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I recently was out on a troubleshooting call to look at a
heating system in one of the housing authorities in the Metro Boston Area. The trouble they were having was excessive
cycling of boilers, under heating of some apartments and daily boiler
lockouts. The housing authorities’
efforts to date had been focused solely on the boiler, which seems logical. If
you don’t have enough heat it must be the boilers right?
The system’s main components where supplied by seven
different manufacturers boilers, pumps, controls, VFD’s etc… add in the
different Manufacturers Representatives and Wholesalers and the housing
authority was stuck trying to piece together the solution. As the head of facilities and I began to look
at the system as a whole we found the following problems.
- The primary or building side of the system was operating at a 50° Delta T, since the heat emitters were all baseboard the units at the end weren’t getting hot enough water.
- The secondary or boiler side was operating on a 10° Delta T and we were returning hotter water to the boilers than was coming back from the building.
- In order to compensate for the lack of heat someone had set the BMS control to target 190° in the building loop.
- The internal safeties kept the boilers from going above 185°, so they were limiting the boilers but the BMS control was sending a 10 VDC signal to the boilers, trying to drive them to 100% modulation.
- Looking at the boiler cycling the units were banging on and off trying to obtain a target that was out of reach. As they would be driven to an internal limit shut off, cool down and start again.
The solution we placed the VFD controls in hand and sped up the
primary or building side pumps to create a 20° Delta T, this in turn created
a 20°
Delta T on the secondary or boiler side as our system was now balanced. The BMS control was set back into a normal
outdoor reset mode and began targeting a lower building water temperature. Since the boilers could reach the target and
we were actually taking the heat off the boilers the BMS began ramping down and
turning off boilers. By the time we left
the building was being maintained by 1 of the 6 modules.
When I followed up the following week the report was no
boiler lockouts and they actually had gotten some complaints about overheating
for the first time so they were working with the balancer to find the happy
medium in the system. This case is a
good example of what happens to us so many times when we get focused on what
seems the logical source of our trouble only to find it somewhere unexpected.
