Boiler supply/return size vs. pipe btu capacity

jmatthewwest

Quick question from a new guy... I'm seeing it referenced that a 1" pipe is capable of carrying 80,000 BTU's as a general rule of thumb. Given that, why would a 120,000 BTU boiler have only a 1" supply/return? And continuing along those lines, if a 10 outlet manifold is meant to supply a load greater than 80,000 BTU's, why again would it only have a 1" supply? Apologies if it's an idiot question, I haven't been able to find a resource with the answer. From another perspective... if a 120,000 BTU boiler needs to provide 12 gpm at 20 degree temp drop... why does it have a 1" connection that should only allow for 8 gpm? Appreciate any wisdom y'all can share.

Jamie Hall

Rules of thumb don't always work. The "rule" that a 1" pipe can only carry 80,000 BTUh is a rule of thumb. The objective here is to keep the flow velocities -- and, therefore, pump requirements, noise, possible erosion and what not -- within reason. Experience has shown that for any significant length of pipe, the rule of thumb works well.

That said, a short length of smaller pipe than the rule suggests isn't going to be a real problem. The velocity in that stretch will be higher, and so will the head loss -- but unless it's a long length, while not trivial -- it's almost insignificant.

There's another minor factor here -- keep in mind that going from 1" to 1 1/2" pipe cuts the velocity in half -- or even slightly less than that -- so if 1" is good for 8 gpm by the rule of thumb, 1 1/2" is good for 16 to 18 gpm. At 12, do we go slightly small? Or slightly large? Hmm...

It occurs to me to add, for the benefit of our steam contingent, that none of the above applies to mixed flows, such as steam...

hot_rod

Also, when you say a 120,000 boiler, is that input rating? So a 120 running 87% efficiency would be a 104,000 BTU/ hr output. Running a 20∆ that would require 10 gpm flow. Your boiler manual generally shows flow requirement minimums at various deltas, sometimes as high as 35∆.

Also, use the engineeringtoolbox website for pipe sizing, they have excellent info there and drill down to types of pipe, even various copper K.L,M types.
Stay under 5 fps velocity in hydronic piping, 4 fps is comfortable.

So 1" M copper could run 12 gpm if needed, 4.4 fps velocity.