SOLVED: MasterTemp 400 Service Heater Light

huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
Hi there,

I have a 4-year of Mastertemp 400. Recently, we discover a family of mice were living in the heater and had urinated all over the control boards internally. The heater would turn on, the blow would turn on, and then the heater would fail to light and eventually light of the 'Service Heater' light. No service codes. I had a service tech come out and they replaced the internal Fenwal Ignition System and replaced the thermal regulator while we were at it. It was back up and running with no issue.

Fast forward 3 weeks again, I turned the heater on to use with the spa and it did the same thing, Service Heater. I opened it up to discover the mice had made this their home again! Well, I took care of the mice problem. I replaced the Fenwal Ignition System again because it was covered in mice urine. But this time it didn't solve my issue.

I noticed after a few days of trying, that the pool heater would light up on the first attempt of the day. That is, it sits overnight and its not used. I go out in the morning, turn the heater on, it lights up and runs fine. When it gets to temperature (100F) it shuts off as per my setting in EasyTouch Panel. However, at the point, the heater won't re-ignite no matter what I do, and eventually the Service Heater light comes back on. I check the internal box and I'm getting a LED light of 2 flashes 'Flame No Call for Heat' when this occurs.

I just replaced the Hot Surface Ignitor thinking this is was the issue and it exhibits the same issue. It will light the first time, stay on until it hits temperature, and then when the spa calls for heat again after cooling off the heater won't reignite.

What else can I replace on this to make it work? None of the wires look chewed on.
I have High Limit Switch, Automatic Gas Shutoff, and a new control board on order to see if this will fix the issue.

Is it possible I have water in the combustion chamber and if I do, is that fixable?

Thanks!
 

JamesW

TFP Expert
Mar 2, 2011
16,599
The Fenwal should have two test points for flame current fc+ and fc-.

Check the flame current using a multimeter that can measure microamps.

Flame Current Measurement

Flame current is the current that passes through the flame from sensor to ground.

To measure flame current, connect a True RMS or analog DC micro-ammeter to the FC+ and FC- terminals.

Readings should be 1.0 µA DC or higher.

If the meter reads negative or below "0" on scale, meter leads are reversed.

Reconnect leads with proper polarity.

Alternately, a Digital Voltmeter may be used to measure DC voltage between FC+ and FC- terminals. Each micro-amp of flame current produces 1.0 VDC. For example, 2.6 VDC equates to 2.6 µA.

A good burner ground that matches the control ground is critical for reliable flame sensing.

When not operating, the flame current should read 0 volts dc and 0 microamps dc.
 

huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
@JamesW I just did a measurement of this. I was able to consistently get a reading of somewhere between 22 and 23v. The heater lit up every time, for 4 times straight.

I've let it come up to temperature, over 100 degrees. It shut off as expected and then when it turns on to heat again, I got the Flame No Call for Heat on the Fenwal box and 0v across the Fc+ and Fc- leads. The only thing that has changed here is that the device came up to temperature after being off overnight. As stated before, cold starts always ignite but hot starts fail.

Do you suspect anything else might be wrong when its at temperature?

Update: Replaced High Limit Sensor - No change in behavior
Update 2: Went back out after an 1hr+ plus of waiting with heater off and the heater lights up again and runs fine. Is something get wet or hot that shouldn't? Again, no errors anywhere except for the 2 flashes on the Fenwal when it doesn't light after its at temperature.
 
Last edited:

JamesW

TFP Expert
Mar 2, 2011
16,599
Flame no call for heat means that the Fenwal is detecting a flame current when it shouldn't be.

Are you sure that the flame current is zero?

Do you have a good True-RMS meter that can test microamps?
 

huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
'Flame no call for heat means that the Fenwal is detecting a flame current when it shouldn't be.'
Thank you for explaining this, I've been looking everywhere to determine what in the world this means.

Attaching a photo of my multimeter, my guess is not?

I ran the spa again this morning, it got up to temperature, then cooled, and now it came back on again after a call for heat. My guess is that on one of these cycles it will go into the Service Heater mode again with the 2 flashes from the Fenwal.

So if my multimeter were to detect microamps, I would see voltage across the leads when it goes into this mode?

If this is the case, do you think it's the ignition system?

Thanks for your help so far, it's been refreshing to talk to someone who knows this.
 

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JamesW

TFP Expert
Mar 2, 2011
16,599
You want a meter that says "True-RMS".

If you're getting 2 flashes indicating flame no call for heat, then the Fenwal is detecting a flame current when it shouldn't be.

When 2 flashes, measure the current with a good true rms meter that can measure dc current in the microamp range.

Also, check for dc voltage.

DC amps and voltage should be zero when there is no flame.

Flame current is generated by an AC voltage being sent to the flame sensor.

When there is a flame, there are ions in the flame that can carry current from the flame sensor to the burner and then through the ground back to the Fenwal.

The current mostly goes in one direction, so it's "rectified" from ac to dc.

This is called "flame rectification" and it's how the Fenwal verifies that the burners actually light when the gas valve opens.

If the Fenwal detects current when the gas valve is closed, the Fenwal determines that there is an error and refuses to operate.
 

huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
Thanks for this information, I'm going to need to get a True RMS multimeter.
After replacing the ignition control, the igniter, and the high-limit switch, the heater now seems to be functioning reliably but not 100%. Yesterday, it consistently turned on every time when there was a call for heat for 6 hours. This morning it ran for a few hours before having the Service Light come back on during a call for heat.
 

JamesW

TFP Expert
Mar 2, 2011
16,599
If you're still getting flame no call for heat, it might be a bad ground or stray current or bad controller.

The burner and the Fenwal need to share a common ground to reliably monitor the flame current.

Since the current is so small, having a different ground reference can cause errors.

You might also have some voltage leaking to ground causing random currents.

Check for voltage between the main equipment ground and the heater chassis and between the bond wire and the heater chassis.
 

huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
I think you hit the nail on the head with this:
"The burner and the Fenwal need to share a common ground to reliably monitor the flame current. "

I went back into the box and sure enough the ground wires connected to the bottom of the box were absolutely covered in mouse urine, even had a hard time disconnecting them. I had never thought this would be an issue, I disconnected them cleaned them with some sandpaper and rubbing alcohol to make sure I had a solid connection to the box.

I've tested this about a dozen times so far with various temperature settings and this thing is lighting up every time. Though, I am going to keep testing it over the next few days before I get too hopeful. I'll post back here in the next couple of days to verify if this was the fix, in addition to the other components I swapped out.
 

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huggablejunk

Bronze Supporter
Nov 14, 2017
10
Marietta, GA
@JamesW Another day later and everything is working as expected, I'm marking this as case closed ;) Thank you for sharing your knowledge of the Fenwal Ignition System, its error codes are all but cryptic in the manuals. You saved me a bundle by being able to diagnose this and fix this myself and I'll be donating to this forum. Hopefully somebody else one day will stumble across this and solve a similar issue.

@ajw22 Mice proof is going in immediately, thanks for the tip. I can't believe the damage and headache this caused me that was totally preventable.
 
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