Hi pool guy!

I’ve got an older Pentair Minimax 75 on my spa. Works great, no functional problems, but it just started making a rattling noise when water is flowing through it. Sounds like it’s coming from the manifold area.

A video of the problem is here:

https://youtu.be/ll_lIO4Q1h4

Makes no difference whether the heater is on or off, simply having water flow through it makes the noise happen. Happens from the minute the water flow starts until it stops. Turning the jets on increases the speed of the rattle, letting it fall back to normal circulation decreases the speed.

I own a stethoscope with a probe, I can’t hear any difference when I probe various areas of the manifold, they all sound the same.

Seems like there’s something loose in the water flow. I looked at the parts diagram and the only thing I can see that could come loose is the flow valve (ref 30 on the breakdown). I took it out and it looked like the hole in the plastic plunger was no longer round – like it had worn – so I thought perhaps the rod in it was rattling around in operation. Bought a new one and replaced the entire assembly. No change. May actually be slightly louder.

The pressure sensor (not shown in the diagram but screwed into the manifold on the left side) looks ok, nothing broken on it. I took the manifold off and looked and can’t see anything loose in it, there is no build up of anything in there or in the heat exchanger tubes. One of the heat exchanger tubes (the center one) had deteriorated to the point where the copper was gone except for a small piece at the bottom, I added a layer of RTV to it (along with the gasket that was there) when I put it back together but no change.

You’ve got a few videos addressing this.

Discusses the water boiling, but my rattle happens with the heat off.

Discusses a similar sounding problem but theirs went away when the heater was off, mine doesn’t. You mention a thermal regulator problem, I can’t see such a part in the breakdown on mine.

https://www.manualslib.com/manual/903553/Pentair-Minimax-75.html?page=24#manual

You also mention scale build up. I bought the heater used so I have no idea of it’s history but I doubt it has ever been descaled. How does scale build-up cause a rattling sound? When I had the manifold off I looked into the tubes of the heat exchanger with a flashlight and could see nothing but copper down the tubes. The heater is old but was not used for many years until recently.

I can descale it “just because”, but doing that is going to mean I will need to buy a second flange assembly to attach the fittings to it, I’d rather not cut the PVC off it, which is what would be needed to attach fittings to it to run a descaling pump through it. Thought I’d ask the question before going through all that…

Thanks!

Todd