Home made swimming pool heat pump

[Gary]

Some years back, a friend asked me to have alook at his fathers pool heater.

A basic set up with a condensing unit, maybe 1hp, situated within his large green house and a stainless steel cylinder with copper discharge pipework coil running through it. It looked like a home made jobby.

When I connected my gauges on the suction rotolock, water came out

When It was stripped in our workshop, the discharge copper coil was disintegrating,

I managed to replace it, dry out the existing condenser, rebuild it and re-install it back into his greenhouse. It's still going strong

I used 316 S/S tubing though, not copper in the re-build.

I haven't worked on any S & T HX's for pools since, only brazed plate HX's

I'm wondering if the corrosion might have been the result of using dissimilar metals.

[Acuario]

I'm wondering if the corrosion might have been the result of using dissimilar metals.

Sounds like it to me - according to nobility table stainless steel is quite anodic and far away from copper which is cathodic with the result being that the copper will corrode.

[nevgee]

Sounds like it to me - according to nobility table stainless steel is quite anodic and far away from copper which is cathodic with the result being that the copper will corrode.

There is always that possibility but the majority of corrosion in swim pool apps is not due to electrolytic wasting but chemical reaction. That's why it is essential the chemical balance of the water is kept spot on.

[frank]

I'm wondering if the corrosion might have been the result of using dissimilar metals.

The corrosion was not at the point where the copper entered the s/s cylinder. It was the majority of the coil within the cylinder which was in constant contact with the circulating chlorine water.

[Acuario]

I've seen this happen with copper cold water pipes that were connected to galvanized iron fittings. I went into a customers garage and there was a fine mist of water being sprayed from the numetous micro perforations in the pipe. How it hadn't burst I'll never know. The whole length was pretty much the same. Water conducts electricity.

Also, years ago my dad worked for the gas company in the UK. In the late 60's/early 70's there was a shortage of copper pipe so they decided to install central heating with steel pipe. Total disaster, it all had to be ripped out and replaced due to electrolysis destroying the steel pipe.