How to size a steam boiler

Unknown · October 2003

when it comes to sizing the boiler for the job , because it's already figured out for me before the install . But we do run into situations where theres a steam boiler with a zone of baseboard heat off the internal coil . What is the proper way to size a new boiler for this situation ?

I know you need the square foot of radiation for the cast iron rads . But how much of a pickup factor do you need for the mains and other variables ? Is the pickup factor already in the SQ FT number in the new boiler specs ? And how do you factor in a baseboard loop ? Thank you .

Unknown · October 2003

Yes

The pickup factor is already figured in, the sq ft rating is the radiator sq ft, with nothing added.

Take a third of that number and that is what you can use for a baseboard zone.

For a water heater, you can use all of it, if you use a priority relay for the DHW.

Noel

Unknown · October 2003

So , if I measure

all the rads and come up with 400 sq. ft. , I can for example , safely use the Slant Fin LD 30 which has a capacity of 421 sq. ft. ? I mean just for the steam part ?

And if I take 1/3 of that number , around 133 sq. ft. , how would I go about converting that number to see if it will handle the connected baseboard loop ? Thank you Noel for the info .

Unknown · October 2003

240 BTU's per Sq Ft Steam

Sorry, I left that out.

Yes, you have the rest correct, too. Right off the rating plate, the number is the max amount of radiation you would connect it to.

Noel

Unknown · October 2003

Aha -

thats the conversion I was looking for , thanks again , Noel .

Oops , one last question - for a baseboard zone off the steam coil , it's sized by a typical heatloss of the room the baseboard is in ?

Unknown · October 2003

GREAT question

I need to find out the exacts, but here is the gist of it.

A coil is rated at how many GPMs it will deliver at a certain temperature rise. That itself is based on intermittant draw (meaning it assumes that the coil starts out at boiler temperature, not at street temp.) so it is inflated a little.

Assuming it figures a 100 degree F. rise, you could multiply it by 5 for a 20 degree rise and be OK. Now the intermittant draw number is a bit optomistic.

On a 100,000 boiler, the tankless rating is about 2 GPM, so that would be 10 GPM at a 20° F. delta T, or over 70 feet of 30 baseboard, if the coil rating were constant. It isn't, so I need to learn more and get back tomorrow to you.

Great question.

Noel

Unknown · October 2003

The math is way beyond me

I'm glad theres guys like you who are around to figure it out , hopefully without getting a headache like I do with math . The situation does not come up alot , but when I'm on a job , I take a quick look at the connected load and the baseboard and make sure the boiler is not grossly oversized . But the baseboard loop always throws me off .

Unknown · October 2003

Baseboard from the boiler water is the easy formula

Direct math.

I'll be back. I need to find out the fudge factor on the coils.

Noel

Unknown · October 2003

Hi Ron,

It turns out simpler than I thought. The coil is 1/2", so the max flow rate is about 1.5 GPM. That's enough for about 25' of Fineline 30 or 15.

I'm glad you asked.

Noel

How to size a steam boiler

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