Heating FAQs

The most common reasons a furnace won't start: the thermostat is not set to HEAT or is set below room temperature, the power switch near the furnace has been accidentally turned off, the circuit breaker has tripped, the ignitor has failed (the most common mechanical failure in modern furnaces), or a safety control has tripped and put the system in lockout. Check the thermostat setting and the power switch first — if those are correct, call a technician for diagnosis.

A furnace running but blowing cold air has several likely causes: a dirty or failed flame sensor (shuts the burner off shortly after it lights), a failed ignitor that isn't lighting the burner, a clogged air filter causing the system to overheat and trip the high-limit switch, or the thermostat set to 'fan only' instead of 'heat.' Professional diagnosis identifies the specific cause.

Short cycling — the furnace turning on, running briefly, then shutting off — is most commonly caused by a dirty or failed flame sensor, a severely clogged air filter causing overheating, an oversized furnace for the home's heating load, or a heat exchanger problem causing the high-limit switch to trip. Repeated short cycling should be diagnosed — it places significant stress on the furnace components.

Banging at startup usually indicates delayed ignition — gas builds up briefly before lighting, causing a small explosion when it does. This can be caused by a dirty burner, low gas pressure, or an ignitor that's slow to heat up. Banging from ductwork is often thermal expansion as ducts heat up. Either warrants a professional inspection — delayed ignition can damage the heat exchanger over time.

A squealing furnace usually indicates a blower motor problem — worn bearings, a slipping belt (on older systems with belt-driven blowers), or the blower wheel making contact with its housing. These issues worsen over time. A motor with worn bearings that isn't serviced will eventually seize, which is a more expensive repair than addressing the bearing issue early.

A brief burning smell at the start of heating season — when the furnace runs for the first time in months — is normal dust burn-off and typically clears within an hour. A persistent burning smell, a smell of burning plastic or rubber, or an electrical burning odor indicate a problem and warrant a professional inspection. Turn the furnace off if the smell is strong or persistent and call for service.

Key factors: furnace age (over 15–18 years old significantly tips toward replacement), repair cost relative to replacement (repairs exceeding 50% of new system cost often favor replacement), and repair history (repeated failures signal the system is wearing out broadly). A cracked heat exchanger almost always means replacement. A failed ignitor on a 10-year-old furnace is clearly a repair. We give you an honest recommendation after evaluating your specific system.

The heat exchanger is a metal component inside the furnace that separates the combustion gases (carbon monoxide, carbon dioxide, water vapor) from the air that circulates through your home. When it cracks, combustion gases can enter the living space — a serious safety concern. Signs of a cracked heat exchanger include a carbon monoxide detector alarm near the furnace, a yellow or flickering burner flame, and unusual symptoms (headaches, fatigue) when the heat runs. Annual maintenance inspection includes heat exchanger evaluation.

Once a year — before the heating season — is the standard recommendation for most residential furnaces. Colorado's long, cold winters mean furnaces run heavily from October through April, and annual maintenance before that season catches developing problems while they're still inexpensive to address.

A professional furnace tune-up includes: heat exchanger inspection, burner assembly inspection and cleaning, ignitor and flame sensor check and cleaning, blower inspection and lubrication, air filter inspection, electrical connections check, thermostat operation test, safety control testing, flue and venting inspection, and documentation of findings. The technician explains any issues discovered and provides repair recommendations with upfront pricing.

Every 1–3 months for standard 1-inch filters — monthly during heavy heating season in Colorado. Thicker high-efficiency filters (4–5 inch media filters) typically last 6–12 months. A clogged filter is the single most preventable cause of furnace problems — it restricts airflow, causes overheating, trips safety controls, and stresses the blower motor. Check the filter monthly and replace it when it looks dirty.

Some maintenance tasks are appropriate for homeowners: replacing the air filter monthly, keeping supply and return vents clear of furniture and debris, and keeping the area around the furnace clear. Professional maintenance covers the components that require expertise, specialized tools, or safety certification — heat exchanger inspection, burner cleaning, gas pressure measurement, combustion analysis, and safety control testing.

Most residential furnaces last 15–25 years with proper maintenance. Colorado's long heating season puts more annual hours on furnaces than milder climates — a furnace running 6–7 months per year accumulates wear faster than one running 3–4 months. Annual maintenance extends service life by catching problems early. The heat exchanger is usually the component that determines end of life — once cracked, the furnace is typically replaced rather than repaired.

AFUE (Annual Fuel Utilization Efficiency) measures how efficiently a furnace converts fuel to heat — 80% means 80 cents of every dollar of gas becomes heat; 96% means 96 cents does. For Colorado's long heating season, 90%+ high-efficiency furnaces usually offer a better long-term return than 80% models, despite the higher upfront cost. High-efficiency furnaces require PVC venting and a condensate drain, which affects installation requirements in older homes.

