John Speake

My HVAC system needs to be flushed after a compressor failure.

I will replace the compressor, receiver/drier and expansion valve.

What's the correct method ? Can the old expansion valve be left in place while flushing is carried out ?

dr bob

John-

Each section/component is flushed individually, so the old expansion valve will be out anyway. Plus the expansion valve is the last restriction, so it and the line in front of it will have whatever made it past the drier.

Take the condenser out completely so you can reverse-flush it with ports down at the end. Do it a few times, until your flush solvent comes out perfectly clean. For commercial/industrial exchangers, a "froth flush" is sometimes used to generate critical velocities for moving crud out. Then follow with a light acetone rinse, blow it out with air, another rinse, until clean. Vacuum will get any solvent out, most important that all the moisture gets evacuated. I leave the pump on the car overnight to be sure.

John Speake

Many thanks Dr Bob !

Mrmerlin

FWIW for anyone reading this later, if you decide to flush the system with everything connected
( I know this is not a good thing to do)
what will happen is that the solvent will get on every O ring seal and damage them,
and cause them to swell ,
it will also do similar things to the flexible lines thus ruining them.

I would suggest that Dr Bobs methods are followed,
simply blowing out the flexible lines with compressed air should be sufficient.
Dont forget to replace the fuel cooler O rings after cleaning

dr bob

Hoses are always a concern. The removable ones are easy, and deserve careful cleaning. Especially the high-pressure hose, the first transporter of grenade fragments. I'm hesitant to solvent flush the hoses since the oil actually plugs the pores of the hose. Since rebuilding with new barrier hose is relatively cheap, I generally like that option better.

SeanR

Get yourself one of these and a gallon or two of brake cleaner.

John Speake

Thanks again for all the comments. I will be dismantling everything for access, but a local air con garage will do the flush.

Randy V

Do you have a rear AC system? That doubles the complexity of a flush.

John Speake

No rear air con. I've never seen a UK market car with it fitted.

dr bob

Someone must have a surplus RHD rear AC system for our friend John. It's the cool thing to have!

dr bob

Most consumer-grade brake cleaner is acetone with something added to slow the boiling. Aerosol acetone is a serious fire waiting to happen, if you need a science project. I'd be tempted to look at a first flush chem that's a higher boiler, then clean that out with the acetone. Outside. Definitely outside.

Awesome slow blue flame from acetone as liquid, haven't tried it with aerosol fog.

Bilal928S4

I am planning of replacing all the O rings in my 1990 S4. I have ordered a flush kit from Amazon. What liquid do I put in the canister?
In my small exposure to automotive wrenching, I have never seen brake cleaner in a bottle and not in an aerosol.

Bilal928S4

Or do I get this to flush?

Rob Edwards

Yes, that's the stuff. It's what comes in the bigger kit that FJC sells.

Bilal928S4

See below