Hydronic pressure loss mystery

[ALD Mechanical]

Hey all, I’m new to this forum but looking for some help. I am the Chief Operating Engineer in a large multi-building complex. We are running a hydronic hot water heating system. The boiler room is located on the 6th floor level with 2 floors above. Heating water from the boilers flow from the boiler room to 4 buildings via 2 - 6” steel mains. Twice in the past year we lost boiler pressure to the point of tripping the low water cut off devices (keeping in mind 2 floors of head pressure above). Initially I was thinking major leak both times, however, after inspection all areas of the building, no leaks were found. I have tested the expansion tank located in the boiler room and it tested good. Within a half hour both times, the pressure returned to the system. As I said, we inspected all mechanical equipment for leaks and none were found. In each of the outlying buildings, heat is transferred through plate heat exchangers for glycol loops feeding the air handling unit coils. The outdoor temps at the time were -20C, but we see about 5 weeks of -50C each year. There have been no issues during the coldest or warmest times of the year.

Any suggestions why pressure would drop and then come back? I’m stumped at this point.

ALD

 

[EnergyProfessional]

Where is water feed into the system (height) and what pressure is that set at? what are the pressures at the expansion tank and where is that located height-wise? Is there enough positive pressure at the highest location of the system where the de-aerators are? Are yo metering make-up water and would know if the system would use up water? Does the RPVZ work properly? Is the expansion tank located upstream of pump (that also is where the water make-up should be)? Is the tank large enough and is it a bladder tank or open?

Is that a new problem, or has it been there "forever"? Could be design issue, could be some devices not working properly.

 

[ALD Mechanical]

We keep the water feed isolated to the system but it is located at the inlet to the expansion tank. Static pressure at the boilers usually sits at 25PSI. Expansion tank pressure is the same as the boiler pressure, 25PSI. The very top of the system is 15PSI, air separators/eliminators are open and functional, however we don’t get anything out of them. The building is 20+yrs old and we have only had this problem twice in the history of the building (we keep history through a computerized maintenance management system and building log books) and Thea happened in the last year. Expansion tank is located before primary boiler and secondary heating pumps. We did confirm a drop in pressure in our other buildings as well through our building automation trend data as we as a drop in temp after the pressure drop due to the boilers failing. As a test when the pressure was rapidly dropping I isolated the expansion tank and pressurized the system to 20PSI which only ended up taking 10gal of water. With the expansion tank isolated, the pressure continued to drop to 0PSI within 2.5 minutes. As the pressure had continued to drop I ruled out the expansion tank and re-opened it. We haven’t seen any pressure fluctuations between +20C outdoor temp to -50C outdoor temp which also leads me to the conclusion that the expansion tank is good. These incidents were both one offs, but I am concerned there may be a larger underlying problem here. The system pressure trend on the automation showed a supply pressure out of the boiler room of 30Psi before the pressure loss, we added the 10gal and after 30 minutes the pressure stabilized to 39PSI. The expansion tank is an Extrol bladder style tank.

As I said the concern here is that we aren’t seeing constant pressure fluctuation as you would see with a failed or under pressurized expansion tank.

 

[LittleInch]

So something transitory is happening.

The question is what and whether your system can recognise it.

The only thing coming to mind is that somehow a very cold room was suddenly connected to the system and dumps a lot of cold water into the loop. So pressure drops as the water is cold then slowly returns as it moves and warms up?

We can't see how complex your system is so a diagram would be very useful otherwise we're operating blind.

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

 

[ALD Mechanical]

Good point Littleinch, I will see what I can dig up for some mechanical drawings. The main heating loop is constantly flowing to provide heat to the heat exchangers, radiant and re-reheat systems, however, I do recall seeing 2 in-floor loops as well as a couple heat trace loops for keeping the plumbing vents thawed. I don’t suspect a cold room due to alarms being set up through the automation system.

 

[GT-EGR]

Twice in the past year we lost boiler pressure to the point of tripping the low water cut off devices - have you ruled out that the low after cutoff is functioning properly?

I assume this 6th floor boiler is above a significant volume of the building loops it feeds correct? You’d have to lose and then reclaim a huge amount of water to some dry source in the loop below? Maybe one loop was fully isolated from your boilers but slow leaked empty and then a valve was opened and filling that whole empty volume instantaneously stole a lot of fluid volume that had to be made up.

Any recollection of the timing or history of these events and what your maintenance team was up to. Were you transitioning between seasons, or servicing something that required draindown, or maybe opened up a portion of the system that had been isolated?

 

[EnergyProfessional]

what type of low-water-cutoff? If they are the electric type (measuring if there is water), that would have meant water actually was gone. this seems unlikely considering you have 2 stories of system above the boilers. is the expansion tank set up and sized properly? the air side of the bladder also needs to be pressurized to the correct pressure, which doesn't always happen. they typically come -re-charged at 12 psi only.

Outside temperatures are not relevant for the expansion tank size. The lowest and highest water temperature are the temps of concern.

 

[lilliput1]

Did the pressure drop when the pumps were turned on?

 

[FacEngrPE]

If you "never get air out of your air vents at the boilers", air (non condense-ables) are likely building up somewhere else. It may be that hydraulic conditions to bring the trapped air to the boiler room only occur occasionally.
Suggestion - locate all high points, if automatic vents are installed periodic inspection verifying function might be needed. If only manual vents are present, scheduled venting is recommended (probably annual, but it depends on the amount of makeup water added).