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Professional HVAC technicians share the 6 critical tasks Denver homeowners must complete before freezing temperatures hit. Prevent breakdowns and stay warm all winter
Once you hear about a cold front coming to Denver, prepping your home for winter becomes a top priority. But while you're gathering firewood for cozy evenings or sealing gaps around your windows, there's one critical system you don't want to overlook: your HVAC.
Not preparing your home's heating system can be a recipe for disaster during freezing temperatures. A furnace that fails at 2 AM when it's 10°F outside isn't just inconvenient. It's dangerous, potentially leading to frozen pipes, property damage, and even health risks for your family.
To know specifically what tasks you should focus on to keep your heating system running strong, we spoke with our professional HVAC technicians who've served Denver Metro homeowners for over 16 years. Here are the six critical tasks to complete before temperatures drop below freezing.
Related: Just as your HVAC system needs winter preparation, so does your plumbing. Check out these 6 essential plumbing tasks plumbers recommend before a freeze to protect your entire home.
How Does Freezing Weather Affect Your HVAC System?
While freezing temperatures certainly affect your personal comfort indoors, they can also push your HVAC system to its absolute limits. According to our technicians, Denver's combination of high altitude (5,280 feet), extreme cold, and rapid temperature swings creates unique challenges most homeowners don't realize.
"When temperatures plummet below 20°F, your furnace isn't just working harder, it's working in conditions it wasn't designed for at sea level," explains one of our senior HVAC techs. "At Denver's elevation, combustion efficiency drops, airflow reduces, and components that would normally last 15 years can fail in 10 if not properly maintained."
But what's particularly concerning is that most furnace failures don't happen gradually. They happen suddenly during the coldest nights when your system is under maximum strain. One moment you have heat, the next you're scrambling to find emergency service while your family bundles up in blankets.
The stress of continuous operation during a deep freeze can expose weaknesses that seemed minor during mild weather. A dirty flame sensor that caused occasional hiccups in October will shut your furnace down completely in January. A blower motor bearing that squeaked a bit becomes a complete motor failure. Small problems become big emergencies.
So essentially, freezing weather doesn't just test your furnace. It exposes every deferred maintenance item, every shortcut in installation, and every component that's nearing the end of its life.
How To Prepare Your HVAC System Before It Freezes
Thankfully, there are proven ways to prepare your heating system for Colorado's coldest weather that dramatically reduce your risk of mid-winter breakdowns.
1. Schedule Professional Furnace Maintenance NOW
Both our senior technicians and industry experts agree: the absolute first thing to do is schedule professional furnace maintenance before the deep freeze hits. Waiting until your furnace fails is like waiting for your car to break down before getting an oil change.
"We can't stress this enough," says our lead technician. "Annual maintenance prevents 85% of mid-winter breakdowns. We inspect the heat exchanger for cracks that could leak carbon monoxide, test all safety controls, clean the burners, verify proper combustion at Denver's altitude, and catch problems before they become 2 AM emergencies."
Professional maintenance includes critical tasks homeowners can't safely perform:
Heat exchanger inspection for dangerous cracks
Combustion analysis to verify safe operation
Gas pressure testing at 5,280 feet elevation
Flame sensor cleaning and calibration
Blower motor lubrication and belt inspection
Electrical connection tightening
Carbon monoxide testing throughout the home
If you haven't had maintenance in the past year, schedule your preventative service immediately. In Denver's climate, this isn't optional—it's essential for safe, reliable operation.
2. Replace Your Air Filter (And Keep Extras On Hand)
The next critical task is replacing your furnace air filter. During Denver's heating season, our dry, dusty climate means filters clog much faster than in humid regions.
"A clogged filter is the number one cause of furnace issues we see," explains our service manager. "It restricts airflow, causes the furnace to overheat, triggers safety shutdowns, and makes your blower motor work twice as hard. In extreme cold when your furnace runs 18 hours a day, a dirty filter will shut you down."
During winter, check your filter monthly. If it looks dirty, replace it. Don't wait for the arbitrary "every 3 months" guideline. Denver's dust doesn't follow schedules.
Pro tip: Buy a 6-pack of filters now and keep them by your furnace. When a freeze warning comes, knowing you have clean filters ready provides peace of mind. Standard 1-inch filters should be changed every 30-60 days during heavy heating season.
