Teledyne Laars heater troubleshooting

JamesR

Well-known member
May 18, 2015
126
Nazareth/PA
I have a Teledyne Laars Lite 400Kbtu propane heater Model LLD 400 that won't light. Ignition module # Fenwal 05-339011-003. If there are any repair guys familiar with this unit, here is my symptom...
Power to the unit = good. Propane to the unit = good. Pressure switch and all interlocks = good.
I have the 24VAC through all of the interlocks. When I call for heat, the 24VAC comes out of the temp control unit and makes it to the TH terminal on the ignition module. I hear the "click sound", so I know the signal is getting there and I measure it on the module. I am not getting the 24VAC back out of the module on the "Valve" terminal that go to the gas valve. Is there anything else the unit is looking for to be satisfied before it enables the gas valve? The only connections on the control module are line voltage which I have 240 VAC, TH from the thermostat which does go high when I call for heat, "valve" that goes to the gas valve solenoid, ignitor wires to the gas ingnitor and ground.
Based on what I see, it just looks like when the switch is set to heat, the 24 volts AC comes from the thermostat to the module which then turns on the gas valve and lights the ignitor. Seems to me a bad module or am I missing something?
 

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swamprat69

Well-known member
Mar 30, 2019
241
Las Vegas, NV
With 24V into the TH terminal on the ignition control the control should first apply power to the ignitor for a specified amount of time so that the ignitor can come up to temperature. At that time it should power the gas valve in order to attempt ignition. After the gas valve opens, it should should look for proof of ignition through sensing of a flame signal. Check ignitor resistance, normally 40-90 ohms. Check power to the ignitor during the ignition sequence. If both are normal and you still have no 24V out to the gas valve during the ignition sequence you should replace the fenwal ignition control. You just had the sequence of operation a little off in that the ignitor and gas valve are not powered up at the same time.
 

JamesR

Well-known member
May 18, 2015
126
Nazareth/PA
Thanks for the info. I did not check the igniter voltage. I will have to do that. I did measure the resistance yesterday though just to see if it were open. So I don't remember the exact value. I think it was 79 ohms??? I will double check after work today. One other thing I noticed is that when I am measuring the TH connector, it goes high (24 vac) when I call for heat but then drops back to zero about 5-10 seconds later. That i did not understand. I figured that signal was dependent on the water temp being below setpoint and the request for heat being on.
I also thought of flame sense, but cant see where that is on the schematic. There are no more connections on the igniter control module.
 

swamprat69

Well-known member
Mar 30, 2019
241
Las Vegas, NV
If the 24V signal to TH on the ignition control drops to 0 within 5-10 seconds it is not giving the ignition control enough time to go through the ignition sequence. Disconnect the wire from your main control to the TH terminal on the ignition control. Attempt to run the heater while observing the voltage on the wire that was connected to the TH terminal on the igntion control. If the voltage on the wire that was connected to the TH terminal on the ignition control drops to 0 while disconnected from the ignition control, then the problem is upstream from the ignition control. If the voltage at that wire remains at a steady 24V, then you may have a mechanical problem internal in the ignition control such as a relay not closing completely, especially if there is no initial output voltage to the ignitor.
 

swamprat69

Well-known member
Mar 30, 2019
241
Las Vegas, NV
When Hot Surface Ignitors originally came out many of them also used the Hot Surface Ignitor itself as the flame sensor. This was not an ideal situation as the hot surface ignitor was more sensitive to deterioration as a flame sensor due to aging and dirt accumulation than a separate steel flame sensor. Since the early days, you will hardly see any products being built that use the Hot Surface Ignitor as a flame sensor also.
 

JamesR

Well-known member
May 18, 2015
126
Nazareth/PA
Good call..... The signal from the temp control unit was dropping out intermittently. I bypassed it by jumping the 24vac right from the output of the pressure switch to the TH terminal of the igniter module to test it. Igniter glowed for about 10 seconds, gas valve opened and the thing lit up. I just ordered a replacement R0011700.
Thank you.