Radiant floor not working as expected


Old 12-22-08, 11:17 AM
petrs's Avatar
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Location: MA, US
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Radiant floor not working as expected


I installed radiant floor heat system this fall hoping for the best; however I am a little puzzled with the result.

When the outside temperature drops to teens (15F) (like it is common in New England) the 1st floor gets only to 60-62 while the 2nd floor goes to 68-70. I played little bit with my Munchkin boiler that allows me to set the temperature up to 180, and ended up with 135 to get the numbers above. I would much prefer to see the temperature to be below 130, but then I would not even get to the numbers above in my 1st and 2nd floor.

Each floor consist of three loops, each loop about 300 ft (+/- 5 ft) using PEX tubing with oxygen barrier. Each floor uses its own pump that is set to the highest speed (3 speed setting). The tubing is stapled under the subfloor, I used aluminum distribution plates (on the 1st floor almost continuously), then reflective aluminum foil, R19 insulation under 1st floor and R13 under 2nd floor. Under the 1st floor I put one more layer of the aluminum reflective foil simply since I had a lot of left.

What bothers me is the return temperature. While I am feeding the 1st floor with 135 the return temperature is about 90-100. If I turn off the 1st floor and do the same test for 2nd floor I am getting the return temperature about 110 that is still not great but much better. Could it be that I have air pockets in the system? I think I can almost eliminate air draft since I was very careful during the installation of the insulation. 1st floor is over unheated basement and garage.

Any suggestion will be appreciated.

Some pictures of the installations are here:

Last edited by petrs; 12-22-08 at 11:32 AM. Reason: adding pictures
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Old 12-23-08, 10:08 AM
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Can you turn the pump speed down? Try the 1st floor on low and 2nd floor on medium.
Old 12-23-08, 07:30 PM
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Thank you for the suggestion. I am not sure what should I expect by doing it? I thought that higher speed gives me a little higher temperature on the return since the water flows faster and does not lose as much heat. If anything I would expect that the 1st floor should circulate faster.
Old 12-23-08, 08:25 PM
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i would think that the lower temp on the return would be better as then you know all of the heat is being transfered from the water to the floor. sounds like you are maybe loosing to much heat to the garage/basement area.

how come common sense isn't common?
Old 12-24-08, 05:37 AM
Join Date: Oct 2008
Location: New England
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Hi petrs, I'm not great at this topic, but can help a little. Your loop length is right at the max for 1/2" pex. Workable, but a lower margin if other issues are present, which seems to be the case. Your heat loss at the higher speeds indicates a greater loss than the temps currently show. I'm not sure why slower is better, but as stated, not my field. My logic would be as yours.

Your insulation job looks very good. The only area I can't really judge is the perimeter exposure. All houses experience a stak effect, where the warm air rises and escapes and pulls cold air in to replace it. The cold air comming in is frequently around that perimeter of your basement. If you did not air seal all seams and the wood to concrete junction, you may be pulling cold air directly into the space you just insulated.

The other end of that air leakage is all of the holes going up through the floor to the house or attic. Those should have all been sealed before you close everything up.

If everything was sealed and the insulation is working as it should, then as u stated, the circulation may not be as indicated, i.e. air in the line or other. A slow moving flow of water would certainly deliver less heat and return colder than it should.

Heating guys, how would he purge any air and verify that he is getting good circulation?

I'll check back,
Old 12-24-08, 06:30 AM
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Hello Bud,

Thank you for your reply and the explanation.

Regarding the higher speed vs. lower speed. I used the higher speed to keep the return temperature higher. I know that this is not good regarding efficiency; however I can at least hope that "some" warmer water reaches also the end of the loop; while if I use slower speed I will know that all the heat has been transferred as Speedwrench pointed out, but it may be very well at the beginning of the loop and thus big part of my floor could be not heated at all.

Regarding the perimeter of the house you are also right. The perimeter in my house is done using bricks stacked next to each other between the floor joists. I tried to spray insulation into every gap I saw between the bricks, but I am not sure how well this worked. The brick seems to be like handmade, (hard to find two same) so there was a lot of gaps. I had also cut extra fiber insulation and I had put it against those bricks as I was worried that the bricks may be weak place where I will experience heat loss.

I want to bleed the system one more time tonight using the "street" pressure (not the 15 psi that my relieve valve allows). If this does not help then I will most likely open the ceilings again and cut 2" foam insulation and tight stuck it around the perimeter of the whole house.