3. Test Your Furnace BEFORE You Need It
A couple weeks before forecasted freezing temperatures, turn your furnace on and run it through a complete cycle. This simple test reveals problems while you still have time to fix them comfortably.
"We get so many calls on the first truly cold night of the season," our dispatcher shares. "Everyone discovers their furnace doesn't work at the same time. If you test it early when temps are still 50°F, you're not desperate, and we can schedule service normally."
What to test:
Turn thermostat to heat mode, 5 degrees above current temperature
Listen for the furnace to start within 60 seconds
Verify warm air flows from all vents within 2-3 minutes
Let it run a full cycle (15-20 minutes)
Check for unusual noises, smells, or cycling issues
If anything seems off—strange sounds, weak airflow, burning smells, or frequent cycling—schedule a diagnostic before the freeze arrives.
4. Clear Vents, Registers, and Outdoor Units
Like plumbers recommend disconnecting outdoor hoses, HVAC techs say clearing all air pathways is critical for freeze preparation.
Inside your home:
Remove furniture, curtains, or storage blocking supply vents
Vacuum dust from all registers and return vents
Ensure at least 80% of vents remain open (closing too many throws off system balance)
Open cabinet doors under sinks against exterior walls to let warm air protect pipes
Outside your home:
Clear snow and ice from furnace exhaust pipes and air intake vents
Remove debris, leaves, or cottonwood seeds from outdoor AC unit (if heat pump)
Ensure furnace exhaust vents near ground level aren't buried in snow
Check that nothing blocks the path around your furnace for proper ventilation
"Blocked vents cause furnaces to overheat and shut down," our technician explains. "I've seen furnaces fail simply because someone stacked boxes too close to the intake. Your furnace needs to breathe just like you do."
5. Program Your Thermostat For Freeze Protection
Denver's temperature swings can catch even smart thermostats off guard. Before a freeze, adjust your programming to protect your home.
Critical thermostat settings for freezes:
Never set below 65°F even when you're away (prevents frozen pipes)
Take programmable thermostats out of setback mode during extreme cold
Set on permanent HOLD around 68-70°F to maintain consistent warmth
Avoid drastic temperature changes that force your furnace to work harder
"We see people try to save money by lowering thermostats to 60°F when they're at work," notes our lead tech. "Then they come home to frozen pipes and a $5,000 repair bill. The $20 you saved on gas isn't worth the risk."
If you're leaving town during winter, set your thermostat to 65-68°F and have a neighbor check your home daily. Smart thermostats with remote monitoring let you verify your furnace is running even when you're away.
6. Know Your Furnace Shut-Off and Emergency Contacts
While this isn't an actionable prep step like others on the list, knowing these two things can save your home if problems arise.
Locate your furnace emergency shut-off switch: Usually a red switch on or near the furnace, or a dedicated circuit breaker. If your furnace malfunctions, you need to kill power immediately to prevent further damage or fire risk.
Save emergency contacts now:
Your HVAC contractor's emergency number: (720) 807-4050
Your gas company's emergency line (for gas smell or leaks)
A backup heating contractor in case your primary is unavailable
"If your furnace fails at 2 AM and you smell gas, you can't afford to be Googling emergency numbers," warns our service manager. "Have these programmed in your phone before problems happen."
Emergency signs that require immediate shutoff and professional help:
Strong gas smell near furnace
Carbon monoxide detector alarming
Visible flames outside the combustion chamber
Excessive smoke or burning smell
Loud banging, grinding, or metal-on-metal sounds
Bonus Tips From Our Denver HVAC Technicians
Add Humidity During Dry Winter Months
Denver's winter humidity often drops to 10-15%, making your home feel colder than it actually is. This forces you to set thermostats higher and run your furnace more.
"A whole-home humidifier is one of the best investments for Denver homes," our tech explains. "Humid air feels warmer, so you can lower your thermostat by 3-5 degrees and feel just as comfortable. That's $150-300 in annual savings on gas bills."
Check Carbon Monoxide Detectors
Before running your furnace constantly, test and replace batteries in all carbon monoxide detectors. This isn't just about code compliance—it's about your family's safety.
Natural gas furnaces produce carbon monoxide during combustion. If your heat exchanger develops cracks or your venting becomes blocked, deadly CO can enter your home. Working detectors are your last line of defense.
