Introduction To Hot Water Boilers And Hydronic Systems

Hydronic boilers, coming from the word “hydro” meaning water, comprise central boilers and water heaters. The hot water or steam that is produced runs through pipes into radiators, which releases heat energy into the room. Once the water or steam loses its heat, it gets sent back to the boiler to be reheated.

An electric-powered circulator sends the hot water to and from your boiler. This is why some people call boilers forced hot-water systems, in contract with a furnace which is referred to as a forced warm-air system.

Fire Tubes

Boilers are commonly made from steel or cast-iron, and use “fire tubes” which carry hot combustion gases. The burner uses fuel (usually gas or oil) to send hot gases into the tubes. The heat from the tubes get transferred to the water, which is then pumped into the radiators. A flue pipe will vent the exhaust gases out of the home.

Expansion Tank

Since water expands when it is heated, boilers require an “expansion tank” to deal with the added volume. It works kind of like an overflow container. In addition to dealing with extra volume, the expansion tank also helps control the pressure within the system. When the water cools, it flows out of the expansion tank and back into the heating system.


The aquastat is basically a thermostat. It tells the boiler how hot the water should be. Typically, 180 degrees Fahrenheit is considered high and 160 degrees Fahrenheit is low. If you have young children at home, consider installing a tempering valve (aka a mixing valve) which reduces the risk of scalding by mixing in cold water.

Safety Controls

The boiler has two main safety controls. The safety relief valve protects your boiler from high pressure and high temperature situations. If the pressure or temperature gets too high, the valve will open to release excess pressure or temperature.

The combination gauge is located on the top or front of the unit, displaying preset water level, pressure, and temperature levels.

A low-water cutoff will shut down the boiler if there isn’t enough water. A float is used to gauge the water level. When the water level drops too low, it can be damaging and dangerous.

Bleeding Radiators

If you have a boiler, you’ll want to know how to bleed them. And no, you won’t need leeches or a blood-letting set. Since water contains some air, it can get trapped in an air pocket, blocking the path for your warm water. When you bleed the radiator, you get rid of excess air, allowing for a smooth transfer of heat.

On each radiator, there is a small air valve which can be opened with a screwdriver, or sometimes a special key. Position a cup underneath the air valve and open it with a twist to the left. You will hear air coming out followed by some water pouring out. When water starts coming out, close the valve!

If one of your radiators isn’t heating up, try bleeding it. Get into the habit of bleeding your radiators each fall and regularly vacuuming the area with your vacuum’s brush attachment.

And don’t forget to schedule your annual heating appointment every fall!

If you have any questions about hot-water boilers, hydronic systems, bleeding your radiator, or anything else related to heating and air-conditioning, don’t hesitate to contact us. Shopping for a new heating system? Learn how to choose a heating system that’s right for you.

Service Champions Heating & Air Conditioning can handle all of your heating needs, including hot water and steam boilers. Contact Service Champions today to schedule your next appointment.

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