Help with radiator heating capacity & boiler sizing calculations

JimmyNJ

Hi all,

I am hoping that someone could help me figure out if my radiator EDR calculations are valid as I am trying to figure out if my current boiler is undersized (we put an addition on and added a couple of radiators to the system).

First things first - Can someone confirm that I actually identified my radiators correctly as "column" type"? (please see attached pic). For the radiator in the picture I made the following calculations:

38" tall, 3 columns, and 4 sections - so the EDR I calculated was: 5sq per section x 4 sections = 20 sq ft EDR for entire radiator = 4800 BTU steam design output (20x240). Did I do this calculation correct?

I followed the same logic for the rest of my rads using the chart in the link below:
http://www.columbiaheatingsupply.com/page_images/Sizing Cast Iron Radiator Heating Capacity Guide.pdf

I also have a couple of 4 tube rads that are 19" in height and one has 42 sections and one has 18. For those I followed the chart below and made the following calculations:

For the 42 section rad: 67.2x240=16,128
For the 18 section rad: 28.8x240=6912

http://www.ocsind.com/cast-iron-radiators.php

And calculated that for all rads the combined steam design output is: 79449 (although I was unable to figure out a couple of my in-wall/recessed rads (please see attached pics and if someone knows how much they would "add" to my total that would be great.

So am I okay with my little Utica boiler for all the rads in the house? It look like it is rated for 90,000 BTU/HR.

Thanks for any help on this!!

Jimmy

JimmyNJ

forgot to attache boiler pic

JimmyNJ

Also forgot to mention dimensions of the in-wall/recessed rads:

Kitchen one is 20" tall and 22" long
bathroom one is 20" tall, 5" wide, and 13.5" long.

nicholas bonham-carter

First figure out the combined total of square feet of steam EDR for the radiators, and hopefully it will be close to the rating of 375 square feet for that boiler. The pickup factor has already been included.
Make sure the piping sizes, layout, etc for the boiler are followed at the minimum.--NBC

KC_Jones

In your boiler plate picture there is a steam sq ft rating of 375 compare this to your EDR calculation directly. You really do not need to convert to BTUs. The calculations you listed look pretty good for what you have. I took a guess on those recessed ones and compared them to this: https://file.ac/LRPlQupV5kE/radiant-product-literature.pdf which gives somewhere around 36 sq ft for the 2 pictured (perhaps someone that has charts for those exact ones can post more accurate numbers). That being said you would be around 367 sq ft which is very close to that boiler rating which is very good. You are neither undersized or oversized by any significant amount. As I said though if someone has more accurate info on your in wall units please post it. Even if those in walls add more than what I said you should be in good shape.

JimmyNJ

Thanks NBC - So I would take the total EDR for each rad and add them up (so with my first example, that Rad would have a total EDR of 20 sq ft correct?

JimmyNJ

If I do that I get to 331 total EDR (not counting the two in-wall rads which I can't figure out just yet).

JimmyNJ

Thanks KC!! So your number would be the correct total EDR (assuming those two in-wall rads have a total combined EDR of 36)...331+36 = 367. So looks like I'm in good shape the size of my boiler then (very glad to hear it!!).

KC_Jones

If you were doing a replacement I would try and get more accurate info for those in walls, but I get the impression you just wanted a "gut check" so you are in the ballpark with those in walls. If they are a little bigger or even a little smaller than 36 you are good either way. Seems like you are fairly well matched and if everything else on the system (piping and venting) is good you should have a nice running system there. I just took a quick look at some of your other posts...were you able to get your main venting done? We always like to hear back how people made out with things.

JimmyNJ

KC - You are correct, just a "gut" check as my boiler looks "small" compared to other people I know (but maybe theirs are oversized). Yes I was able to add main vents (still cannot believe that none existed before!!). I added a #2 Gordon on the longer steam main run (may add another one there) and a Gordon #1 on the short one and everything seems to be running fairly well, I am still not perfectly balanced as the new rad in the family room heats up last but I'm sure I can still experiment a bit with trying smaller air vents on the other larger rads that are closer to the boiler. Thanks for all your help (and to NBC for his comments).
- Jimmy

KC_Jones

Adding more main venting can sometimes help with a rad at the end of the run. Think of it this way if you have rads that are getting warm before the main is full you are warming the house up. That gets the thermostat happy, but also stops the boiler before all the rads get heat. Don't eliminate the possibility of more main venting helping that rad. Oh and over sizing is a very common problem on steam systems, bigger is definitely not better and is most likely what you are seeing in other peoples houses.

JimmyNJ

This rad is not at the end of the main steam line, the take-off is actually fairly close to the boiler, but it is a pretty long supply run (about 20 feet) that runs in a horizontal fashion to the family room rad (it's actually the one I noted above, the 42 section, 4 tube) but has proper pitch so that condensation comes back to main). I've been thinking about putting smaller size air vents on the rads that heat up quicker so that steam can get into that longer radiator better - it does not always heat up, sometimes just a quarter of it gets warm before the boiler stops firing because the thermostat has been satisfied.

KC_Jones

Another suggestion then is put a slightly smaller vent on the rad in just the thermostat room. This will let the system run just a bit longer. It will also tell you if your theory will work. If it does work then you could go smaller on other rads that could potentially be overheating. If it doesn't help that room then possibly a bigger vent on that rad, but take it slow when going bigger...one step at a time. I don't view buying vents as a waste, worse case I have backups if one goes bad....or for when they wear out. We are all different though. Some on here like the adjustable vents and may comment about them. I have no experience with them.