Furnace sizing should be calculated from a Manual J load calculation — not just square footage. Insulation, windows, ceiling height, air sealing, home orientation, and Colorado's altitude all affect heating requirements. An oversized furnace short-cycles, causing temperature swings and premature wear. An undersized furnace runs constantly and can't keep up on the coldest Colorado days. We calculate the right size for your specific home.

Modern cold-climate heat pumps — rated for -13°F or lower — work effectively throughout most of Colorado's Front Range winter. They lose efficiency as outdoor temperatures fall, and may need supplemental heat during extreme cold. For homes in areas with sustained very cold temperatures or at higher elevations, a dual-fuel system (heat pump + gas furnace backup) is often a better fit than a heat pump alone.

Heat pumps produce supply air at 90–105°F — warmer than room temperature but cooler than a gas furnace's 120–140°F output. This can feel 'cool' to the touch of your hand near a vent but is still heating your home. If your home isn't reaching the set temperature, the system is short-cycling, or performance has suddenly changed, those are signs of a system problem that needs diagnosis.

Some frost on the outdoor coil in heating mode is normal — the defrost cycle removes it every 30–90 minutes. Heavy ice encasing the outdoor unit, ice that isn't clearing after several hours, or ice on the indoor coil are not normal. Common causes include a failed defrost board or sensor, low refrigerant charge, or severely restricted airflow. This needs professional diagnosis — don't just keep resetting the system.

A dual-fuel system pairs a heat pump with a gas furnace. The heat pump runs when it's efficient — moderate Colorado temperatures — and the gas furnace automatically takes over when outdoor temperatures drop below a set 'balance point' (typically 25–35°F). This gives you the efficiency of a heat pump for most of the season and the reliable heat output of a gas furnace for Colorado's coldest days.

Twice a year is the standard recommendation — once in fall before the heating season, once in spring before the cooling season. Heat pumps run year-round and accumulate wear in both directions. Annual maintenance at minimum helps catch dirty coils, refrigerant issues, and failing capacitors before they cause a heating or cooling failure.

Repeated pressure loss means water is leaving the system somewhere. Common causes: a slow leak in the piping or radiators, a pressure relief valve that's weeping (releasing below its rated pressure), or a waterlogged expansion tank that can no longer absorb system pressure changes. Simply refilling through the feed valve restores pressure temporarily but doesn't fix the cause. A boiler that repeatedly loses pressure needs professional diagnosis.

Banging or kettling in a boiler is usually caused by mineral scale on the heat exchanger — scale causes localized boiling that makes the banging sound. This is especially common in Colorado's hard water areas. Other causes include water hammer from zone valves closing too quickly or trapped air in the system. Scale-related kettling left unaddressed can damage the heat exchanger, making an expensive repair or replacement necessary.

Well-maintained residential boilers typically last 15–25 years. Colorado's hard water accelerates scale buildup in the heat exchanger, which reduces efficiency and service life without proper water treatment or annual descaling. The most common cause of premature boiler failure in Colorado is scale damage to the heat exchanger from hard water and inadequate maintenance.

A furnace heats air and distributes it through ducts and vents. A boiler heats water and distributes it through pipes to radiators, baseboard heaters, or radiant floor tubing. Boilers provide more even, radiant-style heat with no forced air movement. They require different maintenance and have different failure modes than furnaces. A boiler technician's skill set is distinct from a standard furnace technician's.

For most Colorado homeowners, yes. Colorado's heating season runs from October through April — 6–7 months where you want reliable scheduling and control. Smart thermostats provide remote access (confirm your heat came on during a cold snap), detailed scheduling, and runtime data that shows whether your system is running more than expected. Actual energy savings depend on how consistently you use the scheduling features.

A C-wire (common wire) provides constant 24V power to a smart thermostat's WiFi radio, display, and processor. Without it, smart thermostats use 'power stealing' — drawing small amounts of power from HVAC control circuits — which can cause erratic behavior or damage sensitive variable-speed HVAC equipment. Many older homes lack a C-wire at the thermostat location; we can run one, use an adapter, or recommend models that work without one.

No. Heat pumps require thermostats with specific wiring support: terminals for the reversing valve (O or B), auxiliary heat, and emergency heat. Installing a standard furnace thermostat on a heat pump can cause it to operate only in one mode or damage the reversing valve. Heat pump thermostat selection must account for your system's specific wiring before purchase.

The most common cause is poor thermostat location — near a supply vent, in direct sunlight, on an exterior wall, or in a poorly air-sealed space. These locations expose the thermostat's sensor to conditions that don't represent the room's actual temperature. A drifted or failed internal sensor and air infiltration through the thermostat's wall opening are also common causes.

