Folks with slab heat check in

[nate379]

My house is slab on grade and I have pex running in it. I'm not sure of the spacing or layout, it was done before I bought the house. Based on my neighbors which was built by same contractor it was done correctly. As far as I know there is 2" foam under the whole house.

The trouble I have is to be honest I'm not very satified with the whole system. One of the big reasons I bought the house was for the slab heat. I had always been told that it was so effeicent you could heat for cheap compared to any other type of heat. And it's so comfortable you can keep the house colder.

I never found that the case. The first year I was in the house I saw $150-$175 gas bills each month. To compare my neighbor a few houses down has a forced air system and he told me he has never seen over $100/month gas bill.

ANYHOW...

After that first year I got a wood stove and I have not used the floor heat very much other than in the garage.
A few weeks ago I rented out a spare room. Before they moved in, I just left the door closed to that room and kept the thermostat set to 50-55*. The thermostat controls the heat to that room and also one other bedroom.

Since the moved in we set the T Stat to 68*, which is a reasonable temp. I don't expect them to be cold because I am too cheap to run the boiler you know? As long as the door is left open it will stay 65*-70* in there, all depending on how cold it is outside. Oh course I don't expect the door to stay open all the time either.

It seems when there is a call for heat the boiler runs near constantly to warm the room. I am seeing around 130* input to the pex and 100-110* return. The temp of the floor is about 75* so I don't understand why it would be hard to achieve 68*? I even went ahead and throttled back the water flow to the other bedroom since I have no reason to heat that room (it's used for storage and some gym equiptment)

I'm almost wondering if maybe because I am using the wood stove when the door is open the temp is fine so the slab cools. Then close the door for the night and the slab has to be rewarmed?

His room is running at 2gpm and the other room is at .75gpm.


Any thoughts?

 

[Gasifier]

I have in-floor (slab) heat in my garage. It works great. But one thing that is critical in any heating situation is the insulation. Around and under the slab insulation is very important, but the whole house insulation is as well. And I know you know that. I have closed cell spray foam in the walls and ceiling of the garage. Seals every crack and eliminates air flow.

How is the insulation in your house? I wonder if you are right about the slab having to heat back up again. Could you transfer warm air into that room through a duct w/fan inexpensively and not take away their privacy? Don't know what your set up is but sounds like you might be able to figure something like that out so that you are heating that room with wood. Let us know how you make out Nate. Later.

 

[in hot water]

What type of floor covering? Bare concrete, wood, carpet?

Edge insulation is critical on slab radiant, you should have at least 2" attached and protected around the entire slab. See if you can locate an infrared camera to see where all the energy is going. The camera would be able to "see' the radiant tubing to assure you have enough tube to cover the heat load.

Could be some of the loops are air locked?

Sometimes in bedrooms the furniture can hamper the output. Any dressers, beds chairs, file cabinets, thick area rugs,etc that don't allow the heat to transfer to the room need to be calculated out. It's called "heat flux" Add up the square footage of any furnishings covering the floor and subtract that from the room square footage. The number you get is the amount of radiant surface you have to cover the room load.

Rule of thumb is 1.7- 2 BTU/ square foot of floor for every degree difference between the floor surface and ambient. So a room at 68F with a floor surface of 83F would be about 27- 30 btu/ ft. output.

Radiant slabs are not generally quick responding heat emitters. The thicker the slab the slower the response. Personally I prefer panel radiators in bedrooms for quick, setback-able heat.

hr

 

[bpirger]

How thick is your slab? It sounds to me that indeed, your system is working right, as 130 into the pex and 110 out is a 20 degree drop. Typically 130 into concrete is the maximum desired temp, at least that is what Uponor recommends I think. When the heat comes on, you have to heat up the entire section of the slab you are heating....that's a huge amount of mass I suspect. It of course is a function of insulation, pex spacing and size, flow, etc.

2GPM with a 20 degree drop should be dumping 20K BTU/HR into that slab.
A 15x15 room with a 4" slab is about 3 yards of concrete, or 12,000 pounds.
Specific heat is 0.2BTU/lb/degree for concrete, so 2400 BTU/degree.

You likely need to swing by 10 degrees say, and likely a little more as it is radiating as it is warming, spreading to the asurrounding slab, going down, etc.

So that is 24,000 BTUs needed. So the system would run maybe 1.5 hours to meet that demand? Does that sound about right?

Assuming 4" floors, pure isolation between rooms, no carpets, etc .


Oh, Alaska I see, so how cold is that ground? I wonder how much is going down through that insulation? How is the rest of the insulation? Do you have an idea of the thermal load for that room?

