Lineset chatter sounds like machine gun fire

[Frizzo]

I have 2 older Trane Heat Pumps.
Condenser model TWY036B100A1 with ECM fan motor
Air Handler model TWE037E13FB with Variable speed motor (ECM)

The heatpump with a 40ft lineset that runs in the ceiling between the first and second floors, chatters loudly like an automatic machine-gun for about 6 seconds when thermostat is activated (both in heat and A/C). After about 6 seconds of loud chattering, noise stops, and all is fine. I replaced both (start & run) caps. The run cap. was no good. Any ideas what could be causing this scary noise? I do not have access to the lineset, as it is buried between first floor ceiling and second floor floor. The chatter only occurs on start-up for about 6 seconds and occurs about 25% of the time when starting. Thank you.

 

[WorthFlorida]

You’ll need to have the line pressures checked out. You’re probably get the rattling while pressure is building up ( too low pressure) or the compressor starting up with too much head pressure. There is a timer in the thermostat or the controller board that has a wait period not to engage the compressor so the head pressure has time to drop between cycles. Depending how old the unit is it just be worn out.

 

[Sponsor]

 

[Frizzo]

You’ll need to have the line pressures checked out. You’re probably get the rattling while pressure is building up ( too low pressure) or the compressor starting up with too much head pressure. There is a timer in the thermostat or the controller board that has a wait period not to engage the compressor so the head pressure has time to drop between cycles. Depending how old the unit is it just be worn out.

Thank you for your reply. The problem is definitely not being caused by too high head pressure on start-up. I manually operate the startup times of the heat pumps. I do not rely on thermostat like most people. It either runs constantly or is off for a long time. So, my LOUD machine-gun noise problem is occurring after the unit has been off for a good while. Thus, it is not a pressure equalization problem nor slugging. The problem may very well be what you mentioned (too low line pressure on start-up). The fact that the noise only occurs for about 6 seconds, may indicate proper pressure being built-up. However, why would it be a very intermittent (25%) issue?

 

[WorthFlorida]

Pumps or compressors can captivate. On irrigation pumps with little back pressure you can hear it. For AC compressors it not likely for a scroll compressor but an older piston type it might. Just need to have a set of gauges.

 

[Frizzo]

This is a scroll compressor. If the intermittent loud noise on 20% of start-ups is caused by low line pressure, the reason may be that the sun has heated the cabinet/lines sufficiently to raise the line pressure sufficiently for proper start-up. However, if it was a "low pressure issue", wouldn't the low pressure switch prevent the compressor from starting due to compressor safety? Yep, need to take pressure readings.