Heat Loss from Direct Radiation

acwagner

So, I wanted to get everyone's opinion on this. I've been reading some of the original sources used for the Lost Art of Steam Heating, and in most of them they dedicate a significant amount of text on procedures to determine how many BTUs/Hour are transmitted per EDR for a cast iron radiator. Nowadays, we use what apparently became the standard of 240 BTU/HR per EDR (assuming steam at 215 degrees or 1 psig in the radiator). However, they realized through observation/experimentation that the height, width, and length of a radiator effect the output, in some cases significantly.

Attached is the best summary I've found of their findings. It's great article just bursting with interesting information.

My question is, why did they settle for 240 as the universal standard, when it was obviously demonstrated that one size does not fit all radiators? Convenience? Too complicated? Industry decided that was the standard? It seems like a critical part of the calculation required to meet building heat loss and sizing a boiler to just simplify it to one average number. Does anyone use this technique in the article or something similar when converting EDR?

I ran the calculation given at the end of the article on my radiators just to see the difference it would make compared to the 240 standard. It reduced the output of my radiators by 11%. A surprisingly large amount, in my opinion. Certainly enough to sway a boiler sizing decision.

EBEBRATT-Ed

Design conditions and field conditions seldom match. In most cases being within 10% + or - of design will never be noticed.

Sizing of anything, boilers, pumps, condensate tanks, baseboard, radiation, air conditioners etc are all "supposed" to be tested to a standard to make choices and sizing easier.

If one manufacturer sized his baseboard at 200 deg water and another used 180 deg water how could the average installer compare the two? He couldn't

But guess what? In the real world and I have visited the factories of many equipment suppliers the manufacturers are not above fudging their #s weather it's boiler output, efficiency whatever when they can get away with it. Not all do but some do.

It's all about $$$$

Basically for any pc of equipment they have to decide on some standard to manufacture to

JUGHNE

Go further back to how the EDR was determined, dip a rad section into a vat of paint, weight how much paint adhered to the rad, paint the floor with that weight of paint, measure the floor covered........that is the square footage provided by that rad section.
So how porous was that cast iron, what was the temp of the paint, was it thick or thin and how about the porosity of the floor that was painted for the square footage measurement. It could go on and on.

As far as how many Btu's that may have been calculated by measuring the condensate produced by controlled conditions.
Just too many variables to get an absolute number.

This was all done before our time by people doing the best they could with what they had.
True this probably leads to oversizing both boilers and radiation.

Often I crunch numbers and go for "Spock....what is your best guess??"

R Dougan

In the 1911 book "A Practical Manual of Steam and Hot Water Heating" by Edward Richmond Pierce on page 326
he starts a sentence with the statement: "With steam there is a tacit agreement on the part of the manufacturers to consider 240 Btu per sq. ft. per hour as the value of radiation, but it is different with water, ..."

This qualifier at the beginning of the sentence states clearly that it was the manufacturers who set this as the standard - maybe because the knew it would be easy to meet this standard if it was set lower than their products could produce in real world outputs. Hence no complaints about the product not performing as advertised.

Like a good friend of mine used to always say, "Aim low and over-achieve".

I love reading this stuff, ...what can I say, ...I'm a steam geek. They even cover that whole "Btu loss of metallic paint on the rads" subject. Good find.


Bob D.

BobC

I designed power supplies for a good portion of my life and when doing so 10% accuracy was fine for most of the components. Of course if it was a military unit everything had to be certified within an inch of it's life. You lead some of the young inspectors around the mullberry bush all morning, the older ones would just oversee the testing so we could get a jump on lunch.

Now if your talking woodworking or metalwork, everything has to be right on or you will get the stink eye from the wife.

Bob

acwagner

It's probably not a coincidence that the average value from his tables is around 240. And a 38" two column radiator was probably a popular sized radiator, which also is 240.

I have a copy of the book @R Dougan quotes from (it's a good read!) and earlier in that book he states to be very careful trusting manufacturer standards because of the variance in stated to actual outputs.

I'm a design engineer by profession, and I know the benefits and pitfalls of using general rules of thumb or averages. I just keep seeing tables and explanations like these in all the literature from early 1900 to 1930's. And the numbers are refined each time. Just makes me wonder if it was negligible why the spend so much effort researching and explaining it? Maybe at the residential level it wasn't considered until you got to large systems.

And, I should clarify my 11% number. I was surprised by that because I have radiators of various heights, widths, and lengths. I figured the small ones would balance out the large ones and I would end up overall around the same number as the 240 number would get me. That's just my house. If you take the extremes from his charts you can easily greatly exceed that range.