Switching From AC to Heat: How to Transition Your HVAC System Safely This Fall

Autumn is an unpredictable season, especially in Colorado. It’s not unusual for a day that reached a peak afternoon temperature of 80 degrees to drop into the 30s overnight, or for the first snowfall to arrive in October. Switching from AC to heat after running your HVAC system in cooling mode for months requires more than a simple thermostat adjustment. You’ll need to be strategic about this transition to protect your equipment and prevent no-heat emergencies on freezing Arvada nights.

When Should You Switch from AC to Heat in Colorado?

Colorado’s shoulder season is notoriously tricky, with warm, sunny afternoons that quickly become chilly once the sun sets behind the Front Range. In general, you should switch from AC to heat when overnight lows consistently stay in the 40s or low 50s. At that point, your home will become cold enough overnight that you’ll welcome the warmth in the morning.

Dramatic day-to-night temperature swings force HVAC systems to work harder than they should. To avoid unnecessary wear, try using your thermostat’s AUTO setting or adjusting your smart thermostat so your heating and cooling setpoints are a few degrees apart. Doing this will shield your system from the excess strain caused by flipflopping between AC and heat on the same day.

Don’t worry if you notice a slight smoky smell the first time you turn your heat on. That’s a normal side effect caused by dust burning off the heat exchanger. But if the odor lasts more than a few minutes or smells worse than you remember, it’s a good idea to schedule a furnace tuneup in Denver before colder weather settles in across the metro area.

How to Safely Transition Your HVAC System from Cooling to Heating

Are you confident you know how to switch your thermostat from cool to heat correctly? Follow this checklist to prevent equipment strain, reduce safety risks and feel confident you won’t discover a furnace problem on the first freezing night.

1. Replace Your Air Filter

Summer is the most challenging time of year for your air filter, which may have become clogged with dust, pollen, pet hair and other contaminants after months of running your AC. Most homes need a new filter every one to three months to preserve proper airflow and alleviate strain on HVAC equipment. If it’s been a while since your last filter swap, replacing it before you turn on the heat can prevent problems during startup.

2. Test Your Carbon Monoxide and Smoke Detectors

Gas furnaces produce heat through combustion, making a carbon monoxide detector a nonnegotiable safety feature. Before the fall HVAC transition, test every CO and smoke detector in your house, replace the batteries and confirm each unit is less than 10 years old.

3. Clear the Area Around Your Furnace

Furnaces need breathing room to start smoothly and operate safely. Remove storage containers, paint cans, cleaning supplies and any other flammable items from the surrounding area. Then, walk through your home and ensure your supply vents and return grilles are all open and unobstructed, with no furniture, curtains, throw pillows or rugs blocking them.

4. Switch Your Thermostat to Heat Mode

When adjusting your smart thermostat, don’t forget that fall is an excellent time to revisit your custom comfort preferences and routine. Check the fan setting—AUTO is best for daily heating, while ON will continuously run the fan. Then, set a comfortable starting temperature of 68 degrees, a good baseline for this time of year.

5. Run a Test Cycle Before It Gets Cold

Don’t wait for the first freezing night. Turn the heat on during a mild day, let the furnace run through a full cycle and note signs like:

• Burning smells that persist for longer than an hour
• Strange noises
• Slow startup
• Cold air blowing from vents

Is It Normal for Your Furnace to Smell When You First Turn It On?

Dust collects on the heat exchanger when it sits idle for months, and that’s why switching from AC to heat will cause a burning smell when first turned on. Schedule furnace maintenance in Arvada, CO, if this odor doesn’t fade away on its own or you notice it has sharp, chemical qualities.

You also shouldn’t ignore these smells.

• Burning plastic or electrical odors can indicate overheating wiring or a failing motor. Turn the furnace off immediately and call us for heating services in Arvada.
• A sulfurous smell that resembles rotten eggs indicates a gas leak. Evacuate your property and call 911, then your gas utility provider, to report this dangerous problem.

