Hydronic Heating System Components

Your hydronic heating system is a closed system. The water in the heating system is separated from your drinking water, and you are protected by a one-way valve from heating water back flowing. Water is not compressible. As your system heats up from room temperature the water expands, increasing the water volume. This extra volume needs somewhere to expand to. Hence, we have expansion tanks. The tank contains a cushion of air that can compress as the heating system water expands.

A – Expansion Tank

Charged with a cushion of air. Needs recharging every year.

Residential boilers are designed to work at a certain pressure (usually 12 to 15 pounds per square inch – psi). If the pressure exceeds the working pressure of the boiler it is relieved by the Safety Relief Valve (usually set at 30 psi – about the pressure of a car tire).

B – Safety (Pressure) Relief Valve

Normally 30 psi. Once it has opened (“relieved”)the valve often will not seal and will need replacing.

All hydronic systems lose water from evaporation or leakage not visible to the eye. This causes the system pressure (normally 12 – 15 psi) to go down. Eventually the system would run out of water and components, or the boiler would fail. To prevent this from happening hydronic systems have automatic fill valves. When the pressure drops to 8 to 10psi the sensitive fill valve adds water to top the system back up to 12 to 15 psi.

C – Automatic Fill Valve

The “life saver” of the hydronic system. Needs to be “exercised” and tested annually.All fill valves are not created equal. It is a crucial item to protect your system.

The Importance of Boiler Maintenance

When AB, or C are out of sync, it usually results in disruption to your heating system and component failure. In extreme cases, the boiler could fail. The key to keeping these components working together is annual maintenance.

Here’s an example of what can go wrong:

Say the expansion tank loses its air cushion from a lack of maintenance. The water has nowhere to expand to, so the relief valve goes off every time the boiler heats up. To counteract the continuous loss of water, the automatic fill valve keeps topping the boiler up with water. Eventually you hear gurgling or banging in the system, or you lose heat altogether. We come along and find that A caused B to fail, and B caused C to fail. If you are unlucky, these components caused the circulating pump, or pumps, to fail. In the worst scenario, the boiler fails and the bill is $7,000 to $10,000.

Annual preventive maintenance is essential because:

  • It prevents break downs
  • It saves thousands of dollars


If you live in Marin or Sonoma County and need help with your hydronic heating system, please call John Owens Services, Inc. at or . We work on all types of boiler and radiant heating systems and are a Rinnai authorized dealer and installer for your peace of mind.


Related Posts