Frost stat tripped

[marc5180]

I got called to a Gas fired air handling unit the other day due to no heating.
Once on site i found the frost stat had tripped but i don't know why.
I asked a colleague and he says they are only designed to work in temperatures down to -3ºC, anything lower than that and they trip.

Surely that can't be right can it?

[Brian_UK]

Frost stats on LPHW AHUs make sense.

If the air temperature gets too low then you can split the coils due to freeze ups.

Are there any water coils in the system at all?

Make/model number at all Marc?

[tonyhavcr]

I got called to a Gas fired air handling unit the other day due to no heating.
Once on site i found the frost stat had tripped but i don't know why.
I asked a colleague and he says they are only designed to work in temperatures down to -3ºC, anything lower than that and they trip.

Surely that can't be right can it?

HI

I do need to ask what type of F.H.A. unit is it.

What dose a tripped frost stat have to do with no heat in a gas fired unit? Is it LP or natural gas system.
direct or spark. I would love to know how your system works and why a f- stat.

[Grizzly]

I got called to a Gas fired air handling unit the other day due to no heating.
Once on site i found the frost stat had tripped but i don't know why.
I asked a colleague and he says they are only designed to work in temperatures down to -3ºC, anything lower than that and they trip.

Surely that can't be right can it?

Bare with me Mark because I could be being really thick here.
But are you saying that your particular Frost stat is made N.C at a temperature above -3.c. (Whatever the Frost protection setting is at?)
But when it reaches anything below -3c it becomes open circuit again?
Someone hasn't wired it the wrong way round have they?
Have you any idea what make the stat is?
Or is it simply as you state failing at any temp below -3c (Very strange.. and hence your post!)
Grizzly

[marc5180]

Too be honest i didn't take a note of the make model, since they are the type of units that i come across frequently.

Inside the AHU there is a frost coil that when called for has LPHW going through then it has a DX cooling coil and finally a gas fired heating coil.

I've since been told that when the temperature gets below -3°C, the frost stat trips which shuts the supply fan off, yet LPHW still circulates through the coils to be sure they dont freeze.

Grizzly- yes it is wired as you describe, these units have been in for several years.
Sorry i don''t have the model number i didn't think i would need them .

[sedgy]

there might be a reason for a frost stat in a ahu if it had a lphw coil , but if it is a gas fiered boiler it would depend on a return water stat to keep it safe from frost, or is it a ducted air system?

[Brian_UK]

According to your description Marc the system is working perfectly.

The LPHW frost coil cannot hold the temperature above the stat setting so the plant shuts down to protect the coil.

Not unusual when the coil is undersized.

[Grizzly]

According to your description Marc the system is working perfectly.

The LPHW frost coil cannot hold the temperature above the stat setting so the plant shuts down to protect the coil.

Not unusual when the coil is undersized.

You took the word out of my mouth Brian!
Once Mark had explained it all made sense.
The trip is at the relevant setting and not "out of Range"!
Thanks for the update Mark.
Alls well that ends well.
Grizzly

[sedgy]

if the return stat < or frost stat< senses the system is cool , it swiches the heating on , otherwise the water in the coil would freeze thats why its called a FROST stat

[marc5180]

Ah ok... the frost stat sensor must be positioned after[/COLOR] the frost coil, not before it like i thought.

[marc5180]

Ah ok... the frost stat sensor must be positioned after the frost coil, not before it like i thought.

[Brian_UK]

Had one go a few years ago.

Got a call out at 3:30 in the morning just after a Christmas when there was snow on the ground and a night time temperature of -5°C.

The heating wouldn't come on because of low system water pressure. I tried getting the pressure up but it wouldn't stay so asked the night guard if there was any water around.

"Oh yes", he said, "It's near that big AHU mounted outside."

Walked half way across the factory yard and found a huge, and growing, puddle on the ground that was slowly freezing over. Yep, the coil had blown.

Looked quizzically at the guard and he said "I thought it was the snow melting off the roof."

Not a lot that you can say really, is there?

[Grizzly]

Brilliant! Brian.
I have had similar but I won't bore you all at the moment!
Grizzly

[Magoo]

Hi Marc.
air densities change under zero, could potentially change air gas ratios and be inefficient and potentially hazardous.

magoo

[sedgy]

sorry mark its not in front of a coil , its not behined the coil I would have it strapped to the supply pipe to the coil, it all depends on if there is any fresh air louvers open? all we are doing is saving the water coil from freezing , by bringing on the heating possibly between 0c+5c thanks to all who contributed to this thread