Freezestat Questions Pro/Cons

[CHAMBERS87]

Dear Sir,

Commercial Manual Reset Freezstat application, unit shuts down and preheat valve full open. But some say chill water valve shall open too? Skeptical.

Please help feedback appreciated. Pro/Con's for the units mentioned, they have preheat coil (some steam, some hw coils) and CC in main unit, no reheat at unit:

1) Rooftop AHU's that have SF and RF that serve VAV's.

2) AHU's that have SF and RF that serve VAV's in mers.

3) 100% OA Units. Supply Fan no return. Just exhaust in space to supplement for not having return.

Respectfully need your expertise and experience,
Bob C

[AutomatedFreak]

Chilled water valve should open also. Be sure both hot water and chilled water pumps are running.

[WillyTech]

In freeze condition you should be circulating your chilled water or coil is drained completely. Ideally adding a freeze pump to circulate the water in your chilled water coil.

[bdivell]

I would typically open both valves to verify water is circulating, Hopefully do to the risk of freezing the system that they have added glycol, some don't prefer to add due to the loss of heat transfer.

[tlp261]

Like they said, opening the cooling valve is good but doesn't help without a freeze pump or unless the main pump runs in the winter. You could also command the main pump on when there is a freezestat alarm if there is a BMS and not just local controls.

For what it's worth, Honeywell lawyers (and there were lots of them!) always insisted the device be called a "safety low limit". "Freezestat" could imply it can prevent freezing, which it can't.

[orion242]

Like they said, opening the cooling valve is good but doesn't help without a freeze pump or unless the main pump runs in the winter.

Ditto. It’s rare to see coil pumps on CW coils and even less likely to see the CW pumps running in the winter. Sure you could kick everything on during a trip, ASSUMING your actually monitoring the fstat verses just hardwiring them.

If the HW system has hot water flow and the HW valve fails open, fans off, econ closed….it should make little difference if the CW valve also opens. With no HW flow, the CW coil is likely to freeze regardless here.
Compared to the energy costs of running the CW pumping system all winter, I would think glycol would be a more cost effective solution. Isolating and draining the CW coils is also not an uncommon solution.

At a minimum, we use fail safe actuators on all HW coils with possible exposure to OA and economizers. Hardwire the fstat to the actuators and fans. If monitoring of the fstat is required, it’s a DPDT fstat or a relay on the main shutdown circuit fed back into the BMS. Through software, we open the CW valve if equipped. IMHO this will cover all but the most extreme failures and prevent the coils from freezing.

If they want more protection, we certainly provide it.

[CHAMBERS87]

Thank you all, we have glycol in the system and due to units without OA and IT rooms we have chill water pumps 24/7 all year long.

With that being said, with a standard rooftop AHU with preheat, humid and chw coil. Do you recommend to have the Freezstat open the Preheat full open, and the CHW Valve full open, when the fan shuts down?

[CHAMBERS87]

If you have a chw pump that is running all year long. Do you still reccommend a freeze pump? Maybe on 100% OA units that have Preheat Coil, Hum Coil and Chill Water Coil. No reheats only in space.

Any Negs about opening the CHW Valve too? At all that you can think of?

[orion242]

Any Negs about opening the CHW Valve too? At all that you can think of?

The only con I can think of is cost. There should not be much need for CW freeze pump if you have a properly installed fstat and glycol in the system already. If the existing CW valves are spring return currently, I would spring them open. Otherwise I would leave them as is and worry more about the HW valve, coil pumps, fans, and dampers.

[CHAMBERS87]

Have some steam preheat coils too.

[orion242]

If it makes hot (not gas fired burners), we open it on fstat trip.

[tlp261]

Have some steam preheat coils too.

Be a little careful about having them fail open. We melted some controllers once when the medium pressure steam valve opened on the Unocc cycle.

[tlp261]

If you have a chw pump that is running all year long. Do you still reccommend a freeze pump? Maybe on 100% OA units that have Preheat Coil, Hum Coil and Chill Water Coil. No reheats only in space.

Any Negs about opening the CHW Valve too? At all that you can think of?

If both valves are open for an extended time like overnight, the CHW coil can pick up heat from the HW coil and cause problems.

[theoldman]

You have to be careful when the cooling coil is downstream from the heating coil in the winter even when units are not in freeze. If you have chiller pumps running and a chiller not running and you build heat in the chilled water system because a chilled water valve is also open,or just from pump friction , you can actually build enough heat to blow the rupture disk out of a chiller. I've seen it happen on large centrifugal chillers and it cost a fortune to replace the refrigerant, not to mention the environmental issues with the release.

[n8atk]

If connected to BAS and your chw has proper glycol mixture to prevent freezing coil. Simply close dampers and mod hcv to maintain 15c in the duct.
No BAS you gotta close dampers fail heat open 100% and hope nothing melts.

[CHAMBERS87]

What temp do you recommend not to go over with the above mention issue within the pump CHW System, to avoid the rupture disc from blowing? Thx Bob

[Control Man]

Have lots of units like that in a lot colder enviroment ( - 32'F OA temp the other day )

Heating Valves N O ( Normally open ) Cooling Valves N C ( Normally closed ) on freeze stat trip AHU shuts down OA damper closes ,HEATING valve goes open , Cooling valve goes closed.

[Cphreak]

What I do especially when it is steam heat is on a low limit trip I modulate the heating valve to maintain 45 degf mixed air temp. The heating valves have spring return actuators so on a loss of all control power they fail wide open.

[CHAMBERS87]

What about 100% OA Units? With Steam preheat no reheat. Still recommend just preheat valve to open?

[CHAMBERS87]

What about a) Roof Top Units and b) 100% OA Roof Top Units? With Steam preheat no reheat. Still recommend just preheat valve to open?