Front Range winters have a way of checking your furnace’s homework. It can run fine in October, then fail the first night the temperature slides into the teens. When that happens, it helps to approach the problem like a pro: confirm the basics, read what the equipment is trying to tell you, and work methodically from simple causes toward the less obvious ones. Denver’s altitude, swingy shoulder seasons, and dry air add a few quirks to the equation, especially for older gas furnaces and high-efficiency models that rely on pressure-sensitive safety switches. What follows draws on long, cold service calls across the metro area and plenty of repeat visits in neighborhoods from Baker to Highlands Ranch.
Start where the heat begins: power, gas, and the thermostat
You can save an uncomfortable hour by verifying the inputs before touching the furnace. The thermostat must call for heat. The furnace must have power and gas. If any of those are flaky, the rest of your troubleshooting doesn’t matter.
A thermostat set to “heat,” with a setpoint above room temperature, should tell the furnace to start. If it’s battery powered, weak cells can cause intermittent calls. I’ve seen Honeywell units behave fine for cooling then fail to pull in heat because the call duration is longer. Replace batteries and reseat https://pastelink.net/rw1u7esp the thermostat on its sub-base if you suspect a poor contact.
Power is next. Many Denver basements have a wall switch that looks like a light switch feeding the furnace. It gets bumped during storage shuffles. If your blower is silent and the control board LEDs are dark, check that switch and the service outlet nearby for power. Then check the dedicated furnace breaker in the panel. Breakers can “half-trip.” Move it fully off, then back on. If it trips again immediately, stop and call a technician. Repeated tripping points to a short in the blower or control wiring that needs trained eyes.
Gas supply problems usually reveal themselves by the smell or by multiple gas appliances failing at once. If your water heater or range is working but the furnace is not, the issue is probably inside the furnace cabinet, not at the meter. Make sure the shutoff valve next to the furnace is aligned with the pipe. In newer installs, the flex connector can get kinked during filter changes or cleaning. A kinked connector starves the gas valve. That’s rare but worth a look.
Understand the furnace’s startup sequence
Gas furnaces follow a predictable order. Knowing it helps you pinpoint where things go wrong.
When the thermostat calls, the control board checks safeties, then spins the inducer motor. The inducer clears the heat exchanger and proves proper draft. A pressure switch confirms that the inducer is moving enough air. Only then does the igniter warm up. After a few seconds, the gas valve opens, the burners light, and the flame sensor verifies flame. Finally, the main blower starts to move warm air. If the furnace fails at any step, the board usually shuts things down and may retry. Many boards also blink a diagnostic code.
If you stand next to the furnace and listen, you can usually hear each step: a small fan whir, a click from the pressure switch, the hiss of gas, the whoosh of ignition, then a delay followed by the big blower. Missing a whoosh or hearing repeated clicks tells you where to look.
No heat at all: the common culprits
A silent furnace on a cold night tends to be one of five things: loss of power, failed inducer, stuck pressure switch, bad igniter, or a rollout/high limit lockout. Denver’s dry air and dusty basements also cause flame sensor and filter problems that masquerade as bigger failures.
If the control board is dark, you’re back to verifying power. If the board blinks, count the code. Most units have a legend on the door. Two quick blinks often mean pressure switch open. A single, slow blink might be lockout due to failed ignition. Codes vary by manufacturer, so reading the door chart matters.
A pressure switch code tells you the inducer isn’t pulling enough negative pressure. At our altitude, some furnaces ride the threshold, especially when the condensate trap has partial blockage or the intake is iced. High-efficiency furnaces in Denver often vent horizontally through the rim joist. A wind-packed snow berm on the north side can choke the intake even when the exhaust looks clear. Clearing that intake can bring a furnace to life immediately. If the intake and exhaust are clear and the hose from the pressure switch to the inducer is intact and dry, the inducer wheel may be loaded with debris or the motor failing. You’ll hear it struggle or fail to start. Replacement is straightforward for a pro and not a place to improvise with lubricants or cleaning sprays, which can damage the motor windings.
