Vent Temperature as a Diagnostic

On 90+% high-efficiency furnaces, what should the vent temperature be?

To start this discussion, you first need to understand how the venting system is tested for a 90+% efficient furnace.  Temperature testing is done to assure the materials (ie. schedule 40 pvc pipe) are within the safe working range of the equipment.  This test is always run at the worse case conditions:

• with the furnace in a closet
• at the maximum temperature rise condition
• with the unit cycling on high limit
• and at the maximum outlet temperature

With the furnace working at the above conditions, the median vent material temperature cannot exceed 167 degrees F.  The median temperature is the average of the measured outer surface of the material and the measured combustion gas temperature. I’ll give you an example — with 100 degree F measured outer surface temperature on the vent pipe and a 140 degree F combustion gas temperature, then this would equal a 120 degree F median material temperature which would be within the safe operating conditions for the vent material.

So, how can you use this information as a diagnostic?  To make it simple, the typical combustion gas temperature for a 90+% efficient furnace should be in the 100 to 130 degree F range. This usually equates to a median pipe material temperature value of 86 to 100 degree F.  When this temperature starts to exceed 130 degree F, it is usually an indication that there is a problem.

This temperature should be taken inside the vent pipe, approximately 8 to 12 inches outside the furnace cabinet.  Drill a small hole in the vent material and insert your thermometer so it is about in the center to the pipe and see what the combustion gas temperature is inside the pipe. (When you are done, be sure to seal the hole with some RTV silicon caulk  or a stainless steel screw).

So what does all this mean and how can it help make a diagnostic? Let’s look at a scenario.  You have a job that keeps having repeated inducer failures. All you voltage are correct. All your drains are clear and there is no evidence of water getting into the inducer causing it to possibly fail. Why are the inducers failing?  If you measure your vent temperature and find it up in the 150 degree F range or higher. It is an indication that you are over heating the inducer causing the failures.

So what do you look for?

• Check Manifold Pressure to insure it is within range of furnace’s Rating Plate — Usually 3.5 IWC For Natural Gas and 10.0 IWC for Propane       (LP).
• Check Temperature Rise across the Heat Exchanger and Verify that it AND the Maximum Discharge Air Temperature are within range of Furnaces Rating Plate.
• Check Blower Wheel for cleanliness and centered in scroll housing.
  • If excessively dirty and/or show signs of being used during       construction (Sheet rock Dust) pull blower assembly and check/clean the secondary coil and blower wheel.
• Check Static Pressure (see post on how to do this) Add the Negative Pressure from blower compartment with the Discharge Pressure out of the furnace before the coil. (-0.3 + 0.3 =  0.6 Static) Residential Furnaces are rated at a maximum external static pressure of 0.5” w.c.
• If possible, “Clock The Meter”, due to high gas BTU content, furnace may be over-fired. Follow the procedures in the installation instructions.

Any one or all of the above conditions can cause the vent temperature to rise.  Can you think of other instances where measuring the vent temperature could help  — limit cycling — dirty secondary heat exchanger — dirty blower — high static duct conditons — firing rate?

The point I am trying to make is don’t over look the diagnostic of measuring the vent temperature on a 90+% efficient furnace. It can give you some valuable insight to what is happening with the furnace.