Basics and finer points of troubleshooting gas furnaces

June 1, 2000
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With all of the air conditioners you’ve been installing and servicing lately, heating equipment probably hasn’t been at the top of your mind or your customers’. But once the mercury dips this fall, it won’t be long before you get your first service call.

The following checklist will help jog your memory on how to properly troubleshoot a gas-fired furnace.

In the beginning there was the stat

This may seem obvious, but before you begin troubleshooting difficult.

Note that some digital thermostats may require you to press a button to light up the display, which usually will not reset the system.

Some technicians prefer to bypass this step and check the thermostat from the basement with their meter. Then, as the last step, they check the calibration and operation of the thermostat.

Next, visually inspect the furnace and the basement. Make a note of your findings on your workorder, carefully taking the time to note the accessories or lack thereof.

Is the gas, electric and circuit breaker turned on? Is the vent connector on the furnace and the water heater rusted out, or does it show signs of water damage? Is there any water leaking on the furnace? Is the dryer vented correctly? Are there any duct dampers closed, or is the ductwork damaged?

Now that you are ready to troubleshoot the actual furnace, different steps must be taken depending on why the homeowner called you.

Double-check with the homeowner to be sure you understand the nature of the complaint. Taking a minute to do this could save lots of time later and possibly a callback.

The no-heat call

If it’s a no-heat call, touch the supply plenum. If it is warm, this usually indicates that the burner is cycling but not staying on long enough.

Take the cover off of older furnaces that use combination fan-limit controls. If the indicator dial is wound up to the limit setting, focus your attention on the blower motor and circuit. If it is not winding up past the “Fan On” setting, an ignition or thermostat problem may be causing it to short-cycle.

While the touch method also works on steam and hot water systems, exercise some caution and judgement when touching boiler piping. Since these pipes can get hot enough to burn you, it’s best to hold your hand 1 in. from the pipes for a few seconds to see if you can feel any radiant heat.

Never touch a vent connector unless you know it’s cool. A vent connector can give you a serious burn if you’re not careful.

If the furnace is dead cold, you need to begin a systematic troubleshooting process. The textbook method is to start at the service panel and work your way to the furnace. But since time is money, you can save time by splitting the system at the transformer.

On single-phase residential equipment, checking the transformer for 24 vac between R and C eliminates the need to initially check the line voltage circuit for power.

If you have less than 24 V, check the primary side of the transformer; you may have a weak transformer or less than the recommended supply voltage. You can then decide where to begin the troubleshooting sequence.

One problem with today’s furnaces is interrupting the line voltage power through a blower door interlock switch. This will cause the system to reset when you push the blower door switch back in, which can be frustrating if the system begins to initiate a call for heat and acts like nothing is wrong.

However, furnaces usually lock out for a reason.

The three most common types of lockouts are caused by loss of flame signal, a pressure switch trip, and a limit switch trip. These and other lockout modes will vary from manufacturer to manufacturer. Some control systems may also automatically reset after a specific period of time.

If the control system has a status code light, check to see if there is a service label that indicates what the light is trying to tell you. Some controls use short and long flashes that indicate what or where the trouble is. These lights usually will repeat the sequence until the fault is corrected.

Take the time to verify the correct sequence so you don’t confuse two short and one long flash with one short and two long flashes.

Electrical troubleshooting of line voltage and low-voltage loads is pretty straightforward:

First use your meter to see if you have power to the load. This works well for things like inducer motors and blower motors.

If you power the motor and it doesn’t run, the motor is not necessarily bad since PSC blower motors and some inducer motors have run capacitors on them. It is a good idea to check motors with a capacitor tester or a meter that gives you the direct MFD output of the capacitor.

This is no different than checking run capacitors on a/c condensing units. It makes one wonder how many PSC motors have been replaced because of a defective capacitor.

Circuit boards and switches

Circuit boards are also not that difficult to troubleshoot. Circuit boards are basic power-in, power-out devices that make decisions and select outputs to the loads based on specific inputs in a specific sequence at a specific time.

It is important to understand the sequence of furnace operation to understand how and when an output will occur and what input it is based on. Many installation instructions spell out what occurs during the sequence of operation. If you are unfamiliar with how a particular unit operates, take the time to review the manufacturer’s instructions.

The problem with troubleshooting circuit boards occurs when something in the middle of the sequence interrupts the call for heat.

One thing that will drive any service technician crazy is a switch that “flutters” (opens and closes) in the middle of a cycle. Since it is tough to guess when and what switch is causing the problem when there are multiple switches in series for a particular circuit, having a couple of 24-V test lights on hand can help you identify the problematic switch.