Ignition failures are next on the list. Silicon carbide or nitride igniters crack with age. They may glow weakly or not at all. If you see no glow, check for 120V or 240V power to the transformer and 24V control to the gas valve only after the board commands ignition. If the igniter glows but burners don’t light, you could have a gas supply issue, a dirty burner manifold, or a sticking gas valve. Lighting delays longer than a couple seconds can cause flame rollout at startup, which trips safety limits and shuts the furnace down. That’s not a DIY test. An experienced technician can bench-test an igniter and measure amp draw to catch a weak one before it fails. Replacing an igniter with the correct part matters, especially at 5,280 feet, where furnace inputs are derated for altitude.
Sometimes the furnace lights, runs briefly, then shuts off hard with a burning smell. That’s often a high limit switch reacting to zero airflow. The usual causes are clogged filters, closed supply registers, or a collapsed return duct. In older Denver homes, I’ve seen carpets laid over returns cut into the floor, which starves the blower. If you recently had duct cleaning, verify every register and damper is open. High limit trips can lock the furnace for an hour. Cycling the power clears some lockouts, but the underlying cause remains.
Low heat: when rooms feel lukewarm, cycles short, or the upstairs never catches up
Low-heat problems are trickier because the furnace appears to work. The burner lights, the blower runs, and the house warms a little, but it never reaches the setpoint, or it takes all day.
Start with airflow. Denver’s dry winters move a lot of dust. Filters intended to protect your lungs end up suffocating the blower if they’re too restrictive for your duct system. I’ve measured static pressure well over 0.9 inches of water on systems designed for 0.5, all because an overly dense 1-inch filter replaced a basic pleated one. The result is warm supply air at the furnace that cools too much by the time it reaches the far rooms. The burner cycles on limit, the blower turns off, then repeats. Go with a filter grade your blower can handle. On older systems with small returns, a MERV 8 pleated or even a fiberglass filter in extreme cases performs better than a high MERV that chokes airflow. If you want high filtration, consider a media cabinet with a 4- or 5-inch filter and a blower setup to match.
Duct issues show up as temperature stratification. In split-level homes in Lakewood or Centennial, the basement might be toasty while the upstairs is chilly, and the thermostat sits on the middle level declaring victory. Balancing dampers, if present, might be set for summer. A seasonal tweak helps: partially close basement supplies in winter to push more air upstairs. Don’t close registers completely. Aim for backpressure that stays within the blower’s rated static. A technician performing a furnace tune up denver homeowners can count on will measure external static and adjust blower speed accordingly.
Thermostat placement can mask low heat. If it sits near a return or in direct sun, it ends calls early. Add a temperature reading from a room that feels cold. Compare it to the thermostat. If there’s a 3 to 5 degree difference, consider a remote sensor or relocation. Some modern stats support averaging multiple sensors, which smooths out uneven heating without touching the ducts.
Finally, capacity and derating matter. Furnaces installed at sea level deliver their full nameplate input. In Denver, gas furnaces are typically derated 10 to 20 percent due to reduced oxygen density. Most installers account for this during Furnace Installation Denver CO projects, but when a furnace is borderline for the home’s heat loss, a cold snap exposes the gap. If your system runs continuously on the coldest nights and never reaches setpoint, load calculation and equipment sizing should be revisited. Good contractors will measure heat rise across the furnace, confirm combustion, and assess duct capacity before discussing furnace replacement denver options.
What altitude changes for a gas furnace
Combustion needs a precise mix of gas and oxygen. At 5,000 to 6,000 feet, oxygen is lower. Manufacturers provide orifices and pressure settings that reduce gas input to match available air. If a furnace is never adjusted for altitude, it may run rich, soot up the heat exchanger, and trip on flame issues. High-efficiency units with sealed combustion are less sensitive, but their pressure switches and condensate management rely on narrow pressure differentials that shrink with altitude.
I’ve seen pressure switches that pass bench tests at sea level fail in Denver and Wyoming. For borderline cases, a switch with a lower setpoint can be appropriate if the manufacturer approves it. Otherwise, the root cause is usually restricted intake, misrouted or sagging condensate hoses creating water traps, or a marginal inducer. If your furnace is a 90-plus percent model and you see water in clear hoses near the inducer, make sure the hoses slope steadily toward the condensate trap with no sags. At elevation, even small water pockets can kill the pressure signal.
Flame sensors, dirty burners, and the 90-second furnace
One of the most common calls goes like this: the furnace lights, runs for a minute or two, then shuts down. It retries, runs a bit shorter, then locks out. The symptom often points to a contaminated flame sensor. Denver’s dry air carries fine dust that bakes onto the metal rod. The sensor depends on a tiny microamp DC current across the flame. Any insulating film disrupts that signal.
