Forum / THE MAIN WALL / Btu of heat per square foot

## Btu of heat per square foot (22 Posts)

• ### I like \"it depends\" best...

... but maybe that's because I am a consultant by day. It is my understanding that 30BTU/sq ft/hr should be attainable in most above-subfloor situations if proper installation procedures are followed and there aren't inches of foam carpeting above, etc. However, hack-style staple-up work on 18" centers w/o insulation below or the benefits of plates isn't going to do much. Plus, even if you manage to get the system to put out 30BTU/sq ft/hr, are the occupants going to be comfortable? I'd argue that there are cases such as ski lodges with big windows where radiant floor systems may not provide complete comfort in the absence of 2nd-stage radiators, fan-coils, and the like due to the radiant chilling effect of the windows, even if the radiant system is sized to meet the heat loss. As there are plenty of good calculators out there to help installers figure out just how many BTUs can be squeezed into the space based on insulation, tube spacing, etc. there is no need to guess. Simply put the conditions into the program and let it figure out for you what is or is not attainable.
• If you mean how many BTU's in a square foot of steam, the answer is 240 @ 215 degrees.
• WHAT IS THE RULE OF THUMB OF BTU'S OF HEAT PER SQUARE FOOT?
• ### Or....

if you mean for a radiant floor, the answer is 35 BTUH/sq. ft.

• ### If you mean...

How much heat is required per square foot of a structure there IS NO rule of thumb!

• ### That's O.K., Ken

While Mr. Frederick's question (WHAT IS THE RULE OF THUMB OF BTU'S OF HEAT PER SQUARE FOOT?) is "sterile" or not relevant to a specific energy conversion application, it does imply a radiant application. What that application is remains to be determined. Again, I hear that question all the time from people not familiar with the concepts of radiant heat(or should I say comfort) delivery. It's a general and valid question, considering the vast and varied sources of information available today. My comment about "bait and hook" may have been a knee-jerk one, but; 50 BTU's/H/ft² is NOT a general "rule of thumb". It is a specific floor output, at a given design condition. I have Engineering Tables that can show 77BTU/H/ft² @ 107°F surface floor temperature, and not SIM. Is that a "rule of thumb"? I do hope Mr. Frederick comes back and clarifies his inquiry. But to leave him with an unrealistic number does more disservice, than my questioning that unrealistic number. And, to respond to your e-mail imploring: the answer is NO! Jed
• ### Sorry Jed,

> of Thumb", not potentials, min's & max's. I
> certainly wasn't questioning your statement per
> say; only as it applied to the thread topic. I'm
> confident that you have been asked that very
> question: "What is the rule of thumb output of a
> radiant floor"? I hear 30,35, etc. I have done a
> lot of reading (not the IBR manual,though I may
> get that also). I have been asked that question
> many times, too. My usual short answer is--YUP--
> "It depends".
>
> I happen to be of the opinion
> that THERE IS NO "Rule of Thumb" when it comes to
> radiant heating, unless you set up multiple
> "radiant panel categories" and then apply typical
> floor(ceiling,wall, or whatever) output ranges;
> i.e. slab: x-xx BTU/H/ft². So, would you agree
> that, in the final analysis, it depends on unit
>
> With all
> do Respect
>
> Jed

• If that's a line from "one of the most fundamental" radiant design chapters written, then I guess I see now why there are so many hack jobs out there. That's an absolutely ridiculous figure. Not that you couldn't push that much heat.. you'd just never, ever want to, and it's got no business being anywhere in anything touting itself as a design guide without a big ol disclaimer on it. edit: heh.. it always depends. my bad: 3/4" pipe in uncovered concrete, they are talking about commercial. if it's a workshop kept to 50-60 degrees, then you could do 50 BTUs/sq ft.
• ### A lot of those vintage

copper systems were fed directly from the boiler, typically 180 degree supply. I've been in more than a handful of them :) Some used the early Honeywell dual cap tube outdoor reset control to help manage supply temperaturers. Plenty of BTU output, just don't try to glue down vinyl or lineloum! Most use thick floor covering, or heavy socks, as a means to make the floor "stand-onable" hot rod To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

• ### That \"50\" number

appears in some of radiant manufactures MARKETING pieces. However engineers I know and trust explain output as about 2 BTU/square foot per degree difference. So let's say we all agree 85° is the max surface temperature for a residential application. (Euros prefer 82- 83 max surface for health reasons). If you like an ambient of 70 then the difference times 2 would be 30 BTU/ ft output. To get to 50 you would need to keep the room at 60°, with an 85 surface temperature. You can drive a bit warmer than 85 in shop applications and run a bit cooler air temperature and get closer to that 50 number. Personally I feel the 35- 40 BTU / square foot number is more realistic, for comfort system residential work, and keeps you out of design trouble :) 50 btu/ sq. ft works for me if you move the slab outside. Heck I'd even give you up to 200 btu/ sq. ft outside, under certain 30° F outdoor temperature conditions :) hot rod To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
• ### Got it

Different relationship than upward flux unit load.