Mastertemp 250 new failure

Bazul · Monday at 8:15 PM

Just when I think I know how to fix anything that comes along with my heater a new year brings a new mystery.

Last year I'd had it with mice and attempted to screen the heck out of my heater to keep them out. Every year they get in and have caused progressively more problems. I even left 2 victor traps set inside.

So this spring I fire up the heater and...Yes! It fires right up. I bring the pool up to a balmy 82 and the grand kids actually get to use the pool right away in the spring instead of hearing my excuses of why it's not open yet. It runs a week or two no issues. I'm cleaning up leaves in and around the equipment left from last fall when I think to myself I should check inside the heater to make sure those mouse traps are empty and my mouse proofing really worked. Open it up and both traps have mice in them. Well, at least they didn't break the heater, I empty the traps and close up the heater.

I lower the temp on the pool because it won't be used for a week or two. Upon return, two things start (or stop) happening when raising the temp again. When I bring the temp back up (from Intellicenter panel, the portal or the phone app) the

Heater begins startup (blower starts) but the VSF pump doesn't raise it's speed as it normally would. I have 2 circuits- Pool that usually runs a lower volume (30 gpm) so my SWG operates or Heater that usually raises up to around 3,000 rpm when the heater turns on.
Heater blower is running but doesn't fire up.

So I figure lets take the automation out of it for starters, I remove the wire that goes to the fireman's switch and jumper it so the Heater panel is the only control. Here's the current sequence:

Manually raise VSF pump to 3000rpms just to be safe (I normally don't have to do this by hand).
Green light on the SWG reports flow is good.
Turn on Heater at the heater controls. 128...r=8. Normal revision startup.
On the Heater panel I raise the temp several degrees above current water temp.
Service light flashes a second then the blower starts.
Service light goes out
Heater light starts flashing
About 15-30 seconds
There's a click and the service light flashes a split second but goes out.
Blower keeps blowing.

This appears to keep going indefinitely. I figured it would stop after X number of retries and I'd get some diagnostic lights on the back of the panel but no. It's just keeps going. No diagnostic lights on the underside of the heater control panel and no warning lights flashing on the Fenwal. I never smell a hint of gas and the gas meter indicates zero gas flow. I've checked all wiring, no mouse action that I can see. In the past I've both checked the ground AND added the additional ground wire folks have recommended.

I've re-read the Master Temp further reading and I'm not coming up with a line of investigation other than start replacing each sensor one by one which I'm hoping someone here can help me avoid.

I would expect one of these to help me but I don't see come on other than Service Heater for that split second when I hear the click but it goes out and the heather light goes back to blinking for another 15-30 seconds while the blower keeps blowing.

Diagnostic LEDs:

.....PS (Pressure Switch)
.....HLS (High Limit Switch)
.....SFS (Stack Flue Sensor)
.....AFS (Air Flow Switch)
.....AGS (Automatic Gas Shutoff).
.....Service System
.....Thermistor
.....Heating
.....Pool On
...Spa On
...Service Heater.

Thoughts on sensor that's blocking progress?

ajw22 · Monday at 10:28 PM

Sounds like a flame sense problem.

Heater Flame Sensing - Further Reading

www.troublefreepool.com

How much corrosion is in the heater due to all the mice's urine?

The flame sense current runs through the grounds, and corrosion blocks the low current.

Mice can trash a heater and cause strange problems.

Bazul · Monday at 10:50 PM

I know my grounds are good I went over the top on those during previous mouse urine incidents. Plus we're not releasing any gas at all, we're not even to the point of having a flame to be sensed.

ajw22 · Monday at 11:01 PM

Check if you are getting 24V at the gas valve.

You need to follow the start sequence and see how far it gets.

After the blower runs, the igniter is powered to get hot, then the gas valve is opened. The Fenwal module manages the start sequence.

Using a multimeter you can track the voltages. The Wiki explains the sequence.

Bazul · 2025-05-20T23:39:22-0400

OK. Turns out to be pretty simple I think. It's the HLS (High Limit Switch). It doesn't seem to behave like documented in the Wiki but I think that's because what's documented is probably how the heater acts when the switch is working and signals a problems it's in charge of. Meaning the wiki document more about what to do when the HLS signals a problem. Where my HLS sensor appears to be not working from the get go, so it's behavior is immediately signaling High Limit before any fuel fires and the water is still only 60 degrees. I jumper'd it when I noticed that the HLS diag LED flickered on for an instant then the cycle started again. When it's jumper'd the heater fires up normally.

Question, I ordered another one but since it's failing right out of the gate before anything fires and I know water can't be at a high temp, it's fair for me to assume that there's no reason to think that the other symptoms associated with a bad HLS should be in play (like insufficient water flow, bad flow sensor, etc) right?

While I wait for a good one to arrive how long would you trust a heater to run with the HLS jumper'd if I monitored the water temp? Short term could I rely on the stack flue sensor to signal an overheat?

ajw22 · 2025-05-21T03:12:22-0400

Bazul said:
it's fair for me to assume that there's no reason to think that the other symptoms associated with a bad HLS should be in play (like insufficient water flow, bad flow sensor, etc) right?

While I wait for a good one to arrive how long would you trust a heater to run with the HLS jumper'd if I monitored the water temp? Short term could I rely on the stack flue sensor to signal an overheat?

Nope.

Running the heater with the HLS jumpered can damage it internally if you have multiple problems.

You are focused on the external water temperature and do not know what the temperatures are internally in the heater that the sensors are there to protect.

BTW, the AGS sensor should shut down the heater before the SFS.

However that assumes the other sensors are operating properly.

We don’t recommend operating heaters with sensors bypassed other than for testing.