Evaporator Coil Frosting Around Front Behind Fans

[dixie2005]

I have an evaporator coil that is frosting up around the front of the coil in behind the fans. It is frosting up bad, thick frost that builds up in behind the fans all the way across. It's a small Keeprite coil, 12,000 btu/hr, low-temp (0F), outdoor condensing unit. Paragon mechanical time clock, four 40 minute defrosts/day (electric) with termination and fan delay. Back of the coil defrosting great. 4 fan unit.

So far I've checked
- position of fan blades in shrowd
- tested time clock
- tested termination
- tested fan delay
- checked evap motors and rotation
- checked refrig charge

I completely defrosted approx 3 weeks ago, customer says it is now frosted again. The location is out of town so i would like to go back with a plan.

When I return, I intend on checking:
- superheat
- monitoring defrost cycle
- increasing defrost time?
- increase number of defrosts/day??

Any other ideas? At the moment unfortunately I do not have the model and serial number of the equipment, I have called the customer and am awaiting a return call with that information and when I get it, I will post it.

[Tommy knocker]

Check location of defrost termination. And verify it is switching at correct temp.

[dixie2005]

Check location of defrost termination. And verify it is switching at correct temp.

have you had to move defrost termination from it's OEM location before? (because this is a new coil)

[Tommy knocker]

have you had to move defrost termination from it's OEM location before? (because this is a new coil)

No I haven't. Sorry forgot it was new. I have had to replace them for premature termination though.

[icexprt]

Make sure the drain is clear and flowing properly, drain line heater (assuming this is a freezer) is installed and functioning. Have seen water left over in drain pan at the end of defrost get kicked up by the fans and form a layer of ice on the front side of the coil.
Have you checked superheat? Most coils feed from the front to the back and an underfeed condition or low charge could give you excess frost and ice on the front side of the coil. Also, air leaks in the box could allow too much humid air into the box and form excessive frost on the coil. Make sure the drain is trapped outside of the frozen space to stop warm, humid air from entering the box.
Is someone leaving the door open for extended periods while loading the box? If you ask them, they never admit to it anyway! At the end of all this, you may need to extend the defrost time. The 40 minute setting on the clock is just a failsafe in case the termination thermostat gets stuck, so you may need to get creative and move the termination thermostat to get an increase in defrost duration.

[MAC AIR]

Sounds like airflow problem. Maybe overstocked or fans intermittently cutting out.

[PMARS2345]

Leaking solenoid?

[dixie2005]

I have checked the drain line heater, and yes it is trapped, no ice or water accumulating in the pan, seems to be clearing nicely.

I will check for leakages in the in the cabinet, a local mechanic also suggested I check that.

I will check for the leaking solenoid too.

Here's the list for when I go back:
- check drain line and heater
- check amp draw on fans
- check for cabinet leakages
- check for solenoid leaking by
- check coil superheat
- monitor complete defrost cycle for abnormalities
- analyze system for abnormalities checking various temps and pressures

If after all of that, I don't see any problems, depending on when defrost terminates, I may either move the termination sensor, or increase the time on defrost (or both)...

That's the plan, let me know if there's anything else

[Capz]

How many heating elements are located in this evaporator? Have you checked all of them for correct amp draw?

I've found units not wired correctly too and not heating an element.

[cavalieri85]

i would be looking at all my penetrations. especially the door and frame heaters and threshhold.

[dixie2005]

I will check out all door seals as well however this problem has been happening in the winter (now) which in Canada, the RH is low and humidity/frost is usually not an issue this time of year...

In any event, I'll cover all my bases. Thanks for the advice.

[cavalieri85]

Is it a restaurant? What is the location of the box? Do they have a string on the shelf by the door indicating they tie the door open. Is the freezer inside of a cooler?

[BDOLIN]

The coldest part of the coil is often on the fan side. This is because the warmest air enters the back of the coil and most refrigeration circuiting is set so that the air hits the coldest part of the coil last. This can give a closer approach and more efficient cooling. Terminate defrost when the ice goes away last during a defrost, after all, any other location will leave ice there and it can build up. Adding defrosts or lengthening defrosts may eventually be effective but at the expense of room temp and certainly uses more energy.

[dixie2005]

The coldest part of the coil is often on the fan side. This is because the warmest air enters the back of the coil and most refrigeration circuiting is set so that the air hits the coldest part of the coil last. This can give a closer approach and more efficient cooling. Terminate defrost when the ice goes away last during a defrost, after all, any other location will leave ice there and it can build up. Adding defrosts or lengthening defrosts may eventually be effective but at the expense of room temp and certainly uses more energy.

The termination sensor is in the space that's icing up (front) it's a klixon type that is mounted to the cabinet with the sensor sticking out into the space between fans and front of coil - where can I move it?

[dixie2005]

Is it a restaurant? What is the location of the box? Do they have a string on the shelf by the door indicating they tie the door open. Is the freezer inside of a cooler?

Not a restaurant, outdoor box in Eastern Canada so it's a pretty cold climate this time of year. I was having the issue in the dead of winter as well when RH is at it's lowest....

[DeltaT]

Not a restaurant, outdoor box in Eastern Canada so it's a pretty cold climate this time of year. I was having the issue in the dead of winter as well when RH is at it's lowest....

No head pressure control?

[dixie2005]

No head pressure control?

Yes, headmaster working fine.

[cavalieri85]

I think I would remove the termination switch and install new. I use the ranco with the bulb that you can slide into one of the open passes if the coil. Part # I think is f1127 if I remember correctly.

[BDOLIN]

I agree with Cavalieri85. Placing the sensor in the fins where the ice persists will guarantee a clean coil. We have found that to be very effective. You will have to force a complete defrost to clean up that location to get the sensor placed, but once placed the ice should never return.