I just bought a home and the Janitrol furnace (8 years old) is cycling on and off continuously. I had a HVAC tech out and he said I probably had a cracked HE. This was causing the furnace to cycle of the limit switch instead of the thermostat. He said I'd need a new furnace, and talked about the tax rebate and how this is an excellent time to do it. I had a second opinion out, and he did a complete heat-loss analysis of the house. He said the HE is fine, and it is easy to tell since their is no roll out. He said the intake airflow is 500 cubic feet too low, and the duct work into the furnace (9" metal pipe to 12" flex pipe) needs to be redone. He wants to add two return air mix box plenums, each taking two of the return pipes before connecting to the furnace, and a fifth 9" intake duct to the room above the furnace. He said this will solve the cycling problem. It's an $1800 estimate for this job. Does this sound like a legitamate analysis and solution? The house is 2700 square feet, and the furnace is 90% efficient and 120,000 btu's.

I have an 80% gas furnace that uses intermittent pilot ignition. Normally, once the flame sensor senses the pilot flame, the ignition control will stop sparking and open the main burner valve, and the furnace runs until there is no longer a call for heat from the stat. Lately, sometimes while the furnace is running, the main burner flames go out, while the pilot stays lit. But the control does not start sparking again. After several seconds, the main burners will ignite again. This is strange, since presumably if the main burners go out, it means that the control is not sensing the pilot and is turning off the main burner. But if it isn't sensing the pilot, then it should start sparking again. But it doesn't. The flame sensor is clean, there is always a good pilot flame impinging on the sensor, and the ground and wiring connections are good. I have confirmed that the problem is not the gas valve, as the 24 volts to the valve is dropping out when the burners go out. So it means that the control for some reason stops sending voltage to the main valve terminal on the gas valve. Any ideas on what could cause this? Ignition control going bad? The control is a Robertshaw SP-715A.