Thank you and happy holidays.
Old 12-24-08, 11:43 AM
Join Date: Nov 2001
Location: USA
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The temperature difference between the supply and return should be no more than 25 degrees Fahrenheit. You should be asking this question in the heating forum where you may hear from people who are familar with your system. If floor is insulated and this was my house, I would install more loops for the first floor thereby shortening the loops. This should bring the return temperature up and make the first floor more comfortable.

Again, I recommend you pose this question on the Heating Forum.
Old 01-15-09, 08:57 AM
Join Date: Nov 2007
Location: New York State, Suffolk County
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The aluminum shield you have shown under the pex should be spaced off the pex as much as possible leaving 3 inches or more of air space. The foil is dissipating much of the heat in the pex to your basement and reflecting very little to your floor. Check this web site: http://www.radiantdesigninstitute.com/ for some very good guidance on radiant heating.
Old 01-26-09, 04:39 AM
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Thanks for replies and one more question.

Hello guys,
Thank you for all your suggestion. I think I found one of the major problems. The little entry area of my house has no foundation, and it looks like it has very poor or no insulation. I hung one of the thermal curtains and suddenly the 1st floor is able to hold the temperature around 68F while outside is in teens.

The other major problem I assume will be my garage. The garage door is not fitting very well, and thus the temperature in the unheated, uninsulated garage is very low. When I put the radiant floor heat in the place I put only R19 in the floor above the garage and above the unheated basement, while in the crawl space I combine R19 + R7,5. This is most likely why the area above the crawl space hold the temperature a little better. I am planning in the spring to re-do the insulation. I would like to add a rigid 2" foam insulation around the entire perimeter of the house and also additional R13 below the entire 1st floor. Also I was thinking to add 2 extra 300' loops under the 1st floor. My question is how it will be with the balance of my system. I would have 3 loops (300' each) in the 2nd floor and 5 loops (300' each) under 1st floor + maybe a little baseboard radiator in the basement. Currently I had 3x300' under each floor so when both zones were open the flow was distributed evenly into each floor.

I am also planning insulate basement and fix the garage doors.

Thank you again for all the replies.
Old 01-28-09, 12:07 PM
Join Date: Jul 2005
Location: USA
Posts: 124
I'd hate to think what all of that's going to cost. Do you feel your fuel savings will let you break even before the day you die?

Not to poke fun or anything, but as someone who's been insulating a drafty old house and must continue to do so for heaven knows how long, I'm weighing the cost advantages.

I've already tossed out the 90%+ efficiency boiler idea, after learning what kinds of after-expenses are involved. And your issues with underfloor heating are making me wary about running that route, too.

Old 01-28-09, 03:20 PM
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Thank you for your reply. I had the same question so I run the calculation. It came up all together less than $1500 + my time (5 days). My point is that somehow I need to achieve the comfort temperature. One way is to keep using the thermal curtain and hope that outside temperature around 0F will be just an exception or put the $1500 into the insulation/additional tubing and hopefully be all set.

Overal I am a little upset from this project as I did not feel my expectation; but on the other side if the outside temperature is not below 15F the floor heat is great and I would never go back to radiators. I would certainly do it again but next time I would torn down the walls and check the insulation.

Old 01-30-09, 04:28 AM
Join Date: Feb 2008
Location: Lake Wales, FL
Posts: 463
When you decided to install under floor heating, did you check out the lowest outside temperatures over the last twenty years or so in your area?
Did you take into consideration how exposed your home is, and how it will respond to strong winds that will suck the heat out?
Did you work out how much heat each room will require to keep the home comfortable during the expected periods of cold weather?
Did you work out what size of boiler is needed to run the system?
Did you do a pressure test on your home and fix all the holes.
Did you look at the existing level of insulation and compare the cost saving of running a well insulated home as against one with (by the look of it) very little insulation?
Do your have each floor controlled by its own thermostat?
Where different rooms on each floor require different temperatures are the rooms zoned and each controlled by its own thermostat and motorised valve?
The above are just a few of the questions you need to resolve when fitting a heating system, not the least is, will the next person who owns your home require higher temperatures?
Radiant floors do what the name suggests, they radiate heat in all directions, your heat is mostly heating a space, that is uneconomic to heat, what you need is at least five inches of sheet polystyrene or similar jambed between the joists, below the heating pipes and then to spray foam over the whole of the underneath of the floor to make a monolithic insulation with no holes or spaces. Holes equal loss of heat, they also, more important in many ways lead to wood rot.
It is important to stabilize the moisture content of wood by completely sealing the joists and floor from damp air.

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