Mastertemp 300 - SOLVED! - Cycling -

[bongardjm]

Been fighting a cycling Mastertemp 300 heater for most of the summer season. It would start to cycle once the temperature would reach around 88 degrees, cycle again and then eventually the service heater light would come on. This would occur when heating the spa or pool.
So I:
1. Checked error codes and there were none.
2. Made sure I had good flow, backwashed a few times, cleaned filter...still cycled..uggh!
3. Removed the thermal regulator , seemed discolored, replaced and it still cycled: uggh..
4. Bought a new bypass valve as maybe it seemed like it stuck a little?, replaced...still cycled..ughh
5. Bought a new high limit switch, gotta be bad right?..replaced..still cycled...uggh!
6. New Control Board!..this is it right!, replaced it..still cycled..uggh.


At this point it was starting to get personal!! Ha and a little $$$. But I was determined to fix this thing without calling a service rep. In the back of my mind I always knew it could possibly be the cooper tube sheet being corroded and preventing proper flow of water or heat, but also knew it is a pain to check, but I trudged ahead, took the heater basically apart and below is what I was greeted with:




I went ahead and ordered a new tube sheet, cleaned and de-rusted the bowl it was in, got it installed and now the cycling is gone.
Don't know why I did not get any error codes and my chems are also in line, never had algae etc... Maybe the salt water? Maybe that's just the life?
I don't have a bypass so water is always flowing through it, think maybe I'll add one. The heater is about 5 years old, but none the less it is SOLVED!

 

[JamesW]

Are you sure that there were no LEDs on the board coming on?

If it was cycling, the high limit LED was probably coming on.

Possibly due to scaling inside the heat exchanger.

 

[bongardjm]

I ran it multiple times with it cycling and never saw any LED's. Even replaced the control board and still no LED's came on. It seems to be fixed though so I'll go with it. But like you I wish I would of seen an LED and then replaced the heat exchanger and saw it go away. There was quite a bit of scaling.

 

[ps0303]

I ran it multiple times with it cycling and never saw any LED's. Even replaced the control board and still no LED's came on. It seems to be fixed though so I'll go with it. But like you I wish I would of seen an LED and then replaced the heat exchanger and saw it go away. There was quite a bit of scaling.

I see it happens where you get a unit cycling and no error LED pops.

 

[JamesW]

If the heater is shutting down before it reaches the set point, there's a reason.

Most reasons cause an LED to light up.

The high limit will clear when the water cools. So, you might not see the LED unless you look right away.

Loss of flame rectification will cause a shutdown without an LED coming on.

 

[Munkustrap]

All GREAT ideas and suggestions! For what it's worth, I ran it with all safety switches bypassed, but had FULL monitoring on them, so that I could instantly tell if the flow/overheat/air/140 degree gas overheat switches opened with the cycle - none did, and the cycling continued. As far as I know, this cannot be an overheat issue, as all overheat sensing was totally bypassed, although MONITORED. SOMETHING was causing the burner to cycle on-and-off. Even manually checked the output temp from the thermistor. So my theory is one of the following is happening:

1. Intermittent flame rectification, causing the burner to cycle on and off. This might never give a fault indication, except for the flashing amber HEATING light, which it is doing during the "off" cycle.
2. Bad "board sensing." That is, the control board is thinking that the output temperature, measured at the thermistor, is exceeding the cutoff max temperature. This would cycle the burner without error lights.
3. Bad igniter unit - not properly sensing rectification signal and cutting off gas intermittently.

As there does not appear to be any excessive temperature of the hot water, evidenced by NO tripping of either of the limit switches, I think I can eliminate the probability of a flow or overheat issue.

Thoughts? Thanks!