These test lights have alligator clips on them and usually work in the 0- to 24-V or 0- to 50-V range. These lights can be clipped across limit switches and pressure switches or, if you lengthen the leads, from the load side of the switch to the ground.

For switches that cycle open and closed, such as pressure switches, you can see when the switch closes and whether or not it opens during the cycle. This method is better than having jumper wires hanging all over the furnace.

Remember that if you put the light across the switch, the light will go off when the switch closes. If you go from the load side of the switch to ground with the test light, it will come on when the switch closes.

While it is possible the switch is bad if it doesn’t close or trips in the middle of the cycle, you need to rule out other factors like the inducer motor, vent system, and inlet gas pressure.

To check induced-draft furnaces, first verify that the voltage, current and capacitor output for the inducer motor are correct. If any of those factors are out of range, then the inducer’s rpm might be too low to achieve the proper draft through the heat exchanger.

Surveying the heat exchanger

The next troubleshooting step should be to check all tubing for cracks, kinks, and trapped water. Then clean all ports where the pressure switch tubing connects to the heat exchanger.

Do not stick anything into a pressure switch or gas valve tubing port, as you may damage the switch.

If these items are OK, measure the heat exchanger pressure drop to ensure that it is above the setting of the switch and does not fall below the setpoint of the pressure switch setting during the entire call for heat. This will require an inclined manometer that reads from 0 to 4 in., but there are some new, multirange digital models that also will work well in this situation.

Mid-efficiency furnaces usually reference atmospheric pressure:

1. Connect the negative-pressure side of the manometer to the hose between the pressure switch and the heat exchanger outlet or collector box with a tee fitting. Leave the positive-pressure side open to the air.

2. Condensing furnaces with sealed combustion usually reference a differential across the burner box and the secondary heat exchanger. The manometer needs to be connected between these points per the manufacturer’s instructions.

3. If the reading falls below the switch setting, check the vent system.  On mid-efficiency furnaces with proper pitch, look for a missing or crushed vent cap, a sagging vent, or the correct vent pipe rising off the furnace. The pipe off of the inducer should rise up at least 1 ft before elbowing toward the vent. This will prevent any turbulence or positive pressure from building up at the inducer outlet.

  •  On condensing furnaces, look for a restricted drain and trap assembly. A slow-running drain will cause water to fill up in the secondary portion of the heat exchanger, tripping the pressure switch. Make sure all vent piping slopes in the proper direction to keep water from being trapped in the vent system.

  •  It is important to remember that the heat exchanger pressure drop is also a function of the firing rate. If a unit is under-fired, the flue gas temperature and volume will not be at the manufacturer’s specifications. This may affect the reading on your manometer.

  • It is important to verify the firing rate of the equipment as part of your troubleshooting process, which is covered later in this article.

    Flame sensing

    Flame sensing and nuisance ignition lockouts, two other potential causes of a “no-heat” call, can be difficult to track down.

    It is important to remember that the required flame-sensing current will vary from one manufacturer who uses 0.8 microamps dc, to another who uses 5.0 microamps dc. Be sure to check the specs for this.

    Also, the minimum current required to prevent a lockout could be considerably lower than the nominal flame-sensing current. Make sure that all of the burners are aligned and the ignition is smooth. Check the gas supply pressure and the manifold pressure.

    Using two manometers, hook one to the gas supply side and one to the manifold side, which will let you see if the supply side does not drop below the minimum pressure required for the gas valve to operate properly. Then fire up the water heater to see if the supply pressure drops below the minimum setting.

    Note: You may need to turn on all gas appliances at the same time to verify that the supply pressure is adequate. If all of these steps check out, measure the flame-sensing current. On furnaces with a separate flame sensor, connect a dc microammeter in series with the flame sensor. When the burner stabilizes, measure the flame-sensing current and compare it to the recommended ranges for that equipment.

    Remove the sensor, clean it with fine steel wool, and recheck it. You should then see some improvement of the reading.

    Note: On equipment that uses the igniter as the sensor, it may be necessary to have a test box or special adapter to measure the igniter current while it is in the circuit.

    Both of these systems require proper polarity and a good earth ground to operate properly. When in doubt, make sure the burner and the control are grounded per the manufacturer’s instructions; if necessary, run a new ground to a code-approved grounding point.

    Poor performance = poor indoor climate

    Operation problems require a different approach to troubleshooting. Generally speaking, there is a cause-and-effect relationship to the type of operational problem.

    The best way to approach these situations is to think of the house as a system, with the furnace a subsystem of the house. Other related subsystems would be the duct system, vent system, utility system, and combustion air system.

