Piping & Pumping: Existing Mod-Con Combi Boiler

ILikeEmOlder

I’m looking for advice on how to pipe and pump for a proposed hydronic air handler to be integrated into an existing hydronic system that already has a mod-con combi boiler feeding plated radiant tubing (via a manifold).

This is the current setup:

The boiler, which is located in the cellar, is a Rinnai i060C (with integrated Grundfos 15-78 pump). The manifold currently serves the 1st story of the house (through 5 loops of plated 1/2” PEX, with a 6th loop of plated 1/2” PEX in process).

The plan is to add a hydronic air handler (in the conditioned attic) to serve the 2nd story. Ductwork already exists (and is connected to the existing gas furnace).

My thought was to use a series primary system, with each of the two zones pumped individually. The hydro coil zone would be upstream from the radiant manifold zone.

Here is the proposed setup:

Thoughts?

hot_rod

what temperature are you looking to supply to the AH? You may need a two temperature system. Boiler runs at AH temperature, radiant gets mixed with a thermostatic 3 way, for example

ILikeEmOlder

Hot Rod,

The coil will be oversized for the load to accommodate lower SWTs. At design conditions, the max SWT required should be 140F.

SWT to the radiant manifold is roughly 130F at design conditions. Old house with thick built-up flooring, so it’s closer to mid-temp setup (even with closer tube spacing, shorter loops, and thick plates).

After looking at temp drop numbers across the coil, it appears that no mixing would be required for the radiant floor manifold.

I turn to you guys for your experience, because I know the numbers do not always tell the full story of how things will play out in the real world.