Boiler and radiant heat service — before the cold snap, not during it

Hydronic heating is quiet, even, and once it is working properly it tends to keep working for decades. It is also the system most Colorado homeowners understand least — and the one where a cold snap turns a small fault into a burst-pipe problem overnight.

Boilers and radiant systems are common here for good reasons. Older Denver and Pueblo homes were built with cast-iron radiators and never converted; mountain homes favour in-floor heat for big rooms and long absences; snowmelt loops are close to standard in the high country. All of it is pressurised water moving through a building in winter.

What you actually have

Cast-iron radiators. A boiler heats water, a pump or gravity moves it through large iron radiators, and the room warms by radiation. Slow to respond, extremely comfortable, common in pre-war Front Range housing.

Baseboard convectors. Finned copper tube in a metal enclosure along the wall. Faster, easier to zone, the standard postwar retrofit. Prone to bent fins, blocked airflow and air trapped at high points.

In-floor radiant. Tubing in a slab or under a subfloor, run at much lower water temperature with a mixing valve to blend boiler water down. Exceptionally comfortable, very slow, and unforgiving of anyone cutting into a slab blind.

Snowmelt. Radiant tubing under a driveway, walkway or entry, almost always on a glycol loop through a heat exchanger so it can freeze without splitting. Its own controls, its own sensors, and its own failure modes.

A forced-air furnace that quits leaves you cold. A boiler that quits in January leaves you cold and then starts freezing the pipes that were supposed to be heating the house.

Symptoms, and what they usually mean

One radiator or one zone cold, everything else fine. Most often trapped air at a high point, needing a bleed. Otherwise a stuck zone valve, a failed thermostat or actuator, or a balancing valve someone closed years ago and forgot.

Cold at the top, warm at the bottom of a radiator. Air in that radiator; bleeding fixes it in minutes. If it recurs every few weeks, air is getting in — a leak, a failing air separator, or constant make-up water.

Pressure gauge low. The system is losing water somewhere, and hydronic leaks hide well — under a slab, inside a wall, behind a boiler jacket. A system needing regular top-up has a leak, not a quirk. Leak detection is the next step.

Pressure gauge high, relief valve dripping. Usually a failed expansion tank, or a heat exchanger leak pushing domestic pressure into the heating loop. The valve is doing its job; the cause needs finding, not capping.

When it is an emergency

Treat it as urgent if there is no heat below freezing — especially in an unoccupied property, which describes a great many mountain homes. Urgent too if you smell gas: leave the building and call from outside. And urgent if water is actively leaking; close the isolation valves if you can.

For everything else, a system limping along through a mild spell can wait for a scheduled visit — and you get a better technician and price for waiting a day. What you should not do is nurse it through a winter on fortnightly top-ups. See emergency plumbing for what counts as a night call.

Annual service, and why it earns its cost

A proper annual service covers combustion (burner, ignition, flame sense, combustion analysis, vent and combustion air, altitude configuration), the water side (system pressure, expansion tank charge, air separator and vents, circulator operation, weeping at pump flanges) and controls (thermostats, zone valves and actuators, reset settings, low-water cutoff). On glycol systems, add a glycol test.

Timing matters. Book it in September or October — a boiler that fails on the first cold night competes with every other boiler that failed that night, and parts for older appliances are not guaranteed.

For mountain and second homes, ask about a low-temperature alarm. A cheap sensor that texts you when the house drops below a set temperature is the difference between a service call and a repiping job.

Radiant floors and snowmelt

Radiant deserves its own note because the rules are different and the mistakes are more permanent.

Radiant runs at low water temperature, so it depends on a mixing arrangement to blend boiler water down before it reaches the floor. When a floor runs too hot, that valve or its control is usually the cause — and sustained overheating can damage flooring adhesives.

It is also slow. A slab has enormous thermal mass, so radiant holds a steady temperature rather than responding to setbacks. Drop the thermostat ten degrees overnight and the house never catches up. Set it and leave it.

The permanent mistake is cutting into a slab without locating the loops. A punctured loop in a finished slab is a genuinely difficult repair, and entirely avoidable with a thermal image or a layout plan taken first.

Snowmelt systems add glycol, a heat exchanger, slab sensors and controls. Common faults: a failed slab sensor leaving the system dormant in a storm or running through July, degraded glycol, and controls set so aggressively the running cost startles people.

Repair or replace

Repair makes sense when the boiler body and heat exchanger are sound and the fault is in a serviceable component — a circulator, zone valve, expansion tank, control board or igniter. Those are ordinary parts on an appliance that may have decades left.

Replacement makes sense when the boiler section is leaking, parts are no longer obtainable, the same fault keeps returning, or the appliance is badly oversized — extremely common, since loads were estimated generously for decades. An oversized boiler short-cycles, wasting fuel and wearing components.

If you do replace, insist on a heat loss calculation for the building as it is now rather than a match of the old appliance size — that is where the real efficiency gain comes from — and correct configuration for your elevation, since the de-rating that applies to water heaters applies to boilers too (our high-altitude sizing page explains it). A correctly sized modulating condensing boiler is a real improvement, but only if the system it feeds can run at the low water temperatures that make condensing efficiency real.

• Note the fault code and the pressure gauge reading before you call
• Know where the boiler isolation valves and the water main shutoff are
• Book the annual service in early autumn, not in the first cold snap
• Have glycol tested on any snowmelt or freeze-protected loop
• Fit a low-temperature alarm on any property left unoccupied in winter
• Locate radiant loops before cutting or drilling into a slab

What drives the price

We refer to independent contractors and cannot quote for them, but hydronic work has a recognisable cost structure.

Diagnostic time is a larger share than in most plumbing: a multi-zone system with decades of modifications takes real investigation. Simple component swaps — circulator, actuator, expansion tank, thermostat, a bleed and rebalance — are ordinary service calls. Control boards for older appliances cost more and take time to source.

Leak repair depends on access: a weeping fitting in a boiler room is straightforward, a leak in a slab loop is not. Glycol is priced by volume, and snowmelt loops hold a lot. Full replacement covers appliance, near-boiler piping, controls, venting, gas work, permit, inspection and disposal.

Mountain and remote addresses may carry travel time, and winter access affects scheduling more than price. Annual maintenance, by contrast, is one of the few genuinely predictable line items in home ownership.