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The problem will not be with the flame sensor as the sensors function is just to verify the burners are lit.
The first thing we need to make sure of is that the circulating pump is on when the combustion blower is on. To do this, locate the pump wire on the HM30 TDSI board and move the wire to the L1 terminal (disconnect power to the system first). When you re-apply power, you should hear the pump running (the pump is located on top of the tank but below the combustion blower).
If the pump is running, go ahead and see if the high limit trips with the pump still wired to L1. It is possible that the board isn't sending voltage through the pump terminal (you can check with a multi meter between neutral and pump terminal to see if its getting 120 vac when the combustion blower comes on).
If it's still tripping the high limit the problem can be a bad thermistor. The thermistor is what monitors the tank temperature for the TDSI control board. I'm looking at the manual to try and find the ohm range, but haven't found one yet. I may need to call Lennox tomorrow when they're open to find that out for you.
On a side note, Lennox has a Complete Heat replacement program. Lennox will give you a G60UHV 2-stage variable speed gas furnace and a hot water heater. They will also pay for the labor to replace the water heater (up to $200 I believe). You would need to pay for the labor to replace the furnace, however. This is supposed to be for cracked water tanks, but Lennox has approved it for other problems as the parts are becoming obsolete.
Tried what you suggested to troubleshoot the problem. Cir. pump works and yet the high limit trips after a cycle. It points to the thermister or is it a thermocouple? How
do I test that device. I know that it has a resistance value and pretty good idea on its tank location. Remove two wires and test resistance still mounted in tank?
Can you suggest where to find a replacement part if it is the thermister? I wish that I had a better schematic to work with other than the one shown on the panel cover.
Thanks for the info about Lennox replacement program for that unit. Can you head me
in the right direction to pursue possible replacement.
Here's a good document for you: http://tech.lennoxintl.com/PDFs/9821d.pdf
On page 18 it talks briefly about the thermistor and shows the location. For some reason, none of the engineering handbooks or service manuals give a resistance ratings of the thermistor. You can test the resistance with the thermistor in place, but I need to see if I can get a resistance range for you.
A replacement thermistor doesn't show up on the repair parts program. I'll ask Lennox for a part # XXXXX that as well.
As far as the Complete Heat replacement program, you will need to call an authorized Lennox dealer for that. Put in your zip code at www.lennox.com/residential to find a dealer near you.
I'll get back with you when I know something.