Common Problems with Conventional Gas Furnaces

If your furnace is underperforming and not producing enough heat for your home — or worse, no heat at all and blowing only cool air — it’s one of the most obvious signs that something is wrong.

Potential reasons for lower temperatures include a thermostat setting that has been accidentally nudged out of position or heat registers that have closed up. If the furnace has no power, no heat will be produced.

Beyond these basic reasons, a mechanical failure could be the problem and you should call a professional.

If your gas furnace cycles on and off repeatedly (“short cycles”), it’s a sure sign that something is amiss. This problem is not only annoying — it’s costing you energy and will unnecessarily inflate your monthly bills.

Your furnace should switch off when the correct temperature is reached but should not be doing this constantly throughout the day. If it does, check whether the filters are dirty, exhaust vents are blocked or the thermostat needs attention.

The blower in your furnace should generally only activate periodically (when the system is heating or cooling) unless you want to filter the air or dry out a humid house.

Otherwise, the blower running constantly may be a sign of a problem that could put unnecessary wear on the motor. There are two main reasons for a blower running constantly:

1. The thermostat is set to the FAN setting, in which case you’ll need to change the blower setting on the thermostat back to AUTO.
2. The fan limit control switch (a safety device of your furnace) might be faulty, meaning components in the furnace do not shut off when the desired temperature is reached. If this is the reason, the fan limit control switch will need to be reset or replaced.

If your gas furnace is making loud banging sounds or rattling, high-pitched squeals or humming, it is usually indicative of a problem.

This often-minor issue can turn into an expensive problem to fix if you leave it. It’s usually best to call a professional to replace the filter and oil the blower motor, which may remedy the problem.

Conventional gas furnaces have a standing pilot light that may be extinguished by a strong draft, dirt in the gas tube or a dirty orifice in the pilot burner.

This is relatively easily fixed with some basic maintenance and even if the thermocouple is faulty (which causes the gas supply to shut off), a professional can replace it fairly easily.

Newer furnaces have an electronic ignition for the gas burners rather than a standing pilot light.

Typical problems may result in little or no heat being produced or repeated cycling. These problems need to be fixed quickly but the fix will depend on the type of ignition system in your furnace:

• An intermittent pilot system uses a high-voltage electrical spark to ignite the gas pilot, which then lights the main burners.
• A hot-surface ignition system uses a resistance heating element to ignite the gas burner.

Most simple fixes involve replacing the filter or checking the power and gas lines to your furnace. Otherwise, call a professional.

Thermostat issues can create uncomfortable conditions in the home, with lower or higher temperatures than desired.

The main thermostat problems are a faulty thermostat and mismatched furnace/thermostat, which leads to incompatibility and heating/cooling problems like cycling on and off too frequently.

Most thermostat systems used today are either millivolt, low voltage or line voltage systems. You need to ensure that the right one is installed with your furnace to avoid problems. If your thermostat does develop a problem, a professional should be able to troubleshoot and replace it fairly easily if necessary.