Boiler Heating Question -- Low/No Flow

markHVAC0302

Have a boiler question that I need some help with. This is an older system that has worked perfectly for years, so no design issues to speak of really. Boilers are set up with primary and secondary loops, with lead/lag pumping stations. There's no issue with the primary pumps for the boiler loop. The secondary loop has quite a bit of piping and feeds dual hydronic coils for 7 large AHU's throughout the building.The pumps for the secondary loop are on drives and are lead/lag.
Pump differential pressure at 60hz is 25 psi, and pump discharge pressure is 70 psi. I am not sure of the coil pressure drops on the AHU coils. I can not find any data for equipment as old as this is, but regardless, I only have 10 psi pressure at the farthest AHU hot water supply line feeding that air handler. Also have 10 psi on the return after the coil for that same AHU, so no flow obviously...
That AHU has had no work done to it since it last heated normally, and pump discharge circuit setters have been set since install YEARS AGO and have never been touched so system is balanced. I have searched and searched for a booster pump that may have failed or something for that AHU branch, but can not find anything. I don't know how to calculate required pump head, or any of that other fancy stuff, but everything else has proper flow and is heating normally, except this one AHU and I feel like 10 PSI is probably too low to overcome the coil Pdrop.

PS: All air has been bled from the system and I can get both coils completely hot by closing the return, leaving supply open, and draining water out to a floor drain by a valve just after the coil outlets.

PSS: Newer to commercial and the larger systems, so please be gentle in your comments. If you need any further information, I can get it.

I am at a complete loss as what to do next.... Any help is appreciated.

EBEBRATT-Ed

Any strainers at the air handlers that could have plugged up over the years?

Your right with no PD across the coils you have little or no flow.

Here's the issue you have 70psi at the pump discharge and only 10psi at the farthest coil.

That's a lot of pressure drop.

I would follow the supply line back from the coil that doesn't work check for plugged strainers, closed or partially closed valves and control valves.

Maybe you have other air handlers between the pumps and the 1 that doesn't work where you can stick some gauges along the way. And make sure the gauges on the system are good they could be plugged up and lying to you.

Are the control valve on these AHUs 2 way valves or three way valves?

Are there any 3 way valves on the loop that could be bypassing?

markHVAC0302

They are three-way modulating steppers. I have checked operation of the valve at the AHU and it is full stem up and working normally.

Zman

Have you tracked down the curves for the circs to see what GPM coordinates with the delta P you are seeing?
Do the circs have filters on the intake, They will look like a plate on the suction side with bolts that can be removed.
How do the circs sound?
Are all gauges you are reading trustworthy?
Pictures are always helpful.

markHVAC0302

I am thinking supply strainer plugged as well.

markHVAC0302

@Zman I am not thinking an issue with the pumps because they are providing hot water to the rest of the building just fine. There are no suction strainers for the pumps but there are strainers at the AHU's.

I may have a plugged suction strainer at that AHU now that i think of all the symptoms.

Zman

So your heat distribution is good?
How is the temp delta t?
Do you trust the gauges?

markHVAC0302

I used pete's port gauges of my own and used the same gauge on each test, so I think the gauges are solid for the tests at the AHU. I did use the fixed gauges on the pump differential measurements, but when the pump goes off, they both equalize, so I at least know they aren't completely broken or plugged up, and both pumps in the set show same differential pressure on different gauge sets.

markHVAC0302

DeltaT on water for other AHU's that are heating normally is around 9 degrees currently.

markHVAC0302

This seems to be isolated to this AHU solely. It is the furthest one from the loop pumps. I did not check supply pressure at the other AHU's but I would suspect them to be higher anyway due to head loss from the piping and fittings since the distance from the pump is longer.

EBEBRATT-Ed

@markHVAC0302

If the pipe is sized right the inlet pressure at the ahus shouldn't vary that much. not 70 discharge and 10 at the far end. something seems restricted

markHVAC0302

@EBEBRATT-Ed I agree with that. I am thinking the supply strainer for that AHU is plugged. I couldn't get it loose today and will need to try to break it open Monday.

The only thing that had me puzzled was that a small unit heater (in the AHU room to keep that room tempered) was having the same issue and it was piped BEFORE the supply strainer to that AHU with the heat issue. It also only had 10 PSI inlet pressure and 10 PSI outlet.

That makes me think that maybe the restriction is between the AHU with the no-heat issue, and the next to last AHU that is working as expected in the building.

Sounds like I may be in for a day full of removing ceiling tiles searching for stuff. Thanks for the help!!!

EBEBRATT-Ed

@markHVAC0302

you could be right. my only other thought is some type of bypass bleeding water from supply to return. did this system have VFDs from day 1? Some older systems without VFDs would have a piped bypass with a balancing valve in it that if still there someone could have opened it.

You mentioned 70 psi discharge with 25 psi diff. Do you really have 45psi on the pump suction? Like are you on top of an 80' building??

Could be a control valve broken internally

I think I would move down the line with a pressure gauge until I found the restriction

offdutytech

Is the system controlled by a DDC/BMS system or pneumatic? Do you have VFDs on the secondary pumps controlled by diff pressure? 
If it's a BMS system and suspect a flow issue command the other AHU valves to the closed position leaving your problem AHU valve wide open. If you have VFDs they should back down to the system set point. This would prove that the controls are working for the most part. Then try and find the diff pressure transducer and blow it down. Depending on how it's piped that can be air locked or the pick up tubes could be filled with sludge. When I installed DDC systems it was always a battle not to have the pipe contractor put those taps on the top of the supply and return. Once you can verify controls work. You can trace down pipe. I'm guessing a clogged strainer at AHU. If the AHU coil is piped with drains as most are you can back flush the coil and blow down the strainer. 
Its also possible you may have a " balancing" valve that might me either a fixed circuit setter or a two way control valve that is open in that branch of piping which may shoot you in the foot with diff pressure. Any VAV boxes with hot water heat? 
Pic of the AHU coil might be helpful.