Leave your home immediately — do not stop to gather belongings. Do not operate electrical switches, light switches, phones, or appliances inside the building. Do not use a lighter or match. From outside the building, away from the structure, call your gas utility's emergency line or 911. Do not re-enter until emergency responders confirm the building is safe. Never attempt to locate or repair a gas leak yourself.

A cracked heat exchanger is the most common furnace-related cause of elevated CO in a home. When the heat exchanger cracks, combustion gases can enter the home's air supply rather than being exhausted through the flue. Other causes include a blocked or backdrafting flue, incomplete combustion from a dirty or improperly adjusted burner, or a furnace in a space with inadequate combustion air. If your CO alarm sounds near the furnace, evacuate and call 911.

A properly operating gas furnace burner should produce a blue flame, possibly with small yellow tips at the very top. A consistently yellow, orange, or rolling flame indicates incomplete combustion — the gas isn't burning cleanly. This can produce elevated carbon monoxide. Common causes include a dirty burner, improper air-to-gas ratio, or a heat exchanger problem affecting combustion airflow. A yellow flame warrants professional evaluation before continued use.

Call for emergency heating service when: your home has no heat during a Colorado cold snap (below 20°F, especially overnight), you have elderly family members or infants in the home without heat, a pipe freeze is likely without heat, a CO detector is alarming near the furnace, you smell gas, or you notice signs of a serious system failure (sparks, strong burning smell, visible damage). For gas leaks and CO alarms, call the utility or 911 first — then call for heating service once the immediate hazard is addressed.

If your furnace stops working and temperatures are near or below freezing: keep cabinet doors under sinks open to allow warm air to reach pipes, let faucets drip slowly to keep water moving, locate your main water shutoff in case you need to shut off water if a pipe bursts, use portable electric heaters if available in critical areas (safely, away from flammables), and call for emergency heating service immediately. Do not use gas or propane heaters indoors without proper ventilation.

Fall — August through October — before Colorado's heating season begins. Scheduling pre-season means better appointment availability, any needed repairs can be completed before regular heating demand, and you start the cold months knowing your system was inspected. Colorado's heating season starts early and can include serious cold by late October — don't wait until November.

Yes, significantly. Colorado's Front Range has some of the hardest water in the country. In boilers and water heaters, hard water deposits mineral scale inside the heat exchanger over time — reducing efficiency, causing banging/kettling sounds, and eventually causing heat exchanger damage. In tankless water heaters, scale accumulates inside the heat exchanger and eventually blocks it. Annual descaling or water treatment substantially extends equipment life in Colorado's hard water conditions.

Yes. Gas furnaces are rated at sea level, where oxygen is more dense. At Denver's 5,280 feet — and even more so in mountain communities — the reduced oxygen density affects combustion efficiency. Modern furnaces have electronic controls that automatically adjust for altitude; older furnaces may need manual adjustment to the gas valve. High-efficiency furnaces with sealed combustion are less affected by altitude than standard atmospheric furnaces. When sizing a new furnace, altitude must be factored into the load calculation.

Denver's design temperature for HVAC sizing is typically -5°F to -1°F. Mountain communities see significantly colder sustained temperatures. Heat pumps lose efficiency below 35–40°F and need supplemental or backup heat below 5–15°F depending on the model. Gas furnaces provide consistent heat output regardless of outdoor temperature, which is why dual-fuel systems — heat pump for mild weather, gas furnace for extreme cold — are a practical Colorado heating choice.

Common causes of higher-than-expected heating costs in Colorado: air leaks in the home's envelope (Colorado's dry climate means wood frames shrink more, creating gaps), inadequate insulation (especially in attics), a furnace running inefficiently due to maintenance needs, a heating system that's oversized or undersized for the home, or a heat pump that's running in less-efficient electric resistance mode more than intended. A home energy audit or HVAC evaluation can identify the primary contributors.

Furnace repair cost depends on the failed component. Common repairs: ignitor replacement $150–350, flame sensor cleaning/replacement $80–200, blower motor $300–600, control board $200–600, pressure switch $100–250. A heat exchanger failure may cost more than a new furnace. We diagnose first and provide upfront pricing before any repair begins — you know the cost before we start.

Furnace installation cost varies based on the furnace selected, system capacity, installation complexity, and whether additional work is needed — gas line, ductwork modifications, updated venting, permits. A standard furnace replacement in a straightforward application is very different from a new installation with significant infrastructure work. We evaluate your home and provide upfront pricing before installation.

Heat pump installation cost varies significantly based on the system selected, home size, installation requirements, ductwork condition, and electrical capacity. Some homes need panel upgrades to support heat pump loads. We evaluate your specific home and provide upfront pricing after assessing the installation requirements — not a ballpark that bears little resemblance to the actual job.

Yes. Zenith Home Services offers financing options for qualified customers for larger heating projects — new furnace installation, heat pump installation, boiler replacement, and significant system upgrades. Contact us at (720) 255-9702 to discuss financing options when scheduling your evaluation.