How to troubleshoot a furnace that won’t work.

There are a few things that will cause any furnace to either stop running, or only deliver cold air, so here are a few easy DIY furnace repair tips that may get you warmed up without having to call an HVAC professional. It is at least a place to start, so you aren’t paying someone to come tell you that your thermostat is set too low. 

What to do if a furnace only blows cold air

The most common problem that prevents a furnace from running properly, and the first thing to check, is that the thermostat is set properly. Make sure the temperature is high enough to activate it, and that the thermostat is set to ‘heat’ and the fan set to ‘auto’. If that doesn’t fix the problem then the next likely cause would be the filter.

Dust and dirt restricts airflow, so if the furnace filter is clogged it will slow the movement of air enough that whatever hot air ‘might’ have gotten to its destination at a normal air pressure has simply cooled by the time it arrived. Also, preventing that air flow can cause the heat exchanger in the furnace to overheat and shut off too quickly. Try those two solutions and let us know if you notice any difference. 

What to do if a furnace blower won’t stop running

When the blower on a furnace won’t shut off, the first thing to check is that the thermostat is set to ON, and not to AUTO. If that isn’t the issue, the next most likely cause is a faulty relay. That’s not something the average homeowner would be able to fix, you would need to contact an HVAC professional. 

What to do when a furnace is noisy and rattling

If a furnace seems to be working fine but is making more noise than it used to, first check that the access panel is properly fastened. Over time screws can work themselves loose just from the vibrations of normal operation, or perhaps they weren’t tightened properly the last time it was removed and replaced. See if any screws on the access panel seem to be loose and tighten them, this may be the source of the problem.  

If it's not the access panel, then the noise may be originating from the blower fan. The fan is held in place by one or two set screws. These can also work themselves loose over time, and if the fan is not secured properly it will definitely rattle around. The same quick fix is possible, so check the screws and tighten them if needed and see if that makes a difference.

For another quick DIY furnace noise fix if those don’t work, check to see if there is any debris stuck inside the fan that may be impeding its movement. That about covers the easy DIY furnace fixes, the next most likely cause would be bearings in the fan wearing out.

Over time, worn bearings will cause something like a grinding sound of metal on metal, or maybe sound like a diesel engine idling. It may be intermittent at first, but as time goes on it can be more frequent, and increasingly unbearable to listen to. Whether it is you as a homeowner or a professional that does the work, there is not much besides replacing the motor that will fix that problem. 

Furnace is squeaking from a belt

Some furnaces have a belt drive motor than can wear down overtime, and may start to squeak just like the alternator belt in a car. If the belt is out of alignment or if it’s slipping, it may be something you can fix yourself by adjusting or tightening it. If tightening it in place doesn’t solve the issue then you may need to adjust its position, that is as good a time as any to remove it and clean it first. Only do this if it is within your general wheelhouse of knowledge and that you have the right tools, and be sure to make sure the furnace is shut off.

What to do if a furnace won’t run at all

If your furnace will not blow any air at all, first you need to find out why it is not turning on, and it could be any of the following easy fixes. Like above, the first thing to check is the thermostat; make sure it simply isn’t programmed to be ‘off’. Be sure it is set to ‘heat’ and the fan is set to ‘auto’. 

If that doesn’t activate it, next look in your electrical panel to see that the fuse or breaker is blown. If your electrical panel has fuses, you could find an identical fuse in the panel to swap out for a moment as a troubleshooting measure. If it begins to run, you know you need a new fuse.

If your electrical panel is on a breaker system, first check that it isn’t tripped, but flick it off and back on anyways as that may reset it. Next, locate the furnace power switch that is likely on the side of the furnace and make sure that it’s on in case it has been accidentally bumped to the off position. 

Some units have a cut-off switch that prevents the furnace from running when the blower door is not properly sealed, so you could open that, located that switch if it has one, and make sure it moves freely and that it is activated when you put the door back on. 

Troubleshoot a high efficiency condenser furnace that won’t work

With high efficiency condenser furnaces, water is created as a byproduct and it would have a pump to remove that water. There is probably an overflow safety switch wired to the furnace that will shut the furnace off if the pump fails. There should be a small reservoir you can pour some water into to check that the pump is working. 

Check safety switches on modern furnaces

Newer and modern furnaces will have safety switches that set the furnace into a lockout mode that will shut down the furnace when it is not safe to operate. These include the flame sensor, pressure switch, flame rollout switch and limit switch. If the switch is tripped it will try to cycle on a few times, but if it can’t successfully start it will go in a hard lockdown, possibly for an hour or longer. 

Here is the point where you are going to need your furnace manual to find which system is not operating properly. There are no easy fixes we can recommend, as this is the point where you are likely going to need to contact a professional. There is always a chance you can be walked through a minor fix over the phone, and having your manual on hand as well as a bit of troubleshooting info to offer a service person would be helpful and may spell the difference between a quick fix and waiting for a service call. 

One last important thing - if troubleshooting or fixing a faulty furnace and the furnace runs on gas or propane, be very careful not to attempt any repairs to any of the gas parts like the valve or pilot light area. Leave this to a certified professional. 

And if you smell rotten eggs or hear the hissing sound of escaping gas, it could be a natural gas leak which might be very dangerous, so:

1. Stop what you’re doing. Don’t use your cellphone or landline, don’t smoke, light matches or operate electrical switches or create any other source of ignition. 
2. Get outside immediately. As you exit, leave the door open behind you as well as any windows that may already be open.
3. Report it. Once outside, CALL 911 or your local emergency number, and your gas service provider’s 24-hour emergency line.
4. Turn off the gas tap at the meter outside if you are confident you know what and where it is. 
5. Stay outside, tell the official if you turned off the gas tap, and stay outside until an official says it’s safe to go back in.