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I agree with Icemeister, absolutely. With the condensing unit outside on a roof, in cold temperatures, it should have a head pressure/low ambient valve for proper wintertime operation. I had problems with a WI's, a freezer and 2 coolers. These were installed by an outside contractor during a renovation of culinary arts classrooms. WI's inside, cond units out on the roof. Randell units..?? Freezer especially would warm up sometimes, mostly in cold winter weather. Sight glass would show bubbles. So I started looking into it and found they had low ambient HP valves on them for winter operation. Freezer is 404a, coolers are 22. Field tested the valves as per Sporlans tech bulletin described and found one bad valve. BUT, what i found is that the cond units have a described charge, and additional amount must be added for the specific model unit for low ambient. Seems the installer did not add the additional charges for flooded condenser. Replaced the one bad valve, in addition to recovering the charges on all of them, and charging each with the proper extra amount for low ambient operation. Havent had the problems since. So I would look into the HP valves, checking if they can be added if they are not on the units, and making sure they have the proper charge and additional LA charge.

Good points Icemeister

Thanks everyone, great information.

I agree with bunny about the boss's advice to not clear the sightglass. Clearing the SG is a primary step in charging a system with a receiver.

Pingosize1's questions are typical of what needs to be known about a system before you can reasonably determine how to properly service it.

If a system's air-cooled condensing unit is located in an ambient which never drops below 70ºF, generally no head pressure control method is needed. In a cooler climate where it may get down to 45ºF-50ºF, often a condenser fan cycling control will work OK to maintain adequate head pressure (although the constant pressure fluctuations can drive a TXV crazy ). In either case, you need to have a clear SG and be able to maintain a minimum head pressure corresponding to a condensing temperature of around 80ºF-90ºF to adequately feed the TXV.

The better and preferred method is to use a condenser flooding control, which is usually is a 3-way valve in the discharge line which basically bypasses discharge gas to the receiver causing refrigerant liquid to stack up in the condenser (reducing its effective condensing surface) to maintain a preset minimum condensing pressure. This control a generically referred to as Headmaster (a term coined by Alco many years ago), and when the system is properly charged with the extra refrigerant needed to flood the condenser, they will work down to -20ºF or so.

The key to charging a system with a Headmaster type control is knowing how much "extra" refrigerant is needed to flood the condenser coil. If you're lucky, this may be listed on the unit's dataplate or in the I&O manual. If not, it should be calculated. Here's a link with some very good info on how they work and how to calculate the winter operating charge:

Sporlan Bulletin 90-30-1

Blocking the condenser with cardboard can be a bit risky if the weather suddenly warms up.

It is a short time fix til the boss gets back from vacation. Sometimes you have to conjure a method to keep from losing product. I'd bet that the headmaster is bad, or possibly condenser has multiple fans that are brought on progressively thru pressure switch(es)

Is this a single condensing unit only for this freezer? Does it have a headmaster? Does it cycle the condenser fan in low ambients? What is the ambient temp?

Proper refrigerant charge along with a head pressure control valve (or valves...depending on the application) will provide the same result. The issue is that extra refrigerant is required to properly flood the condenser so that a minimum head pressure can be maintained, and this gets expensive on large systems.

Split condenser applications are typically used in conjunction with head pressure control valves, and offers the same benefit of maintaining a minimum head pressure but reduces the amount of additional refrigerant charge required to properly flood the condenser and reduces the receiver size....that is the only benefit to splitting the condenser.

Covering a portion of the condenser is actually offering the same result that condenser fan cycling would provide....simply eliminating air flow through a portion of the condenser.

"told by boss not to clear sight glass..." bad advice. TEVs don't function at their rated capacity when fed with a mixture of liquid and vapor.

there are controls for that...vs cardboard

There's racks actually piped like that - half-circuits. Year-round condenser and a Summer condenser, in low ambient only the Year-round has flow through it. What you where asked to do is similar, only on a smaller scale.

Partially covering condenser to raise head pressure

A four door reach in freezer that is set to 0 degrees was up to 30. It was cutting off on low pressure, cycling on every couple of minutes and cutting back off in about 30 seconds when suction got back down to about 10-15 psi. This was a few hours ago in the dark on a roof, so not looking for the leak until next week. Added some 404 (told by boss not to clear sight glass, just watch my pressures). At about 155 head/32 suction I was told to cover about half of the condenser (taped cardboard to it) to get my head up to about 190-200. Head pressure went up to right about 190, freezer temp coming down nicely. I didn't ask my boss why (he knows his stuff and was on vacation talking me through it on cell), but am really curious if this is something you guys have done before. I'm going back tomorrow morning to check the temp first thing and he will be back in town to address the issues Monday, this was just a quick fix so the customer wouldn't lose product (actually so it could still be used to sell product out of). So what's the scoop on doing this?