I have a 44x32' floating slab, 6-7", and it takes a few hours to bring the thing up 5-6 degrees. I have 5/8" tubing about every square foot, a bit more around the edges, and 2" of polystyrene under the whole thing. I love it. When it is warm, I agree with what you said initially, it feels incredible and it doesn't have to be as warm in the house on the t-stat to feel nice and warm. As for efficiency, I can't really give you any numbers there...

 

[ISeeDeadBTUs]

I have ~1800[] of pexed slab. They are a b|tch to heat. As with other zones, try to set the thermostat and walk away. Speed is NOT radiant's strong suit.

Do you have outdoor control, which will effect the temp of the H2O? Though as someone else said, sounds like the water going in/out are at the appropriate temps.

Jimbo

 

[jeff_t]

Based on your posts about the Princess, it sounds like you could heat the room with an electric space heater for next to nothing. Or maybe a small dog.

 

[nate379]

R60 attic, R21 walls, decent double pane windows.

No easy way to get warm in that room without leaving the door open. To compare I leave the master bed door open and I don't think the heat has come but maybe a couple times this winter. I have the T Stat set to 65*

How is the insulation in your house? I wonder if you are right about the slab having to heat back up again. Could you transfer warm air into that room through a duct w/fan inexpensively and not take away their privacy? Don't know what your set up is but sounds like you might be able to figure something like that out so that you are heating that room with wood. Let us know how you make out Nate. Later.

 

[nate379]

Carpet with pad.

Yes 2" edge insulation plus 2" apron insulation 3-4 ft out as well. I did it like this: http://inspectionprotection.com/info/BasementInsulation.pdf Look at page #9.

What type of floor covering? Bare concrete, wood, carpet?

Edge insulation is critical on slab radiant, you should have at least 2" attached and protected around the entire slab. See if you can locate an infrared camera to see where all the energy is going. The camera would be able to "see' the radiant tubing to assure you have enough tube to cover the heat load.

Could be some of the loops are air locked?

Sometimes in bedrooms the furniture can hamper the output. Any dressers, beds chairs, file cabinets, thick area rugs,etc that don't allow the heat to transfer to the room need to be calculated out. It's called "heat flux" Add up the square footage of any furnishings covering the floor and subtract that from the room square footage. The number you get is the amount of radiant surface you have to cover the room load.

Rule of thumb is 1.7- 2 BTU/ square foot of floor for every degree difference between the floor surface and ambient. So a room at 68F with a floor surface of 83F would be about 27- 30 btu/ ft. output.

Radiant slabs are not generally quick responding heat emitters. The thicker the slab the slower the response. Personally I prefer panel radiators in bedrooms for quick, setback-able heat.

hr

 

[nate379]

Slab is 4" I believe.

Take much much longer than 1.5 hours. I just got home from work and with the door closed it's only 66* in there. Not sure how long it's been running, I would guess 4-5 hours.

How thick is your slab? It sounds to me that indeed, your system is working right, as 130 into the pex and 110 out is a 20 degree drop. Typically 130 into concrete is the maximum desired temp, at least that is what Uponor recommends I think. When the heat comes on, you have to heat up the entire section of the slab you are heating....that's a huge amount of mass I suspect. It of course is a function of insulation, pex spacing and size, flow, etc.

2GPM with a 20 degree drop should be dumping 20K BTU/HR into that slab.
A 15x15 room with a 4" slab is about 3 yards of concrete, or 12,000 pounds.
Specific heat is 0.2BTU/lb/degree for concrete, so 2400 BTU/degree.

You likely need to swing by 10 degrees say, and likely a little more as it is radiating as it is warming, spreading to the asurrounding slab, going down, etc.

So that is 24,000 BTUs needed. So the system would run maybe 1.5 hours to meet that demand? Does that sound about right?

Assuming 4" floors, pure isolation between rooms, no carpets, etc .


Oh, Alaska I see, so how cold is that ground? I wonder how much is going down through that insulation? How is the rest of the insulation? Do you have an idea of the thermal load for that room?

I have a 44x32' floating slab, 6-7", and it takes a few hours to bring the thing up 5-6 degrees. I have 5/8" tubing about every square foot, a bit more around the edges, and 2" of polystyrene under the whole thing. I love it. When it is warm, I agree with what you said initially, it feels incredible and it doesn't have to be as warm in the house on the t-stat to feel nice and warm. As for efficiency, I can't really give you any numbers there...

 

[nate379]

Yes. Right now it's -15* outside and it's calling for 200* water.

