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# Flow rates/velocity for panel radiators

Member Posts: 51
Hello,

I’m trying to design a radiator system for my house, and I am not quite sure I understand how to get the proper flow to each radiator. I will have 9 radiators with trvs and plan to use a grundfos alpha. I will use pex to radiators to a manifold. Total heat loss at design temp of 34* is 23,000 btus. The heat loads for the radiators range from 1,200 btus for a small bathroom with a tiny window up to 4,200 btus for a large room with a lot of windows. The necessary flow to supply the btus with a 20* delta t ranges from .116 gpm up to .42 gpm. I think I have three main questions:

1) should I use 3/8” pex to keep the velocity of the water up? An issue of caleffis idronics suggests that velocity should be 2 ft/s to purge air, and the smaller diameter pex will have more velocity. But I’ve read multiple posts on here saying there’s not much downside to up-sizing to 1/2” for more available fittings. If I use 3/8 I’ll probably use hepex, and 1/2” I would probably use fostapex.

2) if I install a manifold with balancing valves and flow meters, would I balance flow based on btu demand, or just equalize flow (I guess it’s the same as equalizing head at that point) to all the radiators with the trvs wide open and then let the trvs pull whatever they want in terms of flow in the course of normal operation?

3) is it likely that small radiators with such a low flow requirement will actually heat up and work ok? If things aren’t working, could I increase flow to lower the delta t to correct any problems?

Thanks!

• Member Posts: 7,341
I'd run 1/2 pex or PAP to all the radiators. With TRVs you adjust the flow setter on the rad's and the manifold all the way open and let the TRVs do the rest. The exception would be trying to get enough flow to a very large rad on a long runout - something you don't appear to have.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 51
Thanks, Bob. Is air less of an issue with radiators since you can vent them at the "top" of the system, rather than rely on velocity to carry the air bubbles back down to the air eliminator?
• Member Posts: 21,820
Home run piping, TRVs and a ∆P circulator will work great together.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,341
OaklandNS said:

Thanks, Bob. Is air less of an issue with radiators since you can vent them at the "top" of the system, rather than rely on velocity to carry the air bubbles back down to the air eliminator?

Once any air gets to the rad, it's going to the top of it and will need to be vented manually.

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 51
Thank you.

I just realized from your comment that you suggest keeping the manifold valves "wide open." Does that mean I should just skip the expense of a manifold with flow meters/balancing valves? That would save a few hundred dollars on an already-expensive system.
• Member Posts: 7,341
OaklandNS said:

Thank you.

I just realized from your comment that you suggest keeping the manifold valves "wide open." Does that mean I should just skip the expense of a manifold with flow meters/balancing valves? That would save a few hundred dollars on an already-expensive system.

You'll loose the ability to isolate and purge each loop individually unless you make provision for that. I've always found that your better off in the long run with quality manifolds.

Do it right and you'll do it once.

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.