# radiant heat question



## kendall j (Mar 30, 2007)

I'm thinking about redoing my floors, and when I pull them up I am considering running radiant heat using pex. The problem is, if I do this, I will most likely have to rehang my exterior doors as clearance will be a problem. Neeldess to say, I am not looking forward to that prospect. 

Would placing the pex between the insulation and the subflooring work almost as well, or is putting the pex between the subfloor and the hardwood going to be worth the extra trouble? 

I may be able to get by if I can use 1/4 inch pex and 1/2 inch osb to offset the floor rather than 1/2 inch pex and 3/4 osb. If I go that route, will I be able to get enough flow for there to be a heating benefit? 

Thanks,

Kendall


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## GREENCOUNTYPETE (Jul 25, 2006)

i have seen retro fitted houses with the pex placed under the floor from the basment it is fastened to the underside of the plywood then a reflective insulation is used to cover it and to put more heat into the house less into the basment.

but honestly heat rises and a bit of heat in the basment would be desirable in many applications 

so yes it would work between the insulation and subflooring maybe some loss but much easier to service should something happen to the pex 

i have an aunt who has the radiant baseboard thru the first floor rooms of the 2 story house the propane hot water (boiler )for lack of a better term at the moment,it sits in the garage with just the tank and a few feet of pipe it keeps the garage a pleasant tempratire with outdoor temps in the tewnties farenhiet (garage is a newer atached and isulated )and even with only baseboards on the exterior walls of this 120 year old house insulated with rockwool and 120 year old windows single pane with single pane storm windows it stays very comforatbal and if the door is opened and cold air rushes in it comes back to temp very quick as the walls are kept warm by the base boards and it keeps the temps very even , enough heat rises to the second floor bed rooms that they are fine with no heat to them.


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## Highground (Jan 22, 2003)

The doors shouldn't be a problem. Trim off what you need from the bottom and put them back up on the same hinges.


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## WisJim (Jan 14, 2004)

The regular way to retrofit radiant heat if access is available to the underside of the subflooring is to run the tubing in aluminum sheets that are fastened to the floor from underneath. The aluminum sheets have channels in them for the tubing to fit in, and help spread the heat over a wider area of the floor.
Here are some fancy expensive ones:
http://www.pexsupply.com/categories.asp?cID=208&brandid=
The people that I know that do this stuff (radaint in-floor hot water heating) for a living usually use flashing weight or similar aluminum and bend it around a steel rod of the right diameter to make their own much cheaper plates.


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## SolarGary (Sep 8, 2005)

Hi,
Putting the PEX under the subfloor is called a "staple up" -- they are used a lot.

As Jim says, the alum heat spreader plates help make this more efficient. There is a cheaper version that are just formed out of sheet metal -- I think they are made by Watts. Or, as Jim says, you can make your own.

The reflective layer below the PEX (not touching it) with insulation under this will help the efficiency, especially if its an unheated area under the floor.

There are some pictures of doing a staple up and other stuff on radiant floor heating in this section -- you will have to rummage through the links and find, but its there:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Active

On my house I added the PEX above the floor using half inch PEX and 5/8 plywood spacers between. I had to trim a little off most of the interior doors, but the thresholds on the exterior doors were high enough to not need trimming.
There are some pictures of my floor retrofit here:
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/House.htm

Gary


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## kendall j (Mar 30, 2007)

Thanks all,

Gary, the thresholds on my exterior doors may not be high enough to accomodate 5/8 inch spacers. Would using a slightly smaller pex to allow for 1/2 or 3/8 spacers work, or would flow be reduced too much to do any good as far as heating is concerned? With interior doors I have no problems trimming, unfortunately my exterior doors are metal and not solid wood. 

Thanks,

Kendall


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## SolarGary (Sep 8, 2005)

kendall j said:


> Thanks all,
> 
> Gary, the thresholds on my exterior doors may not be high enough to accomodate 5/8 inch spacers. Would using a slightly smaller pex to allow for 1/2 or 3/8 spacers work, or would flow be reduced too much to do any good as far as heating is concerned? With interior doors I have no problems trimming, unfortunately my exterior doors are metal and not solid wood.
> 
> ...


Hi,
They do make a 3/8 PEX -- I would guess that its outside diameter is about a half inch.

I'll check my book later and see if it has anything on whether you need to make the loops shorter, or how you need to change things between 1/2 and 3/8 PEX.

Gary


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## kendall j (Mar 30, 2007)

Thanks Gary I appreciate the help.

Kendall


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## SolarGary (Sep 8, 2005)

Hi Kendall,

There is a 3/8 inch PEX -- here are the specs:

3/8 PEX:
ID = 0.346 inch
OD = 0.5 inch (so it would fit in half inch plywood spacers)
Pres loss/100ft at 1gpm = 6.62 psi 15.4 ft of water

1/2 PEX:
Id = 0.485 inch
OD = 5/8 inch
Pres loss/100ft at 1 gmp = 1.13 psi = 2.6 ft of water

The pressure drop through the 3/8 inch is quite a bit higher than for the half inch. This means that for the 3/8ths you have to use more shorter loops (to keep the pressure drop down), or use a pump with more pressure drop capability to push the water through longer loops. 

Back when I was doing my floor, I used some software from IPEX to size the floor loops for the heat loss. I tried both half inch and 3/8 PEX.
For the case where I used the same loop space and loop length, everything comes out the same for the 3/8ths and 1/2 inch except the pressure drops over the loops. The maximum pressure drop for the half inch loops was 3.9 ft of water, and the maximum pressure drop for the 3/8 PEX for the same loop was 20.2 ft of water. This was for a 200ft long loop.
In the end, I ended up using some loops of 250 ft, so the pressure drop would have been even larger.

In the end, I decided to live with the 5/8 OD of the half inch PEX rather than the high pressure drops for the 3/8ths PEX. 
The larger pressure drop limits your choice of pumps, but Taco does make at least one pump that will do this. It also increase the pump power consumption.

If you decide to go with the 3/8ths, make sure you can get all the components you need in the 3/8ths size -- 3/8ths is not as common and things like manifolds may be harder to find.

Gary


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## kendall j (Mar 30, 2007)

Thanks for the info Gary.

Kendall


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