Air Conditioning - Loud hammering when ac compressor shuts down

I am a general contractor (jack of all trades … master of none). Great forum. Anyhow, I live in So Cal, unit is 30 yr old General Electric with 10 yr old rebuilt compressor. Inside is a Carrier furnace with who knows what evap coils. Cools great but hammers like crazy the hotter the outside temp. The best description of the noise is that it sounds similar to water hammer noise in plumbing. Hammering lasts 5 to 20 seconds after compressor shut off. Have grabbed the copper lines on both the compressor & evap ends and cannot tell what is happening. This is obviously a pressure equalization or bleed back from a high pressure to low pressure side. I’m gonna guess a bad expansion valve? Any other ideas?

It is possible if it gets noisier as it gets hotter outside that the condenser coil is dirty or there is a problem with the fan.
To get some idea if this is so you could feel the smaller of the two lines going to the indoor coil.
The smaller line should be in a range of about 100 degF. This would feel like the surrounding temp. If hotter you may have a problem.

Could also be several other things.
Might need to call a tech to install a set of guages to diagnose it for you.

Thank you … I wonder why other forum responders did not suggest this simple test … yes … the high pressure line runs so hot, it will burn you! … blower fan runs and discharge ducts seem to supply lots of cooled air … outside compressor coils ez to inspect look clean … inside condenser coils will require serious work to get thru the sheet metal … is this what I should do? … strip away the sheet metal & clean the coils?

When I say the smaller line going to the indoor coil I mean the small line that comes from the outdoor unit and into the house.
If this line is so hot as to burn you then the outdoor fan is likely not working.
If you are feeling the discharge line on the compressor and is inside the outdoor unit this would be normal for it to be that hot.

Outside unit (compressor) small line is very hot to the touch when running (blower fan works and blows like crazy), no obstructions and clean coils

Inside unit (condenser) small line is very hot to the touch when running (blower fan works and blows like crazy) mucho air moving thru ducting & vents, cannot see how dirty coils are without pulling the unit or removing sheet metal

There is a bit of confusion on a few terms I will try to clear up.

If you have a typical central airconditioning system you will have the following components.

The part that is outside is often referred to as an outdoor unit.
It is actually called a condensing unit.
It contains among other things a compressor, condenser and fan motor and blade.
Within the cabinet you will find tubing when running that gets hot, cool and mid temps. Do not worry about the temp of tubing within the cabinet.
There is tubing running to the house which is called a line set. There are two tubes in a line set, a larger and small one.
It is the smaller of the two I have asked the temperature of. If the smaller tubing that runs to the house is hot you have a problem outside.
The part that is inside the house is called an evaporator coil.
Any lines inside the house that are hot on a typical airconditioner would again be from a problem outside.

Unless you actually have a heat pump and that would be a different story.
If the outside coils are clean and the fan spinning, a hot liquid line would likely need to be fixed by a licensed tech.

Thanks GregH for clearing up terms for me. No … not a heat pump. When examining the line set … the smaller line is very hot at the evap coil inside the house

The problem could be that the fan is not running on the outdoor unit or the coils on the outdoor unit are dirty.
The outdoor coils could look clean on the surface but be plugged between the fins.
Turning the unit off and then washing with a garden hose should clean it.

Another possibility is that the unit is overcharged with refrigerant.

GregH is right on his assessment. You stated it is a GE so it probably has brush fin coils. Be very careful cleaning them as they are easily bent and at 30 YOA will probably easily fall apart. If the unit is very dirty it can be a bear and time-consuming to clean properly. Also those compressors tend to get very noisy as they age.

Yes, has brush fin type coils, looks clean, but I will power wash anyhow, compressor is not noisy, because it blew up about 10 yrs ago and I had it replaced, keep in mind that the unit still cools great and is very quiet running ... only hammers at shut down … does not hammer at 70 to 80 weather ... some hammering above 80 ... sounds like a wrecking crew at 90+ … it gets hot hear in So Cal … can’t wait for global warming!

I wouldn't power wash it. Get some foaming coil cleaner, spray it down good and use moderate pressure from a garden hose to wash it. Pressure washing would certainly bend and destroy the fins.

