Primary/Secondary to Parallel Reverse Return

Alex Ma

I want to design a pretty large system. The main will be in Parallel Reverse Return mode and I want to hook up multiple boilers to the main in Primary/Secondary mode. The boilers will be the secondary loop and main loop is parallel reverse return. Is it going to work? any opinion?

Mark Eatherton1

Absolutley...

That's one of the neat things about P/S piping. You can have "input" secondaries and "output" secondaries. I usually start with a primary loop in the mech room, with secondary inputs into the loop, and then have a set of output tees going to the individual loads. In your case, the space heating loop could be the primary, and the boilers set up as secondary inputs.

Here are some examples of what you can do, and the possibilites are endless. A note of caution, this is just about the ONLY time that I'd pump towards the boilers. Otherwise, I'd pump away.

Enjoy!

ME

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Tom Anderson

Why Reverse Return Loop?

The primary/secondary sounds fine, but why use reverse return loop distribution?

Unless the piping layout is for a square or rectangular building where the piping will be run along the perimeter, a direct return would be less expensive to install and work just as well. After all, the purpose of a reverse return loop is to equalize differential preesure across each terminal control valve. So what is really happening is is buying and installing a lot of avoidable piping for balancing.

Reverse return loop piping was developed as a way to equalize flows, BEFORE manual balance valves were invented.

Mark J Strawcutter

question about valves

what is the purpose of the valve you put on the primary in between each pair of secondary supply/return Ts?

Jim N.

Valves in primary line

High Mark, haven't seen you since the Hyatt.

I'd recommend you leave out the valves between the supply and return on the secondary loops, particularly when you have multiple zones on the secondary as in your first illustration. The purpose of the short pipe section (less than 12" or 4 pipe diam) is to ensure that each of the secondary pumps sees a constant suction head. with the balancing valve there the head seen by the secondary pumps will vary with the flow thru the valve whenever a pump turns on or off (or a valve out in the system).

I'd also recommend the secondary boiler loop shown by Weil McLain in their manual (Don't have ACAD on this machine or I'd try to post it from memory, probably a bad idea) It ensures that both boilers see the same return temperature whereas this system the second boiler will see the mixed water output of the first and the system return.

Boilerpro

hey mark

I am always a little concerned about pumping toward the boiler in this application, because you could end up popping the relief valve. The pressure the pump creates is added to the fill pressure and might be just enough to pop the relief valve when the system is operating near its high temp. Not very likely, but just something to think about. Also, I'm not sure if you ever caught any of my discussion with Glenn Stanton about the potential to "hot shock" the second stage boiler with this piping layout. This occurs when the system is up to full temp., let's say 170F, an increase in load occurs, and the second stage bioler is at room temp, say 70F. The second stage boiler pump kicks in at full volume and dumps 170F water into a 70F boiler. KA BLAM....you've go 100f differential across the boiler at full flow. If memory serves me right, Glenn said Burnham engineers are presently working on a solution to this problem. Just a word of warning, I've seen several 50 year cast iron boilers dead in little as 7 years with this piping layout.

Boilerpro

Steve Eayrs

secondary input loop not pumped away?

Was wondering why you would not want to pump away on the secondary input loop?
Also a comment was made about high temp. boiler shock, when the second boiler kicks in and/or you switch the lead boiler. On bigger cast iron boilers do you let the boilers go down that cold that their is a problems with shock? How about maintaining a boiler min. temp (triple aquastats?), even when its not the lead boiler, or indoor outdoor reset controls which would allow lower lead boiler temp.? I realize there is some extra standby heat loss (both residual and stack), when running the boilers hotter, but don't know if there is an option, when piped like drawn.

Steve

Jim N.

pumping toward the boilers

I don't really have a problem pumping toward the boilers in this situation. What "pumping away" really means is pumping away from a constant pressure point established by the expansion tank. If the system is close coupled the location of the expansion tank will establish that satisfactorily.

That said, it is even more justification for my previous post about eliminating the valves in the main line between the secondary input loop supply and return. These valves will cause a pressure drop which will defeat the common constant pressure point.

I agree with your comments about this piping system and thermal shock of the second boiler. Again I prefer the Weil-Mclain method. (Oh to have my library)

Boilerpro

I believe the Weil-Mclain mehtod has the same problem

they still let off boilers get cold and then when they are cycled on get shock from hot water entering at full flow. The solution I am using now is to use standing pilot boilers with stack dampers (the boiler usually stays about 120F from the heat of the pilot), or, with electronic ignition, drilling a small hole in each of the boilers check valves to allow a small amount of water to circulate from the on boiler to the off. I just did this today and hope to have the job finished in a few weeks...I plan to post pics and I'l tell you how it works, then. You could also constantly circulate all the boilers like Hydrothem does, I think they figured this stuff out long ago since they have been doing modulars since the 50S's.

