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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.