Cast iron radiator output tables are wrong?

That is all exactly right. As I've been learning from the pros here like yourself, Ed, Jamie, Hot Rod, etc, I've made numerous posts over the last two years or so with observations that boil down to almost exactly what you said. And I've modeled the building heat loss a number of ways, so I'm highly confident in my numbers, which I could summarize here as:

165,000 BTU/hr input (1.18 gph @ 140,000 BTU/gal)

82% dry gas efficiency, less 7% latent heat loss, so actual combustion efficiency = 75%

Then net output = .75 x 165,000 BTU = 124,000 BTU/hr net output

Envelope heat loss at zero degrees outside air temp requires 50,000 BTU/hr input. So assuming 75% efficiency, actual envelope heat loss = .75 x 50,000 = 37,500 BTU/hr at zero degrees outside air temp.

Radiator area = 482 sq. ft.

Estimated pipe area = 100 sq ft.

Estimated water volume = 150 gallons, including boiler, piping, and rads.

(This is all for one of two identical heating systems, each heating identical halves of a 100-year-old building near Boston).

Your breakdown of the numbers is a close parallel to a post I made a year or two ago showing where the BTU's from a typical 45-minute boiler run go. Some into radiation, but most is stored in water and cast iron for slow release over hours.

https://forum.heatinghelp.com/discussion/193741/understanding-heat-flows-in-high-mass-heating-systems-in-old-houses/p1

Possible, but our envelope is fairly tight. Windows are original 100-year-old double hung wood, but have been professionally weatherstripped with spring bronze air seals and silicone rubber bulb seals, plus semi-crappy aluminum storm windows that do help some. But the windows themselves are quite tight now.

So while it is colder near windows, it's mainly because of the low R-value of the window assembly, and not air leaks per se. So maybe deduct 5 degrees from avg room temp at the rads, ie 63 vs 68.

Yes. In reality, the heat loss on an average cold day is only maybe 20,000-30,000 BTU/hr, so about 15% to 25% of boiler net output. So boiler needs to run only 30 minutes or so every 2-3 hours on most cold days.

The only reason for the one long 80-minute burn in the morning is to recover from a 3-degree overnight setback. And during that burn, 60% or more of those BTU's are being stored in the water and cast iron as delta T, not being radiated.

Sorry, no complaint, just a question.

When I consult the BTU output tables for cast iron radiators, the table says cast iron radiators will radiate 110 BTU/hr/sq ft at 150 degrees avg water temp. But from measuring my system at steady state, at 150 degrees avg water temperature, the radiators (and pipes) have to be radiating closer to 220 BTU/hr/sq ft.

That's because at 150 degrees avg water temp, with the boiler firing continuously, at a known output, the water temperature stops rising. That means the radiators and pipes are now radiating the boiler's entire net output, in a thermal equilibrium.

Then dividing that known boiler net output, corrected for known combustion efficiency, by the radiator square footage, says that the radiators must be outputting 220 BTU/hr/sq ft. But the tables say they should only be radiating half that, at the 150 degree avg water temp.

So the question is, why is there a factor of 2 discrepancy? Are the cast iron radiator BTU tables intentionally "sandbagged" to be conservative? If so, why such a big fudge factor? It seems the BTU output tables for finned baseboard are quite accurate, by contrast.

@EdTheHeaterMan I agree with everything you said. 🙂

Let me try again, as I evidently didn't express myself very well.

What I was trying to say is, that during a zero-degree day, in order to maintain a constant interior temperature, the boiler burns an average of .36 gallons per hour. So at 140,000 BTU/gallon, that's a BTU input rate of about 50,000 BTU/hr.

This is from measured boiler run times during actual zero degree days, so it's not theoretical. It's what the boiler actually consumes.

Now, we know the boiler isn't 100% efficient. It's not actually putting 50,000 BTU/hr into the house. It's losing 20% or more up the flue. And what's left over is what's actually heating the house. And it's the "what's left over" that is the actual heat loss of the house envelope.

So the question becomes, "what's left over?" Because that's what the actual heat loss is, not the 50,000 BTU/hr. We don't know exactly "what's left over" after flue losses, because that depends on combustion efficiency and jacket losses. But we can get a good estimate. Our dry gas combustion efficiency is known at 82%, and then I subtract 7% latent heat of vapor loss (a ballpark figure for oil), so overall combustion efficiency is maybe 75%.

There are also jacket losses, but since those are small and whatever heat is lost through the jacket goes into the house anyway, I ignore that for simplicity. So I estimate that, of the 50,000 BTU/hr the boiler consumes during a zero degree day, 75% of that is what is coming into the house as useful heat and ultimately being lost as heat loss through the envelope. So I figure that our envelope heat loss is actually .75 x 50,000 = 37,500 BTU/hr.

I phrased my earlier post the way I did because people will have different opinions of what our boiler efficiency actually is. Someone will say, no, your boiler is only running at 70% efficiency. OK, well, if that's true, then we're only getting .7 x 50,000 = 35,000 BTU/hr into the house, and that's our actual heat loss, not the 37,500 BTU/hr I calculated.

So I was trying to remove the uncertainty by saying that, on a zero-degree design day, the boiler consumes oil at an input rate of 50,000 BTU/hr, and if you believe my 75% efficiency estimate, that means you agree our envelope is losing 37,500 BTU/hr. If you don't believe my efficiency number and you think the boiler is only running at 70% efficiency, then you're implying our envelope heat loss is closer to 35,000 BTU/hr.

I don't know what the exact right envelope heat loss number is. Opinions will vary. What is a known fact is that the boiler uses an average of 50,000 BTU/hr input during a zero degree design day, and then we can infer the envelope heat loss from there if we know the exact boiler net output, which we don't. But we can make an educated estimate based on a reasonable estimate of boiler efficiency.

Sorry, I didn't grasp what you were saying the first time around.

Let's run some numbers. There are only two "storage" media to absorb the BTU's: water and cast iron.

At 150 degrees average water temp, the water stops "storing" BTU's, because the water temperature stops rising.

So the only other media that can continue "storing" BTU's is the cast iron. Based on the number and size of rads, and the boiler weight, I calculate a combined rad and boiler weight of 4600 pounds. Let's throw in another 1400 pounds of piping and call it 6000 pounds.

Cast iron absorbs 0.11 BTU/lb/deg F. So the total heat capacity of our cast iron is, say, 6000 x 0.11 = 660 BTU/deg F.

So I'm trying to account for 60,000 BTU/hr of "missing" heat loss. It's not being stored in the water as delta T, because the water temp has plateaued at 150. So let's assume you're right, and that those 60,000 BTU/hr are still being soaked up as delta T in the cast iron as it heats up more slowly. That implies a rate of temperature rise of 60,000 BTU/hr divided by 660 BTU/deg F, which equals 91 degrees/hr temperature rise in the cast iron, or about 1.5 degrees/minute temperature rise.

That sounds quite possible. I'll check the rate of temp rise in the cast iron tomorrow morning.

I'm not sure which is worse: wasted brain power being used, or used brain power being wasted.

🤔