Defrost stat

[marc5180]

Hi guys, I've been working on a large packaged heat pump unit and I noticed the supply air was between 15-20C which is too low.
The bottom of the evaporator was heavily iced.

On the evaporator is a defrost stat that was fitted by the original installers at a later date.
The defrost stat has a bulb that sits in-between the coil and switches the 4way rv when ever it senses the set temp.

However I'm unsure as to what it should be set at.
It was originally set at 0C but if I leave it at that setting then the unit constantly goes on defrost (every 5 mins or so)
Could this defrost method work without a timer or is there something I'm missing.

[sedgy]

hi marc,
what % of fresh air have you got ? and whats the ambiant ?

[marc5180]

I'm not sure what percentage the fresh air is to the coil as it goes through a recouperator that looks like it mixes with the return air from the building.
I'm going to try to speak to the manufacturers tomorrow regarding this.

The ambient temp is -1C.

[sedgy]

hi marc,
well in this weather I would shut down all external air, what is the axilary heating?

[chilliwilly]

Does the fan shut off whilst its defrosting? Maybe set the defrost stat to 10 deg C as a starting point, then monitor the ambience over 15 minute intervals. Defrost intervals for any fridge or aircon system, have different taylored parameters for each system. But I know that you already know that.

[marc5180]

I dont think its's an option to shut off all the external air although i will try this when i go back. I think this may cause lack of airflow across the coil maybe i'm wrong but i'll check this.

The auxillary heating is by an electric heater just after the fresh air inlet ( this was on 100%)

[marc5180]

Does the fan shut off whilst its defrosting? Maybe set the defrost stat to 10 deg C as a starting point, then monitor the ambience over 15 minute intervals. Defrost intervals for any fridge or aircon system, have different taylored parameters for each system. But I know that you already know that.

No the fan doesn't shut off. As this is an after market frost stat fitted it is very basic and the control logic is as simple as when it senses the bulb temp is at the stat set temp then it switches the 4way rv.

It doesn't use a timer, ambient temperature sensor or even switch the fan off to control the defrost.