Furnace Pressure Switches

As we start the heating season, we already have been getting questions about the vent pressure switches in furnaces.  I usually hear something like this, “I’m getting a flash code 3 – open pressure switch.  I jump the switch and the furnace works.  I’ve changed the switch twice and still have the same thing. Do you have a bad batch of pressure switches?”  What’s wrong with this diagnosis?

What’s wrong is that the tech did not do a complete diagnosis of the issue.  Flash codes are present to give the tech a direction. It is then up to the tech to see why that flash code is occurring , to use the proper tools, and see why the switch is not closing.  In the case above, all he did was jump the switch out.  He never found out why the switch was staying open in the first place. As with any safety, it is there for a reason and to respond to some unsafe condition.  Just by-passing the safety and seeing the unit work is not a diagnosis. It just verifies that the safety is doing it’s thing.

So, what do we need to know in order to make a proper diagnosis of pressure switches?  First and foremost, pressure switches react to pressure, in this case the vent pressure or the intake pressure. If it is out of range, the switch will not allow the unit to work because there is an unsafe condition in either the intake or vent or somewhere in the total vent process.  All pressure switches have a setting on them which is either the open or close point. It is usually listed as PF (pressure Fall) or PR (pressure rise) along with the setting number. ie: 0.9 PF would open if the vent pressure dropped below 0.9 IWC.  This is usually within a tolerance of 0.05 IWC (so the range in the example above is 0.85 to 0.95 IWC). 

Since these safeties react to pressure, then the first diagnosis is to measure the pressure. To do this, you need a magnahelic, manometer, or something that can read pressure. You need to do it while the system is operating so you need some tees to put into the pressure switch tubings so the switch is active while performing your test.  A typical hook up would look like this:

With this arrangement, you can now see if there is enough vent pressure to close the switch based on its trip point rating.  Without knowing the pressure, all you can do is guess.  If the pressure reading is below the trip point, there is something wrong in the  system and the switch is doing what it is designed to do.  This could be a restriction in the vent pipe, a restriction in the intake, a faulty inducer, a blocked drain,  a heat exchanger problem, sagging vent pipe creating a trap and holding water reducing the size of the pipe, lenght of the pipe, type of elbows used, size of the pipe, and so on.  Since 90+% furnaces are a category 4 appliance, the whole system includes the primary and secondary heat exchangers, the inducer, the vent and intake pipes, the drains, anything that the products of combustion have to travel through to get to the outside or bring combustion air into the furnace.  If any one or more of these has a problem, then the pressure switch is responding correctly and not letting the unit run.

If the pressure is above the set point on the switch, then we need to verify that the contacts in the switch are really open.  In order to do this with the unit running, you need to use the voltage drop method to check the switch.  The voltage drop method is a basic application of Ohm’s Law.  It simply states that the drop in voltage measured across a resistive load (in this case, the pressure switch) is proportional to the resistance.  Therefore, by measuring the low A/C voltage across the pressure switch terminals, we can determine: 

1. If the pressure switch contacts are closed, and
2. That the pressure switch impedance is low.

When the pressure switch contacts are closed, the meter should read no more than 0.8 VAC or 800 mVAC. (reference Figure 3 below)

When the pressure drops below the pressure switch setting of the switch, the contacts open and the meter will measure approximately 24 VAC (nominal furnace voltage) across the switch. (Reference figure 2 below)

In order to perform the voltage drop test, connect your meter leads to the pressure switch terminals as shown below:

By checking pressures switches this way, you can see if the switch and the internal contacts are good and not corroded causing a voltage drop that the control board could read as a bad switch.  If you have the correct pressure, and the switch is not closing or is showing more than 0.8 VAC voltage drop, now you can replace the switch. 

Some techs remove the leads and check continuity of the switch with the system running.  The problem with this is most digital meters are “auto-ranging” and can show a closed switch , when in fact, the contacts are corroded and dropping voltage. The preferred method is to always do a voltage drop test across the terminals of the switch.

So, to properly diagnose pressure switch problems, you need to  (1)  CHECK THE PRESSURE AT THE SWITCH and (2) do a voltage drop test across the switch. 

Keep in mind, the voltage drop test can be used for ANY SAFETY — limit switches, rollout switches, pressure switches, or anything that opens and closes contacts.  If you use this method to check switches, you will make a  more accurate diagnosis of the problem and be able to correct it instead of making multiple trips back and changing parts that really may have nothing wrong with them