Potential High Limit Switch Issue

[frustrated]

I have an old Hayward H300 natural gas heater. Despite being 21 years old (there is no digital display on the unit for example, that is how old it is), It still ignites faithfully every morning and heats up my pool to the mid 80s without an issue. However, when I switch to spa mode on my Jandy Aqualink, the spa only heats to about 95/96 degrees and then the heater shuts off and cannot be re-ignited for a few minutes. The spa temperature is clearly set at 102 degrees. I suspect that the issue is with a High Limit switch. However, my heater has two high limit switches: one for 135 degrees and another for 160 degrees.

My question is: which one is most likely to be the problem?

Any insight would be much appreciated. Thanks in advance.

 

[ajw22]

Replace both high limit switches. No way to guess which one.

You can test the switches using a multimeter, a candy thermometer, and a pot of water on a stove, and see when they open.

 

[frustrated]

Thanks AJW22. I will try those tips for troubleshooting and most likely replace both.

 

[1poolman1]

I would suspect your temperature control before the limit switches. Regardless of the setting on the Aqualink, the heater will only heat to the temperature that the heater itself is set. That's why the temp settings on a pool heater are set to the highest possible setting when automation is installed. Set the heater to 90 and the Aqualink to 100 and you will only get 90.
The limit switches are in place in case something internal to the heater water management system goes bad and water starts coming out of the heater at those temperatures.

A good way to tell is to put you hand on the heater outlet pipe while it is running. It should be warmer than the inlet, but not hot.

 

[frustrated]

Thanks "1poolman1" and "ajw22." Well, I replaced both high limits this weekend and that clearly was not causing the problem as the heater cut out again when the spa temperature reached 95 degrees. However, I did have the time to do some extensive trouble shooting over and observed/heard something which I am hoping will be helpful in diagnosing the problem:

I physically sat next to the heater as it attempted to heat the spa to its set temperature of 102 degrees (please note this temperature was set on the Jandy Aqualink and the dial on my heater was turned up to maximum - I only have a dial on my heater, not a digital display). When the temperature rose (I was monitoring the temperature level via my iPhone using the Jandy Aqualink app) to 95 degrees, I clearly heard the sound of a valve or switch (opening and/or closing) within the heater unit itself. About 10 seconds later, the heater cut out. The heater then made a few attempts on its own to reignite. It would successfully reignite, stay lit for about 5-10 seconds and then cut out again. This happened two or three times and then it cut out for good. It would not/could not be reignite(d) for at least an hour or so after it cooled down.

I tried the exact same experiment another time and observed the exact same behavior. Based on this information, do you have any idea what the problem may be and what part I should try replacing next?

Any insight or suggestions would be appreciated. Thanks!

 

[ajw22]

This is not a matter of replacing a part. You need someone to diagnose why your heater is overheating someplace in it and triggering one of the safety systems to shut it down. Could be something like a scaled heat exchanger. We can only guess.

You need someone who can dig into the heater, disassemble it, and look for what may not look correct.

There is a lot of information on the heater operation in Section 6 of the H300 Manual.

Maybe @swamprat69 @ps0303 has ideas.

 

[1poolman1]

Thanks "1poolman1" and "ajw22." Well, I replaced both high limits this weekend and that clearly was not causing the problem as the heater cut out again when the spa temperature reached 95 degrees. However, I did have the time to do some extensive trouble shooting over and observed/heard something which I am hoping will be helpful in diagnosing the problem:

I physically sat next to the heater as it attempted to heat the spa to its set temperature of 102 degrees (please note this temperature was set on the Jandy Aqualink and the dial on my heater was turned up to maximum - I only have a dial on my heater, not a digital display). When the temperature rose (I was monitoring the temperature level via my iPhone using the Jandy Aqualink app) to 95 degrees, I clearly heard the sound of a valve or switch (opening and/or closing) within the heater unit itself. About 10 seconds later, the heater cut out. The heater then made a few attempts on its own to reignite. It would successfully reignite, stay lit for about 5-10 seconds and then cut out again. This happened two or three times and then it cut out for good. It would not/could not be reignite(d) for at least an hour or so after it cooled down.

I tried the exact same experiment another time and observed the exact same behavior. Based on this information, do you have any idea what the problem may be and what part I should try replacing next?

Any insight or suggestions would be appreciated. Thanks!

Try and locate the source of the sound. If it is in the intake/return manifold it would probably be the internal bypass (all heaters have them).
Every heater school I ever attended starts with the fact that (literally) 90%+ of all heater service calls have nothing to do with the heater but are a water-flow problem. In my experience at least half of the rest are a user error. Then we have automation issues.

Are you using a VSP? May just need to increase flow to solve the issue. At 21 years, it really is time to start thinking about a replacement.

 

[frustrated]

Thanks everyone for your suggestions. "1poolman1:" to address your question directly, I do not have a VSP. I will continue to troubleshoot and report back here with anything notable I may discover.

 

[ajw22]

When was the last time you cleaned your filter?

 

[frustrated]

Thanks again ajw22. I disassembled the filter about one month ago and cleaned out all the baffles with a garden hose. So, I don't think that is the issue as the pressure has been fine. I think my next step will be to replace the bypass valve (plunger, spring, valve, etc.) since that is relatively inexpensive and seems like a somewhat easy repair.

 

[ajw22]

Have you done a Overnight Chlorine Loss Test?

Algae can easily slow flow through a filter in a month.

 

[Poolbreh]

If you have an external bypass close it. Like mentioned above if flow is good you will probably need to tear into it, check sticking unitherm and bypass is intact. Make sure exchanger doesn’t have soot build up triggering the limiter switches, ventilation is adequate.