Consider a Backup Heat Source
During Denver's 2022 deep freeze, we saw hundreds of furnace failures. Families with backup heat sources (electric space heaters, fireplaces) stayed safe while waiting for repairs.
Keep at least one quality electric space heater stored for emergencies. If your furnace fails during a freeze, a space heater can keep at least one room warm and prevent pipe freezing until help arrives.
Don't Cover Your Heat Pump's Outdoor Unit
Unlike air conditioners that sit idle all winter, heat pumps run year-round. Never cover a heat pump's outdoor unit—it needs airflow to operate. Clear snow away from it, but don't insulate or wrap it.
The Bottom Line: Prevention Beats Emergency Repairs
Preparing your HVAC system before a freeze isn't just about comfort—it's about safety, preventing property damage, and avoiding the stress and expense of emergency repairs during the coldest weather.
Our technicians have seen too many families suffer through freezing homes, expensive emergency service calls, and dangerous situations that could have been prevented with simple preparation.
"Every winter, we get calls from people whose furnaces failed at midnight when it's 5°F outside," reflects our service manager. "They tell us they kept meaning to schedule maintenance. They noticed the furnace making noise but thought it could wait. Now they're desperate, cold, and facing premium emergency rates. Don't be that person."
Take action now, before the freeze:
Schedule preventative maintenance to ensure your furnace is freeze-ready
Get a diagnostic if your furnace showed any issues during test runs
Request an estimate if your aging furnace needs replacement before winter hits hard
At MoJo Home Services, we've served Denver Metro homeowners for over 16 years. We understand how altitude, climate, and extreme weather affect your HVAC system. We're here to help you prepare, whether that's maintenance, repairs, or honest advice about when replacement makes more sense than another repair.
Don't wait for a freeze warning to think about your furnace. Call (720) 807-4050 or contact us online today. Let's make sure your home stays warm and your family stays safe all winter long.
Frequently Asked Questions
What does the "4% per 1,000 feet" derating rule mean in practical terms for a Denver furnace?
Derating means intentionally reducing a system's rated output capacity to match what it can actually deliver given the available oxygen at altitude. A furnace rated at 100,000 BTU at sea level loses approximately 4% of effective output for every 1,000 feet of elevation — arriving at roughly 80,000 BTU of effective output at Denver's 5,280 feet. This matters for sizing decisions: a contractor calculating your home's heating load must account for this derating when selecting equipment, or they'll install a furnace that appears correctly sized on paper but consistently underperforms in practice. A 100,000 BTU furnace chosen to meet an 85,000 BTU heating load will fall short at altitude without derating factored in — which is why load calculations for Denver installations must be altitude-adjusted from the start.
What is a high-altitude conversion kit and when is one needed versus factory pre-adjustment?
A high-altitude conversion kit is a manufacturer-supplied package of components — typically including smaller burner orifices, adjusted pressure regulators, and updated installation documentation — that converts a furnace calibrated for sea-level operation to perform correctly above a specified elevation threshold, usually 4,500 feet. Some premium brands like Trane and Carrier ship Denver-bound equipment with factory pre-adjustment already applied, meaning the orifices are already sized for altitude before the unit leaves the manufacturing facility. Others ship standard equipment with a kit included that the installing contractor must apply during installation. The practical implication is that homeowners should confirm with their contractor whether the specific model being installed is factory pre-adjusted, kit-converted at installation, or field-adjusted — and request documentation that whichever method was used has been verified with combustion analysis before the system is commissioned.
Why do cooling coils specifically lose 14% capacity above 5,500 feet and what does that mean for AC sizing in Denver?
Cooling coils transfer heat from indoor air into refrigerant by relying on air mass flowing across the coil surface. At altitude, thinner air carries less thermal mass per cubic foot, so even with adequate airflow volume, the actual heat transfer rate drops — accounting for the roughly 14% capacity loss above 5,500 feet. For Denver at 5,280 feet, the effect is slightly less severe but still meaningful. A 3-ton AC unit (36,000 BTU) effectively delivers closer to 31,000 BTU of cooling capacity in Denver than it would at sea level. This is why contractors sizing AC systems for Denver homes should apply an altitude correction factor to their load calculations rather than simply using the manufacturer's sea-level rating — and why a system that appears correctly sized for a home's square footage may still struggle to keep up on Denver's hottest days.
