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Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

(OP)
Hi All. First time posting and looking forward to reading more on this valuable site.

Here is my current problem. This 5 story 5 family building with 5 bathrooms, 5 kitchen sinks and dishwashers needed to have a Gould Aquaboost II installed 7 years ago because the City water pressure was only 32 psi. This 1HP HMS pump (model 2HM1E2D) variable speed constant pressure pump is set to give me 50 PSI at the pump outlet.

We just changed our boiler out and have a new steam boiler with a power burner and a hot water coil. The attached drawing shows the domestic hot water board mixing panel. A circulator was installed as you can see in the pictures. It is a Bell and Gossett NBF-25 103418LF1M61. I can supply pump curves. The circulator pump has 3 speeds and it was left at speed two in the beginning. What we noticed was that for some reason, on some occasions ( which I am trying to pinpoint what is happening in the system when we see this) the pressure gauges around the mixing valve , p2, p4 and p5 ( but not p3...probablyt because of check valve. would go to 100psi on the gauges.... scary! since they looked pegged. When hot water was used the pressure would get immediately releaved and drop back down to 50 ish psi.

The guy who built the board came in and dropped the speed to 1 on the circulator. Now I see it goes to around 70 is PSI on those three gauges.

Can anyone see what in this system could be causing that?? Is the circulator the problem? Or how the system is piped? The guy who built the board would leave some of the valves cracked instead of open as you can see in the picture of the board.

I am very concerned that this may open up some joints in the hot water system running up to the apartments.... any suggestions or ideas are very much appreciated!

RE: Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

Nothing attached.

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RE: Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

OK,

Not the simplest of system but some points for you.

The recirc pump is vey low power and incapable of pumping your system up to any great extent. The difference between point 1 and 2 is about 2 psi so itys nothing to do with the pump.

I don't understand what the valve is doing D/s of the pump which connects the presumably hot recirc to the incoming cold supply. Should that be normally closed?

Anyway you kind of answer your own question "When hot water was used the pressure would get immediately releaved and drop back down to 50 ish psi" and "Could this just be thermal expansion and need an expansion tank? It seems like a significant delta P..."

If you have no expansion tank on a water system which is subject to temperature rise the pressure impact is very significant.

On a sealed system you can see something like 50-60 psi per deg C. My guess is that your system isn't really sealed so the pressure rise is limited as the volume to relieve the pressure is very small.

If you insert either a pressure relief valve set to 60 or 70 psi then it doesn't matter or a sealed expansion tank is much better.

Why the minor change in flow from the re-circ pump is a bit of a mystery unless he did something else at the same time.

I don't understand why the recirc line mixes with the cold supply before going into your boiler. Does the mixing valve discharge connect to the domestic return?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

I think you definitely need an expansion tank in your system. When you have a water heater full of cold water after filling a bath or a long shower, there will be a great deal of expansion as the water is reheated. The tank will also keep the pressure booster pump from running continuously or short cycling.

RE: Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

(OP)
Thank you all for great replies. I posted a picture of the mixing valve board so you can see the position they left the valves in. PS. I have the drain valve in the wrong position after the recric pump, it is like the drain on the other side... I am going to run a series of tests tomorrow and report back.

http://files.engineering.com/getfile.aspx?folder=8...

RE: Disimilar Pumps in Parallel, Pressure problem. Pumps or piping issue?

It's taken me a while to figure this one out, but what I think is happening is that your issue is at no flow out from the loop mixing valve discharge then returning to domestic return.

Now at no flow out the flw path is basically through the pump, through the NRV and then vertically up the bypass past P4 and through your mixing valve and back up the riser.

If the temperature isn't high enough then a bit will dribble through the water coil and heat up the loop up to your 60C. I don't know what flow state you took the photo but it has a 50C return temperature.

Hence at no flow out, your loop will gradually heat up and hence will create additional pressure given you have no expansion tank.

Now why it does it less at the original setting I'm not sure, but it could be that the lower flow / head isn't enough to force flow through the heating coil and maybe that partly open return valve has something to do with it. That valve should be closed - it's not doing anything for you other than perhaps creating a local hot loop. Just add an expansion vessel.

Either way this is NOT a dissimilar pumps in parallel issue, it's a lack of expansion tank issue.

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Also: If you get a response it's polite to respond to it.

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