A boiler heats water and distributes that hot water — or steam — through a network of pipes to radiators, baseboard units, or radiant floor loops throughout your home. Unlike forced-air furnaces that heat and move air, boilers deliver radiant warmth that many homeowners find more comfortable and consistent. The basic principle is simple: a gas burner (or electric element) heats water in a sealed tank, and a pump or pressure differential moves it where it needs to go.

The Core Components of a Hydronic Boiler System

A residential boiler system has several key parts working together. The boiler itself is a sealed, insulated tank where water is heated. A gas valve and burner assembly control the flame, while a heat exchanger transfers combustion energy into the water without mixing flue gases into the supply. A circulator pump moves heated water through the distribution piping, and a thermostat signals the system to fire when indoor temperature drops below the set point. An expansion tank handles the slight volume increase as water warms, preventing pressure spikes, and a pressure relief valve is a critical safety device that opens if pressure climbs dangerously high.

Hot-water boilers (the most common residential type today) typically operate between 140°F and 180°F. Steam boilers, found in older Denver homes built before the 1950s, work at lower water volumes — they heat water to the point of steam and distribute it through one-pipe or two-pipe systems. Steam systems require more careful maintenance and balancing but are still very much serviceable.

How Altitude Affects Boiler Performance in Denver

At Denver’s elevation of roughly 5,280 feet, atmospheric pressure is about 17% lower than at sea level. This matters for gas-fired boilers in two ways. First, combustion efficiency is slightly reduced because there’s less oxygen per cubic foot of air — boilers need to be properly adjusted (de-rated) for altitude or they burn rich and produce more carbon monoxide. Second, water boils at approximately 202°F rather than 212°F at altitude, which affects steam systems specifically. Any boiler installed or serviced in the Denver metro should be set up by a technician familiar with high-altitude combustion requirements — it’s not just an efficiency issue, it’s a safety one.

Boiler Efficiency Ratings and What They Mean

Modern condensing boilers achieve Annual Fuel Utilization Efficiency (AFUE) ratings of 90% or higher, compared to 80–85% for standard atmospheric boilers. A condensing boiler extracts so much heat from the combustion gases that the flue gases cool enough to condense into water vapor before exhausting — that condensate is a byproduct of capturing heat that would otherwise go up the flue. High-efficiency condensing boilers can vent through PVC pipe rather than a traditional metal flue, which also simplifies installation in many retrofits.

Common Boiler Problems to Watch For

  • No heat or uneven heat — often a circulator pump failure, air trapped in the system, or a zone valve issue
  • Pressure too high or too low — could indicate a failing expansion tank, waterlogged expansion tank, or a leak somewhere in the system
  • Pilot or ignition failure — modern boilers use electronic ignition; a failed igniter or flame sensor is a common service call
  • Kettling noise (rumbling or banging) — typically caused by scale and sediment on the heat exchanger, common with Colorado’s hard water
  • Leaking from the pressure relief valve — a sign pressure is running too high and needs immediate attention

Maintenance Keeps Boilers Running Efficiently

Annual boiler maintenance typically includes inspecting the heat exchanger for cracks, testing the pressure relief valve, checking the expansion tank, bleeding air from radiators or baseboard units, and verifying combustion is clean and properly adjusted for altitude. Colorado’s hard water can cause scale to form on the heat exchanger over time, reducing efficiency noticeably — a technician can address this before it becomes a serious problem.

If your boiler is over 15–20 years old, it may be worth discussing replacement options with a heating specialist. Newer condensing boilers offer significant efficiency gains that can lower your gas bills noticeably over a Denver winter. Done’s heating team handles boiler repair, maintenance, and installation throughout the Front Range. Learn more on our boiler repair page, explore our full heating services, or contact us to schedule a visit.