Tips for Heat Pump Owners

Your seasonal transition will look slightly different if you choose heat pump installation in Denver because you have a single appliance handling heating and cooling. Here’s how to safely move into heat pump heat mode and enjoy efficient performance all season.

Unlike homes with a separate AC and furnace, a heat pump reverses its refrigeration cycle to provide warmth. You won’t have to remember to ignite a pilot light or turn on a secondary unit. Instead, your thermostat will trigger your heat pump to switch modes when temperatures drop.

Most heat pumps have two backup settings.

• Aux Heat turns on automatically in response to frigid outdoor temperatures or when the heat pump needs help reaching your set temperature.
• EM Heat, short for “Emergency Heat,” is a manual setting that bypasses the heat pump entirely and runs the backup heat source only. EM Heat should never be your default setting because it can lead to sky-high energy bills. Only use it in genuine emergencies, like when the outdoor unit is malfunctioning or frozen solid.

If you want a closer look at how these modes work, Lakeside’s technicians can explain everything during a routine maintenance visit, or you can check out our heat pump guide for more details.

Heat pumps run year-round, and covering the outdoor unit traps moisture, restricts airflow and can cause corrosion or mold. Leave it uncovered and promptly clear away snowdrifts and windblown debris to prevent them from piling up. Maintaining at least two to three feet of clearance around all sides will help your heat pump run more smoothly.

Why a Fall Furnace Tuneup Makes the Switch Easier

A smooth transition from cooling to heating starts with making sure your furnace is ready for the workload ahead by scheduling a professional furnace maintenance visit from Lakeside Heating & Air Conditioning.

During a fall tune‑up, our technicians complete a thorough fall HVAC maintenance checklist, which includes:

• Cleaning burners and the flame sensor for reliable ignition
• Checking safety switches to ensure proper shutoff and system protection
• Inspecting exhaust piping for blockages or damage
• Testing for carbon monoxide and gas leaks to keep your home safe
• Verifying airflow and electrical components for smooth operation

Preventive care reduces the chance of a no-heat emergency, improves efficiency and prolongs equipment lifespans. For year-round peace of mind, Lakeside also offers a comprehensive HVAC maintenance plan called the Residential Energy Savings Agreement. This plan delivers ongoing tuneups, priority service and other maintenance plan benefits that keep your heating and cooling systems running reliably in every season.

FAQs About Switching from AC to Heat

Is it bad to switch from AC to heat on the same day?

Not necessarily, but frequent seesawing can strain your system, especially during Colorado’s temperature fluctuations. Setting the fan to AUTO mode and putting your heating and cooling temperatures a few degrees apart will prevent unnecessary cycling.

Why does my house smell like burning when I turn on the heat?

Dust collects on the heat exchanger in spring and summer and burns off during the first few heating cycles. If the smell lasts longer than an hour or resembles burning plastic or electrical wiring, turn everything off and call a professional.

What temperature should I set my thermostat to in the fall?

Most Colorado homes remain comfortable at around 68 degrees in autumn. A programmable thermostat that automatically adjusts temperatures throughout the day will save energy during mild weather.

Should I cover my outdoor AC unit for winter?

We do not recommend covering your outdoor AC unit. Covers trap moisture, invite rust and can attract pests. Clear away snow, ice, leaves, branches and dirt so the unit stays clean and ready for spring.

How often should I get my furnace serviced?

Once a year, ideally in early fall. Annual maintenance keeps your system safe, efficient and in good condition all winter.

Transition From AC to Heat with Less Stress

Use the tips in this guide for a smooth, safe and worry‑free switch from air conditioning to heat. The transition doesn’t have to be stressful. If you notice anything amiss, Lakeside Heating & Air Conditioning is here to help.

For the most reliable start to your heating season, schedule a furnace or heat pump tuneup with our family-owned team in Arvada. We’ve served the Denver metro area since 1971, and it would be our honor to keep you and your family comfortable this fall and winter. Call 303‑421‑3572 or request service online to book your appointment.