If your home uses hydronic heat, you rely on a boiler to provide hot water to your radiators, baseboards, or other heating elements. Most modern homes use hot water boilers instead of steam boilers. Despite the name, these appliances heat water to sub-boiling levels. In other words, a central heating boiler is usually more like a water heater.

Unlike steam boilers, hot water boilers can't rely on gravity to do the hard work. Instead, a circulating pump moves water throughout your home while cold water flows back to the boiler along a return circuit. While some steam systems use a single pipe for both supply and return, hot water hydronic systems need separate pipes for each purpose.

What Does a Circulating Pump Do?

Your circulating pump has a surprisingly complex and essential role in your home's hydronic heating system. The pump must provide enough pressure to supply the entire system, but it must also accommodate the heating load within a heating zone. In other words, the pump needs to move hot water to your heaters rapidly enough to replace water as it transfers its energy to the air and cools.

The circulating pump has another essential role: it maintains the appropriate level of heat transfer in your boiler. Every boiler has a required flow rate in gallons per minute (GPM). This rate ensures efficient heating and prevents water from remaining in the heat exchanger, where it may become overheated and begin to boil off.

What Happens If Your Circulating Pump Fails?

Your circulating pump may seem like an "extra" component, but it's a critical part of your boiler. If your pump fails or begins to wear out, the flow rate through your system will decrease. As a result, water will spend more time in the heat exchanger, allowing it to absorb more heat before circulating through the rest of the system.

In severe cases, you may hear rumbling or boiling noise from your boiler. This noise occurs when water in the boiler overheats and converts to steam. Hydronic heating experts commonly refer to this sound as "kettling" since it sounds like water as it begins to boil in a kettle. Kettling is inefficient and may eventually damage your hydronic plumbing or the heat exchanger in your boiler.

What Should You Do If Your Circulating Pump Fails?

Diagnosing and repairing a faulty circulating pump is a job you should leave to an expert. Once you recognize the signs of a bad pump, you'll want to call in a boiler repair HVAC technician as soon as you can. Not only will repairing your pump help restore efficient heating to your home, but it will also prevent the problem from causing further damage to your hydronic heating system.