Parallel Primary Secondary vs. Reverse Return Primary Secondary

hydronicsmike

Hello Guys,

Just looking for your feedback. I was always under the impression that Parallel Primary Secondary was the best Boiler piping method that results in the lowest stand-by losses, same return temperature to each boiler and propper flowrate across each boilers heat exchanger.

Now, in one of my recent schools, one of the guys claimed that Reverse Return Primary Secondary (as shown below with a Pump on each Boiler) was the better way to go, where I don't really see the benefit of using this piping method over Parallel Primary Secondary. He claimed that using Reverse Return Primary Secondary would cause the same pressure drop across each boiler due to equal length runs, where Parallel Primary Secondary does not.

I could see his point there, but even if piped in Parallel Primary Secondary, the piping size to each boiler and the header above should always reflect the flowrate (and pressure drop) it has to accomodate and as long as this is kept in mind there should be no difference between one way or another. Except for the Reverse Return requires more piping and two more fittings.

Is there any other reason why you may chose to go one way over the other? If so, pelase let me know.

I see some of the wall-hung modulating boiler manufacturers show the Reverse Return Primary Secondary Piping method, but the ones I asked about it, couldn't give me a good answer as to why they show it that way. what do you guys think?

Thanks,

Mike

Unknown

Both right

Since all three are pumped, it isn't really a balance problem.

I see it as an "equal head" problem. He's right in that the head will be different. You're right in that the difference in each boiler's head is so small that it isn't worth the time it takes to figure it out.

My $0.02

Noel

dconnors

Reverse return

i wouldnt bother unless we are talking about using one circ for all three boilers.

Unknown

I also agree

Since each boiler is pumped and its pressure drop is somewhat negligable, either way should perform the same provided the common piping is sized orrectly. I also agree that if a common pump was used then Reverse Return may be a better way, but with that kind of setup we would end up with a boiler raising temperature and the other two giving up temperature where you need it the least....the boiler room. Hope this helps.

Glenn

hr

About all the reverse

return will do is cost you more pipe, fittings, and insulation Unless the boilers are far apart from one another KISS

hot rod

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hydronicsmike

thanks guys (nm)

Boilerpro_3

Both can be bad news depeding on the boilers used

Let's say you have good old iron boilers installed and the lead boiler is already running with a 180F temp with the system. The other boilers are sitting there at room temp...say 85F. Then the load increases and the pump kicks on for an off boiler. That pump dumps at full flow 180F water into an 85F casting, shocking it.
I've seen several cracked cast iron modulars where this probably was the chief cause.

Each boiler needs it own shock protection or must be kept at reasonable temp to prevent this. Burnham now has brought out a control to do this in response to the problem. I generally use standing pilot boilers with stack dampers that keep the boilers up near 120F when idle or I drill a small hole in each boiler's check valve to allow a small amount of flow to keep off boilers warm.

Biolerpro