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# What is the BTU/h of these old non-cast iron radiators?

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Member Posts: 34
I’m looking to size radiators to match (roughly) the original radiators from this building. The original radiators aren’t cast iron; they have weird fin shapes:

For radiator #1, the box with the fins is 46¾ inches wide, 8 inches deep, and 2 inches high. For radiator #2, the box with the fins is 35 inches wide, 8 inches deep, and 2 inches high.

Is there a good way to determine what the BTU/h of these radiators are?

Thank you.

• Member Posts: 23,324
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It's not rocket science. Remember what "EDR" stands for: equivalent direct radiation. Or, to put it differently, equivalent flat plate radiation. Now to a usable first approximation, you can calculate the EDR of those contraptions. Find the area of one side of one fin. That will be its length times its width, and you should subtract the area of the pipe going through it. Now double that -- there are two sides to each fin, after all. Then multiply by the number of fins and convert to square feet.

That's it.

To get BTUh out of that, if it's normal residential steam multiply by 240. Hot water is messier -- but your setup is steam.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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Do you know who made these units? If not, get a copy of @DanHolohan 's book "E.D.R.", there should be something close to your units in there.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
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@Steamhead Excellent recommendation; I ordered the book so let’s see.
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i have some questions about the location of the element on that trv...
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I have in my possession drawings from Beacon Morris on the specifications for their convectors. You can go to"www.beacon-morris.com" and click on residential, then convectors to get specs on their convectors. From there you can get an idea of the output of all their convectors utilizing steam or hot water. They may not be exactly the same as your units but they will give you a place to start. More information is needed to get an accurate BTU output of your convectors such as the height of the cabinets. Hope this helps.
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My copy of EDR came in, and boy am I wiser. Some replies.
mattmia2 said:

i have some questions about the location of the element on that trv...

Oh yes. That struck me too. You’re sticking a TRV inside of the convector? Yikes.

More information is needed to get an accurate BTU output of your convectors such as the height of the cabinets.

Indeed.

When I ran the numbers five years ago, the consensus of the BTU calculators that I found was that my living room needed 12–15k BTU/h and my bedroom 6–7k BTU/h.

Meanwhile, looking at the original convectors installed in the ’50s or so, the cabinets run from the floor to just under the windowsills, meaning they’re about 24″ tall. The elements are 8″ deep, and either about 48″ or 36″ wide. I didn’t find any exact match for the convectors in the EDR book, but all convectors of that size are roughly the same capacity, so we can go with rough numbers.

This seems to mean that the living room convector is about 14,000 BTU/h, which is in line with the calculation. However, the bedroom convector doesn’t make sense. It’s sized to be ¾ of the size of the living room convector, or around 10,000 BTU/h. That seems to be way too high.

I wanted to know what the original convector sizes were so that I could revert to the sizing of the original system. However, the more that I think it through, the more the original sizing seems like nonsense.

Is there something that the old designers knew that I’m not knowing to size the bedroom radiator like that? Or is it the kind of thing that I can write off as them being on a deadline, doing some math wrong, not double-checking it, and just moving on?
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As much as anything they had a few stock sizes and wanted one which fit as well as might be the application.

So. Is it the bedroom one "too big"? Quite possible. So what? This is two pipe steam. There's no rule anywhere that says you have to run a convector or radiator flat out -- and on two pipe steam, the inlet valve makes that really really simple. Just close the valve enough so you get the amount of heat you want out of it. You can even put a thermostatically controlled valve on the inlet if you like to control things at a lower temperature than flat out would be.

Too small would be a problem. Too big, within reason, not a problem.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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I can't quite tell from the pictures but I think it is one pipe. If it is, the valve must be all the way open or closed or the convector will trap water and won't heat. The trv will stop it from heating if the room is already warm enough, the speed of the vent will control how fast it heats.
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mattmia2 said:

I can't quite tell from the pictures but I think it is one pipe. If it is, the valve must be all the way open or closed or the convector will trap water and won't heat. The trv will stop it from heating if the room is already warm enough, the speed of the vent will control how fast it heats.

Um. Might be right. On a second look I can't honestly tell either -- what I thought might be an inlet valve in one picture might also be a thermostatically controlled vent. If that is the case... you can still throttle back the bedroom convector by using a very very slow vent. Won't be as satisfactory, but will work.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 34
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This is indeed one-pipe steam.

As much as anything they had a few stock sizes and wanted one which fit as well as might be the application.

Honestly, that sounds like the most reasonable answer.

I guess that’s the guidance size-wise that I was looking for.

Thank you, all!
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