Have you ever thought about replacing the unit if it is 30 years old it is at most a 10 SEER (seasonal energy efficiency rating) the new models are 14 SEER units and would save you about 30% in electricity costs..

Just wondering I have an old unit myself and will be replacing it this spring when the new City rebates come out,, the town I live in has rebates of up to $300 for replacing stuff in your home with higher efficiency stuff; doors, windows, furnaces, a/c's ect,,

I wouldn't power wash it. Get some foaming coil cleaner, spray it down good and use moderate pressure from a garden hose to wash it. Pressure washing would certainly bend and destroy the fins.

if you do do this make sure to spray from the inside of the unit out, spraying from the outside of the unit in will only compact the filth more

It’s getting hot here in So Cal this week … 90s the last few days

Started up the ac and working fine, no obstructions in ducting, clean filter, all fan motors working fine … results as follows:

Cooled inside of home to 73
Outside ambient temp = 85
Loud hammering sound at shut down for about 1 minute
Following temps when running

Large outside line = 65
Small outside line = 125

Large inside line = 55
Small inside line = 100

All temps taken with infrared temp sensor

The temperatures you have read are not out of range for an infra-red thermometer so I would say that you need to have an assesment done by an a/c tech.

If the operating pressures are within the normal range then your compressor could possibly need to be replaced.

I will almost bet there is a broken spring mount inside the compressor or your valves are leaking , allowing liquid refrigerant to feed through on shut down causing detenation.

At 30 years old I would agree with the other poster that a new high eff system will pay itself off in a short time.

Read back over theses posts. Know one seems to listen to what this guy is saying.

His compressor sounds like a train wreck when it shuts down. It's a 30 year old system with a 10 year old compressor. The system works fine according to Mike.

What's happening on shut down is the high pressure liquid is turning the comprssor in reverse because Mikes dischrarge valves are failing. The compressor of course does not like this.

Has anyone ever heard an early scroll compresor on shut down before the addition of check valves?

I see that the man has 3 choices.
1 Ad a liquid line solonoid valve to prevent the hp liquid from returning to the compressor (noise gone) and have the coil cleaned.

2 Change the compressor and have the coil cleaned.

3 change the whole thing.

I said clean the coil because althought you didn't provide pressures, from the temps you gave your condenser coil is dirty or the unit is overcharged (GregH).

Read the post!

Chris

Problem completely solved due to the knowledge, suggestions, and downright helpfulness gleaned from the tradesmen, contractors, and moderators who contribute to this forum. I can appreciate a guy who humps it for 8 hours, gets home dog tired, and can still find the energy to set at a computer and go thru this stuff.

Anyhow, somewhere in the posts, I read that my line temps didn’t seem right. So, I removed the can surrounding coils of the outside unit – compressor. I had previously replaced the fan motor and the inside of the coils were very clean. Upon removing the can, I immediately saw the problem. The entire outside of the coils were coated in a 30 yr mixture of fine sand, adobe dust, etc which had solidified into a layer of adobe … it looked like someone had applied a layer of stucco to the coils. A line pressure hose removed about 95%, I then sprayed it with a degreaser, and then gently power washed the coils (a previous post warned of possible damage to “feathered coils”, but nothing blew off. Someone might ask “why did you not notice this earlier?” The can on this compressor is louvered and you cannot see the outside of the coils without disassembling the can. From inside the can when replacing the fan motor everything was so clean, I just did not look further … my mistake.

The unit runs so well now that it almost puts frost on the interior walls and runs about half the time.

A lot of times the problem can be so simple that you can't see even though it's right in front of you.

One of my favorite stories is the Tech who puts his gauges on a unit and finds it's very low on gas and the unit was charge not long ago. He spends hours trying to find the the leak he is possative is there, somewhere.

So he finally calls an older Tech in his company with years more experiance. The older guy listens and asks "Did it ever occure to you that when you attached your gauges you stopped the leak?"

Sure enough he takes off his gauges and finds the gauge ports are leaking. Problem solved.

Chris