grinding noise on startup

I have 2 ac units that grind on startup (one worse than the other) It only lasts for a second then stops. I had the hvac guy here to check it out and he’s stumped. amp test proved normal. Things I can thing of are bent fins, bad blower motor (or low/no oil) and low amount of refigerant although the air is cool as it blows into the home. Any other thoughts?

motor bearings maybe? You would be suprised at how much damage some units can have and still run.

You can rule out low oil/no oil and low freon would not be your problem.

You stated that both units were making similar noises leading me to believe that both units were serviced and or installed by the same person and could have an overcharge with some liquid lying in the crankcase on startup and this would make a noise.

A bent or broken internal compressor mounting spring could also make a noise but the probability of both compressors having that same problem is un-likely.

If it is not the fan or the freon charge but is an internal noise from the compressor there is no exact way to determine this on a hermetically sealed unit and you will probally never know the reason until failure.

Refrigerant lines that rise to a vertical height of more than what is stated in the installation manual will cause this.

ie. Compressor on ground and air handler in attic of two story house can exceed the limit.

Will sound like gravel churning for a few seconds.

They sell a kit I think to solve this, check valve or something?

Bruce where would you put a check valve in what freon line and what would be its purpose.

I personally have never read any MFG instulation instructions that restricted verticle height they will require the size of the liquid and suctions lines to be increased for length but not necessarly for height. Have installed tons of units on two story homes with no problems

This may also be a simple rain shield on the fan motor.

Bruce & Charlie,

I think you are on to something there.
It’s impossible to hear the grinding over this board so this is a WAG not a SWAG.

Seeing that both units are doing it, it should not be a defect. It is unlikely that both would have the same defect at the same time. (ie. fans, motors rubbing, bearings, blower wheels)

Also, scroll compressors have a tendency of being loud (starting, running and especially when stopping as they run backwards during pressure equalization).

Now, on to you guys.
Refrigerant migration could be a factor.

Evap up, cond down: If the refrigerant metering device is other than a TXV it may flood the compressor as it allows the liquid refrigerant to pass through the metering device rapidly until equalized. You guys hear this surging of refrigerant for a while after the unit shuts down from up in the house at the air handler?. A TXV will “slam shut” when the compressor suction stops (most residential valves still have a small equalizing orifice inside the valve).

Creating a “Pump down” or just a “shut down” configuration by installing a solenoid valve in the liquid line will correct this. However, a hard start kit may be needed if the compressor doesn’t have a start capacitor as the compressor must start against the head pressure of the refrigerant caused by the elevated AHU.

Excessive refrigerant charge (due to system design): The amount of refrigerant in a system must be kept to a minimum. Arbitrarily installing over sized lines may cause this. These flooding conditions are more prone in over sized refrigerant circuits.

No crankcase heater in a scroll compressor (or reciprocal compressor where slugging is occurring): Mfgs think the scroll is tuff and doesn’t need an accumulator, start capacitor or crank heater. Refrigerant has an infinity for oil and during shut down it will migrate to the crankcase of the compressor. At start up the refrigerant (and the oil) are sucked up through the compressor. It makes one hell of a noise! Look for a compressor in a cooler location than the AHU (like the AHU in a 200 degree attic space).

Installing an accumulator in an A/C that doesn’t have one will help with a flooding situation.

The HVAC Tech should be able to identify this stuff.

I looked it up, a LSV, liquid line solenoid (not check valve) is needed for lines longer than 50 feet or 20 ft vertical rise. (The manual had 10 ft printed but the engineer told me it was a misprint, 20 ft. is correct.)

LSV goes between filter drier and indoor coil on a Bryant heat pump and as close as possible to the filter drier.

Now you are making sense to me:) with the exception that I have always installed a LSV if required just prior to the entry of the A-coil box.