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# Reverse piping an indirect HWT

Member Posts: 55
edited July 2020
I saw this diagram in an article by Siegenthaler, about reverse piping an indirect tank and using it as both an HWT and a buffer tank, but there's something I don't understand. It makes sense that if all the circs are operating, they would not interfere with each other due to the oversized headers, but it seems that if P1, the boiler circ, were operating alone, it would have to draw at least some flow through the circuits of P2 and P3. The indirect tank and the circuits of P2/P3 share the same oversized headers, so why would P1 draw only from the indirect tank?

Presumably there's a lot more head loss in P2 and P3 to overcome, compared to the indirect tank, but is that enough?

Here;s the original article: https://www.pmmag.com/articles/101984-john-siegenthaler-a-synergistic-system

Would appreciate any enlightenment. Thanks.

• Member Posts: 7,013
I think that if you assume the check valves are spring checks and the pipes going into the tank are sized for < 4 ft/sec, there would be no ghost flow. There is just so little pressure drop over the few inches of pipe between the Tee and the tank.The minuscule amount of water that would take that path would not have enough force to open a swing check.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 55
So if I understand you, the diagram implies (but doesn't specify) that the check valves shown have an opening pressure that is greater than the expected pressure that P1 can exert through the common piping?

If so though, in such a working system, would you ever know if a small amount of fluid were making it through the check valves into the boiler circuit? It would be losing some efficiency at heating the indirect tank, in favor of heating the load unnecessarily, correct?
• Member Posts: 2,437
edited July 2020
dave123 said:

...but it seems that if P1, the boiler circ, were operating alone, it would have to draw at least some flow through the circuits of P2 and P3. The indirect tank and the circuits of P2/P3 share the same oversized headers, so why would P1 draw only from the indirect tank?

The check valves need P2 and/or P3 to operate in order to open the aforementioned check valves. So P1 path of least resistance is thru the tank, if P2 and P3 are not operational.

Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
• Member Posts: 2,437
The configuration of the Tee fittings also helps in the resistance. straight thru the tee is less resistance than taking the turn. Every little bit adds up. Selecting the proper check valve is important
Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
• Member Posts: 15,070
the concept of hydraulic separation is part of what is going on. properly sized that "header" has very little flow restriction. So the various flows can move inland out, and around inside that heater without inducing flow in the circuits not being pumped. this drawing shows what should be happening inside a separation pipe, chamber.

Almost impossible to get zero, but note this example with a 50 gpm flow, notice the flow velocity from top to bottom .44 fps.

A rule of thumb is 3X the diameter of the barrel to the pipe size.

Or calculate the maximum flow in that "header" section, use pipe flow tables to select a pipe size for under 2 fps at the combined flow rate in that section.

Check valves or flow checks can provide check protection against unwanted forward flow, and also against reverse flow conditions.

In some instances checks are needed on both supply and return to prevent two directional flow in a pipe, usually vertical runs, where the buoyancy of the hot water can use ghost flow.

The concept of gravity systems where hot water rises, cooler water falls
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 55
edited July 2020
Ok, now it makes sense.

It is an interesting arrangement, because it removes not only a separate buffer tank, but also the circulator needed to then pipe the indirect in parallel to the boiler circuit.

Thanks everyone.
• Member Posts: 15,070
Pros an cons to all the different methods. With reverse indirect you need to maintain the tank temperature sufficient for DHW
I know Thermo 2000 has a software simulation that shows required tank temperature for desired DHW. The tank models with more coils allow lower tank temperature, better mod con efficiency.
Make sure you have adequate tank insulation, some early models only had a 1/2 fiberglass wrap

It would be nice to see tanks with
R-15, 3” or more insulation
You can also add external wraps for more R.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream