Mastertemp 250 - short cycling, low exhaust gas temp

[dw886]

My Mastertemp 250 was installed in the fall of 2016, and is on it's 6th swimming season. Last year I replaced the SFS, the Thermal Regulator, and the ignitor (mostly in part to the heater short cycling and the breaker tripping, two different issues that I had during the year).

This year after starting it up, it's short cycling again. I can set my watch by what's going on, it ignites for 15 seconds, stops for 35 seconds, and repeats. There's quite a bit of corrosion on the exhaust (and the landscape rocks below the exhaust), which I'm thinking is also in part to the short cycling last season and this season and condensation from low water temp and the cycling.

For troubleshooting, I started looking at the exhaust gas temp (by pressing the Pool button for 10 seconds). It never gets beyond 156, and from what I've read this isn't considered in the "normal" range. I've seen plenty on what can cause Exhaust gas temps to be high (and short cycling), but not much on what can cause them to be low. Any thoughts as to what would cause it to short cycle with a LOW exhaust gas temp?

I'm trying to determine if I'm just at a point where I pull the trigger on a new heater, or if I continue to replace parts on it. It functioned fine for the first 4 seasons, but has been having issues both last season and this season.

 

[ajw22]

Disassemble the header and check the bypass valve.

You have an internal water flow blockage in the heater and with little water flowing through it the heater quickly overheats and shuts down.

 

[dw886]

Thanks @ajw22 . Just pulled it off, and there was definitely some copper in there. Not sure where this came from as the ends of the heat exchanger were all still in tact.

I put it back together, and it's still short cycling, and the exhaust temp is low both by the display (120 - 150F) and by an IR heat thermometer pointed at the exhaust (~130-140F).

 

[ajw22]

What did your bypass valve look like?

What does the water flow channels in the header look like.

Did you flow water through the coils and confirm they are not clogged and flow water easily?

It looks like most of the water flow is bypassing the heat exchanger and flowing in one side of the header and out the other.

Even if the bypass valve looks good I would replace it.

 

[JamesW]

Check the gas pressure.

 

[dw886]

The manifold all looked good (outside what I washed out of there, which I'm assuming is copper from something), and so did the bypass valve. I didn't think to flow water through the coils. I'll disassemble it again and do that to double check.

@JamesW - Gas Pressure is something that I wanted to check as well - What's the proper way to check the gas pressure? Line is high pressure to the pad, and then I have a 7-11" WC regulator about 6" before the gas pipe enters the heater.

 

[dw886]

OK, so I pulled it apart again. Here's some pictures from the manifold in case something looks out-of-whack. It all looks good to me. The bypass valve moves freely, etc.

Here's a video of the water flowing through the manifold, seems to move freely to me: 20220529_132941.mp4

 

[JamesW]

 

[JamesW]

You can get a digital or analog manometer.

It should come with a 1/8" NPT brass barb fitting that will screw into the gas inlet tap port.

Don't do anything unless you are sure that you can do it safely.

 

[dw886]

The video makes sense. The part that I wasn't understanding is how to test line pressure vs what the heater is consuming, but hooking the nanometer up and then letting it cycle and watching for drop makes sense.

One other observation that I'll throw out there - the heater exhaust not only feels "wet", when the heater does cycle it makes *a lot* of water vapor - like it looks like it would if it was 20 degrees outside when you can see your breath. It continues to make water vapor throughout the whole cycle. Not sure if this is a clue, but I've never noticed this before. I'm sure this is why my exhaust shows so much corrosion and why it's always wet below the exhaust on the landscape rocks.

 

[JamesW]

Remove one of the bottom manifold bolts to see if water comes out.

If water comes out, the exchanger is leaking and filling the combustion chamber.

Did water come out of these two bottom holes when you removed the manifold?

They look wet to me.

If yes, the heat exchanger is probably leaking.

 

[JamesW]

This shows the gas pressure being tested at the inlet tap test port.