Pulling and cleaning the flame sensor with a fine abrasive pad or steel wool restores readings. Don’t sand aggressively. You’re polishing, not reshaping. While you’re in there, note the flame pattern. Burners should light smoothly from the igniter across the manifold. Lazy flames that lift or roll suggest dirty burners or misaligned carryover ports. Burners can be removed and brushed, but gas manifold service should be done by someone with experience. A furnace service denver visit that includes flame signal measurement, burner inspection, and heat rise checks often prevents that 2 a.m. lockout.
Filters, returns, and the myth of closing vents to “save heat”
Homes in the city’s older neighborhoods often have undersized return paths. You can add a high-efficiency furnace to an old trunk-and-branch duct system, but if the return grille area stays small, static pressure rockets when you install a dense filter. The furnace overheats at the heat exchanger and trips the limit. People respond by closing supply vents in rooms they don’t use, which seems logical but raises static pressure further. The furnace runs hotter, cycles harder, and shortens its life.
Opening all supplies and returns and using a modest filter reduces resistance. If you want room-by-room control, talk about proper zoning or an ECM blower tuned for your system. During a furnace maintenance denver appointment, ask for static pressure readings and a duct assessment, not just a filter swap and a quick visual. The numbers matter. A system that breathes easily rarely fails on the coldest night.
When the problem lives outside the furnace
Not every no-heat call is a furnace failure. Smart thermostats lose Wi‑Fi and power differently than old ones, and some models rely on a common wire that your system might not have. A power-stealing thermostat can cause erratic heat calls if the control board isn’t compatible. If you notice heat failures coinciding with thermostat reboots or flaky app behavior, the solution may be a clean C‑wire pull or a different thermostat.
Another external cause: tripped float switches on condensate pumps. High-efficiency furnaces produce water as they run. That water must drain by gravity or pump. If the pump fails or the drain is frozen in an unconditioned crawlspace, the float switch opens the safety circuit and the furnace goes silent. If you see water near the furnace, don’t bypass the switch. Clear the blockage, thaw the line, or replace the pump.
Finally, carbon monoxide safety devices and smoke detectors can interact with homeowner behavior. I’ve seen flues disconnected by accident during attic storage or roof work. If multiple gas appliances struggle at once or CO detectors chirp, evacuate and call for help. A good furnace tune up denver technicians perform includes flue verification and combustion analysis, which catches drafting problems before they turn dangerous.
Quick triage you can do safely
Here is a short, safe checklist that often resolves no-heat or low-heat calls without tools or panels removed:
- Confirm the thermostat is on heat, set above room temperature, with fresh batteries if applicable. Check the furnace switch and breaker, and verify the door is latched fully to engage the door switch. Replace or temporarily remove an extremely clogged 1-inch filter to test airflow, then install a proper replacement. Inspect outdoor intake and exhaust terminations for snow, leaves, or windblown debris, and clear them. Look for standing water around a high-efficiency furnace or a failed condensate pump, and restore drainage.
If none of these changes behavior, stop opening panels. Furnaces have multiple safety interlocks for good reasons. The next steps involve electrical measurements and combustion checks.
What a thorough professional visit looks like
A competent gas furnace repair denver technician doesn’t just swap parts. They gather data. Expect electrical checks at the control board, inducer amperage, igniter resistance or current draw, flame signal microamps, gas valve operation, and temperature rise across the heat exchanger. On high-efficiency models, they’ll test pressure switch operation under load and inspect and flush the condensate trap. They’ll also measure static pressure and confirm blower speed matches the duct system’s capacity.
Denver’s better shops build these checks into a furnace service denver offering and not just emergency repair. It’s routine work that pays off in reliability. If any readings are out of spec, you’ll get options: clean and adjust now, replace a marginal igniter before it fails in January, or consider upgrades if the system can’t meet the home’s load under altitude derate.
When repair gives way to replacement
No one loves the timing of a failed heat exchanger or a blower motor that quits at midnight. But there’s a point where continued repair costs eclipse long-term value. Age, part availability, efficiency, safety, and comfort all factor in.
If your furnace is 20 to 25 years old, original to the house, and shows a cracked heat exchanger or repeated safety trips, it’s time to consider Furnace Replacement Denver CO options. Heat exchangers sometimes fail earlier if the system has been running against high static for years. A cracked exchanger is not a debate point. It risks carbon monoxide leakage. Replacing it in a very old furnace rarely makes sense economically.
