Sizing steam radiators

BDRaff

: I need help sizing a steam radiator. the current radiator in the room is 16 sections, 26 inches high, 5 regular tubes. I'm trying to find out how many BTU's this radiator is and I want to replace it with the highest radiator available so I can maximize wall space. please help

thank you

ethicalpaul

Since you're replacing it anyway I would recommend doing a heat loss on the room to see how much you actually need. Old radiators were often larger than they needed to be.

Then look for a modern radiator that provides that BTU

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1810063#Comment_1810063

CAUTION: changing the radiator size from oversized to the correct size can backfire if the rest of the radiators are also oversized. If you place the correct size for the load in the room with the thermostat, then that room will heat normally, while all the other rooms with oversized radiators overheat. If you place the correct size in any other room, the room with the thermostat will heat correctly but the room with the correct size radiator may not get to temperature because the oversized radiator in the thermostat room may stop the boiler from making steam before that new, smaller radiator has time to heat that space.

The real question is "are there any rooms that get too hot or too cold?" then adjust the radiators accordingly. Ig your home is balanced with the radiator you plan to replace, then stick with a similar size. To your point… to select the radiator based on output… use the EDR of the existing radiator to match it to a different design radiator of the same EDR.

EDR stands for Equivalent Direct Radiation, which is a fancy way of saying how many square feet of surface area a radiator has. You can find charts for most of the common radiators online. I used the Weil McLain boiler sizing guide whan I would size new steam boilers. Unless you have something that is really unique, this should help.

https://www.weil-mclain.com/sites/default/files/field-file/Weil-McLain_BoilerReplacementGuide_WM2012-web_0.pdf

This is on page 10.

You should have a professional that experienced many types of radiators help you out on this. I have experienced a DIY homeowner use the wrong chart from this page to calculate his radiator incorrectly because their experience was limited to the radiators in his home. They had nothing to compare the thin tube to the standard tube with the column type radiators. OR…

Take a picture and post it here with the dimensions hight x with x length and I'll let you know the EDR. Then you can look at local scrap dealers for a replacement radiator that may fit your needs.

ethicalpaul

I don't see it, Ed. Each radiator will fill according to its vent. The rest of the system doesn't know the radiator is smaller.

If this was a concern then any TRV or turning off a radiator would wreak havoc

mattmia2

On short, infrequent cycles you may be able to balance very mismatched radiators with venting but on a long cycle the oversized radiators are going to heat completely and proportionally put much more heat in those rooms than in the room with the radiator sized to match the heat loss.

ethicalpaul

What do you mean "mismatched radiators"?

You're saying if I have oversized radiators in every room except for one, that is going to cause some kind of problem in the system?

The call for heat will end when the room with the thermostat gets warm enough to satisfy it…how warm one of my bedrooms, for example, with the correct size radiator in it will have no effect on that.

Yes, if the OP changes the radiator in the room with the thermostat, it will affect the system balance, but this is no different than any other normal system balance issue.

Do you have an answer for my point about a TRV or a turned-off radiator?

mattmia2

The trv or turned off radiator just doesn't heat that room at all and that is presumably what is desired by turning off the radiator or using a TRV to turn off the radiator if the room is too warm.

Let's start with the simple case. You have 2 identical rooms each with a radiator in them and each has a heat loss of 5000 btu/hr at design conditions.

It is an average 10 degrees warmer out than design conditions for the next 4 days.

Room 1 has a radiator that was sized when the house was built with no insulation and single pane windows. Its output is 8000 btu/hr.

Room 2 had a new radiator installed that was sized for the current heat loss after insulation was added and storm windows were added and some air sealing was done. Its output is 5000 btu/hr.

After say 20 minutes in to a cycle both radiators are fully heated.

The heat loss of both rooms is let's say about 3500 btu/hr at 10 degrees above design conditions.

The radiator in room 1 needs to run 43% of the time to provide that amount of heat.

The radiator in room 2 needs to run 70% of the time to provide that amount of heat.

Whichever room has the thermostat in it will be comfortable and the other room will either under or over heat. You can make up some imbalance through different venting but you won't be able to make up almost a 50% difference through venting alone, especially when some radiators are sized close to the actual heat loss so they need to run most of the time as you near design conditions so even a radiator with a slow vent will spend a significant portion of the cycle fully heated.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1810089#Comment_1810089

If most of the house has oversized radiators, the room with the correct size is going to be cooler.

If the house has all correct sized and one is oversized the oversized room will tend to be hotter. Especially on cooler days.

If It's a single pipe steam system a TRV and a reasonably sized vent will fix an oversized radiator but not an undersized one vs the rest of the system.

BDRaff

Thank you everyone for your comments. Here is the picture of the old rad I am replacing. You can find my dimensions with my first comment.

20240909_191932.jpg

Steamhead

That's an Oriole, made by Republic Radiator Co right here in Baltimore, hence the name.

it's a standard type of large-tube radiator, at 3.5 square feet per section, so that's 56 square feet. Get a replacement close to that value and you should be OK.

ethicalpaul

@mattmia2 and @ChrisJ I want to thank you for taking the time to explain your thinking about radiator sizing. I didn't see any major issues when I resized my living room radiator quite a bit smaller, but New Jersey is pretty moderate nowadays. Thanks again.