Two pipe buffer - Placement of Large Pipe ?

Dave Carpentier · October 4

System schematics are one thing, placements and piping is another.

What are the drawbacks of placing the Large Pipes somewhere other than directly onto the buffer tank ports ?

I would prefer to mount my Large Pipes vertically and closer to the wall. This would reduce stress on the buffer tank ports as well as leave the piping a bit more open to future changes of buffer tank. Planning on using 1.5" tees for the Large Pipe.

Jamie Hall · October 4

Well… in your sketch it appears that the "large pipes" are manifolds; they could — in principle — be placed anywhere. The idea is that they allow separating the flow from one pipe into many — or vice versa. In the more complete diagram (which I presume is the designed system) I see several problems with the overall arrangement, and I'm rather led to wonder if perhaps you have misunderstood the function of the "large pipes" in other systems, where they are not only manifolds, but hydraulic separators allowing the flow rates in the boiler loop to be decoupled from that in the heating loops although that will happen as the buffer tank is connected between the supplies and the returns.

The expansion tank should be on the line from the return manifold to the boiler, as close to the pump on that return as is reasonable.

hot_rod · October 4

Maybe a better definition of "generously sized header. The goal is very low flow velocity in that short large section of pipe. Ideally less than 2 fps. So that close to tank piping and the tank itself presents very little head loss.

Screenshot 2026-10-04 at 12.42.50 PM.png

So determine the total gpm at that point. Then look at the flow tables for pipe, or tube.

Lets say you have a total of 8 gpm. You could flow up to 9.3 gpm in 1-1/4 pipe and stay in that 1- 2 fps window. No harm in going larger.

Caleffi hydraulic seps are designed to keep the flow velocity in that chamber, "generously sized pipe" below 2 fps.

This assures flows from various pumps and variously sized pumps do not interfere with flows.

Screenshot 2026-10-04 at 12.52.51 PM.png

Dave Carpentier · October 4

@Jamie Hall I think I had originally thought to attach the expansion at the buffer as it is the point where all three circs are pumping away from. I can alter the plan to attach it and the Axiom fill box to the return line before the Dirtmag if that seems like a better location. The vent is probably best on top of the buffer, I think (reusing the top anode port).

@hot_rod My boiler loop will flow 6-7 gpm and the load loop would be anywhere from 0.5 to 4 gpm. At 4 , the excess flow (unused boiler flow) would be 2-3 gpm. If only one zone was open at 0.5, the excess would be 5.5-6.5 gpm. Hopefully Im looking at this the right way ?

So if I made the Generous Header with 1.5" tees and placed them on the back board across from the buffer ports, then stung out to the ports with <12" long 1" pipes, it might be close to workable ?

hot_rod · October 4

Ideally a tank with a 1-1/4 or 1-1/2 ports.

If not these swage nipples make a nice increaser. Screw the 1-1/2 tees right onto it, close nipples between them, it for a short, fat header.

Connection for the HP, air handler, radiant, maybe a spare.

Expansion tank could be on any of the 3 ports as the whole tank becomes the PONPC. On the upper header where the pumps locate works well.

The tank can mount anywhere, just the connection needs to be at the manifold, pex to the tank location.

I doubt you can build a header with tees and nipples for the price of a "speedy header"? They come in an assortment of sizes.

Connect the header to the swage nipple on the return with this valve. It becomes a coupling and the purge/fill port for all zones

Two pipe buffer - Placement of Large Pipe ?

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