Do you have outdoor control, which will effect the temp of the H2O? Though as someone else said, sounds like the water going in/out are at the appropriate temps.

Jimbo

 

[711mhw]

Nate, I'm not sure if it's "cool" to recomend another forum but since there is no radiant section here, check out greenbuildingtalk.com They have such a section there, one of the guys that does a lot of responding there (Rob - NRT) designed my radiant system and I am very pleased with it. I'm heating 2500 s/f of shop and a little 1000 s/f living space. Very comfortable and efficient. Sounds like you have a good insulation pkg. so hopefully it's something simple like a circ. pump but I don't venture a guess. (I did intstall my sys. but all I did was follow the drawings.)

 

[tom in maine]

-15F in November makes me love coastal Maine!

You might be a candidate for a good energy auditor.

I suspect the thermal envelope of the building is marginal. Certainly in that environment, one can make a great case for much higher
slab insulation than what you have. Heating a concrete slab to 110-130F with R 10 insulation on the edge is a big energy deal.

I think the simplest solution is a small heater for the rented room. You can make a great case here for a pellet stove or vented gas heater like a Rinnai.

 

[huffdawg]

Carpet with pad.

Yes 2" edge insulation plus 2" apron insulation 3-4 ft out as well. I did it like this: http://inspectionprotection.com/info/BasementInsulation.pdf Look at page #9.

What type of floor covering? Bare concrete, wood, carpet?

Edge insulation is critical on slab radiant, you should have at least 2" attached and protected around the entire slab. See if you can locate an infrared camera to see where all the energy is going. The camera would be able to "see' the radiant tubing to assure you have enough tube to cover the heat load.

Could be some of the loops are air locked?

Sometimes in bedrooms the furniture can hamper the output. Any dressers, beds chairs, file cabinets, thick area rugs,etc that don't allow the heat to transfer to the room need to be calculated out. It's called "heat flux" Add up the square footage of any furnishings covering the floor and subtract that from the room square footage. The number you get is the amount of radiant surface you have to cover the room load.

.



Rule of thumb is 1.7- 2 BTU/ square foot of floor for every degree difference between the floor surface and ambient. So a room at 68F with a floor surface of 83F would be about 27- 30 btu/ ft. output.

Radiant slabs are not generally quick responding heat emitters. The thicker the slab the slower the response. Personally I prefer panel radiators in bedrooms for quick, setback-able heat.

hr

I would rip out the carpet . It is an insulator not a conductor. Use either slate, ceramic tile or engineered hardwood . I heat a 3000 square foot house on propane and infloor heat also DHW. for an average of $ 275 a month. propane is 84 cents a litre. OUCH. Hence the EKO 40 soon to be firing up. And again before you do anything drastic take the carpet and pad out and see what happens.

Huff

 

[nate379]

I did some math and $275 of propane at your price is 7,981,00 BTUs.

The first year I was at my house I heated just with the floor heat. My bills where around $125-150. That equals 12,887,500 btus.

The total space I heat is about 2100 sq ft.

 

[huffdawg]

I did some math and $275 of propane at your price is 7,981,00 BTUs.

The first year I was at my house I heated just with the floor heat. My bills where around $125-150. That equals 12,887,500 btus.

The total space I heat is about 2100 sq ft.

I am also heating hotwater for a family of 5 with 2 daughters that will run a 50 gal indirect cold when they have a shower.
The outside temps where I live are a lot milder than where you live.
Seems that you might be losing a lot of heat somewhere though.
What do you pay for a gal. of lpg.
We keep the house at 72 °F all interior walls and the ceiling between the first and second floor are insulated.
Exterior walls are r20 and the attic is r40
Its the house in my avatar

 

[PassionForFire&Water]

did not read all postings here, but how is the pex getting into the concrete.
I assume it's from a manifold.
Check the length of each pex loop; there should be numbers in feet on the pex tubing.
If there is a hugh difference in length some pex-loops will get much more flow (heat) then others.
Do you have flow meters on each loop?
once you know the length of each loop and the flow in each loop, you can balance it

 

[ISeeDeadBTUs]

I would rip out the carpet . It is an insulator not a conductor. Use either slate, ceramic tile or engineered hardwood . I heat a 3000 square foot house on propane and infloor heat also DHW. for an average of $ 275 a month. propane is 84 cents a litre. OUCH. Hence the EKO 40 soon to be firing up. And again before you do anything drastic take the carpet and pad out and see what happens.

Huff

+1

We have NO carpet. I was always told thats a no-no with radiant. Fleas and ticks also like carpet.

