Quick answer
Have a qualified technician inspect your boiler every year before heating season, as the CPSC recommends for all fuel-burning appliances. Ask the technician to check the venting, pressure relief valve, system pressure and controls. Angi's 2026 data puts boiler repairs at $190 to $660 and replacement at $3,600 to $8,400. ENERGY STAR gas boilers are rated at 90 percent AFUE.
Updated · By the Denver Plumber Pros editorial team · How we research
Why does boiler care matter in Denver?
A boiler that fails during a cold snap is two problems at once: no heat, and a risk of frozen pipes. Denver's average first freeze is October 7, so late summer or early fall is the right time to book a service. Hot water and steam systems also need different care than a forced-air furnace, so make sure the technician you book works on boilers.
What should an annual boiler service include?
The Consumer Product Safety Commission has long advised homeowners not to service their own fuel-burning appliances and to have a qualified plumber, heating contractor or gas company technician inspect them every year. Angi's maintenance guide likewise recommends a yearly professional inspection. When you book, ask the technician to walk you through what they check and to note it on the invoice. Useful questions:
- Is the venting and chimney or flue clear and properly connected?
- Does the pressure relief valve work, and is the system pressure in the manufacturer's range?
- Are there signs of leaks, corrosion or sediment?
- Is the circulator pump, expansion tank or low-water cutoff showing wear?
- Do the burners and controls operate as designed, and is there any sign of combustion gas spillage?
The CPSC also recommends a carbon monoxide alarm that meets Underwriters Laboratories standard 2034 as an added safeguard.
What can you do between service visits?
Angi's boiler maintenance guide (updated July 2026) suggests:
| Task | How often |
|---|---|
| Check the water level and pressure gauge is in the safe range | Weekly during heating season |
| Bleed trapped air from radiators | Before each heating season |
| Professional inspection | Annually |
| Full system flush | Every several years, or when water looks dirty |
| Oil the circulator pump, if your model needs it | Start of the season, and midway if it runs often |
To bleed a radiator, turn the boiler fully off, hold a pan under the bleed valve and open it until air stops and water appears. Your boiler's manual gives the correct pressure range; if pressure keeps dropping, call a licensed technician because it can mean a leak.
What does boiler efficiency mean?
AFUE, annual fuel utilization efficiency, is the share of the fuel's energy a boiler turns into heat over a typical year; a higher number means less fuel wasted. ENERGY STAR certified gas boilers are rated at 90 percent AFUE, about 6 percent more efficient than the federal standard, and ENERGY STAR estimates about $780 in savings over the product's lifetime. For oil boilers, the ENERGY STAR level is 87 percent AFUE. When you compare replacement quotes, ask each contractor for the AFUE of the unit offered and whether it is ENERGY STAR certified.
What are the carbon monoxide rules in Colorado?
Any fuel-burning boiler can leak carbon monoxide if its venting fails, which is why the CPSC pairs its annual inspection advice with a recommendation for CO alarms. Colorado law also sets alarm requirements. Under C.R.S. 38-45-103, when a home with a fuel-fired appliance or attached garage has permitted work done, including fuel-fired appliance replacement, a carbon monoxide alarm must be installed within 15 feet of the entrance to each sleeping room, or where local code specifies. A boiler replacement is permitted work in metro cities such as Arvada, Lakewood, Thornton and Westminster, so expect the inspector to check for alarms. If you smell gas, leave the house and call your gas utility or 911 from outside.
Repair or replace?
Angi's 2026 figures put professional boiler repairs at about $190 to $660 and a complete replacement at about $3,600 to $8,400. A repeated repair on an old, low-efficiency unit is often the point to price both options. Our boiler repair guide covers common failures, and the Denver cost guide has broader price context.
Who should work on your boiler?
Boiler work involves gas, water and venting, and metro cities require permits for replacements; Lakewood, for example, requires mechanical or plumbing registration with the city to replace a boiler. Ask for the technician's credentials and check plumbing licenses in the Colorado DORA license lookup. See how to choose a plumber for the full checklist.
About this guide
We are an independent homeowner guide, not a heating or plumbing company, and we do not perform work. If you want help finding a licensed local technician, email us a request with the boiler make, age and symptoms; we reply with next steps and, where a licensed contractor is available, an introduction.
Frequently asked questions
How often should a boiler be serviced in Denver?
Once a year, before heating season. The Consumer Product Safety Commission recommends an annual inspection of fuel-burning appliances by a qualified plumber, heating contractor or gas company technician. Between visits, Angi suggests checking pressure weekly during the season and bleeding radiators before the season starts.
Do I need a carbon monoxide alarm if I replace my boiler in Colorado?
Very likely. C.R.S. 38-45-103 requires a carbon monoxide alarm within 15 feet of the entrance to each sleeping room when permitted work, including fuel-fired appliance replacement, is done on a home with a fuel-fired appliance or attached garage. Local codes may specify additional locations.
What AFUE should a new gas boiler have?
ENERGY STAR certified gas boilers are rated at 90 percent AFUE, which ENERGY STAR says is about 6 percent more efficient than the federal standard, with estimated lifetime savings of about $780. Certified oil boilers are rated at 87 percent AFUE. Ask for the AFUE in writing on any replacement quote.
Sources
Independent homeowner guide operated by SWKM GmbH. Not a licensed contractor. We do not perform plumbing work. More about this site.