kwilcox
Active Member
As I suspected, you're cycling on the limit switch due to limited airflow through the heat exchanger which in turn is most likely caused by all the vents you closed. :angry: This is very bad and can result in a cracked heat exchanger. I'd immediately open up those vents again.
Zoning a forced air system w/o just venting excess air when takeoff or register dampers close is difficult. Even when done properly, dumping excess air into the basement doesn't result in any cost savings.
One way to do it is to increase airflow in non-dampered takeoffs with duct fans and then have the home automation system intelligently control those fans as well as the dampers to keep supply plenum pressure balanced. Results here can be mixed however as the furnace may still short-cycle (on the thermostat this time) because the heated area is now less than what it was originally sized for. This isn't as bad for the furnace but you will waste a lot of fuel because its never coming up to its fully efficient operating temperature. Kind of like the way a car wastes gas when it is only used for short trips.
edit:
Removing the filter fixed the problem because a filter normally provides some restrictions to the return air. These + the vent closures added up to insufficient airflow through the heat exchanger to keep the limit switch from cycling. You may be able to find a filter that flows more but don't depend on this for the fix as any clog or even normal dust accumulation will cause you to start limit cycling again. You may not notice this right away.
My system is AI controlled in the manner described above and the energy savings are impressive. However, to insure that everything is working correctly, I built a simple relay circuit that senses when the limit switch engages then latches the big red LED on my VisionPro thermostat. That way I can tell if the system has any issues. So far so good; it's only engaged once due to an unknown pre-filter still in the return side from an old electrostatic filter....
Zoning a forced air system w/o just venting excess air when takeoff or register dampers close is difficult. Even when done properly, dumping excess air into the basement doesn't result in any cost savings.
One way to do it is to increase airflow in non-dampered takeoffs with duct fans and then have the home automation system intelligently control those fans as well as the dampers to keep supply plenum pressure balanced. Results here can be mixed however as the furnace may still short-cycle (on the thermostat this time) because the heated area is now less than what it was originally sized for. This isn't as bad for the furnace but you will waste a lot of fuel because its never coming up to its fully efficient operating temperature. Kind of like the way a car wastes gas when it is only used for short trips.
edit:
Removing the filter fixed the problem because a filter normally provides some restrictions to the return air. These + the vent closures added up to insufficient airflow through the heat exchanger to keep the limit switch from cycling. You may be able to find a filter that flows more but don't depend on this for the fix as any clog or even normal dust accumulation will cause you to start limit cycling again. You may not notice this right away.
My system is AI controlled in the manner described above and the energy savings are impressive. However, to insure that everything is working correctly, I built a simple relay circuit that senses when the limit switch engages then latches the big red LED on my VisionPro thermostat. That way I can tell if the system has any issues. So far so good; it's only engaged once due to an unknown pre-filter still in the return side from an old electrostatic filter....
