In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.

# Btu of heat per square foot

Member Posts: 1,640

And despite our disagreement about "rules of thumb," which is based more on semantics, than scientifically accurate information that's been known and understood for almost 50 years...

I'm sure Dan would agree, if you want to be a knee-jerk, or merely the latter, we would all prefer you stay away from arguing your flawed case with me personally, in this public forum. I have invited you to attack my sources or me personally if you must, via private e-mail.

• Member Posts: 1

WHAT IS THE RULE OF THUMB OF BTU'S OF HEAT PER SQUARE FOOT?
• Member Posts: 52

If you mean how many BTU's in a square foot of steam, the answer is 240 @ 215 degrees.
• Member Posts: 79
Or....

if you mean for a radiant floor, the answer is 35 BTUH/sq. ft.
• Member Posts: 6,928
If you mean...

How much heat is required per square foot of a structure there IS NO rule of thumb!
• Member Posts: 1,640
If you were referring to

how much radiant can you get out of a S.F. the number's about 50 BTU's/S.F. for a floor - without being uncomfortably warm

• Member Posts: 781
That begs for explanation

So, Ken, you're saying the "rule of thumb" unit load of a radiant floor is 50 BTU/H/ft²? Never read ANY engineering or design manual suggest that number. That's your bait, what's your hook?

Jed
• Member Posts: 781
35 BTU/H/FT² only for the uneducated (NM)

• Member Posts: 6,106
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

• Member Posts: 781
Got it

Different relationship than upward flux unit load.
• Member Posts: 1,640
Then you

And I never said that was some rule of thumb. It is probably the max you should design for.

The source? Page 68 0f I=R=R Guide 200. The book is probably older than you are; but one of the most fundamental and widely read radiant design chapters written. The radiant heating system in Dan's house and mine was dependent on it.

Quote, "An uncovered concrete slab floor panel having 3/4" pipe or tube on 12" centers with an average water temp of 120°F will deliver 50 BTU's per square foot of panel."

So Jed, there's the hook.

Better luck next time.

• Member Posts: 6,106

invest in one of these RadPads featured on sale at the front page of this site!

Slide the output to 50 BTU/ sq.ft and you will see a floor temperature of 93° at a room air temperature of 70°. A bit high for bare feet, on concrete floors.

Over 40 btu per square foot requirements, look into radiant walls or ceilings. A radiant wall with a mean radiant temperature of 120° in a room at 68 could release close to 100 BTU/ hr/ sq. ft

ASHRAE would have some more surface temperatures and info on comfort ideals.

hot rod

• Member Posts: 214

The so called "2" is the heat transfer coefficient and the units are Btu/hr/sf * deg F and comprises of both radiant and convective output from a heated floor.

For efficient structures the 2 Btu/hr/sf * deg F = 1 (radiant) + 1 (convective)which is why when the heated surface is a wall or ceiling the HTC changes. The convective component virtually disappears in a heated ceiling or a cooled floor.

However, it is a nominal number in that it isn't exact and it is influenced by such metrics as the average unheated surface temperature (AUST), radiant asymmetry and air velocity.

This first link takes you to a spreadsheet that you can play with. Scroll Down to the file called "Room Environment Parameters" when you start playing with the values make note of the HTC numbers at the bottom as you change the values at the topalso make note of the radiant  convective split.

As a matter of good practice you should right mouse click  save to disk then scan with anti virus software before running the spreadsheet although we scan all our files several times before uploading to the address its still good practice.

Also if you right mouse click and "open in a new window" you can have the Wall still running while you play...that way you can just shut down the link and come back and discuss what you discovered.

This second link takes you to a free
ASHRAE Journal Article with the HTC for walls, floors and ceilings in both the heating and cooling modes. Note: this article was written for the ASHRAE audience so be forewarned.

The graph below shows the minimum and maximum recommended surface temperatures to satisfy 90% of the occupants in the room. At 84 deg F and a dry bulb temp (assumed equal to operative for this example) of 70 deg F there would be a 14 deg F delta T x 2 Btu/hr/sf * deg F = 28 Btu/hr/sf output based on comfort. When 68 deg F is used the potential output becomes 32 Btu/hr/sf which is where the rule of thumb comes from.
However the higher one drives the floor temp the greater the output but at the risk of making people uncomfortable... eventually if the temperature becomes to hot it becomes unhealthy. ( see European Association for Phlebotomies Study from 1966 ) and if one operates the room at 68 deg F with over 3 ft/s air velocity the folks will also start to complain about draftsand people think this stuff is just so darn easy

Anyways believe it or not thats the short story.

Thanks Dan.

RB
• Member Posts: 781
Not to be argumentive,

Ken, but the thread subject was all about "Rule 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 load required vs. unit load supplied?

With all do Respect

Jed
• Member Posts: 1,640
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

• Member Posts: 1,640
Jed,

Take a look at the e-mail I sent you.

And Jed? What you wrote was nothing but argumentative, even after your edited your comment to seem slightly less so.

• Member Posts: 6,232
take your thumb and put it in a book *~/:)

• 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.
• Member Posts: 781
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.

Jed
• Member Posts: 6,106
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

• Member Posts: 3,782
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.
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