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I have 2 Laars Mighty Therm Lo Nox heaters on the roof at my laundry. They heat the water. Everything fine for about 6 years, now the flame roll out or fused link keeps breaking. The first time it lasted about 6 mo. Then 2 mo. then a week or 2 . Now they seem to last about half an hour. Couold the heaters be dirty (carbon build up)? Thanks for any help. model # PW0400Cn22NASN...Don

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Hi Don and welcome to FixYa, I am Kelly.
I first have to ask a question: Is there any sign of water being introduced into the firebox? Fused links open due to high current flow. High current flow is caused by several things:
1. Chassis grounds
2. Failed components
3. Corrosion on terminals
4. Hot terminals of wiring connections (even main power) on terminal blocks or components.
5. Clogged flue or litle air flow out the flue so the unit is getting very hot inside the chassis.

Since you have 2 units... I would be doing some serious resistance comparisons between the 2 units.

Take a look at Figure 16 on page 22 of this manual: (All 24 VAC related)
The fused link is the fuse for the fan switch, flow switch, Ignition BD, Gas valves and relay via the pump switch to K3. You need to compare the readings between the 2 units in this circuit. Somewhere you have a very LOW resistance that is cooking your fuse link. I think your going to find the problem is either with the ignition BD or the gas valves. If you suspect the ignition board as the probable cause move the boad between units. If the fuse link fails on the other you you know the board was bad. If the fuse link fails on the unit with the original symptom suspect a problem with the gas valves or relay K3.

Thanks for choosing FixYa,

Posted on Mar 30, 2011


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