Sizing, conversion to gas

DavidM

After help from this forum last year, my 1950's or 60's GE boiler has been working well. However, heating oil in Oregon is now exceeding the price of natural gas by a factor of 4. So I am converting to a new Crown gas fired steam boiler. The question is sizing:



The installer calculated the radiator surface as requiring boiler size at about 100,000 BTU/hr input, but to be sure was planning to use a 138,000 BTU/hr boiler. The present oil boiler is fired at 1 gpm, or about 140,000 BTU/hr input, but perhaps it is only 75% efficient (or even less as the tubes have not been cleaned in years) compared to 81% for the new boiler. The steel piping of the system is at least 50 years old.



With the present boiler, the system works nicely at 1/2 psi when it starts up, but after a while the pressure rises to 2.5 psi and holds there. I could not confirm it today, but my impression is that after a while it goes even higher, and trips the high pressure cutout. Is this because steam is coming into the returns? Does this indicate that the boiler is oversized?



Crown also makes a 103,000 BTU/hr (input) boiler. Should we specify that size rather than the 138,000 unit?



Many thanks,



David

BobC

Match the EDR to the boiler nameplate.

A lot of old boilers were oversized and that wastes fuel. It all comes down to matching the sq footage of radiation to the nameplate steam rating. If the house is reasonably well insulated and the windows are in good shape that should do it.



This link will take you to the Crown sizing chart -



http://www.crownboiler.com/educate/sizeheat.asp



Bob

crash2009

Sizing the boiler

 is simple like Bob said.  Match the EDR of the of the combined radiators to the EDR of the boiler.  The "but to be sure" is all-ready built in. 

I wouldn't trust anyone to make the sizing decision for me.  It would be kinda like sending my wife to buy me a suit.

 What kind of radiators do you have?  Post a picture of one of your rads and we will help you figure it out. 

Here is one I measured a while back, and the document, here in the library, that told me how many EDR/section.

DavidM

Can't get proper value for finned tube baseboard

Thanks for the comments so far. I have, as suggested, run my own calculation. The problem is that on line, for 1.25 copper -aluminum finned tube baseboard, I find roughly 1100 BTUH/foot, but the Crown Boiler chart uses only 600 BTUH/foot. Furthermore, I doubt either of these numbers contains the 30% fudge factor that is normally included for radiators. This discrepancy has a huge effect on the size of the system needed! Help please.

DavidM

Can't get proper value for finned tube baseboard

Thanks for the comments so far. I have, as suggested, run my own calculation. The problem is that on line, for 1.25 copper -aluminum finned tube baseboard, I find roughly 1100 BTUH/foot, but the Crown Boiler chart uses only 600 BTUH/foot. Furthermore, I doubt either of these numbers contains the 30% fudge factor that is normally included for radiators. This discrepancy has a huge effect on the size of the system needed! Help please.

BobC

This is what i found

I'm not very familiar with finned heat exchangers on steam systems but I did find this -



http://www.embassyind.com/pdfs/embassyind_compakbroc.pdf



Make sure you are measuring the length of the element not the length of the enclosure. Also the IBR rating of a boiler already has a 33% pickup don't add another 33%.



Bob

Charlie from wmass

The rating crown has for baseboard

is for a hot water system. Do you have a photo of your baseboard?

DavidM

Thanks to all, decision taken.

I have confirmed the finned tube rating to my satisfaction. The Crown rating must be for hot water, not steam. My totals for the home suggest that the size selected by the installer is the correct one. It allows only 20% for piping, not 33%, but it will put out the same BTUs as the old oil boiler does now, which has been sufficient.



FYI, the old GE boiler had already been down-fired by 20% from a 1.25 gal/hr nozzle (on name plate) to a 1.0. So that is another indication that it is correctly sized, or only slightly oversized.