Just before the pandemic, FM-PRO was wrapping up a large boiler replacement project at UHN’s Bickle Centre when commissioning revealed some very strange issues…
For context, I will provide a bit of background information on the boiler replacement. As part of the project, Energy and Environment worked closely with the project team to ensure various efficiency goals were met, including high efficiency boilers, sub-metering of gas and electricity, new heating water pumps with variable frequency drives, and connection of all major equipment to the building automation system (BAS). The project was expected to provide many benefits including renewal of equipment, 90tons of CO2 emissions reductions, 184,000kWh saved, and about $35,000 of utility cost savings. Another project happening in parallel to replace all the air handling units in the facility also helped with data and information. Now on to the mystery…
After the new boiler plant was up and running, the commissioning agent found that the heating water pumps could never meet the differential pressure setpoint, even during low load periods and with both pumps running full blast. In fact, even turning off a pump yielded no change to the differential pressure (image below). As a temporary solution, we manually set the pump speed to stabilize the system, but this was defeating the purpose of our variable speed design. This situation reminded me of a previous story from Bickle facilities about the old pumps burning out and having to be throttled back. Any ideas yet?
New heating pumps P10 and P11
Although I suspected it was a larger issue due to the huge amount of bypass flow generated by the two pumps, I wanted to rule out some of the potential easy fixes first:
- we brought a BAS tech on site to verify the DP sensors were working correctly
- checked to make sure the minimum flow bypass valve wasn’t installed backwards or leaking
- checked to make sure the AHU valves weren’t leaking
Were these items on your list to check?
With heating season in full swing, facilities began to note that they were observing reversed flow on some AHU coils. Armed with a infrared temperature gun, a camera, and with the benefit of more information on the BAS and new gauges on all the AHUs, I went on site to investigate further and things started to become a little bit clearer.
The image below from a day in early February showed a supply temperature to the heating coil of 22C and a return temperature of 66C. Hmmmm… what is going on? Backwards flow and a very high temperature delta, suggesting much lower flow than design. Got any ideas about the cause?
The next photo shows a 10 inch return line from the basement mechanical room, temperature reading 75C. The boiler supply temperature setpoint is 80C. That’s odd… Lots of flow must be bypassing from supply to return, but we already checked all the typical major sources of bypass.
Have you figured out the problem yet? Keep reading for the answer…
Since we couldn’t find the source of the water bypassing back to the central plant, we decided to visually trace the supply and return lines from the central plant to the main mechanical rooms. We followed the lines from the boiler room to the north mechanical penthouse, then down the mechanical shaft from the penthouse to the basement. Everything looked normal until we reached the basement mechanical room and found that the 10 inch supply and return lines had been crossed in the ceiling space before going into the mechanical room! This meant that almost the entire flow of water from our boiler plant was bypassing the building and going straight back up to the boiler plant! And the building had been operating this way for over 40 years! It is amazing that such a fundamental issue could be missed when the building was originally constructed and it is a testament to the ingenuity of the building operators that they managed to keep the building comfortable over the years.
After the piping mix-up was discovered, we waited for warmer weather and corrected the piping. It took a few hours to get the lines filled back up and the heating system back online. The system is now properly under control and working efficiently. The pumps are able to deliver heating at the proper temperature and pressure throughout the building. UHN’s energy team will be monitoring the utility consumption at Bickle this winter as we should see some savings from reduced pump power consumption, reduced boiler short cycling, and better control in general.
I think the moral of the story is that data is powerful stuff – without the added information that was available from the boiler and AHU projects, inefficient and difficult to control heating would likely have continued for many more years. Thanks to Facility Manager Joe Angione and BAS specialist Allan Wu for being instrumental in solving this mystery.