Efficiency gains also matter. Swapping an 80 percent unit for a 95 percent can trim gas usage by 10 to 20 percent in practice, depending on duct and envelope. That’s not magic, and load calculation still rules, but winter bills will drop. During Furnace Installation Denver CO projects, reputable contractors will adjust for altitude, set proper fan speeds, and size the intake and exhaust correctly through the rim joist or roof. They’ll also evaluate ductwork to make sure the new blower isn’t pushing into a brick wall of static pressure.
Finally, lifestyle and comfort goals play a role. If you’re adding a basement office or finishing an attic, better airflow and zoning might be part of the conversation. This is where furnace replacement denver planning overlaps with duct changes, thermostat strategies, and possibly pairing with a heat pump for shoulder-season efficiency. Gas prices and electric rates move, but a dual-fuel setup in Denver’s climate can deliver comfort and flexibility.
Maintenance that actually prevents heat loss
Maintenance becomes meaningful when it is specific. A drive-by filter change and a flashlight peek aren’t enough. A practical furnace maintenance denver plan includes:
- Combustion inspection and, when possible, analyzer readings adjusted for altitude to verify proper burn quality. Heat rise measurement against the nameplate, along with static pressure to catch airflow issues early. Flame sensor cleaning and burner inspection, with flame signal recorded in microamps for trend comparison. Condensate trap cleaning and hose routing checks on condensing furnaces, plus pressure switch testing. Electrical checks on blower and inducer, including amp draw and capacitor health where applicable.
Documented readings allow pattern recognition. When a flame signal falls from 3.5 microamps to 1.2 between visits, you can schedule cleaning before an outage. When static pressure creeps up season after season, you’ll know it’s time to address returns or change filter strategy. That’s the difference between maintenance and a courtesy visit.
A few Denver-specific quirks to watch
Cold snaps and chinooks stress furnaces in different ways. Rapid temperature changes cause vent icing overnight and sudden melt that dumps water back toward the furnace in the morning. Make sure exterior termination points are slightly pitched away from the house, and that screens are intact but not fine enough to catch frost easily. If your intake and exhaust exit on a north wall that never sees sun, consider protective hoods that reduce wind-driven snow intrusion.
Basements with sump pits or damp corners increase dust and corrosion around burners and controls over time. Simple housekeeping helps. Keep storage off the floor around the furnace. Don’t lean cardboard against the cabinet. Leave clear space for airflow and service. If you run a humidifier, set it conservatively. Over-humidification promotes rust and can create condensation on the heat exchanger in shoulder seasons.
Finally, remember the altitude derate. If you bring in out-of-town equipment or parts, verify that they’re correct for 5,000-plus feet. Gas orifice sizes, pressure switch ratings, and even igniter specifications can differ. A good local supplier and an experienced tech will keep you out of that trap.
When to call, and what to tell the dispatcher
You can speed up gas furnace repair denver response by describing symptoms clearly. Share whether the inducer starts, whether you see the igniter glow, how long the burners run before shutdown, and whether there are any blinking codes on the control board. Mention any recent work: roof replacement that disturbed vents, thermostat changes, or filter upgrades. If you’ve noticed water near the furnace, say so. The more specific the report, the more likely the technician arrives with the right parts in the truck.
If the furnace is safe but underperforming, ask for a combined diagnostic and tune-up. Many shops fold furnace tune up denver tasks into a repair visit, which saves time and catches underlying system limits that a quick fix alone would miss. If your equipment is older and struggling, it’s reasonable to request a side-by-side estimate for repair versus replacement. A clear price to restore reliability today and a separate price for a properly sized replacement gives you control of the timeline.
Reliable heat in a Denver winter isn’t a luxury. It’s the baseline that allows everything else to function. When you approach no-heat and low-heat problems with a clear sequence, respect for altitude, and attention to airflow, most furnaces settle down and do what they were built to do. And if the time has come to retire a veteran unit, a careful Furnace Replacement Denver CO project, done with the duct system and home load in mind, will carry you comfortably through the next two decades.
Tipping Hat Plumbing, Heating and Electric
Address: 1395 S Platte River Dr, Denver, CO 80223
Phone: (303) 222-4289