Boilerpro

Jim N.

Problem with all of those methods is you still have the boiler standby losses. On a large system I like to use a 3 way in the boiler loop. On startup circulate the boiler before firing and let the boiler come up to temperature before beginning to take load.

Mark Eatherton1

Easier to purge...

and if a pump fails on a weekend, you could choke the ball valve and provide SOME circulation. Primarily ease of purging.

ME

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Mark Eatherton1

See my post above Jim...

these valves are only used during fill and purge. After that we remove the handles with the valves wide open. We "hide" the valve handles under the cover of the boiler. No one EVER looks there:-)

What Hyatt are you referring to?

ME

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Mark Eatherton1

Proper pump sizing ...

is critical in this application, and yes, you are correct that if the system height and fill pressure are such that the operating pressure is real close to the relief threshold, you COULD have a problem. Not likely, but possible.

I've never had a problem with hot thermal shocking of a boiler, although I can see where it could happen. What would you do to avoid it?

Always willing to learn.

ME

ME

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Mark Eatherton1

The PONPC...

is the primary loop to which the expansion tank is attached. If you are pumping away from the boiler, you'd be pumping towards the PONPC, and the pump would generate negative pressures across the boiler, which COULD cause (high pressure drop boilers) the water to flash to steam and cause noise and other havoc with the system.

As I previously stated, this is a rare moment when pumping towards the boiler is acceptible. Pumping away from the point of no pressure change is more important.

ME

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Mark Eatherton1

Reverse return versus Direct return piping.

Tom, your assertions are correct, however, and this is a BIG however, all it takes is some young unknowing hydronician to come in and open up all your balanced valves, and viola, you are no longer in balance, and you furthest circuits suffer. For the little extra cost associated with the reverse return method, the system is pretty much automatically self balancing, and you don't have to worry about someone throwing the whole system out of whack.

Been there and done that too many times to remember.

For me and my money, I'd rather go reverse return and eliminate any future balancing problems for once and for all. Mo bettah flow and even comfort.

JMHO

ME

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Jim N.

Actually it was the Sheraton then, now the Tech Center Hyatt. I and Don Croy were CAER Engineers. We proposed on some energy studies with you and your brother.

Dana Zaichkin

Motorized mix valves

Do you have any "druthers" on selection of motorized mix valves for constant circ/reset on the load loops (ie your #5 photo) - any that control particularly well when matched with a tekmar?

Thanks & Best Regards

Dana

Jim N.

Here is a link to Weil-Mclains piping diagram, page 11.

http://www.weil-mclain.com/FTP/88_Boiler_Manuals/88Manual.pdf

Mark Eatherton1

Oh sure yeahh...

Now I remember. My dad used to do one of your plumbing, Don I think. Good to see ya!

ME

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Boilerpro

It's more common than you think...

It happens with almost all step fired modular/multiple boiler setups unless you pump all boilers continously or have very low system supply temps (outdoor reset helps). The solutions in my post above seem to work, I trying out the second one this week. You could also install a theromostatic mixing valve on the return of each boiler piped p/s. It will not open up the return line into the boiler unless the boiler has brought the bypass up to temp...Used this before too. Or go to boilers that are shock resistant... copper tube and others.

Boilerpro

Boilerpro

Done that too with ESBE thermix

But I do like standing pilots because they keep the boilers nice and dry and clean over the summer..especially when I am using cast iron. Usually lose less than 1% seasonal efficiency, and I'm sure I gain more than that back by the heat exchangers staying very clean. Checked on boilers a few months back that have 2 and 3 years of use and between the Esbe on the boiler return with the boiler loop piped p/s to the system (converted gravity) loop and the standing pilot, there was not a spot of rust in the burner tray. REAL NICE.

Boilerpro

trbodden

where is drawing

where are the drawings associated with this post



http://www.heatinghelp.com/forum-reply/740081/Primary-Secondary-to-Parallel-Reverse-Return

Mark Eatherton

Jeez.....

That post goes W A Y back. I seriously doubt you will find it here, and if I had one back then, not sure I'd still have it now. THat looks to be 2 or 3 computers ago,



What in particular are you looking for?



ME



PS, Notice the 1's in my posting time. Nothing but ones......:-)

World Plumber

Valves

If you use full-port valves, shouldn't that be the same resistance as a piece of pipe? Help clarify my thinking. It sure is nice to be able to isolate all the loops to purge the system! I learned that the hard way.



MC 

Mark Eatherton

Yes Mark...

Full port ball valves make the differential between the tweener tees zero.



Be sure and remove the handles and hide them under the cover of the boiler where no one will find them.



ME

hot_rod

what about a separator?

instead of a primary loop.

Mark Eatherton

That'll work too...

See how far we've come since back in ot 2 :-)



Hydronics IS progressive ;-)



ME