Welcome! Here are the website rules, as well as some tips for using this forum.

# BTU's per foot

Member Posts: 13
how many btu's per linear foot do you get out of 1/2" in floor pex @ approx 140 degrees or 160 degrees F

• Member Posts: 6,868
There are many other factors that are involved in determining the output, and the entire floor is the actual emitter, not just the tubing.

Flow rate, tube spacing and length, floor construction and type, installation method, R value and covering, conductivity of the flooring materials, insulation, etc.

We seldom design a floor that would require more than 120* SWT at design temp.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 13
wasn't real helpful to me, maybe I'd understand if I were an engineer. don't know what SWT means. guessing the WT means water temp but i give up on the S. but anyway, ok lets say 120 degrees, what is an approx btu per linear foot
• Member Posts: 13
i think i've figured out that S is for Supply
• Member Posts: 6,868
Let me simplify it like this: btus are put into the pipe by the boiler and carried by the water. Putting them in is one side of the operation or equation. Getting them out, or extracted, is the other side.

Heat is transferred, or extracted, by three ways:
1. Radiation. Heat rays carry heat in every direction and give up that heat when they meet any matter or mass (walls, floors, bodies, whatever).
2. Convection. This basically involves the movement of air over the heated emitter (pipe, radiator, etc.).
3. Conduction. Heat is transferred by intimate contact between two substances. Each substance (conductor) has a certain amount of conductivity or ability to conduct heat. This is the best method of heat transfer. An example would be holding your hand six inches from a hot radiator. You would feel some heat from radiation, but if you touched the radiator, then you would feel a lot more heat. That's conduction.

Now I said all that because your question is basically about how much heat is extracted from a 1/2" pipe, correct? But to know that, we would have to know what method(s) is being used to extract it.

Is the pipe in a radiant floor? Is it a concrete slab or wood floor? If wood, what type of heat transfer method or system is being employed?

If it's not a radiant floor, are you just asking how many btus it would emitt hanging in free air with no blower?

SWT= Supply Water Temp.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 13
Thanks for that Bob, that was great. I see I did fail to mention it is in a concrete floor. Sand base, 2" foam board insulation, 4" concrete. Just curios as to approximately how many btu's per foot I would/could get @ 120* SWT or the difference if 140* SWT. This is in a 220 square foot breezeway that hooks the house to the garage that I recently installed for my youngest son. When he asked me how many btu's he would be getting out of the 300' x 1/2" pex loop, I can't seem to get an answer from our local suppliers/engineers here in Michigan. I would also appreciate your opinion on whether or not it would damage the concrete if running the SWT @ 140* vs 120*. I'm only asking because I assume I would get more btu's per foot at the higher temp which I may need based on all the glass my son/daughter in law decided to put in this breezeway.. thank you for responding --Denny
• Member Posts: 7,330
Oh, That's different.
I assume the tubing is embedded in the concrete? What is the spacing? Do you know the approximate flow rate? What is the floor covering.
There are a bunch of guys here that can spit out that info.

BTW, I recently read the attached article about sand under concrete. It was an eye opener for me.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 13
Yes, embedded in the concrete, 12" on ctr, do not know flow rate, concrete is stamped to look like wood floor so there is no floor covering
• Member Posts: 13
read the article above, I did put a vapor barrier between the sand base and the concrete
• Member Posts: 5,839
Denny, there is only one correct answer to any question pertaining to hydronics. That answer is "It depends"... To many variables to be able to simply spit out a number. If the slab is cold, it will chew up every BTU that you throw at it. If the slab is stabilized at a given temperature, then it is something that can be calculated. The first part is called slab acceleration, the other is called slab maintenance. Maintenance is much less energy intensive than is acceleration. What are the operating conditions? For flow rate, I'd assume 1/2 to 3/4 GPM. The simple estimation math would be using the universal hydronic formula. GPM X pounds X delta T = btuH. So if we as assume .5 GPM flow rate and a ten degree differential in fluid temperatures, then .5 (GPM) time 500 (8.33 lbs times 60 minutes per hour) times a 10 degree F differential would equal 2500 BTU's per hour. If the tube length is limited to the 300 foot maximum recommended length, then that would equate to 2,500 divided by 300 = 8.3 btu's per lineal foot. Radiant floors are capable of delivering a lot of energy, but the floor surface temperature limitation as dictated by the codes, tells us that they can't be more than 85 degrees F, which equates to 30 btuH/square foot.

Why are you asking? Are you trying to size a heat source for your slab?

ME
It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
• Member Posts: 13
I have approx 290'-300' of half inch pex in this 220 sq ft area and was just curious as to how many btu's i might be capable of getting out of it
• Member Posts: 5,839
Will there be human contact with the slab? If yes, then 30 btuH/sq ft per hour is the maximum recommended by code, which equates to a surface temperature of 85 degrees F. If not, then "It depends" kicks in. Hope that helps. Google Uponor CDAM (7th edition) for more detailed technical information.

ME
It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
• Member Posts: 9,514
Like Mark said. Especially in terms of cold start up, and maintenance of the floor.

There is also the difference of air temp verses floor temp. The larger that delta the more btus the floor will produce. Hot goes to cold, and the greater the difference in temps the faster the transfer of btus. So as mark pointed out from cold start you are chewing up btus, but as you approach setpoint the output diminishes as floor temp, and room temp delta narrows.

BTUs per square foot is used because that is what is looked for in terms of floor output.

If your tubing is 1' centers every foot of tubing is covering 1 square foot of floor. However the actual btu output is not evenly dispersed over the whole square foot. Depending on the imstallation detail the floor will be warmest at the tube center. By tightening up tube spacing you can achieve the same output with lower water temps, and you even up that output across the whole square foot of floor.
• Member Posts: 7,330
So, Ballpark calcs, .93 gpm 12" spacing R-0.3 floor covering.
These numbers should apply once the slab is up to steady state operating condition. As mentioned above, a cold slab is very thirsty.

160 SWT ~ 50 BTU/ft
140 SWT ~ 40 BTU/ft
120 SWT ~ 30 BTU/ft
100 SWT ~ 16 BTU/ft
80 SWT ~ 5.5 BTU/ft

Bottom line, use outdoor reset and far lower temps than you probably think.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 13
Thank you guys, now we've gotten somewhere. Very helpful.
Just so everybody knows, the heat source is an outdoor wood boiler. thanks again for the info,
Denny
• Member Posts: 7,330
Outdoor wood boilers with a high mass concrete slab gives you some interesting options.

The boiler's weakness is that it just wants to run nice long hot cycles, it will love having all that concrete to heat.

A high mass heating system's down side is that if you overheat the slab, you will end up with some big fluctuations in indoor temps. Think of that slab as a "thermal flywheel".

The trick is to use outdoor reset so you heat the slab slowly. Not so slow that the boiler can't run efficiently and not so fast that you get wild fluctuations in indoor temps.

I think if you use a outdoor reset mixing arrangement and you get a feel for how much wood you need to burn for how long, this should be workable. Keep in mind, there will be a significant lag between throwing the wood in the boiler and the inside temp rising. A slab temp sensor might not be a bad idea. Even if it is just a cheap wireless barbecue grill setup that you can visually monitor.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 9,514
I assume there is more than 220 sf of radiant the odwb is heating.
• Member Posts: 13
Yes Gordy, the existing 1800 sq' home is heated with in-floor joist heating. This is just a recent 220 sq' breezeway addition.