Different Estimates for Boiler Size

Al FitzSimons

Dear Dan,
I have a 20 year old Burnam steam gas fired boiler input BTU 385,000. It has developed a crack in the upper portion. I have gotten several estimates for replacement. I have been reading on the internet that right sizing is very important and over sizing is common. The house was built in 1931, has 8 built in convectors on the first floor and 11 on the second. It is a two pipe system. They are Trane convectors with orifices on the input side to limit/help equalize steam throughout the house.(I think)
By the way it takes 11 minutes to steam up and shut down based on 4 PSI pressure(not thermostat) and cycles about every minute to 90 seconds until temp is reached. It heats pretty evenly.
My problem is one company measured the interior size of the recessed "cabinets" where the convectors are located, determined the square footage of each and totaled it to 761 square feet which he said was my steam requirement. (roughly -which he matched with a Burnam Independence IN10 at 812 steam sq ft which has an input of 315 MPH.
Company #2 carefully measured the convectors themselves - did not use the cabinet sq ft - and determined (by some mysterious and proprietary method not disclosable to me) I needed 587 sq ft steam which they equated to a Burnam IN8 at 633 steam sq ft and 245 MBH input.
Can you help me decide which is better? Are there any other methods I could use to tell which one is correct?
Thanks
Al

Paul Mitchell

Well Al

I sure am not Dan so I would wait for his input. But I would measure the "cast iron". Not the enclosures. Then add a few differant factors in depending on pipe insulation etc. That would give you total EDR and then you multiply by 240 for BTU. The boiler of choice would equal or be a little bigger than rquired. Never smaller. I think it is short cycling because the pressure is too high and possibly you may have some traps with some problems.

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Al FitzSimons

Thanks Paul! I just downloaded the Trane Radiation Convector manual and am looking at the type RK floor standing recessed unit which is very similar to mine. Table PD-5 gives the various "box" dimensions and their EDR rating which they multiply by 240 to get BTU for standard conditions. I assume this figure is designed to engineer how much radiator one needs in a particular lossy room. But couldn't I reverse it to determine the BTU needs of all existing convectors in my house, throw in a little loss and get close to the boiler BTU output rating?

Paul Mitchell

You probably should look at some of the Q&A

on this site to gain some more knowledge. Add up all radiation. Total the EDR. Multiply by 240. Add appropriate pick up factor.(That is what you need to read about) That is that. Doesnt matter what your actual heat loss is gotta go by metal you have..

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scrook_2

it should be noted...

that "the metal you have" i.e. the total steam radiator/convector size, was likely oversized when it was installed and w/ later improvements to insulation, windows, etc. it will becomes even more oversized now, so the actual heatloss of the building is immaterial. The important thing is the steam condensing capacity of the rads and convectors, hence the need to size to them (plus the pickup factor for the pipes).

Given the old system short cycled (even at 4psi) it was too large, only careful measurement will tell how much though.

OTOH, with hot water, no condensing of constant temperature steam occurs so the boiler *is* sized for the heat loss of the building iinstead, and the water temperature then can be adjusted up or down for the capacity of the radiators/convectors at various tempertures to match them to the building heatloss.

Steamhead

On an Orifice Vapor system like your Trane

the convectors or radiators were only filled to about 80% capacity. This kept the steam from reaching the dry return.

The new boiler should have a Vaporstat rather than a Pressuretrol. This type of system typically runs at 8 ounces or so. Also be sure the steam mains and dry returns are vented properly.

Get a copy of Dan's book "E.D.R." available on the Books and More page of this site. I'm pretty sure your Trane convectors are in there. You can size the new boiler from the info in the book.

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