    Duct system problems are usually indicated by either a complaint of not enough heat, too much heat in one area, noise, erratic cycling, and/or short cycling. Duct systems are sized to match cfm and velocity requirements of the house and the equipment capabilities.

    Duct-related operational problems can develop if these requirements or equipment capabilities are not met.

    Sometimes, multiple complaints about a system arise from one problem within the duct system.

    1. Begin checking the duct system by looking at the firing rate. Low input translates to low output, resulting in a low temperature rise and not enough heat.

    2. Next, check the temperature rise. (The rise rate is usually listed on the rating label.) Verify that the air filter is clean (check for multiple filters in different locations, or prefilters on electronic units); increase or decrease the heating speed to the midpoint of the temperature rise.

    Remember: An increase in fan speed increases the chances of a noise complaint. On the other hand, if the input is too high and the fan speed too low, limit trips could occur during the cycle, creating a lockout condition.

    If you add a short-cycle condition because of improper control settings or operation, you could end up with a limit trip that occurs after the call for heat has ended, resulting in delayed burner startup on the next call for heat.

    An undersized supply and/or return duct creates a high-static condition in the duct, reducing the volume of air to each room. This, in turn, could cause limit trips and/or other operational problems, such as noise.

    The best way to determine the correct airflow is to measure the external static pressure at the furnace and compare it to the manufacturer’s performance charts. Other methods include using the temperature-rise method, or measuring the velocity at each register and converting it to cfm with the chart supplied with the measurement equipment.

    Check for other duct items that may need to be corrected.

    • Has a “high-performance” air filter system been installed? Some have a high pressure drop and are so restrictive that when used on a marginal duct system, problems develop.

      Possible solutions include increasing fan speed, reducing duct restrictions, or replacing the filtering system with a lower-pressure-drop model.

    • Another potential problem is flex duct that is too small, kinked, or bunched up. Flex duct is a great product but like anything else in an hvac system, it must be sized and installed correctly.

      It is important to remember that flex duct does not move as much air as smooth metal duct of the same length. You may have to increase the size of the flex duct to get the same performance you would from metal duct.

      Vent, combustion air

      Vent safety switch trips are usually related to the vent itself, the combustion air supplied to the furnace, or both.

      • The vent size and installation need to conform to code requirements in the customer’s geographical area, the National Fuel Gas Code, and manufacturer requirements.

        Long, horizontal vent runs may require a larger-diameter vent connector than what is on the furnace. Type B vent is still the best choice for a vent connector on mid-efficiency furnaces. Condensing furnaces and alternative venting systems are usually required to follow the manufacturer’s instructions.

        • Other areas to look at are the vent cap, location of the cap, and height of the cap above the roof. Vent manufacturers can supply you with information regarding optional vent caps for their venting systems.

        • If the cap is close to the peak of the roof, it may be necessary to extend the cap above it. In some cases, adding 1 ft to the vent pipe is all it takes to correct a vent problem.

        • With construction methods and materials improving, air infiltration from the outdoors has been reduced and the number of exhaust devices has increased. One related problem may occur if the house goes into negative pressure, as air will come down the vent, spilling flue gases into the house.

        • To check if you have a negative-pressure problem, close all the doors in the house, turn on all venting fans in bathrooms, oven ranges, etc., and turn on the clothes dryer and water heater. Inspect draft hoods, vent connections and vent pipes for leakage of flue products.

        • Severe cases may require more sophisticated tests and possibly adding a heat or energy recovery ventilator (HRV/ERV) to offset the household air lost through exhaust fans and fireplaces.

        • If the return air grille(s) are in the same area as the furnace, such as in an adjacent hallway, then that portion of the building may be going into negative pressure. This will result in poor vent performance as well.

        • In some cases, placing transfer grilles over the door or undercutting the door to allow free air passage will help.

          The utilities

          Utilities are also a part of the system. Low voltage and low gas pressure may occur at regular intervals when demands on the system are at their peak. While rare, these situations have been known to occur.

          •  If the system typically fails or locks out during a specific time of day, try to arrive prior to that time to try to discover the cause of the problem.

          •  If the problem occurs when the unit comes out of night setback, try setting the thermostat staging to stagger the recovery period. If the problem goes away, one of the problems mentioned previously may be the culprit.

          •  A slow condensate drain or improper airflow, combined with a long run time, may cause the unit to shut down until normal operating conditions return to the furnace.

          While there are still many other steps that go along with troubleshooting a furnace, the most important thing is to understand how all the system components function and relate to the other components.

          Try to think beyond a failed blower motor or a nuisance trip and develop an understanding of what caused the failure or problem to develop in the entire hvac and house system.

          With that in mind, you can minimize troubleshooting efforts, satisfy the customer on the first visit, and help your business beat the competition.

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