Antonette's Lessons Learned Story #2: Murphy's law

VAV story:
Recently, on a routine Variable Air Volume (VAV) system complaint we encountered an interesting investigation because as Murphy's Law would have it, many simultaneous things were going on that were hiding the major culprit. The system was working well when a couple of occupied spaces along the exterior wall progressively climbed in temperature as the day progressed. Not too troublesome, it could be any number of situations easily corrected.

So off we go to verify the space conditions. The space is 80F and climbing. You really have to commend these people for not complaining, in their desire to be a gracious tenant.

System Checkout:
The first thing is to verify there is sufficient cool air. At this point, the room thermostat is 10 degrees above set point and trying to bring the space temperature down, so I'm looking for cold air of 55F to dump into the room and plenty of it. But it is warm. Not good. Problem1.

I check the heating coil on the VAV unit, suspecting the three-way mixing valve is stuck and the coil is hot. Being 5'2" and standing on my ladder in an OSHA approved manner, I am able to reach the bottom of the coil and feel the hot water supply line to the coil and it is cool.

I leave to check the computer monitor for the air handling unit discharge temperature. It's a little higher than I would expect but it is reacting to its program parameters utilizing the outside air for free cooling. It is riding on the cusp and caught between the program dead bands so I lower the control set point to encourage the unit to utilize a DX compressor and lower the discharge air. All other system parameters look o.k. except to verify the warm space temperature. At this point the room starts to come back but too slow for our liking, and we are suspecting Murphy's Law is still at work. After another hour goes by to let the room stabilize, we are still not satisfied and Jimmy of Total Control Concepts arrives onsite. He is checking the space with my temperature gun, a man with a toy, great zeal and a mission. He notes the baseboard is doing an above average job of sweltering the room. A look at system drawings indicates that this baseboard zone is controlled by the thermostat located in a room down the hall behind a locked door. A key later and wa-la, we enter into a very empty dark room. So just our luck a slave room to a VAV box being controlled by a heavily utilized room is calling for lots of cold air to this unoccupied, (no people, or computer equipment heat load) dark room (no lighting heat load). To re-clarify, this artic room for added irony contains the thermostat that is driving the exterior baseboard heat, which according to the room conditions needs all the heat it can get, much to the chagrin of the other users. So we depress this particular zone set point and once again we leave to allow the system to stabilize. But while we are waiting, I go to a meeting, and Jimmy looks into a recent development in our space. Annalisa is warm and the back rooms are warm so they make use of the time, and thankfully investigate this new development. Again, in the back of my sleep deprived mind I'm thinking something is up.

And now part two unfolds. Jimmy and Annalisa, trek off to check the thermostats, the VAV box controls, and the coils. The coil is also cool at the bottom supply feed, but with more due diligence, perhaps a taller ladder, they determine the coil on top with the return piping on top is hotter than he__. So I'm asked to verify and do sanity check. Is this line piped correctly to the bottom feed? And for those of you who know me your probably wondering where are her famous labels!!!!! Unheard of, there is no clear indication of utility pipe description and arrows. Well, a condition that will obviously be corrected. Have marker will travel!!! And so they torture me with "well oh mighty one what's that all about"?
Upon checking the main hot water supply and return lines, we found the supply lines were colder than the return lines. Since the system was recently drained and refilled, the lines were checked for closed isolation valves, but all valves were open. No supply pressure, lets check the boiler room, so they we are, bumping the motor and watching the rotation and yes everything looks good except we aren't really buying the pump is pumping, but guess what, yea, no discharge pressure gauges; another item for rectification. So Jimmy valves off the suspecting pump-less wonder and he activates the spare pump. And guess what, yep, the hot water supply goes to the bottom of the coil and the control valve works and the cold air enter the spaces and life is good. Now if you were like me, either sleep deprived or confused you are wondering what just happened.

When the pump failed there was no supply pressure and so the hot water in the common return filled the upper portion of the cooling coil. Even when the VAV box controls were in cooling mode, the coil was inadvertently partially warm. Combined with the overzealous baseboard heat and the tempered discharge air the space could not maintain the space temperature conditions.

After a call to the mechanical contractor J. Moore who arrived in a jiff and confirmed the coupling was Capluey, a highly technical term for the impeller separated from the shaft, the problem was identified, and repair is under way.

Lesson learned. MUST HAVE PUMP SUPPLY AND DISCHARGE PRESSURE GAGES!!!!! It would have alerted us to the problem during a routine inspection in the beginning of the problem investigation. A pump motor status using a current switch as we have just shown doesn't give true indication of flow verification. Differential pressure switches, wired to the monitoring system would have been a better application.