Boiler sizing, 1920s two-pipe steam

Mike Cascio

With a 33% pick up factor and all radiation installed, your boiler would be 154,000btu. Even at 154,000 btu you would notice a big difference because the newer boiler will steam much quicker and efficiently.

If you say that you keep one living room radiator off, you can either disconnect the rad all together or leave it permanently off. This would bring your load from 485edr to 425edr which translates with a 33% pick up factor to 135,000btu. Make sure with a 33% pick up factor that all pipes are fully insulated.

Michael J. Cascio

Garret

Howdy gentlemen... Some of you may have read about my two-pipe steam boiler replacement project.. mostly in this thread:
http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=229995

I'm making progress... last weekend I swapped out all of our steam main & return vents. Had to dig out the pb blaster and some elbow grease I didn't know I had, but eventually I got them all replaced with gorton #2's.

I've also found a few good local contractors, and plans are quickly coming together. We are going to stick with steam, for one thing. I think we're going to hold off on TRV valves for now, though they seem great on paper, I don't think we have enough "overheating trouble" to justify the cost.

What I'm still hung up on is how to size our replacement boiler.

Based on every estimate and rule of thumb I can find, I believe that our house heat loss isn't above 75,000 BTU/hour.

It's probably closer to 60,000 BTU/hour, and is likely to get better over the next few years as we improve the house in various ways.

Now I know I'm not supposed to look at home heat loss when sizing a replacement steam boiler, but I just can't help it. I'm willing to remove radiators, install orifice plates, etc., whatever it takes to reduce our installed radiation, so we can put in a boiler that more closely matches our house's heating needs.

According to my math, we have 485 sq ft EDR of installed radiation. You can find my measurements and pics here:

http://www.worksongs.net/photos/2006-02-Boiler-Replacement/rad-sizing.txt

http://www.worksongs.net/photos/2006-02-Boiler-Replacement/


If my math is correct, according to Dan's sizing approach (pages 75-86 of the Lost Art) we need:

485 sq ft ER * 1.50 * 240 = 174,600 D.O.E. heating capacity

As you can see, that's a ton, compared to what I think our heat loss is. And looking at this the other way around... What do we have currently? We have an old cast iron boiler from the 1920s, with a gas conversion burner that was put on probably 30 years ago. I figure we're getting 60% efficiency at best. I've timed our gas meter: it's burning 150,000 BTU/hour. So at 60% efficiency, the output is 90,000 btu/hour.

So the rule-of-thumb replacment boiler is almost twice the size of our current boiler!

As far as I can tell, our current system works as well as I could expect. The rads all heat simultaneously, there's no water hammer, we don't have any hot/cold rooms, I don't think the boiler is short-cycling or running nonstop, etc. Our only complaint is the gas bill.

I've been thinking about how a tremendously oversized set of radiators could be working so well. Now, first off, I could be wrong in my sizing math. If someone wants to double check my numbers, I'd greatly appreciate it...

Assuming my numbers are correct, however, let's start with our 485 sq ft of radiation.

Is the boiler really seeing all that?

-- We've been running with one of our living room window units shut off as long as we've been in the house. It's probably been that way for years... So can we subtract 60 sqft EDR for that rad?

-- We also have sheet metal covers on three of our window rads. I've read that poorly designed covers like ours reduce the output of a radiator by 30%. Those three rads total 172 sq ft EDR.

That puts us at 485 - 60 - (172 * 0.3) = 373 sq ft EDR, or 373 * 1.5 * 240 = 134,000 D.O.E heating capacity.

Much closer, but still 50% more than what our current boiler can put out.

I really should disconnect a radiator to see for sure... but I'm wondering if orifice plates in our radiator valves are the secret. Our radiators rarely heat all the way across, and have return lines that barely rise above room temperature. We have a few rads with modern (hot water) replacement valves. Those rads have very hot return lines... I think the lack of orifice plates is to blame.

I read somewhere in the Lost Art that typical orifice plates reduce the sq ft EDR by 20%. If this is the case, our 373 sq ft EDR from above is really 298 sq ft EDR, which makes the math look much better for our 90,000 btu/hour (output side) boiler...

298 sq ft EDR * 1.3 * 240 = 93,000

298 sq ft EDR * 1.5 * 240 = 107,280

My plan right now is to put in a Weil Mcclain EG-40, D.O.E. heating capacity 101,000 btu/hour.

Am I nuts? Is that horribly oversized/undersized? (and you're welcome to say "stop obsessing and go back to your day job, mr. homeowner.")

Cheers,
-Garret

Garret

bump

Garret

bada bing, bada bump

Mike Cascio

Garret,

You are losing a very important point, the boiler under every circumstance unless radiation is REMOVED must be sized according to all radiation. If it is undersized, your gas bills will soar. Perhaps this is why your gas bills are so high, your boiler is almost 80years old, and operates at 60% efficiency and UNDERSIZED. Don't undersize the boiler, you will be the only one who loses. Also if all of your pipes are insulated you only have to add a 33% pick up factor not 50%. And lastly, the poorly designed radiator covers DOES NOT reduce the amount of radiation you have by 30%. Those covers reduce the output to the room by 30% but not the amount of steam heating those particular pieces of radiation. It really makes your boiler work 24% harder to get the same job done.

Best of luck,

Michael J. Cascio

Garret

Thank you very much for taking the time to read over my admittedly long post, Michael.

You make some very interesting points. I'm just trying to make sense of it all from an engineering point of view, so we make the best possible decision. I've found some good contractors in my town, but they're recommending very different boiler sizes. I think between 'em all, I have everything from 80,000 to 200,000.

Our supply pipes are insulated, but I'm sure I could improve them. Only the mains are insulated right now... I could insulate the takeoffs where they're exposed in the basement.

The radiator cover issue makes sense, the way you put it. I guess I was thinking of a steam system as dynamic thing where the boiler keeps feeding steam as needed to replace whatever condenses. I.e., as water in the rad condenses, it tries to create a vacuum in the radiator, which in effect "pulls" more steam from the supply (steam that otherwise would go elsewhere, or get limited by the pressuretrol). If water doesn't condense as quickly, you'd think the rad would get less steam to replace it. But I guess I'm just mistaken about that one. I think I'll go re-read Dan's extra notes about "undersizing boilers" in the Lost Art and see if it sinks in this time.

What I'm hoping to do is to use a boiler that is sized somewhere closer to our heat loss needs, and use orifice plates in the radiator valves to balance out the load so the system functions fine and all of the rads heat evenly.

I don't mean to put words in his mouth, but I think that's a strategy Boilerpro has had good luck with. (Dave, please correct me if I'm wrong).

Best,
-Garret