Help me understand: send/return loop

JohnnyCB

Hi All,



Since my old oil boiler was just replaced with a new mod/con gas boiler I thought I'd try to do my homework and learn how all this works.



One thing I'm confused about is that the installers placed a line from the send side to the return side.  There's a ball valve (half open) on this line, presumably to limit flow rate.  There's also a circulator there between the ball valve and the large return line from my three zones.   There are other circs too - three Taco 007s - one for each zone also on the return side.



My question is this:  This seems to me like it would be done to raise the temperature of the return water.  Wouldn't this be counter-productive to the condensing function?  I can tell you that the send and return pipes for the zones are both rocket hot (I'd expect the return to be cooler enough to "notice by hand" - but maybe not).



So, what the heck does this piping circuit really do?  Pic below.



Thanks in advance,



-JohnnyCB

Al Letellier_21

send return loop

Johnny, its hard to tell from the angle of the photo but most mod/con boilers are piped primary/secondary as it looks like yours is. The horizontal pipe appears to be the heating loop connecting the returns to the supplies (send) loop and the boiler is taking water out of that loop and heating it so it doesn't have to heat all the water in the system, it just maintains design loop temp. Not sure why the restricting ball valve but it probably is used to restrict flow. You should talk with the installer about why he put it there and why it is adjusted where it is.

It most likely is not mixing with the return piping as the flow would appear to be from right to left in the main and the pump is taking water out and sending it thru the boiler and back into the main. More photos would be necessary to verify my assumptions which I never like to do but unless you are having operational issues, I would not be too concerned. Congratulations for wanting to understand your system better as most people just ignore it as long as it is working. Call your installer. If he cares enough to use Viessmann, he most likely will be happy to help you understand the how and why of his work.

Good luck.

JohnnyCB

I think I get it....

Thanks Al!



Since you've given me the correct terminology for this setup I've been able to research it a bit.



So if I understand it correctly, the short parallel loop around the boiler helps maintain a constant loop temp and flow rate without moving all the water in the system.  Most of the examples I found had the circulator pushing from the supply side but I imagine since my zone circs are on the return side this one had to be there to, pulling into into the return and keeping the water going in the same direction in both loops.



Am I correct in assuming that when that short loop temp falls below a certain temp the boiler itself both fires and switches that circ on?  Does it behave differently if it is getting a heat call from the heating zones?  i.e. just trying to satisfy the demand and *not* switching the short loop's circ on?



Thanks again,



-JohnnyCB

 

icesailor

Return Loop

That boiler requires from Veissman that the boiler pump be piped in such a way that it has full flow through the boiler. The system should be piped with closely spaced tees or a Hydraulic Separator. There are many drawings in the installation. I looked but didn't see one with a bypass valve, 1/2 open on it. Maybe the installer knows something that I and Veissman don't know.  But the circulator definitely does belong pushing into the return of the boiler.

icesailor

Return Loop

That boiler requires from Veissman that the boiler pump be piped in such a way that it has full flow through the boiler. The system should be piped with closely spaced tees or a Hydraulic Separator. There are many drawings in the installation. I looked but didn't see one with a bypass valve, 1/2 open on it. Maybe the installer knows something that I and Veissman don't know.  But the circulator definitely does belong pushing into the return of the boiler.

NYplumber

Output of the boiler

As habit we install the boiler pump on the output of the boiler especially when running loop pressures near 20psi in taller homes. The boiler pump thus doesn't add its pressure to the 20psi in the loop. This keeps the pressure relief valve farther from its 30psi breaking point.

icesailor

Pumps on return

NYP,

You better check the Veissman schematics for Vitodens 100 and 200's. They show the pump on the return, pumping into the boiler. They want the pump between the closely spaced tees and the boiler for maximum flow through the boiler.

JohnnyCB

Just read...

...the install manual the installer left with the boiler.



It would seem my system is piped exactly like "System Layout 3" in the instructions, and it definitely says right below the diagram "Primary Pump L must pump into the boiler (as illustrated)"



As pointed out earlier though I can't find a reason the ball valve would be there.



I'll email the installer and ask him what it's for.



Thanks everyone for all the great info so far!



-JohnnyCB

Jean-David Beyer

Closely spaced Ts

I am just a homeowner with a mod|con boiler who has actually read the Installation manual. The original contractor read only part of the manual, unfortunately, which is why he is my former contractor.



My boiler must be piped primary-secondary. They have about 8 examples on how to pipe it, and all are primary-secondary with the boiler circulator in the return to the boiler. Mine has closely spaced Ts, not a low loss header, but the same principle applies. The idea of closely spaced Ts is to have no pressure drop between the Ts, so as to have the flow in each loop uncoupled from the others; allowing pressure drop defeats the purpose of primary-secondary piping.. The instructions with mine specify that they should be spaced not more than 12 inches apart (in one-inch piping). My contractor used 1 1/4 inch pipe up there, and spaced them 5 inches apart. I read in John Siegenthaler's book that they should be not more than 4 pipe diameters apart; mine are exactly that. Putting a valve in between, even if it is fully open, just adds to the pressure drop, and closing the valve part way increases it still further. My guess is that the installer did not know what he was doing.

JohnnyCB

Searching for clues...

I've been doing some googling to try to find an example anywhere of why a ball valve would be placed in the primary loop.



I was actually able to find numerous examples citing that it made purging easier - but that it had to be a full port valve to avoid a pressure drop in the primary when open.



That might explain why it's there in my case - but not why it's half-open.



I just asked my installer.  I'll report on his response later.



-JohnnyCB

JohnnyCB

The response was...

...that they usually leave it half-closed to make purging easier and because they feel it contributes to a performance improvement but that I could certainly leave it fully opened if I wanted to.



I opened it.



-JohnnyCB

Gordan

Good on ya.

What I heard was "duh, we forgot to open it back up, but darned if we're going to admit it."