Cast iron radiator output tables are wrong?

I know the abbreviation EDR is commonly applied to steam boilers, but the principle is the same for hot water. You add up your radiator square footage per the tables, look up the BTU/hr from the hot water table, and calculate your predicted BTU output totals. Take a look at the pdf I linked above. It's all for hot water radiators.

I've tripled-checked and quadruple-checked the math. If there's a 2x math error, my 40 years of working as a mechanical engineer are failing me.

I know the boiler input in BTU/hr based on known oil input rate of 1.18 gph for a 1.0 gph nozzle running at 140 psi. And I've confirmed that input flow rate indirectly by taking boiler run time data from our ecobee smart thermostats, converting the run time to hours, calculating the expected oil consumption over months, and comparing that result to known oil delivery amounts. And there's perfect correlation to within 3% accuracy. So there's no doubt the boiler input rate is 1.18 gph, or about 165,000 BTU/hr.

I also know our dry gas efficiency (82%) and latent heat of vapor loss (about 7%, standard for oil), so total combustion efficiency is 75%. Then when the boiler is running at steady state, there is some jacket loss, but a few percent at most. So the boiler is still outputting pretty close to 75% of input as hot water. There may be a few percents off, but not a factor of 2 off. So even if I assumed 70%, or even 65%, efficiency at that point, there would still be almost a factor of 2 missing somewhere.

I've even confirmed the net output by known delta T across the boiler (20 deg), flow rate from the Taco 007 curve and our estimated head loss to get an estimated gpm (12 gpm), and when you plug those numbers into the delta T equation, you get 12 gpm x 20F x 8.3 lb/gal x 60 min/hr = 120,000 BTU/hr, which is almost exactly what I calculated the boiler net output to be based on known input and efficiency.

Which is twice the BTU/hr output of my cast iron radiators based on what the tables say.

So my suspicion here is that the cast iron radiator tables significantly understate the BTU/sq ft emission rate for a given average water temp, because they know homeowners are going to put enclosures over them, dry their laundry on them, or whatever.

If that's the case, then it appears they've sandbagged the numbers quite significantly. In my case, my un-enclosed radiators appears to be emitting around 220 BTU/hr/sq ft at 150 degrees avg water temp, which is almost exactly twice the 110 BTU/hr/sq ft published number.

Ah, interesting that you mentioned this. So let's assume the impossible "best case," in which all the radiators have 160 degree water, the same temperature I'm measuring coming right out of the boiler.

In that case, the tables say the rads should be emitting 130 BTU/hr/sq ft, which works out to about 72,000 BTU/hr for both the rads and a 15% pipe factor. That is still 52,000 BTU/hr short of the 124,000 BTU/hr the boiler is putting into the supply water. So now it's not a factor of 2 wrong, it's "only" a factor of 1.7 wrong, or still 70% too low.

I suspect we're in the same territory as the magical 33% steam pickup factor, where the steam boiler mfrs derate their steam boilers by 33% to account for unknown installation conditions. But in this case, it looks like the cast iron radiator table people have "derated" the cast iron radiator BTU tables by 50% or so, perhaps for similar reasons (people putting enclosures over rads, etc).

I'm always willing to be proven wrong, but someone once said "numbers don't lie." Those 124,000 BTU's/hr are going somewhere…if you have a better theory where they're going, I'm all ears. 😉

That could well be. But for my case, I measured the triangular faces of my radiator columns and calculated the resulting surface area, and it came surprisingly close to the published area numbers. So while those might vary depending on radiator config, I'd guess the published numbers are within 10% of my actual radiator areas. Which still leaves the BTU/hr/sq ft published numbers as the most suspect.

Thanks Ed, we never hit high limit, because the combination of cold start, high water volume, and massive radiation means even these 3x oversized boilers can't ever hit the high limit. They max out at 160!

And they only ever get to 160 during morning recovery from a 3 degree setback, when they run about 80 minutes. Before I programmed in the setback, they only ran maybe 45-50 minutes max, with even lower max temps.

See answers above. I will repeat for the nth time that the boiler NEVER HITS HIGH LIMIT, EVER.

I've literally studied these boilers for hundreds of hours (sad, I know). I've even added up all the ecobee run times for an entire year, calculated the total call for heat hours, multiplied by the expected 1.18 gph input rate based on nozzle size (1.0) and pressure (140 psi) and calculated an expected total oil usage, and it came to within 3% of our actual oil usage for the entire season, based on known tank levels before and after, and known delivery quantities.

So I can state with absolute certainty that the BURNERS NEVER SHUT OFF during a call for heat.

But don't let the high limit thing distract you. It's not relevant.

What matters is that, in the morning when the boiler has run continuously for over an hour and the supply temp has finally crept up to 160, I can stand there with an IR thermometer on black tape on the supply line, with the boiler firing continuously, as I can hear and see through the inspection port.

So while the boiler is still firing continuously and the supply temp finally pegs out at 160 after 1+ hours running, I can guarantee you that:

1. The boiler is still running, has been running, and could continue running until next Sunday without ever hitting the high limit. Or until the air temp gets so hot that the rads can't reject the heat fast enough, and we have a China Syndrome. 😅
2. The supply temp gets to 160, and does NOT climb any higher. The boiler is cranking out those 124,000 BTU/hr, but the pipes and rads are rejecting them just ast fast. So that's the actual "high limit."