions, or higher water temps.

I've starred in the movie- I'm ripping out the thin plates under my lossy family room and am replacing them with extrusions to be able to keep up at design-condition, rather than boost the water temp above condensing range just for one room.

The biggest heat-transfer problem in radiant is getting the heat out of the PEX with such little surface area. Extrusions grip the PEX very well, and because of the higher cross sectional area, present a lower temperature to the PEX/aluminum interface for higher heat transfer rates at any water temp. Thin plates don't grip as reliably and transfer less heat at the same delta-T. Staple-ups have very little contact between PEX & subfloor, which leads to noticeable temperature-striping on the floor when you crank up the water temps to be able to deliver much heat. Systems that use tubing in grooved material sandwiched between the subfloor and finish flooring (eg WarmBoard) with thin-metal plates work VERY well with low water temps and are very responsive- you can use overnight setbacks and get similar response as with baseboards or radiators, etc.

At the water temperatures you're talking the radiated heat out of the PEX is actually pretty small, even in a staple-up, and just snugging up cheap low-density fiberglass batts works better than any kind of reflective goods. For 180F water and suspended tube, maybe, but its kinda academic at that point. With tubing in any kind of plate, extruded or sheet metal, the low emissivity of the aluminum renders radiant-barrier type goods useless at any water temperature.

Without a careful heat loss calc, you'll have no idea what you really need, but unless you have a very low window/wall fraction and R19+7.5c.i. walls or better you're probably looking at sheet-metal heat spreaders at a minimum. If this is batt-insulated 2x4 construction with lots of U0.34 or higher windows you'll likely need extrusions. For reference: My family room is 2x6 R19 construction, no exterior foam, and a rather large glazing fraction- it doesn't quite cut it at sub 15F with just the thin plates, though it's still pretty comfortable sitting on the warm floor even when it's 62-63F in the room, which is about where it hit's when it's 0F outside. With extrusions it should keep up just fine without boosting water temps.

Best,

-AC

[randen]

I thought I would offer some experience here. The staple up PEX beneth the subfloor dosen't work no matter what temp you run thru. From experience the bedroom I tried to heat with the staple up was bone chilling cold. The bathroom was the same, cold. The only saving grace was the bathroom was in the centre of the house. The bedroom was over a heated garage and three outside walls with R30 wall insulation and R50 in the ceiling. The floor was 5/8 plywood with plush carpet. Yes the floor felt warm lying on it or with your hand but the room was cold. After we changed to the solar hot water and Geo-thermal there was no chance of heating the space. water temp average 100F. The system that works is the warm board with a ceramic tile over-top set with a thinset mortor. If you need a carpet in the room a small area rug over the tile.

The solar heated or HP heated water (low temp. heating)warming the ceramic tile by near direct contact by the pex tube augmented by the thinset is the game changer. The radiating effect of the tile is amazing the room will warm readily.

The feeling of a bare foot on a warm ceramic floor is therapeudic as the cold winter wind is howling outside. The other thing that will make you smile is to know this therapy is only costing you pennies.

Randen

[Exeric]

You are comparing apples to oranges. If you use staple up you must extensively insulate from below. Perhaps you did and just failed to mention it. There seems to be some confusion about this whole subject. If you mount pex from below without a heat spreader you won't have good heat conductivity to the floor. That's just physics. However, you can get away with it if the ratio of R value below to R above is high enough.

Heat doesn't just disappear because you have insulation from layers of subfloor above the pex. It has to go some where. If one has no insulation below it will just go down because the ratio of R value above and below isn't good. It will twke the easiest escape route. But even without a heat spreader if you increase the ratio high enough of R value below to that above it will still work well. But it might work more like a concrete floor in that it will take a long time for that heat to work through the upper layer(s) of flooring.

I myself would go for pex mounting below with a heat spreader so I wouldn't have to wait so long. If you didn't heavily insulate below without a spreader then that was probably a mistake.