Crossing Your Deltas: T vs. P

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Our Facebook page is often a place of great conversation.  Just this last week the following response was posted by our friend Brad White in response to a conversation about zoning with circulators. When asked in the thread his preference on ΔT & ΔP this is what Brad shared with us all.  You just never know where the conversation will go, thanks Brad!


Crossing Your Deltas: ΔT vs. ΔP

I had that set-up on a larger system a few years ago, switchable between DP and DT. A control contractor obliged me as an experiment.

Flow rate was roughly 500 gpm, 6″ pipe, that magnitude; University building. First off, my initial (not-well-thought-out) premise was that both modes would work together to whichever need was greater. That if Delta-T was fine but DP fell short, DP would take priority for example. It did not work out in practice for various reasons and we abandoned the DT approach after a few weeks.

Mind you, this kind of experimenting in an occupied new building is risky because commissioning is ongoing and new occupants if uncomfortable or inconvenienced, have their minds take a “set,” and are honed to be sensitive to things not just right. We had a little of that, but manageable in the name of energy conservation. See how many times I get to play THAT card!

Anyway, the DT setup was for 30 degrees across the S and R mains. As the DT narrowed, pumps slowed. DT widened, pumps sped up. Works in principle. Design was with condensing boilers for 140 HWS, 110 HWR, serving a mix of radiation at 20 degrees delta and coils at 40 degrees delta, unit heaters at 30 degrees delta, so was averaged to 30 degrees.

DP setup was by a DP transducer at the most remote representative branch, pretty much the end of the line. The balancing contractor verified flow at that terminal (a vestibule unit heater) and we backed off the pump VFD until that flow rate started to drop off, with the control and balancing valves wide open.

When we first started it up with a shared priority mode, the controls bounced back and forth between modes and locked up, which did no one any good. So we started fresh with DT mode. The first major branch (4″ mains as I recall), went to the air handling penthouse. There was an especially high initial delta T in that branch. The remainder was radiation, unit heaters and reheat coils. Upon AM warm-up, the water would start out at nearly room temperature from a cold start and return about 10 degrees cooler (60F coming back after a long weekend). Pump tended to slow down to widen the DT. This made for prolonged warm-up times. When the set point was reached after a couple of hours, the ventilation air was introduced. The DT widened from 30 F to about 50 F for a time. Pumps sped up but went to 60 Hz limit, lest they over-draw the boilers. This eventually settled but the heating was quite uneven, even though balanced. It took a few hours to settle out. (Outdoor reset did not help; it actually retarded the warm up times even more.)

We programmed over-rides to boost during warm-up then went back to ODR and disabled the VFD auto control to default to 50 Hz. constant speed, as a DP limit. This defeated DT basically.

In the end we went back to DP control but also implemented Automatic reset of the DP set point. For example, if 80 percent of the control valves were less than 70 percent open, we reduced the DP by 0.25 psid until at least 80 percent of the control valves were at least 75 percent open. In other words, if control valves were closing off too much, that meant there was excess DP and back off a bit.

In summary, I think DT is a great idea for a small system with large piping/low head such as gravity HW and single temperature differences. DP tends to be over-used in small systems where pressure differences are not that extreme. Small tube systems with coils and other higher PD emitters, DP control has more value.

Brad White is an HVAC engineer at a Boston consulting engineering firm with more than 35 years experience in the design of HVAC systems, in addition to energy auditing, enclosure testing and system commissioning. Brad is a LEED Accredited Professional (BD+C), a Certified Energy Manager and Certified Energy Auditor (CEM and CEA) with the Association of Energy Engineers and has developed and taught the environmental systems course at Boston Architectural College and workshops at various Boston area local organizations.

Brad is married to The Lovely Susan and lives outside Boston with Susan and his two lovely step-daughters and two cats. He does not stop BTUs from escaping but slows them down, inconveniencing them as much as he can.

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