sizing replacement boiler

humhead

for my house.   total EDR is 376 sq feet.  I included two radiators that were removed from my system, they are still in my garage.  All basement piping is insulated and all piping in outside walls are insulated as well. I have done a lot of work on my house and know where all the pipes are.   When I did my BTU calculation i ended up with NET I=B=R rating 90,240, Gross rating 90,240x1.33= 120,019  I have had two contractors give estimates on replacement boilers and they both have sized the new boiler one size larger. I am concerned with having an over sized boiler.  I  have read Dan's books and understand the reasons for using 1.5 pick-up factor.  I have included my calculation so my work can be checked.  I am NOT a Pro, so what have I missed?   I have an oil-fired Weill-Mcclain that I want to replace with a gas fired system.  I have received bids for burnam I6 and Weill-Mcclain eg-50.  When I looked at the slant-fin galaxy gxha-160 That looked like the correct sized boiler for my system.  If I wanted to add a condensate loop for some baseboard heating how would that affect my boiler sizing?

Thanks to all who take the time to read and answer home owner's questions.  You guys are a great help.

Dave in QCA

boiler sizing

You are correct in your assumptions.   Here are some additional points to try to clarify things for you.

All Steam boilers have several ratings:

Input rating: is the amount of fuel that is burned.

Output rating: is the gross amount of heat that the boiler can provide to a system.

Steam Sq. Ft.:   is the total EDR that can be connected to the boiler and have it heated propertly.  (note that the piping and pickup factors are already figured in)

Net Output Steam:  is the size of the system in BTU that can be heated by the boiler.  (EDR x 240 BTU/sq ft)  They have already done the calcs for you, and of course this number also accounts for the 1.34 piping and pickup factor.

So, in a case like yours, where the piping is insulated, does not have any unusual piping or pickup consideration, and where you have accounted for the radiators tha are temporarily disconnected, it should be simple.  You do not want an oversized boiler.  You could be able to size to the EDR or the NET rating on the boiler, going the nearest size that is greater than your system, assuming that there is not a boiler that is rated exactly at 376 sq ft. or 120,019 Net Btu Steam.

Now, you mention that you might connect some finetube baseboard as a hot water loop out of the boiler.  This must also be factored in.  If you are going to insulate the piping, I think you would be fine to account for the additional load by the following.  What is the total system load of the fin tube that you will add?  Figure the number of Lineal feet X the BTU rating per lineal at 215F, it will be 240 BTU per sq ft EDR, if you have the EDR/lineal feet number.  When you know the load of the fin tube that you might add, I would multiply it by 1.15, (the piping and pick up factor for hot water), then add that net number to your steam system net of 120,019 sq ft.   You should be OK.

I am sure that others will chime in on this thread and give you additional clarification.