Mystery: Minimax 400 heater short-cycling, but flow valve replaced in Nov!

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aasarava

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Dec 28, 2015
32
San Diego, CA
How quickly can a flow valve go bad?

Back in November I had a short-cycling problem with my Minimax 400 (original Triton brand) heater. I replaced the flow valve which looked like the spring had elongated and no longer fit right. I also replaced the pressure sensor, which was rusted. Everything seemed to be back to normal.

Now, the heater is short cycling again. Just like last time, it happens when the temperature of the spa gets close to the set temperature. For example, if I set the spa to 100F with the starting water temp at 75F, everything works fine for a while. Then when nearing 98F or so, the heater cuts out -- the sensor light comes on briefly, cold water keeps circulating through the heater for a while, and then the heater comes back on a couple minutes later.

I checked the flow valve and the spring looks good and nothing is corroded.

Yet, when I measure the temperature of the steel manifold on the outlet side, it's getting up to about 150F -- which is about when the heater cuts out. This says to me that the high limit sensors are working fine, and that the water is getting too hot on the outlet side. (When measured on the inlet side, the water temp is around 95-99F).

So do all signs point to a bad flow valve again? Or is there some other potential culprit here?
 
Have you tried to increase the pumps speed to see if that changes anything? I also wonder if you have build up inside the heat exchanger that is restricting flow. That is a rather older unit and it's probably going to start getting finicky with things.
 
Hmmm. It's already pretty high -- 2700rpm. I bumped it up a tad to 2800 and it didn't make a difference. I can try higher tomorrow. But I will note that I haven't noticed any flow problems or decrease in the other pool functions, like the waterfall, all of which require flow through the heater (when it's off) because of the way things are plumbed.
 
Well if for some reason you are having water bypassed from the heater, you might not notice and issues with flow in the pool. Is the element still working as it should?
 
There's no bypass around the heater. All water has to go through it on the way back to the spa or the pool.

As for the element, I have to apologize -- I don't know what you mean by that specifically. The heater itself does come on without a problem and stays on until you get close to 100deg spa temp / 150deg water leaving the heater.
 
Actually it's called "power element" and it is located at the end of the flow control valve where the spring is at. When it gets hot, the internal pin moves to open or close the valve. Is this part working correctly? You can test by removing the valve and using a lighter, heat it up and see if the pin moves freely like it should.

pentair_minimax_power_element.jpg
 
Hi Paul,

OK, so the power element seems to be working fine. I put the whole flow valve in a pot of 115F water and the element pin extended and caused the valve spring to compress. After taking the valve out of the water and letting it cool for a minute, the pin retracted and release some of the tension on the valve spring.

So, the question remains, what could be causing the water to heat up so much that the high-limit sensors trip and shut the heater down? You mentioned possible build up in the heat exchanger. Is there a way to check and clean that out?

- - - Updated - - -

By the way, as I'm sure you know, there are two thermostat dials on the front of the control panel. One is for pool and one is for spa. This might be a dumb question, but would turning down the spa dial help with keeping the internal heater temp from reaching 150F?

(Would be nice if the solution is that easy, huh?)
 
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