[Oregonian]

My Carrier 8000 gas furnace was used to replace the oil furnace which had 4"ducting. The ducting was replaced with 6" but the 4" registers were not changed. I replaced half of the 4" registers with 6" and the short cycles stopped. Now after 5 years they have returned. I found I have to be careful and not install a filter that is too restrictive or that contributes to the short cycles. Is thare anything else that could be done to stop the short cycles? My return vent is 14" x 25"
Thanks,
Dave

[drtom4444]

Yes, adjust the anticipator on the thermostat. It is a sliding variable resister where a metal piece is against a winding of Ni-chrome wire in the t-stat. On some it is circular and on others it is rectangular. Sliding away from the source of power shortens the cycle and towards the source lengthens the cycle, but slide too close and it will burn out. Look at: http://www.howstuffworks.com/home-thermostat2.htm http://homerepair.about.com/od/heatingcoolingrepair/ss/adj_anticipator.htm http://www.inspectapedia.com/heat/Thermostats6.htm
The anticipator adds a small amount of heat to the thermostat to shut off the heat after a certain amount of time. It has numbers to respond to the amperage of the gas valve coil or relay coil. When you changed units the amperage was changed and this caused the anticipator to be out of adjustment. There is also an anticipator for the AC which is a high value resistor behind the bi-metal strip to bring on the AC after a while. You cannot adjust the AC anticipator.
For the heater anticipator to work at all it will have to be adjusted. DrTom4444

[Oregonian]

Thanks for your reply Tom,
I can see my posting wasn't clear. Let say I turn off the furnace when going to sleep. I awaken to a house that is 62 degrees. I turn the funace back on when I wake up by turning the electronic thermostat up to 68 degrees. The furnace starts a cycle and instead of continually running until the house warms to 68 deg, it runs for 3 minutes, overheats and shuts down. It cools down and the call for heat is still on so it stats another cycle, runs for 3 minutes, overheats and shuts down again. This will reapeat for an hour until it finally warms to 68 deg. What can I do to keep the furnace running continuesly until it reaches 68 deg and stops the call for heat?

[drtom4444]

Your unit is cycling on the high limit switch then. You will have to provide more air flow to unit. The heat exchanger is getting too hot and the safety is shutting it down. Your return needs to be at least twice to three times the footprint of the furnace. Measure the width and length of the furnace and figure the return size. Three times the footprint with pleated filters would be best. You should duct in another return to increase the return airflow and enlarge the air ducts on the supply side. Generally, we figure 7" ducts on regular bedrooms and 8-9" ducts on the master with two 9" ducts in a living room and a 9" in a kitchen with a 4-6" in a bathroom. Bigger ducts are quieter as are bigger boots and grills. You are way undersized. An 8" duct will move about 1 ton of air for 400 square feet. Add all of that up to get the main plenum and go to http://www.engineeringtoolbox.com/air-duct-calculator-d_813.html to calculate the size you need. It's important to guide the air and use turning vanes and scoops to move it where you need it. DrTom4444