Simplest Pressurized Storage Solution Heating from Storage Question

[SteveJ]

Thanks again for the great post https://www.hearth.com/talk/threads/19086/

Question about the "Theory of Operation" when "Heating from storage"

If the load circulator is on and both boiler circulators are off, why is there only flow through storage and not through either boiler?

If the shortest path for the water was through one of the boilers, then wouldn't the flow be through that path?

I thought when circulators are off (even IFC) that the impeller spins freely in the installed direction of flow.

So, if the storage path was longer (say installed next to the wood boiler in an out building) than the oil boiler path (say installed in basement next to zone manifolds) then when the load circulator only is on wouldn't most of the flow be through the wood boiler?

I am trying to do something similar with unpressurized storage and it seems that a zone valve on the return loop between the storage and the oil boiler that was closed when both boilers were off would prevent any "path of least resistance flow" when heating just from storage.

Somewhat confused???
Steve

 

[DaveBP]

I'm totally new to hydronics as of this May. I've worked through almost half of Siegenthaler's 'Modern Hydronic Heating' now and suddenly, 2 days ago, I could read Nofossil's schematic and see what was going on. And I was puzzled by the same thing. What was going to stop a good deal of circulation going through the boilers when they were "off"? My first take was that I should read more and I would learn to see what must be obvious to everyone else. So there's at least 2 of us waiting for Nofossil's answer.

 

[Dune]

There should be a check valve for each boiler.

 

[Nofossil]

The assumption is that there are three things that diminish the tendency for there to be any flow through the boilers:

1) The flow path through storage is shorter and lower flow restriction than the path through the boilers.
2) The tees are oriented so the straight path is through storage, and you have to make two additional 90 degree bends to get through the boilers.
3) The IFC check valves offer at least a bit of additional flow resistance.

It's probably important to have larger diameter plumbing with few if any elbows for the path through storage. To be absolutely safe you could add an isolation valve between storage and the boilers, but it would want to be a reasonably large orifice and therefore expensive. A better approach might be a spring type check valve. I haven't researched the flow characteristics, but any such valve should easily have a high enough opening pressure to ensure that there would be no unwanted flow. If the storage is plumbed with 1 1/4" or 1 1/2" iron, there could only be a tiny pressure differential from outlet to inlet at the flow rates that you'd see in this application.