boiler sizing

STEAM DOCTOR

I was recently hired to do a consultation on an old house renovation. The homeowner doesn't trust the people that the general contractor is bringing in and I am getting the impression that the GC also doesn't trust the people that he is bringing in. They are replacing the  old massively over sized oil boiler with a new gas boiler. They only want to go with burnham because the local gas company gives a rebate for on the burnhams. I did an EDR survey and came out with 148 mbtuh. I told them to go with the IN6 which is rated for 144mbtuh. The problem with the IN6 is that the manual only requires one 2" riser from the boiler. My gut tells me that this is to small. There is no way that I will be able to  get the boiler installer to exceed the manufactures recommendations.I am wondering if it is worthwhile to tell the installer to use the IN7 which on one hand would be over sized but on the other hand would require him to use two 2" boiler risers as per manufacturers recommendation. Thanks in advance for the advice and input.

Dave in QCA

BTU net, gross, or input??

The ratings always cause confusion.

When you did the survey, what was the total sq ft?  I am guessing that it was about 617, and thus your number of 148,000 NET BTU.   sq ft x 240btu.

If this is correct, then you need to be sizing according to sq ft, or net btu steam.  In which case you would need an IN8.

STEAM DOCTOR

gross

Sorry bout that. The btu's are the gross output. The edr is aprox 465(465x240=111600x1.33=148428)

Dave in QCA

IN6 is correct

Ok, you got the right model, and if the manufacturer says it is acceptable to pipe it with one 2" riser, then it should be fine, as long as all of the other little details are correct. 

I note that the IN12 has 2-2" risers and it is more than twice the capacity than the IN-6, therefore the velocity in the risers will be even greater than that IN-6 with one riser.

It is very important that all other details of the piping is exactly correct in order to deliver dry steam.   No reductions in the horizontal piping, make sure the swing joint / double 90 is present coming off of the riser, etc. 

YOu should be OK 

STEAM DOCTOR

exit velocity

The IN6 has a steam velocity of 42.85 feet per second. The IN7 with the two boiler risers has a steam velocity of 25.7 feet per second. The IN7 also calls for a 3" header as opposed to the IN6 which only calls for a 2" header. My gut would tell me that even though the IN7 is approximately 25mbtu over sized it would still do a better job in heating the home at lower cost.

Steamhead

The IN6 is rated 450 square feet EDR

while that's not much less than your 465 square feet, I think I'd go with the IN7 and down-fire it a bit. You won't be working the cast-iron so hard- remember, an atmospheric boiler has all the cast-iron above the flame rather than surrounding the flame as on a power-burner unit. So the heat is pretty concentrated in a small area.



And the minimum specs would force them to install a better header.



Make sure the installer tests the boiler with a digital combustion analyzer before you finish paying them!