Sizing Question: EDR --> BTU Calculation

KarlW

Question:

1 EDR=240 BTU at 215-degree steam at 1psi.

Is there a conversion for BTUs for 215-degree steam at 0.5 psi? Is it relevant?

Background:

My attic bedroom has a 5-column, 7-section radiator that I rate at 35 EDR. Using 1 EDR=240 BTU means the radiator puts out a nominal 8,400 BTU.

Simple room calculators say the room requires around 7,650 BTU to heat, but this room is cold/losing too much heat, and even with my boiler fully operating cannot keep up on cold/windy days. Although insulation may be a better solution; my attic is fully finished, so a larger radiator may be a more cost-effective answer - I have the steam to spare in my system.

bburd

No, the math the first part of your question is not relevant. At 0.5 psi the steam temperature would be very slightly lower, ~213.5° F.

What got my attention was your description of the radiator: "5 column, 7 section". Almost certainly what you have is a tube radiator rather than a column radiator; and there are small tube and large tube types. All of these have different ratings per section. Can you post photos (front and side) and dimensions of this radiator? We can help you identify it.

Other issues may be that the radiator is not getting enough steam due to sagging pipes or venting imbalance. Also the room may be losing more heat than you've calculated; there are variables in the calculation, the greatest of which is the outside infiltration rate.

delcrossv

at 0psi its 212, at i psi it's 215 interpolating at .5 psi it'd be 213.5.

Does it make a real world difference? No. I'd think the EDR is off or the rad isn't filling completely (latter being most likely)

ethicalpaul

I would say the error is in the simple room calculators compared to your actual room.

in short, if you need more heat, you need more heat. It doesn't really matter what the EDR to BTU conversion is.

I also wonder if your radiator supply is in a cold, uninsulated, drafty stud bay.

KarlW

Here’s the radiator, it’s a 7-section, 5-column radiator 38/39” tall.

It's two-pipe steam, and the smart TRV is doing its thing without issue and this valve will be fully open when 20-22 degrees Celsius (68-72 Fahrenheit) can't be met.

It's on an interior wall in a fully finished plaster and lathe room. Perhaps a little too tucked in, but nothing too bad.

As to the air vent - I put this on last year when the radiator wouldn’t heat unless the boiler had been on continuously for 45 minutes or more - I posted a whole thing on this forum last year - thanks to this vent, this radiator heats up as fast as any other radiator (it is the highest in the system and furthest from the boiler). It doesn't leak (noticeable amounts of) steam.

delcrossv

Does the whole thing get "steam hot" during a long call?

EBEBRATT-Ed

@KarlW

You don't have enough radiation for the room.

You stated your heat loss is 7650 BTU…Per hour.

Thats how many BTUs the rad will output if it is steam hot for 1 hour that is constant steam for 60 min.

The thermostat is not in that room. The boiler does not run for that room specifically

If the rest of your system lets say the rads are sized to 130% of what the room needs and this rad in the cold room is sized to 100% of what the room needs then this room is going to be COLD

No amount of venting or TRVs or anything will fix this.

Do a room x room heat loss of the house and find out the average %oversized the other rads are and then size this rad accordingly.

Unless I am mistaken you will find all the other rads oversized for the heat loss in those rooms…..this one is not.

KarlW

Yes it does. Right now with an outdoor temperature of 29 F it maintains the room just fine. It just fails bad in comparison to the rest of my house at 10-20 degrees and windy (and why insulation may be a more appropriate fix). It actually worked just fine when the whole house needed heat in the single digits as well.

I asked the question to see if a lower pressure was something I needed to worry about if I were to size and buy a new radiator, new or used.

KarlW

@EBEBRATT-Ed

I had come to this conclusion before posting. I was looking for confirmation/better understanding of EDR before moving forward. Your part about BTU/hr is especially insightful.

As I mentioned it in my time delay post, when my system is all on demand with TRVs calling for heat, it fails when only one or two radiators call for heat while the rest of the house is maintaining temperature. Insulation is clearly the best solution so all rooms act similar. I was just curious to explore radiator options.

I already have answer for this winter: unplug the air conditioner (cooling is actually a worse problem than heating in this space) from its dedicated circuit and use a space heater for the occasional days the system doesn’t keep up.

This attic bedroom was built as the “maid’s apartment” and I’m sure the radiator was sized to get the edge off of the cold, not necessarily to make it comfortable.

bburd

As I thought, that is not a column radiator but a large tube pattern radiator. The EDR is considerably less.

I have seen undersized radiators in servants' quarters in other old homes.

EBEBRATT-Ed

@KarlW No. For residential the temperature of the steam does not vary enough to matter.

In fact with steam it is not the temperature so much as the latent heat of the steam which heats the rad. Latent heat is the heat given off by condensing steam to water.

The lower the pressure the more latent heat the steam gives up. 970btu/lb of steam or water. Most round this off to 1000btu/lb for simplicity.

1 lb of steam =1 lb of condensed water so your 35 EDR rad would condense about 8.6 pounds of water if it was hot for 1 hour or about 1 gallon of water.

KarlW

@bburd I’m quite sure it’s an EDR of 35, I wrong by calling it a column, but i did size it right:

5 tubes at 38”, 7 sections=35 EDR.