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Sizing ancient boilers

M Lane
M Lane Member Posts: 123
I am providing a budget to a HOA so they can assess the cost of upgrading their boilers. The complex consists of 4 separate buildings plus the center one that houses the mechanical room. I will do a cheesy heat load calc (this is only a budget report) to devise a quickie btu/square foot design criteria.

However, the 2 existing boilers are a couple of Cranes from the 30's that are coal to gas conversions. No rating plates.

Denver will want to know what they have existing as part of the permit process.

So I was going to clock the meter; something I have not not had to do for at least 6 years; so I'm rusty.

My plan, now that it is not really heat season out here now, is this:

Turn them both off. Let them cool for at least a day. They operate on a simple set-point only control.

Once pretty cool, fire one up, read the CFM at the gas meter for 1 minute.

Obtain the multiplier from our gas supplier to covert CFM to BTUH.

Double that, and there is current capacity.

Domestic hot is a separate system, btw.

Does that sound right to you guys?


  • Jamie Hall
    Jamie Hall Member Posts: 23,275

    that should give you the gross input of the boilers -- only I'd let them run for five or ten minutes, rather than one, to reduce the error in clocking the meter.  But that's just me.

    You may be able to get an approximation as to their efficiency by the usual measuring the stack gas approach (and might as well check the adjustment while you're at it!), which will help getting at least in the ballpark on net output.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England