What is a thermal shutdown on an HVAC blower motor and what triggers it at altitude?
A thermal shutdown is a built-in protection mechanism inside the blower motor that cuts power when the motor's internal temperature exceeds a safe threshold. All electric motors generate heat as a byproduct of operation — and they rely on moving air across their windings to dissipate that heat. At altitude, the blower motor is simultaneously working harder to move adequate air mass through thinner air and receiving less cooling benefit from that same thinner air passing over its windings. The result is a motor that runs hotter under normal operating conditions than the same motor would at sea level. When restricted airflow from a dirty filter or duct obstruction is added on top of the altitude baseline, thermal shutdowns become more frequent. A Denver homeowner experiencing unexplained furnace or air handler shutdowns that reset after a rest period should suspect motor overheating as the cause — and address filter condition and airflow restriction before assuming a component failure.
Why are condensing high-efficiency furnaces particularly well-suited for Denver's altitude compared to standard 80% AFUE models?
Condensing furnaces extract heat from combustion gases through a secondary heat exchanger, cooling the exhaust to the point where water vapor condenses out — which is how they achieve 90 to 98% AFUE. This two-stage heat extraction process is advantageous at altitude for two reasons. First, it wrings more usable heat from each BTU of gas, partially offsetting the efficiency losses from altitude-adjusted derating. Second, condensing furnaces operate with induced draft systems that actively control combustion airflow rather than relying on natural convection, giving them more precise control over the fuel-to-air ratio at altitude. Standard 80% furnaces using natural draft are more susceptible to altitude combustion issues because they have less active management of the combustion process. For Denver's seven-month heating season, the combination of higher base efficiency and better altitude-tolerant design makes condensing furnaces a particularly strong investment.
How does Denver's combination of altitude and extreme temperature swings specifically compound equipment wear compared to either factor alone?
Altitude causes HVAC components to operate under sustained elevated stress — motors running hotter, compressors working against different pressure conditions, combustion producing more soot. Temperature swings add a separate mechanical stress: thermal cycling. Every time your system starts up after sitting idle, metal components expand from cold; when it shuts down, they contract again. Denver's 40 to 50-degree daily swings mean this expansion-contraction cycle happens more dramatically and more frequently than in cities with moderate, stable climates. Heat exchangers are particularly vulnerable — the combination of altitude-driven soot accumulation inside and aggressive thermal cycling outside creates the conditions most likely to produce the micro-cracks that lead to carbon monoxide leakage. Neither altitude nor temperature volatility alone would stress a heat exchanger as severely as both operating simultaneously across Denver's seven-month heating season.
What does it mean that altitude adjustment is required by code in Denver and what happens if it's skipped?
Colorado building code requires that all gas appliances operating above 2,000 feet elevation be adjusted for altitude as a condition of permitted installation. This means a permitted furnace installation in Denver legally must include altitude adjustment verified before the city inspector signs off. When altitude adjustment is skipped — which can happen with unpermitted work or contractors cutting corners — the consequences range from inefficiency to genuine safety hazards. An unadjusted furnace running rich at altitude produces elevated carbon monoxide concentrations in flue gases, and if the heat exchanger develops even minor cracks, that CO enters the home's air supply. Beyond the immediate safety concern, an unadjusted installation that later causes a carbon monoxide incident creates serious insurance and liability complications for the homeowner. The permit and inspection process exists specifically to catch this — which is one of the strongest arguments for insisting on fully permitted HVAC work in Denver.
How does zoning specifically address altitude-related performance challenges in a multi-story Denver home?
An altitude-derated system is already delivering less capacity than its nameplate rating — meaning it has less performance margin to overcome the natural temperature stratification in a multi-story home. Heat rises and collects on upper floors; a single-zone system responds to the thermostat's main-floor reading and shuts off before the upper floor is adequately conditioned. The system then has to restart and run another full cycle to address the upstairs temperature, accumulating the startup energy inefficiency repeatedly throughout the day. Zoning solves this by giving each floor its own temperature feedback loop, allowing the system to continue conditioning the upper zone even after the lower zone is satisfied. For a Denver home where the system is already working at a capacity disadvantage due to altitude, eliminating the wasted energy of repeated short-cycles driven by single-zone thermostat blindness can recover a meaningful portion of the altitude performance deficit.
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