Furnace Installation
A new furnace sized from a heat-loss calculation, derated for altitude, vented and permitted: furnace installation from MoJo in the Denver metro.
Swapping a furnace looks simple from the outside: old box out, new box in. The details decide whether it runs quietly and efficiently for the next two decades or fights the house from day one. At MoJo Home Services, a family-owned HVAC company, our installers handle furnace installation and replacement across the Denver metro, from the first measurement to the final startup check.
When to Replace Your Furnace
The furnace is 15 to 20 years old and starting to need major parts.
A technician found a cracked heat exchanger, which makes it unsafe to keep running.
Repairs every season, each one costing more than the last.
Rooms that never get comfortable, or a system that runs constantly on cold nights.
Gas bills creeping up on an older standard-efficiency unit.
Choosing the Right Furnace
Efficiency. Standard-efficiency furnaces, around 80 percent AFUE, send hot exhaust up a metal flue or chimney. Condensing furnaces, 90 percent and up, pull so much heat out of the exhaust that it can vent through PVC pipe, and they produce water that needs a drain. Going high-efficiency in an older home sometimes leaves a gas water heater alone in an oversized chimney, which may then need a liner. We check that before recommending a model.
Staging and blower. Single-stage furnaces run at full fire every cycle. Two-stage and modulating models spend most of the winter on low fire, which means steadier temperatures, quieter operation and more filtering because air moves longer. A variable-speed blower also helps if you run central air in summer.
Capacity. Bigger is not better. We size the furnace from a heat-loss calculation for your house, not from the label on the old one, because older furnaces were often oversized from the start. We also confirm the return air can feed the new blower, since a starved return is a common reason a modern furnace runs loud or hot. If you are weighing a furnace against a heat pump or a dual-fuel setup, see other heating system options.
The High-Altitude Derate
At this elevation a gas furnace cannot simply be installed the way it ships. Manufacturers call for a high-altitude derate, which can mean different burner orifices or a lower manifold pressure, and in some cases a high-altitude pressure switch. Skipping that step leads to an over-fired burner, sooting, nuisance lockouts and a shorter heat exchanger life. We set the gas pressure and verify the temperature rise on every unit we put in.
The Day of the Swap
Most furnace swaps take one day. The crew lays down floor protection, shuts off gas and power, and removes the old unit. The new furnace is then set and connected to the gas line, venting, condensate drain on high-efficiency models, ductwork and thermostat wiring. If you have central air, the furnace is matched to the existing indoor coil, or the coil is replaced when it cannot be matched. Furnaces in attics or garages get extra attention: a condensing model's drain line can freeze in a cold garage, so we route or protect it, and an attic unit needs a drain pan and safe service access.
At startup we measure gas pressure, temperature rise and airflow, check every connection for leaks, and walk you through the thermostat and filter changes. We handle the permit, and the city or county inspector signs off afterward. It is also a good moment to make sure you have working carbon monoxide alarms near the bedrooms.
New Furnace Rules in Colorado
Since January 1, 2026, every new gas furnace sold and installed in Colorado has to meet the state's ultra-low NOx emissions limit. A furnace that already works doesn't have to be replaced because of it. Separately, the ENERGY STAR label now requires 97% AFUE in Colorado, and a federal 95% minimum is scheduled for furnaces made from December 18, 2028, though that date could change. Our guide to the 2026 furnace rules explains what each one means for your choice.
What Affects the Cost
Two installs rarely match, so your final price depends on the efficiency and staging of the furnace, its capacity, venting changes (especially a switch from chimney to PVC), duct or return-air corrections, whether the AC coil has to be replaced to match, how hard the furnace location is to reach, and permit fees. As a starting point, a new furnace starts at $3,050 installed, and regular prices run from $5,360 for a standard 80% furnace in a smaller home to $12,160 for a 97% variable-speed furnace in a large one. Our 2026 furnace replacement cost guide lists prices by home size, and the free HVAC price calculator gives you a range for your home in about 60 seconds. Your written estimate lists every factor so you can compare options. Incentive programs for efficient furnaces come and go, so we look up the ones that fit your model while preparing the quote. If you would rather spread the cost out, financing options are available.
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Help Center
Frequently Asked Questions: Furnace Installation in Denver, CO
How long does it take to replace a furnace?
Most straightforward replacements are done in one day. Switching from chimney venting to PVC, correcting ductwork or replacing the AC coil at the same time can add hours or a second day.
Is a high-efficiency furnace worth it in Denver?
With a long heating season, a condensing furnace usually pays back over its life, especially in a larger home or one that runs the heat hard. It does need a drain and a route for PVC venting, so in some houses an 80 percent model is the more practical fit. We price both when it makes sense.
Do I need to replace my air conditioner at the same time?
Not always. If the AC is fairly new, we match the furnace to the existing coil. If the AC is aging or uses an older refrigerant, replacing both together avoids a mismatched system and a second installation visit, and it is a good time to look at a heat pump.
Does altitude change which furnace I should buy?
Most current gas furnaces can be set up for high altitude with the manufacturer's conversion parts, so altitude rarely rules a model out. What matters is that the derate is actually done at installation and checked at startup.
How long does a new furnace last?
With a filter change every few months and a yearly heating tune-up, a gas furnace commonly lasts 15 to 20 years. Skipped maintenance, poor airflow and incorrect altitude setup are the most common reasons one fails early.
How much does a new furnace cost in Denver?
At MoJo, a new furnace starts at $3,050 installed. Regular prices run from $5,360 for a standard 80% furnace in a smaller home to $12,160 for a 97% variable-speed furnace in a large one. Our furnace replacement cost guide lists prices by home size. For a quick range for your home, try our HVAC price calculator.
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