How to Test a Colorado Furnace or Boiler for Leaks

Test a Colorado gas furnace or boiler for leaks in three steps: a visual check of every connection and the area around the unit, a listening pass for hissing or bubbling while it runs, and a look at your gas bill and boiler pressure gauge for unexplained changes. If you smell rotten eggs (the additive in natural gas) or your carbon monoxide alarm sounds, leave the house and call your gas utility's emergency line before calling a technician — that is not a wait-and-see situation.

Colorado adds two wrinkles that homeowners in milder climates do not deal with: altitude, which changes how a furnace is supposed to be set up in the first place, and a state law that makes a working carbon monoxide alarm a requirement, not a nice-to-have, in most homes with a gas furnace.

How do I test my furnace or boiler for a leak?

Start with the furnace or boiler off and cooled down. Walk the visible components — the unit itself, exposed gas and water lines, connection fittings, and the venting — and look for moisture, rust streaks, or a soapy-looking residue at joints (a mix of dish soap and water brushed on a gas fitting will bubble at a leak). Run the system and listen from a few feet away and then from other rooms for hissing, whistling, or bubbling. On a boiler, check the pressure gauge against the manufacturer's normal range, in most cases 1 to 2 bar when the system is cold; a gauge that keeps dropping between refills points to a leak somewhere in the loop.

What are the signs of a heating system leak in a Colorado home?

• A rotten-egg smell near the furnace, gas meter, or basement floor.
• A gas bill that jumps with no change in how you are running the system.
• Hissing or bubbling sounds from the unit, especially right after it kicks on.
• Water stains or pooling around a boiler, which signals a water-side leak rather than gas.
• Pilot light or burner flame that burns yellow or flickers instead of a steady blue.
• A carbon monoxide alarm that goes off, which is treated as an emergency, not a maintenance item.

Does Colorado law require carbon monoxide alarms?

Yes. Colorado's Carbon Monoxide Poisoning Prevention Act, C.R.S. 38-45-101, requires a working CO alarm in most homes and rentals that have a fuel-burning appliance (a gas furnace, boiler, water heater, or fireplace) or an attached garage. The alarm has to be within 15 feet of each bedroom entrance. A seller in a home sale is expected to have a functioning alarm in place, and a buyer's inspector will check for it — a missing or dead alarm is a fast, cheap fix that should never turn into an inspection objection item.

How does altitude affect a gas furnace in Colorado?

Combustion needs oxygen, and there is less of it per breath of air at 5,280 feet in Denver than at sea level, and less still in Colorado Springs, Boulder, and the mountain corridor. Furnaces and boilers rated for sea-level performance lose roughly 4% of their output for every 1,000 feet of elevation, so a unit installed above about 2,000 feet needs a high-altitude conversion — commonly a smaller gas orifice and an adjusted manifold pressure — to burn cleanly and without damage instead of running rich and producing excess carbon monoxide. Any HVAC contractor licensed to work the Front Range should set this correctly on install; it is worth confirming on a home built or system replaced outside Colorado, or on an older unit that has never been serviced.

How commonly should a Colorado furnace be inspected?

Once a year, ideally in September or October before the first cold snap. An annual inspection checks the heat exchanger for cracks (the most serious failure, since it lets combustion gases mix with the air the blower pushes through the house), tests the CO output at the vent, and confirms the altitude setup is still correct after any parts have been replaced. A Denver metro inspection and tune-up runs $100 to $200; a full heat exchanger replacement, when one is needed, runs $500 to $1,500 depending on the unit, and is commonly close enough to the cost of a new furnace that a contractor will walk you through both options.

How long does a gas furnace last at altitude?

A well-maintained gas furnace lasts 15 to 20 years along the Front Range. Furnaces that are undersized, incorrectly set for altitude, or skip annual service tend to fail earlier, and a heat exchanger crack is the failure mode that matters most, since it can let combustion gases mix with the air the blower pushes through the house. A furnace past 18 to 20 years old is a common inspection flag even if it is still heating the house, because insurers and inspectors both treat age as a risk factor on its own.

Can a leaking or aging furnace affect a home sale?

Yes, in two ways. A confirmed gas or CO leak stops a transaction cold until it is repaired and re-tested, no negotiation. An aging furnace that is not actively leaking still shows up on the inspection report as a system near the end of its rated life, and buyers along the Front Range increasingly ask for either a service record or a credit. Our guide to HVAC systems and Colorado home sales covers how much a documented, recently serviced system is worth in negotiations compared with one that has no paper trail.

What should I do if I smell gas or my CO alarm goes off?

Leave the house immediately, do not operate light switches or anything electrical on the way out, and call your gas utility's emergency line from outside. Xcel Energy, which serves most of the Denver metro, runs a 24-hour gas emergency line for exactly this. Do not go back in until the utility or fire department clears it. This is the one heating system issue where "wait for a technician's regular appointment" is the wrong call.

What's the difference between a furnace leak and a boiler leak?

A furnace leak is almost always a gas leak, since a forced-air furnace does not carry water through the house. A boiler runs both gas (to the burner) and water (through radiators or in-floor loops), so a boiler "leak" more commonly means a water-side drip at a valve or joint, which is a repair issue rather than an emergency, unless the smell of gas is also present at the burner itself.

Where to go next

• Why an efficient HVAC system helps a Colorado home sell
• Preparing and adding value before selling
• Cutting summer electric bills in a Colorado home
• Denver utility providers and costs guide
• Explore homes for sale in Denver
• Search every home for sale in Colorado

Talk to the Kenna Real Estate Group

Buying or selling a Front Range home and want to know what an aging furnace or boiler means for the deal? Call or text 303-955-4220. A live person answers. Not a robot, not a phone tree. Ready to look at options? Search every home for sale in Colorado.