Boiler

A boiler heats water and circulates it through radiators, baseboard, panel radiators or in floor tubing. Despite the name, most residential boilers never boil water. They run a supply temperature well below boiling, set by the emitter type and the design conditions.

1. 1A thermostat or zone control calls for heat.
2. 2The boiler proves safeties, starts its burner sequence and fires.
3. 3A circulator moves water from the boiler through the distribution loop.
4. 4Heat transfers from the water to the space through the emitters.
5. 5Cooler return water comes back to the boiler and the cycle continues until the call is satisfied.

Outdoor reset control lowers supply water temperature as outdoor temperature rises, which improves comfort and, on a condensing boiler, keeps return temperatures low enough to condense.

• Air trapped in the loop, which stops circulation in a zone and produces gurgling.
• A waterlogged or failed expansion tank causing relief valve discharge.
• A circulator that has failed or is airbound.
• Low water pressure from a leak or a failed fill valve.
• Short cycling from oversizing or from a control setting that does not match the emitters.
• Condensate drain blocked on a condensing boiler, tripping the appliance.

Boiler installation, venting, relief valve discharge piping, gas piping, expansion tank sizing and permits vary by jurisdiction, adopted code edition and the manufacturer instructions for the specific appliance.