Friday, December 11, 2015

A Nutty Furnace Problem

A common fault in condensing furnaces is a draft switch shutting down the furnaces. Before changing that switch, check to see if the switch is just doing its job. Most draft safety switches are open before the draft inducer fan starts. The ignition board looks to see that the switch is open before starting the inducer blower. If the pressure switch is closed before the draft inducer starts, the process stops right there and the board will flash a diagnostic code or report a fault in the case of communicating systems. Disconnect the tubing and wires going to the draft switch and check resistance between the two electrical connections, usually labeled “common” and “NO” for normally open. You should read infinity (OL). If you get any reading besides OL, the switch is bad. Assuming the switch reads open (OL), you want to check to see if the switch is closing, and at what pressure.  Put a tee and short piece of tubing in line with the tubing connected to the draft safety switch and connect a magnehelic or digital manometer to the branch on the tee. Remove the wires connected to the pressure switch and use alligator clips to connect meter leads to the switch connects and set the meter to read continuity. A meter with a continuity beeper works best so you don’t have to watch the meter. Start the furnace and observe the pressure reading when the meter beeps. You are trying to determine the pressure where the switch closes. Once you have that, compare it to the rating on the switch to see if the switch is opening at the right pressure. The correct pressure is usually on the pressure switch.    This can be pretty tricky because it happens quickly. Connecting the pressure switch to a tool that provides a controlled vacuum works better, but involves another tool.  There are electronic vacuum tools as well as simple rubber bulbs that can be used to produce a vacuum. If the pressure switch does not close and the pressure reading shows a vacuum of at least the pressure switch rating, the switch is bad. If the switch does not close but the pressure is not at the rating, the problem is that you don’t have enough draft. This can be caused by leaking vacuum hoses, bad inducer fan motors, loose or rusted induced blower wheels, or stopped up vents. Recently I learned about a condensing furnace whose vent was stopped up with acorns and plants. Squirrels were hiding acorns in the vent, which was discharging horizontally out of the house. When the acorns got wet from the furnace vent gasses they started to sprout, so the vent was stopped with plants and acorns. One way to prevent this would be to add a large mesh screen over the vent outlet. However, many manufacturers recommend against using a screen over the vent. They are worried that water will condense on the screen and freeze, causing an obstruction. In areas with a lot of squirrels, you may still want to add a large mesh screen (1/4” between wires) over the vent pipe because it is easier to remove an obstruction on the vent outlet than down inside the vent pipe. Removing the squirrels also works, but is easier said than done in some areas. There are more squirrels than people in my neighborhood.  


  1. Have also seen spider webs in vents. Also check for condensate drains that may be draining slowly or blocked completely.

  2. Have also seen spider webs in vents. Also check for condensate drains that may be draining slowly or blocked completely.

  3. This is really useful information. I kept getting strange readings and I did not know what they meant. I wasn’t sure about whether I should have the draft switch replaced or not. I have a handy friend and I’m going to have him come look with me since reading the correct pressure can be tricky as it happens so fast.

    Andy Jones @ AQS Comfort

  4. These are some excellent pointers to help find a diagnosis with tricky furnace problems! Another one that I personally encountered myself was with a clogged drainpipe. The furnace was not draining correctly since the water line was plugged up with dust, fur balls from our dog, and so on. Once I vacuumed these away, we were up and running. Just another thought!

    Lane Pemberton @ Metcalfe Heating & Air Conditioning

  5. That is a rather hilarious problem! I wonder if instead of using a mesh screen over the vent if perhaps creating a decorative grate around the area would be less risky for complications with condensation? I had to do this with my dryer vent, because leaves would stick to it and obstruct the vent, which already had a cover.

    Roxanne Vaughn @ Total Plumbing Inc.