Steam Boiler Sizing

rwilson2526 · November 2017

Hi everyone....first time poster.. have been reading on here lately due to imminent failure of my existing boiler. I will start another thread on why I only got 9 years out of my new boiler (corrosion above the water line) but I'm wanting to possibly get a new boiler ordered while this one is still going.

I've been in the house 14 years...5 years in the original boiler failed . water on the floor one day....never really looked into how bad it was and cant recall how old it was.

As it was winter I scrambled to find a boiler in stock local. Installed a Crown BSI138 138K in 85,000 Net I=B=R rating. I cant recall what the original boiler was but I remember the Crown had more capacity but I don't recall it being ridiculously more.

As I want to do this installation correctly I want to start off with proper sizing on the new one.

Using this site I totaled up all my radiators, number of sections, 2 columns...etc and came out with 268.75 ft with a BTU requirement of 64,500. (268.75X240)

I know boiler published performance for AHRI includes a 1.33 pick up factor.

My basement horizontal mains and returns are all insulated well. (the header not too well but I can fix that) The risers to the 2nd floor are not insulated. There are 5 risers to the second floor. 3 of them are exposed inside the rooms below and 2 of them are between the exterior wall and inside wall.

My question is that there seems to be a cutoff in the next size boiler on many of the brands I was looking at at 64,000-65,000 with the next size being in the mid 70,000's to 80K

It seems to be the general consensus that oversizing a steam boiler really is a waste of money so I was wondering opinions on if the1.33 is sufficient correction for my setup or is getting a 65,000 AHRI boiler cutting it to close.

Price difference doesn't seem to be much of an issue....just looking for optimum fuel use.

Thanks in advance and stand by for more threads on my existing near boiler piping....

acwagner · November 2017

Perhaps best to start with questions on your current boiler. Is this one pipe or two system? How does the system perform currently--as in does it cycle on pressure and how often? What kind of pressure does it operate at?

Fred · November 2017

To avoid any confusion, look at the Net Sq. Ft. of steam on the boiler plate of any boiler you are looking at and match that as closely as possible to what you calculated your Radiator EDR to be (269 Sq. Ft.). The Pick-up factor of 33% gives you a little bit of play, if you pick a boiler that is rated at say 260 Sq. Ft. of Steam but stay as close to your connected EDR as possible.

Is this an oil or natural gas boiler? If oil, the Burnham Megasteam is the only way to go for a quality boiler that will last for many years to come. If gas, there are numerous options available, Crown being a good option too.

Whatever you do, if you know why your current boiler only lasted 9 years, fix that issue before or by the time you install a new one and by all means have the new boiler and near boiler piping properly installed, by the I&O manual, at the very minimum.

New England SteamWorks · November 2017

1st step is to determine why your boilers are dying prematurely.
Pictures of your current setup would be very helpful.

mikeg2015 · November 2017

I agree. Figure out why this one is failing first.

I also think that you generally should size on the smaller end. A boiler that never really builds pressure or even needs vents/traps except main vents, is really the ideal setup. I think you need 10% for piping losses and jacket losses, but that's about it. Plus if you are a little light on sizing, you can then add in some TRV's in places that get overheated easily like bathrooms and kitchens.

Next look at some recent installation on here and find a contractor that can install like that, with a nice drop header, 2 connections off the boiler, and proper tapping and trim.

Fred · November 2017

@mikeg2015 said: I think you need 10% for piping losses and jacket losses, but that's about it.
I think 10@ Piping and Pick up is too low to recommend to anyone unless we know exactly what their piping configuration is and we are doing the work with a written guarantee that it will function as intended, down to design day. Our opinions don't count for much when someone else is paying the bucks. Manufacturers build 33% overhead into their steam boilers. I would never be comfortable that anything less than 20% is "enough" until I ran the numbers on each specific installation. It's not worth the risk, especially since we know the 33% works very well. JMHO

mikeg2015 · November 2017

I think it depends too. 33% pickup on a 100k boiler is not the same as a higher mass system with a 300k boiler. My boiler has 75% more output than radiation, but still takes 45 minutes from a cold start, 30 minutes from a warm start to build any real pressure. Thermostat is always satisfied before that point, except with recovery from setback. If the system was smaller overall, and smaller header, it would have less forgiveness I think.

I think my concern Is that 1.33 puts you between 2 sizes, and most contractors, if they even sized it correctly, goes up. But they really should go down. I think there will be more issues being a little big than a little small (but still above EDR).

rwilson2526 · November 2017

Thanks for the comments guys.....sorry it took me so long to get back...I'll post some pics as soon as I can get them from I-Pad to here.....to correct, I was doing it from memory ...edr was calculated at 260.6 with btu's of 62,542.

Yes I definitely want to find out why current boiler was only 9 years....to answer first responders questions it is a 1 pipe system.

Since I installed the crown boiler I have had the dissappearing water problem that has been talked about on here...water level drops slowly and boiler usually cuts out on lower water cutoff.....sits for a few minutes and then water returns and fires again.....

I am putting a new gauge on this week....current gauge either reads 0 or 4 psi when cold and goes to about 6 when firing.....never can really tell if its shutting off from low water or pressuretrol switch. (switch set at 1 and .5)

Once I get the new gauge piped into the pigtail I will post some more reliable data.

Over the years I have been neglectful of the new boiler....about 4-5 years ago after wiring into a generator during hurricane sandy to get heat I fried the honeywell pressuretrol .....boiler pressurized to 15 and blew the safety valve...(I happened to be in the basement at the time and required changing of drawers!)

As a result cracked a few big box store radiator valves.....small leak in the 18 section radiator in the living room....had it off line for a season.....

point being is even though its been 9 years I dont have a good handle on how the existing boiler heats the house due to varying conditions year to year....House is comfortable...radiators heat up reasonably fast.....with the current leak I have in the top (which seems to be better now that it is hot regularly) its going to be hard to tell how it holds pressure but I will post info as soon as I get the new pressure gauge installed.....

stay tuned for pictures

rwilson2526 · November 2017

Hopefully these pictures post.....

First , yes I know...the boiler is facing backwards....As I said it was winter when the old one went and I bought what I could find in stock....EVERYTHING was exactly freaking opposite of the old one, so in it went...

2nd ....all that copper was existing ....

Lastly, in a perfect world i'm hoping to get through the heating season and redo the whole thing correctly in the spring....I'm kind of in a unique situation....I'm a sheet metal contractor at a larger corporate customer ....we share office and warehouse space with pipe fitters, electricians, carpenters and GC.....I have access to all the equipment and expertise in the world for performing the work with exception of the design.....we all build to engineered drawings and when it comes to residential boilers nobody knows jack..! I can follow a mfg drawing but with the limitations of the space and location of the header/returns I'm hoping for some guidance on how best to pipe it in.....

Fred · November 2017

You have plenty room there for a good header and better yet, a great drop header. shouldn't be a problem. All boilers come with piping and installation instructions. Sounds like you work with people who know how to read them. You're ahead of the game.

rwilson2526 · November 2017

Also attached are pics of corrosion on heat exchanger. I am also contemplating just putting in a new heat exchanger for 1k investment. It's a 5 section unit. Swap out looks pretty easy. The two outside chambers are the ones corroded. Inside 2 chambers look pristine. I filled with water and top section on one side started seeping water.

dont really know if it corroded from inside out or outside in once the steam leak formed.

Steam Boiler Sizing

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