Blower Works with AC but Not Heat (7 Reasons + How to Fix Them)

If your blower works with AC but not with Heating, then you’ve got a furnace problem or air flow problem that we can troubleshoot and fix, sometimes DIY. Let’s look at reasons the blower works with ac but not heat. Solutions are included.

Note: If your burners continue to fire and the blower isn’t starting, shut off the furnace immediately and don’t use it until it is repaired. This is rare, since your furnace is loaded with safety features to prevent such occurrences, but if it happens it can obviously be a fire hazard.

Reasons Why the Blower Works for AC – Not for Heat

There are a number of possible reasons why your blower motor worked just fine all summer long cooling your home, but now that winter is coming, the furnace blower motor is not pushing out any heat.

Start working your way through these common issues. It begins with those most likely to be the problem when the blower works with AC but not heat.

Faulty Thermostat or Wrong Setting

The most common and easy fix is that your thermostat is not set on “auto” or “heat,” but on “cool” instead. Just check to see if you need to switch the settings on the thermostat to get the necessary heat you desire.

If the thermostat is older or a cheap model, or both, then it might just be broken and need replacing. If you’ve been considering a smart/intuitive or WiFi thermostat, this might be the time to install one.

Ignition Problems

Spark Ignition

If the gas igniter for your furnace is not working properly, your furnace will not supply heat to your home. First, check to make sure that the gas to the furnace is turned on. The gas cock, or valve handle, should run parallel to the gas line. If it is perpendicular, it is off.

Take the door off your furnace, and you should find a viewport – a small hole in the sheet metal with a glass window. Through it, you might be able to see the igniter heat up – it’s also called a glow plug, and you’ll see why. Also, if there is a safety switch that pops up when you take the furnace door off, you might have to hold or tape it down for the furnace to operate.

Resetting the ignitor, if it has a reset button you can push, might solve the issue. If resetting the ignitor does not energize the igniter and you are getting gas/propane to your unit, you most likely will have to replace the ignitor to get heat to your home. You can also try cleaning the igniter, a pretty easy DIY task, and it might solve the issue.

Furnace Not Igniting? 6 Common Causes & 5-Minute Fix

By the way, if you have a furnace with a pilot light, it must be lit for the burner to work and the blower to come on. And it might also be time to look into getting a new, efficient furnace that is under warranty.

Clogged or Dirty Filters

Dirty or clogged air filters will cause major problems with the entire heating and cooling system in your home. You might be able to still get air conditioning to circulate throughout your home, but not heat. It could be that dirty filters reduce air flow to the point that your furnace overheats, and the high limit switch, another safety device, shuts down operation, possibly before the blower comes on.

You should also have the right type of furnace filter for your system. If this issue began just after you replaced the furnace filter or filters, perhaps the new filters might have a MERV rating too high to allow adequate air flow – and as a result, causing the furnace to shut down prematurely. Learn more in this Pick HVAC guide called When you Shouldn’t Use a High MERV Filter.

Bad Draft Motor

The draft motor is the first thing to start, the first noise you hear, when the thermostat calls for heat. The motor ensures that the burners can be fed oxygen and also that the path for exhaust to leave your home is clear. Listen for the draft motor – a quiet hum – as soon as the thermostat calls for heat. If you don’t hear it, then your draft motor isn’t working. If it doesn’t work, nothing else will.

Here’s information on the draft motor, how to replace it and replacement cost.

Flame Rollout or Bad Flame Rollout Switch

When air is properly flowing into the burn compartment and out of the furnace, a draft is created that pulls the flames into the furnace where they belong. If there’s a poor draft, the flames will “roll out” of the burner ports into the furnace cabinet where the heat will quickly be detected by the flame rollout sensor, and its switch will shut down the gas valve often before the blower turns on. If your furnace has a view port where you can see the flame, watch it as it starts. If it “backs up” rather than being sucked into the burn chamber, you’re seeing flame rollout and should start by checking for a clear furnace vent. Dirty burners might also be the issue if your furnace hasn’t been cleaned in several years.

Cleaning furnace burners isn’t fun or easy, but it might help and can save you money. Here’s how to do it.

A bad flame rollout switch can “think” you’ve got flame rollout even when you don’t. It will shut down the burners all the same. For diagnostic purposes only, and if you’re comfortable working on a gas appliance, you can briefly bypass the flame rollout switch to see if it’s the issue. The switch is easy to replace and affordable.

Plugged Exhaust

If the draft motor starts, but the burner and blower never start, it could be that the sensors are showing that there’s a blockage somewhere in the system. Check the vent for debris or a nest. If the unit is vented through the roof, check for a buildup of ice. The exhaust heats snow, which then might freeze when the furnace is off, causing a blockage.

Dirty Furnace

If you have a lot of soot or dust built up around the furnace blower then it might not work properly. A dirty blower motor will not be able to push the air over the heat exchanger. This will result in the blower overheating and not pushing out the desired amount of heat.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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