Tertiary Coil Pumps for Freeze Protection 1

[BronYrAur]

I haven't seen coil pumps in a while but recently ran across them on a job. I have the opportunity to remove them and "correct" the piping if needed.

There are 2 air handlers with 2-way valves on their hot water preheat coils. There is a bypass line with a check valve to allow the coil water to circulate even if the 2-way valves are closed. The coil has an inline tertiary pump sized for the full flow of the coil. It runs whenever the 2-way valve is open or when ambient drops below 35 deg F.

Similar situation on 12 other air handlers, except they have 3-way valves. The coil pumps again run when the valve is calling or ambient drops.

My overall task is to convert the now constant volume hot water system (which is the secondary side of a primary/secondary) into variable volume. I will be replacing all 3-ways with 2-ways, adding VFD's on the pumps, etc.

So back to my original question of the tertiary pumps. I see plenty of installations without them, but I know they are a good backup. If the AHU's don't already have freeze stats, I will be including them. The job is in New Jersey.

Any thoughts?

 

[mauricestoker]

My first thought is that 35* F is low for freezstat setting.

 

[walkes]

We normally run pumps on coils when using water systems (they may be excluded on glycol systems).

With a 2-way valve and pumped coil you can maintain variable flow in your secondary circuit and maintain the freeze protection with the pump at the coil (tertiary circuit).

The coil-pump only sees the head loss of the coil and not the control valve, so depending on how the 3-way valves were piped you may reduce the head loss at the coil pump by changing to the 2-way valves.

 

[ChasBean1]

I agree with walkes. One advantage of the pump setup: you minimize water temperature needed to maintain the required air temperature because flow at each coil is at the full design value at all times (conservation of Q; delta-T is lower and m-dot is higher). Under this condition, you reduce stratification that could cause coil freeze or freezestat trips. If you eliminate the pump, you’ll have hotter water entering at lower flow. This condition could cause 90% of your heating to occur at the bottom 30% of your coil. Cold air could therefore channel through the top (outlet part) of the coil and hit a section of your freezestat.

All this said, as walkes iterated, your secondary loop flow should be the same with or without the coil pump and may even be marginally less, with pump work added to fluid, reduced fluid boundary layers inside coil piping so better heat transfer, etc.