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@jesmed1 @PC7060

hmm. that's all interesting. Total EDR is approx 660, 2700 sq feet.

You're still over-radiated for a modernized old house. Some numbers for comparison. Our circa 1920 4-unit condo building in Boston was built originally as a 4-apartment house by an elderly man who wanted his adult children to live with him in their own separate apartments. We have EDR=960 and 4800 sq ft. That's an EDR/sq ft ratio of 0.2. Our total design heat loss is 96,000 BTU/hr at zero degrees outside design temp. That's 20 BTU/hr/sq ft.

Your EDR/sq ft=660/2700=0.24. Which is even more EDR than we have, in this massively over-radiated house. So if your heat loss was the same as ours (20 BTU/sq ft), you could heat with even lower water temp than we do. Because we have large boilers, way too big for our design heat loss, they run about 45 minutes on, then 3-6 hours off depending on outdoor temperature. Since they're cold start, the water temp starts at 65 end ends up at 130-140 only at the very end of the cycle. So the average water temp during a heating cycle is like 100 degrees. For just 45 minutes out of 3-6 hours.

That's how low your radiator water temperature could be with your EDR/sq ft=0.24. But only if your heat loss is similar to ours, ie 20 BTU/hr/sq ft. Even if your heat loss is double that, say 40 BTU/sq ft, you could still heat with water temps that low, but your boiler would just run twice as long, say 90 minutes every 3-6 hours.

Here's where I think the disconnect is. When I calculated our total radiator EDR and then calculated our total design heat loss for the building, the radiator table I was using said that we'd need 140 degree water temps in our radiators, to give us about 90 BTU/hr/sq ft to heat the building. But in practice, even on a zero-degree design day, the boilers run only half the time, and the average water temp is maybe only 120.

I haven't taken the time to figure out why the predicted water temp using the radiator tables was wrong. Maybe the radiator table I used is wrong, or maybe I miscalculated our EDR. I'm reasonably certain I used the right radiator dimensions and calculated the EDR correctly. But for whatever reason, using that radiator table vastly over-predicted our required water temperature.

So I'm wondering if you have the same over-prediction problem, because by our EDR/sq ft comparison, you have more EDR/sq ft than we do, so you should be able to run similar water temps unless your heat loss/sq ft is massively more than ours.

BTW, this is the radiator table I used which over-predicts our water temp:

https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf

Your real bottom line is how many BTU's you burn over a heating season, or over one coldest month in the season. Keep track of your oil or gas usage, then calculate your actual design heating load using this method:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

That method is very accurate. Then once you know your actual heat loss number and your radiator EDR, you can estimate your required water temperature. But as I said, the radiator table above seems to over-predict our water temperature at 140, where in reality we're averaging well under 120.

Also, I recommend this article by Ron Beck. Read the section headed "Cast iron radiation," where he says that, contrary to popular belief, cast iron radiators are low-temperature systems in modernized old houses:

https://www.usboiler.net/outdoor-reset-doesnt-work.html