Furnace Gas Pressure and Ignition Control Diagnosis

An intermittent lockout is the furnace call that eats a day. The furnace runs fine while you stand there, then locks out at 3 a.m. Good diagnosis means understanding the control's logic and measuring the margins, not waiting to see a failure. A pressure switch that barely closes or a gas line that sags under load passes every simple check. Gas pressure work and live testing must be done by a qualified, licensed technician. The figures here are typical values, and the rating plate and the manual control every job.

What You Will Be Able to Do

• Measure inlet and manifold pressure, and know which one you are looking at.
• Tell a gas supply problem from a furnace problem.
• Explain how an ignition control proves flame and when it locks out.
• Test the safety circuit without bypassing it.
• Find the margin that causes an intermittent fault.

Inlet and Manifold Pressure

Two pressures matter, and they answer different questions.

• Inlet pressure is the gas supplied to the valve. It tells you about the piping, the meter and the regulators.
• Manifold pressure is the gas leaving the valve to the burners. It sets the firing rate.

The inch of water column, or in. w.c., is a small pressure unit. Twenty-eight in. w.c. is about 1 psi, so gas valves work at very low pressure. Two-stage and modulating furnaces have other values at low fire, so use the rating plate and the manual.

Measuring Pressure

1. Use a digital or U-tube manometer that you have zeroed.
2. Turn off the gas to the unit and connect at the correct tap on the valve.
3. Open the gas, and leak check your connection with a gas detector or a soap solution made for the job.
4. Measure inlet pressure with the furnace off. This is static pressure.
5. Measure inlet pressure with the furnace firing and every other gas appliance on. This is the working pressure.
6. Measure manifold pressure at steady state, at each firing rate.
7. Close the tap, and leak check it when you finish.

Warning: Every tap you open can leak. Leak check every fitting you touched, with gas flowing, before you leave. If you smell strong gas, stop, get everyone out and call the gas utility from a safe place outside.

Reading a Pressure Problem

Watch what happens to inlet pressure when the load comes on. A small drop is normal.

Worked example: a natural gas home reads 7.0 in. w.c. static. With the furnace firing it reads 6.2 in. w.c., and when the water heater and the range join in, it drops to 3.8 in. w.c. Static pressure was fine, so a simple check would have passed it. Under load, inlet pressure falls below the minimum on the plate. The furnace may then fire weakly, make CO or lose flame. The cause is on the supply side, such as an undersized gas line, a failing regulator or a meter that is too small.

Fixing this means changing the supply, not turning up the manifold. Call the gas utility for anything on its side of the meter.

If the valve has no voltage during a call for heat, the fault is upstream, in the board or the safety circuit.

Adjust manifold pressure only within the range the manual allows, and only to reach the plate value. It is not a way to derate a furnace for altitude. High altitude needs the orifice or kit the maker lists, and fuel conversion needs the listed kit and a new label.

How the Ignition Control Thinks

The control board runs a fixed sequence, and each step must be proven before the next one starts. Exact timings vary by board.

1. Thermostat calls for heat.
2. Safety circuit is checked. Limit and rollout switches must be closed.
3. Pressure switch must be open at rest, which proves it has not been stuck or jumped.
4. Inducer starts, and the pressure switch must close.
5. Prepurge runs to clear the heat exchanger.
6. Ignitor heats up, or the spark begins.
7. Gas valve opens for a short trial for ignition.
8. Flame must be proven within that trial, or the valve closes.
9. Blower starts after a delay.

If flame is not proven, the board retries. Many boards try three times and then lock out for about an hour. Some then reset themselves, and others need the power or thermostat cycled. Read the board's label and fault history before you reset anything, because a reset can erase the history. If the furnace runs fine after a hard lockout, the question is why it failed before.

Flame Sensing

The control proves flame by flame rectification. It sends AC voltage to the sensor rod, and the flame carries a small DC current back to ground through the burner. That current is measured in microamps, and a healthy signal is commonly 1 to 6 microamps. The maker's value decides.

A low signal has several causes:

• A dirty or misaligned sensor rod.
• A poor ground. The burner must have a good path back to the control's ground, and a loose or rusty ground screw is a common cause.
• A weak or lifting flame, from low gas pressure or poor air.
• A cracked sensor insulator.
• Reversed line and neutral on the supply. Many boards are polarity sensitive and show a code for it.

A furnace that lights and then drops out after a few seconds has failed flame proving, so measure the flame signal. If cleaning does not raise it, measure gas pressure and check the ground.

Testing the Safety Circuit

Limit, rollout and pressure switches are wired in series on many furnaces. They are safeties, and each one is telling you something.

Warning: Never jumper a safety to run a furnace. A jumper across a limit or a pressure switch can lead to fire or carbon monoxide in the home. Find the open switch with a meter, and find out why it opened.

With the system calling for heat, a qualified technician checks voltage at each side of each switch. Voltage on one side and none on the other shows the open switch. Then ask why it opened.

A switch is not a suspect until you have proved its signal is good. Measure the actual pressure with a manometer, then compare it with the rating printed on the switch.

Finding the Margin

Intermittent faults usually live in a small margin. Measure how close each item is to its limit.

Worked example: a pressure switch is rated to close at 0.60 in. w.c. On a calm day, the manometer reads 0.66 in. w.c. That barely passes. Wind at the vent or a partly filled trap can cut that reading by a few hundredths. The switch drops open and the board shows a pressure code. The switch is fine. The margin is too small, and the cause is a dirty inducer wheel or a partly blocked vent or drain.

Use the fault history, and write down what you measured so that the next visit starts ahead.

Replacing a Control Board

A board is rarely the first cause. Before you replace one, check that it has correct voltage in, the correct input signals, and no output where there should be one. Then look for a reason it failed, such as a shorted gas valve coil, a shorted wire or a failing transformer. Set any jumpers, match the model, and test all stages.

Worked Diagnosis

A furnace locks out every few days. The fault history shows failed ignition, with repeated flame loss. The technician before you cleaned the sensor twice.

The signal fading as the burner box heats up points to a poor connection. After you clean and tighten the ground, the flame signal holds at 3.2 microamps. The sensor was never the fault.

Common Mistakes

Check Your Understanding

You measured inlet pressure once, with the furnace off, and it was in range. What is missing? A working reading, with the furnace and every other gas appliance on, because a supply can sag only under load.

During a call for heat, the gas valve has 0 volts. Is the valve bad? Not yet. With no voltage, the fault is upstream, in the board or the safety circuit.

A furnace lights, then drops out after 3 seconds. What do you measure? The flame signal, then check the sensor, the ground and the gas pressure.

A pressure switch closes and the furnace runs. Why measure the pressure anyway? A switch can pass with a thin margin. Compare the measured pressure with its rating to see how close it is.

A natural gas furnace seems to overfire, and inlet pressure reads 14 in. w.c. What do you do? That is above the usual range and points to a failed regulator. Stop and call the utility.

Key Takeaways

• Inlet pressure tells you about the supply, and manifold pressure sets the firing rate.
• Measure inlet pressure both static and under full load.
• Never raise the manifold to make up for a poor gas supply.
• Read the fault history before you reset anything.
• Low flame signal can be a sensor, ground, polarity or gas pressure problem.
• Intermittent faults live in thin margins, so measure them.