Why does my furnace sometimes turn off at night?

Why does my furnace sometimes turn off at night?

Shivering under a blanketOne Sunday morning, there were five different HVAC service companies on the same block of a residential street at homes all with Carrier 90% furnaces. All had condensation draining issues and I still had three other clients to get to that day.

A 90% efficient condensing furnace produces condensation. All 90% furnaces are piped outside through two-three-inch PVC pipes. One pipe is the air intake needed for combustion to the main burners. The second is the exhaust pipe to expel flue gases…much like a car. These furnaces only lose 90 to 100 degrees of heat through the exhaust when compared to older furnaces losing four to five times more heat through the flue. With this in mind, the 90% furnace can’t heat the flue piping as the older furnaces, thereby creating condensation, hence the PVC piping (plastic piping).

The condensation, with design, simply drains back to the furnace and is routed through a series of rubber hoses to an integrated condensation trap. These furnaces have a motor strong enough to pull in fresh air and expel flue gases. So a ‘trap’ is needed to keep the furnace from pulling negative pressure on the draining of condensation and monitored by a pressure switch, a safety device.

Herein lies the concern- the intake pipe is always pulling air inward for combustion to happen and at the same time, moisture, dust, dirt, and leaves can be pulled in as well. This debris travels through the piping, into the combustion chamber and is mixed with the condensation. As mentioned everything drains to the ‘trap’. Over time, especially during the summer, this debris and condensation sit at the bottom of the ‘trap’ decaying, drying-out and forming an almost cement-like blockage.

Here's Why Your Furnace Keeps Turning Off

These 90% furnaces may start-up and run fine at the beginning of the season as the blockage absorbs the condensation and may even drain with no negative results. However, as the winter season gets colder or during the coldest parts of the day, usually in the wee hours of the morning is when our furnaces work the hardest, the furnace may shut down as it cannot drain the condensation as fast as it is produced. Of course at this point one is frantically searching for a service company to get the heat back on. By the time a technician arrives, the furnace may have already started back-up and running fine. Again, it has had time to drain slowly and doesn’t produce as much condensation. If it happened once, it will happen again within the same time frame.

All the name brand systems, Carrier, Lennox, Rheem, Amana, etc., all have these condensation traps. However, from experience, Carrier seems to have the biggest design flaw or issues with these ‘traps’. With the same token, the only real prevention is annual maintenance as recommend by all manufacturers.

Other reasons why your furnace may keep turning off include:

  • Your furnace is too large for your home and is short cycling
  • Your furnace's heat exchanger is overheating due to low airflow
  • Your furnace's flame sensor is dirty

If you are experiencing concerns with your furnace,
call Sunshine Plumbing, Heating & Air at (303) 622-5526 to schedule an annual or repair heating service.


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