How to size a steam boiler

Unknown

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

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

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

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

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

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

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

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

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