Freeze Protection Pump

Matt_49

I have an air handler with a hot water coil that is designed for 75 gpm at full load. The coil can see outdoor air. There is a freexe protection pump that circulates 75 gpm of water through the coil at all times when the outside temp is below 35 deg F. It has a 1-hp motor.
Can I use a smaller pump (less flow) and reduce the power consumption or does it need to be the full 75 GPM?

How do I size it?

Brad White_9

Typically

I design the pump for full flow.

This does two things, the first being freeze protection. The second is that with constant circulation and the coil being in effect a secondary loop, I can inject/bleed in primary hot water for superior temperature control.

If you were to use straight valve control, low load periods when the valve was barely cracked open would stratify the coil face temperature leading to sensor error and possible freeze-stat tripping.

Can you get away with a smaller pump? Sure you can. Just be aware of the reduced capacity of the coil based on a reduced average water temperature. Keep above laminar flow also.

By the way- while moving water has less probability to freeze it still can. Running such a pump should be part of a larger control strategy. When the freeze-stat trips, we specify that the OA damper closes, the RA damper opens (spring return to default positions), the HW control valve fails open and the fan shuts down and this circulator runs if not already running. This belt and suspenders approach has served us well.

Hope this helps.

Brad