I copied the following from a forum several years ago I have never tried it but may do so this summer, Sorry I don't remember the name of the original poster*
Steve W.
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"Ok, I've been refining this process now for a couple of years, and am ready to share it with the forum...
This works for ANY heat exchanger that can be isolated - genset, engine primary, transmission, and your AC system!
You will need:
1. Inexpensive 500gph bilge pump with a 3/4" outlet. 20 feet of 16ga wire and a lighter plug
(or suitable means to connect it to ship's 12V power)
2. About 15' of 3/4" hose.
3. A 5 gallon bucket.
4. 1 gallon of Ph-Ospho-RIC (from Home Depot, found in the paint department - about $13) This is phosphoric acid and a detergent.
5. Chemical resistant gloves and safety goggles are highly recommended.
To clean the H/E:
1. Close the seacock going to the H/E you intend to clean.
2. Remove the hose connections to the H/E. If it is a genset or A/C unit, there is a good chance the connection is 3/4" hose.
If so, you're in like Flynn. Mains will have a larger hose inlet typically; 2" is popular.
3. If the inlet is up to 1", you can get nylon hose-barb adapters at most hardware stores to adapt 3/4" to up to 1".
Buy the ones you need for your specific H/Es once. If the inlet is LARGER (e.g. 2") take one of the
hoses to Home Depot or Lowes and come up with the PVC fittings to make up an adapter; 2" hose, for example, uses a 1-1/2" PVC coupler,
a 1-1/2 to 3/4" threaded adapter, and a 3/4" hose barb to MIP nylon adapter. Fit up the pieces dry in the store and buy what you need.
PVC fittings require PVC cement. Make up the fittings you need to adapt to the hose sizes in question.
4. Connect the output of the bilge pump to the OUTLET of the H/E using the adapter if necessary and half the 3/4" hose.
Take the other half and connect it to the INLET, and run the hose back into the bucket. Place the bilge pump in the bucket.
THE BUCKET MUST BE ABOVE THE H/E YOU ARE CLEANING TO INSURE THAT ALL AIR WILL BE EXPELLED.
5. IMPORTANT: Remove all zincs from their plugs, and reinstall the BARE PLUGS. DO NOT LEAVE THE ZINCS IN THE HEAT EXCHANGER!
5. Add roughly 2.5 gallons of clean water (half full). Turn on the pump and insure that the pump retains prime while circulating the water.
6. Add approximately 1/2 to 3/4 gallon of the Ph-Ospho-Ric to the bucket.
Allow the acid to circulate through the H/E until the foam and bubbling on the surface of the bucket's liquid STOPS.
The amount of time this will require depends on how badly fouled the H/E is; it is not uncommon for this to take an hour or two.
Phosphoric acid will "parkerize" steel and iron along with removing all the rust (a good thing!), and will ignore the cupero-nickel
used in heat exchanger bundles. It will dissolve the organic, mineral and salt deposits, which is what you want.
The solution will turn dark as it works. This is normal.
When the bubbling stops, turn off the pump. Drain the solution back into the source bucket.
Reconnect the original hoses and immediately start the engine or other device, allowing it to run, to flush any remaining residue out of the system.
Make sure to flush the hoses and pump with clean water when you're finished, and reinstall the zincs in the plugs.
You're done."
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-- Edited by Steve on Monday 16th of May 2011 07:11:18 PM