Do I need to increase the size of my boiler piping?

Joe107

I have a hydronic system with 12 zones each fed by .5" pex coming off of a 1.25" copper manifold. The 1.5" output on the boiler was reduced down to 1.25" and 1.25" copper piping was used for the supply and return lines between the supply and return manifolds and the boiler. I am going to be adding a .75" pex loop for baseboard radiators in my basement. I am planning to install a tee into the supply piping for the basement zone before the manifold. With adding this new zone, should I re-pipe the boiler using 1.5" pipe and reduce to 1.25" between the new tee for the basement and the existing manifold? I don't want to do all this work and then have inadequate water flow when multiple zones are calling for heat. Thanks for your help!

Alan (California Radiant) Forbes

I think you will probably be alright, but we need more information. Do you have the total BTU heat loss number for the entire area to be heated?

Ironman

What’s the btu rating of the boiler?

EBEBRATT-Ed

@Joe107

That's like reverse engineering. The way to design a system is to do:

1.a heat loss room by room of the building
2.size the # of radiant loops & supply water temp and td
3. then the water flow will be known and the circulator(s) and boiler can be picked

But 1 1/4" pipe will transfer at least 140,000btu/hour (more for a radiant system)

Joe107

Thank you all for the replies. The system was sized several years ago when we built the house but before we considered finishing the basement. The heat loss for the house is 73,000 BTUs according to calculations done at that time. I calculated 11,000 BTUs for the basement and the net rating on the boiler is 103,000 BTUs. We also have a 56 gallon indirect water heater with priority zoning connected to the boiler. The house is heated using ultra-fins below the subfloor and a high limit temperature of 175 degrees.

Alan (California Radiant) Forbes

With those numbers, 1¼" is fine. Frankly, 1" would work.

.........and a high limit temperature of 175 degrees.

This makes sense for a high limit; hopefully, your operating limit is around 120°.

Great product, Ultra-Fin.

Joe107

Alan, Thanks for that info. Glad to hear I don't have to redo all of the piping.