 

[nate379]

Yes I have 4 Rehau manifolds. 3 of them are in the garage. They are for the garage, living room/kitchen and master bedroom. The other loop has a run of larger pex, 3/4" maybe to another manifold which is in the closet of one of the bedrooms. That one is for the 2 bedrooms.

They are all set to 2gpm (max) other than the one unused bedroom which I set to I think .5 gpm

Not sure on the lengths if there are markings on the pex. I will check.

I like carpet in the bedroom. The rest of the house is laminate flooring

 

[huffdawg]

[quote author="NATE379" date="1321914696"]Carpet with pad.

Yes 2" edge insulation plus 2" apron insulation 3-4 ft out as well. I did it like this: http://inspectionprotection.com/info/BasementInsulation.pdf Look at page #9.


I was reading this . It sounded like you had carpet with pad in your cold bedroom that you were renting]

 

[nate379]

Yeah all 3 bedrooms have carpet.

I think the issue is either need to let the stove heat the house like I have been doign the last 2 years or let the slab come up to temp and heat the house that way. I'm sure the carpet doesn't help things either, but it's just a few years old so it will be many years before I decide to remove it.... and quite honestly I would rather have carpet. I might do a short pile and thin pad though.

I think maybe what happens is the room is warm from the stove when the door is open. Close the door and after a bit the temp drops some. It ends up taking the night or longer to get the slab warmed up. The line that runs to those 2 rooms goes from the garage across the master bathroom, laundry closet, hall and then to almost the outside wall to the manifold, then to the floor in the two bedrooms.

I know the first year I lived in the house the power had gone out for 2 days when it was -10* outside. I went from 68* to 63* two days later. The slab took that long to start cooling down.

 

[JrCRXHF]

Mine is setup a little different i had a leak in the baseboard that ran in the floor so i ditched that and put down plywood with reflective foil under it and then put 7/16 pex every 6" off center in the plywood and foiled the tubes then put hardwood floor over that i have been really happy i would say in the winter if i am running on gas it saves me about 100.00 a month over baseboard running on the outside of the walls and the rooms feel so much more even with heat.

 

[ISeeDeadBTUs]

I think maybe what happens is the room is warm from the stove when the door is open. Close the door and after a bit the temp drops some. It ends up taking the night or longer to get the slab warmed up. The line that runs to those 2 rooms goes from the garage across the master bathroom, laundry closet, hall and then to almost the outside wall to the manifold, then to the floor in the two bedrooms.

You might want to check with one of the pros here. . . but my unprofessional observation . . . tubes in CC need to run on some sort of timed cycle, instead of air temp 4' above slab. But again, Viessmann is designed to CONSTANTLY circulate, so that would probably 'fix' the whole situation.

 

[bpirger]

Do you have a slab sensor on the concrete? I can set a minimum temperature for the slab. Before the Garn, I heated with the woodstove primarily, but also would maintain the slab at a minimum temp, usually about 67. I'd burn maybe 100 gallons in a winter to maintain this. The woodstove would keep the house at 74 or so...too warm....but it also maintained the floor I think as well. Certainly now with the Garn I use only the floor...I have bare concrete at the moment. I can bring the slab up 10 degrees in about 4-5 hours or so.

I don't have any zone control, it's the whole 1400 sq ft slab...upstairs at the moment has no heat, but the staple up with extruded plates will someday service that. With the radiant now, downstairs will be 72-73 and upstairs is usually 63. Great for sleeping.... The woodstove of course used to keep upstairs 72 or so. The house is very open.

A slab sensor holding a minimum would help keep the slab at a reasonable temp, reducing your bring up times.

Last night I cranked the floor up to 83....and it's about 16 hours later and still no heat has gone into the floor. Now, it's only 30 outside last night and today at 35 or so....

 

[nate379]

No slab sensor. Would be nice but at this point would involve too much demo to install. If/when I remodel I will probably add that though.

 

[Sawyer]

Do you have a slab sensor on the concrete? I can set a minimum temperature for the slab. Before the Garn, I heated with the woodstove primarily, but also would maintain the slab at a minimum temp, usually about 67. I'd burn maybe 100 gallons in a winter to maintain this. The woodstove would keep the house at 74 or so...too warm....but it also maintained the floor I think as well. Certainly now with the Garn I use only the floor...I have bare concrete at the moment. I can bring the slab up 10 degrees in about 4-5 hours or so.

Bruce, what brand and model slab sensor do you have? I have been thinking of installing one in my workshop to replace tht wall thermostat. I agree, wall thermostats are less than desirable for in-floor.