That hot/cold scenario that you’re describing is what I call “hunting”. It usually has to do with a unit that has a thermal expansion valve as a metering device. It is associated with refrigerant starvation (for whatever the cause). The thermal expansion valve senses the temperature of the suction line, when it gets warm the valve opens up wide and floods the evaporator with refrigerant. When there is not enough liquid refrigerant in the condensing unit to maintain a constant load, hot gas abruptly discharges through the condenser and down the liquid line without adequate sub-cooling.
There can be other issues concerning this scenario as well, but as you say we are limited to what we try to evaluate just simply try to understand it. Also as you said, insufficient data!
As for your question; between you and I a Megometer is an HVAC engineers “crystal ball”!
Yes I do use them ( on all old HVAC equipment I inspect during a home inspection w/ HVAC diagnostic ancillary inspection).
The refrigeration circuit (including the compressor) is a closed system that you must have certification to access. Contaminants in the refrigerant occur for many reasons, the Megometer can identify the presence of these contaminants in the refrigerant system as well as determine the installation capacity of the compressor motor windings. As the compressor starts, is under a considerable load and there is considerable flexing of the windings of the motor. This eventually breaks down and causes leakage which the Megometer can detect and identify.
In cases where contamination is the cause, there are numerous mitigation processes that are available and not that expensive that will stop the erosion of the compressor and extended its expected life by stopping the accelerated deterioration. Contaminants cause coking of the oil which turns to an acid which eats away at the windings of the compressor. Even when the insulation is still intact, contaminants in the refrigerant and oil will trigger a bad meter reading on the Meg. Additional testing of the refrigerant oil/refrigerant can be performed to determine whether it’s contamination or damage to the insulation on the compressor (which cannot be repaired). If it is contamination, mitigation practices will alleviate some of the situation.
My first Meg, I confiscated from the company’s tool locker along with numerous other test devices that nobody knew how to use. Last year I purchased my third Meg. They are not cheap, but I consider them a worthwhile investment for how I do home inspection.
As far as bubbles in a slight glass, ensure you realize that this means absolutely nothing as far as refrigerant charge! Here I go again! Another practice that is futile!
You never charge by slight glass!
If you don’t perform adequate analysis on the equipment to determine if a flashing slight glass is appropriate or not, you will overcharge the system.
I don’t know how many commercial 25 to 150 ton air conditioning compressors that I have come across where refrigerant was added because the slight glass was flashing. By the time I got there, the oil slight glass in the compressor was foaming and the compressor was shutting down on low oil pressure safety devices. Excessive refrigerant charge was washing the compressor oil out of the crankcase tripping the oil pressure safety switch. I remember many cases where I removed as much as 60 to 80 pounds of refrigerant from the system to get it back online again.
Hard start capacitors are another touchy subject. There are some cases where hard start kits are required because of system design. When Lennox came out with the first Copeland scroll compressor, they thought it was indestructible. No accumulators on heat pumps. No pressure equalization safety measures while using accurators versus internal equalizing thermal expansion valves. They refused to let us add hard start kits. They required compressor replacement under warranty!
As a home inspector and you show up to a home inspection and see a hard start Kit (a start capacitor with a potential relay) what is the reasons for its installation?
In many cases when the equipment is old, it takes a lot more to get the compressor started because the bearings are wearing and other issues. In some cases the brand-new equipment should have had one from the factory but they didn’t put it on.
At least note in the inspection report that these devices have been added as an aftermarket product and it just may be an indication of future failure of the equipment due to the age of the equipment.
Please note: I never recommend replacement of equipment because of its age. If you haven’t noticed I do not respond to “what is the age of this equipment” request on this website.