Pool heater cycles on and off incessently - how to fix?

nick_shirleyJanuary 12, 2010

I have a Stayrite Max-E-Therm 400 gas heater that was installed when I bought my house in March 2009. It's a fairly powerful heater and when working correctly, it'll quickly get the spa or pool to a nice temp.

In recent months it's been doing something odd. The heater will kick in as expected right after the switch is flipped but after about 45 seconds or so, the "Service System" light goes on and the burner shuts off (but not the blower). After about 45 seconds, the red service light goes off, the green "Heating" light flashes, and then the burner kicks in again (the heating light goes solid at that point). This cycle repeats endlessly, significantly extending out the time needed to get the spa or pool to the right temps.

According to the manual, a "Service System" light is a good indication of some sort of flow problem. While I do have a variable speed pump that is generally set at a low flow rate for purposes of economy, this problem occurs regardless of whether I'm running the pump at the lowest speed or in high speed vacuum mode.

The manual also talks about the heat exchanger possibly being blocked through the accumulation of "scale formation on the plaster or in the heat exchanger tubing". This sounds plausible. My salt generator gets significant build up on the electrodes in just a few months, so much so that I have to dunk it in an 4:1 water:acid mix bath every once in a while to get the hard white deposits out. The culprit is overly alkaline water but despite dumping a whole lot of muriatic acid into the pool on a regular basis, I'm only at about 7.8 ph (and getting it that low was a relatively recent development).

So what I've wondering is whether I should try go see if there is a way to clean out the pipes in the heat exchanger. It wouldn't surprise me at all if over a period of almost one fiscal year the high pH water would have deposited enough crud into the heater's internal pipes to interfere with the flow and triggering the above mentioned cycling.

Kicking up the pump to high doesn't help dislodge the accumulation. Unfortunately, I can't see any easy way for getting access to the heating coil. If indeed there is no access to the coil directly, I was wondering if I should try blocking the heater's outlet line (the salt generator is in-line past the heater exit and should be easy to remove it and plug the pipe), removing the PVC pipe to the inlet line, and then filling up the interior with a mix of 3:1 water\acid. After letting the acid mix do its stuff, I'd drain the heater with the hope that any junk would have dissolved.

Does this sound like a good idea? Got any others?



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Pouring acid in the heat exchanger is not a good idea, ever. The acid will eat it up!

Post the rest of the pool's chemistry numbers.

I suspect that you have a very high calcium hardness level combined with a high pH. This causes scale to form. With high calcium hardness in the pool, I like to see the pH down around 7.3, give or take a tenth. This will also help your reduce your frequency of cleaning for the salt cell. This will help keep the calcium in solution.

Are there any yellowish/off-white deposits forming on the pool walls?

Test your fill water too. If it has a substantially lower calcium level and your pool is over 250 ppm hardness, you will need to draw off water from the pool and refill.

Salt cells tend to push a pool's pH higher. Adding MA is a normal and expected process to help keep it in check.

The only way to check this heater's exchanger is by a nearly complete tear down by a trained technician. This is not a DIY job. Given what you said about only recently gotten your pH down to 7.8 suggests you had an extended period where the pH was substantially higher, a condition that makes calcium form scale deposits fairly quickly.

If you did clog the heat exchanger with scale, it will need replacing and it won't be under warranty. No brand of gas fired heater that I know of would be since scaling is a poor chemistry problem.

I suspect though that the issue isn't scale in the heat exchanger yet. I suspect the the flow switch is possibly scaled and having having difficulty making it's contacts. Another potential is a clogged filter causing low flow.

A great resource for understanding pool chemistry is found at Trouble Free Pools web site. While I don't subscribe to the use of the BBB system, particularly if pets and small children are present, that they suggest, the rest is very,very valid and useful. I highly recommend it.


    Bookmark   January 12, 2010 at 6:23AM
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Pull out the thermal regulator and look behind for scale. If you have scale built up in the heat exchanger you will see scale deposits after you pull the regulator. It is located behind the threaded plug between the inlet/outlet connectors. You can unthread the plug with a screwdriver shaft. The thermal regulator may also be stuck closed. If you like you can re-install the plug without the regulator to see if the heater quits cycling. Another culprit may by the bypass which is located above the regulator,but hard to test without pulling the whole header.

    Bookmark   January 12, 2010 at 8:07AM
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High limit switch. You probable need to clean or replace the sensors. You could jumper it at the thermal sensor temporally to troubleshoot. Most heaters have led indicators on the back of the circuit board that will tell you which interlock is tripping.

    Bookmark   January 14, 2010 at 10:56AM
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If the heater is short cycling most likely your thermal regulator has failed due to bad water chemistry/corrosion or electrolysis issues. Replace thermal regulator and look at the old one to see if it has turned green or corroded. If this is the case you have to make sure your heater is electrically bonded and check your water chemistry on regular basis.

    Bookmark   January 14, 2010 at 10:21PM
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This has something to do with your pool water, have you check your water chemistry?

Here is a link that might be useful: Spa Heaters

    Bookmark   January 26, 2010 at 8:28AM
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I have the Sta-Rite Max-E-Therm pool heater as well, and it was cycling on and off every 45 seconds exactly as described in the first post of this thread (nick_shirley). I replaced the high limit switch based on another blog I found, and that didn't do it. Turned out to be the thermal regulator (part 77707-0010), which was stuck closed by scale as described by dapooltec above. Takes 5 minutes to pull it and replace it with the new part. Part sells for $55-57, not including shipping/tax, at several sites on internet. Scaling is definitely a water chemistry problem, but I suspect you're going to get some scaling on this part after 4-5 years even if your alkalinity/pH has been kept in good range. I have a concrete/rock pool and probably should have been adding acid more frequently after the pool was first installed, when the concrete is leaching heavily and pH tends to be very high/alkaline.

    Bookmark   October 4, 2010 at 6:27PM
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You folks are great! I finally decided to pull the thermal regulator (really just a small automobile-type thermostat) and tested it. Didn't move at all even when I dunked it in boiling water. I was hoping that just removing the gunk that had built up on it would help but after a while I realized that there was a tiny hole in the cylinder. Ordered one on-line for about $42 including shipping, swapped it in, and now my heater works perfectly. The replacement, as boatbuilder306 indicated, was a snap. Just unscrewed the access plug, pulled out the long spring, replaced the thermostat, and screwed it back in.

In reviewing my original post, I see that I claimed the "service system" light was on. Maybe it was at the time but since then it was the "service heater" light that was coming on when the heater kept cycling every minute or so. For those of you trying to diagnose this problem, the key is the intake water temp. When the spa or pool is 55 or 65 degrees, the heater works perfectly because the outlet temp is maybe 85. You think everything is OK and it has fixed itself. But when you get the pool up to 85 or above, the outlet temp starts getting near the point where the safety temp switch kills the heater. If the thermostat is working, it mixes the super hot water out of the heater with some cool water to keep it under 120 degrees. If the thermostat isn't working, the heater just turns off, then tries again, then turns off, etc.

This was probably related to the pool chemistry which had a horribly high pH for quite some time before and after my purchase of the house (the hole in the piston was probably corrosion related). I only added acid in small amounts as recommended. That might be a good practice for long term maintenance but when you are dealing with pH so high that Leslie's can't give you a read, then you need gallons of muriatic acid. I'm fine now and I expect that things will be OK from here on out.

Thanks again for your help everyone!

Nick in Palm Springs

    Bookmark   December 13, 2010 at 12:33PM
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