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Posted by Harold on December 15th, 2002 11:44 AM
In reply to Lennox G32 gas furnace by Bill on December 15th, 2002 09:17 AM [Go to top of thread]

What Dodgeman and I would do would probably be different than most other folks. I would put a thermometer on the furnace bonnet and measure the rise across the heat exchanger to assure that the warrantee temperature is not exceeded regularly.

The heat exchanger is cooled by the air flowing thouogh the furnace. When it is reduced, the heat exchanger has less air coolant to release the heat of the flames. this forces the air temperature through the exchanger up toward or to the limit control which then works far more than it ever was meant to do, which is never, because the high limit is an emergency switch to be saved for the occasion of inadequate air flow - which should be never.

If the furnace reduces its input flame when you reduce the air so the air is heated to a temperature below the high limit, then you have 'gotten away' with your intent. You certainly would not want to stand next to a register with 150F air coming out or have plastic that melts at low temperatures nearby.

Some furnaces are over 87% efficient, which means they condense water. When the temperataure is forced over the condensing temperature to the high limit, they stop condensing; then a 94% efficient furnace becomes an 85% efficient furnace, so it loses 10% efficiency. It is possible to gain 10% fuel savings by shutting doors and using less fuel, but to lose it back by dropping out of condensing mode.

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