How Does Radiant Heat Work?

Radiant heat systems are among the most comfortable ways to heat your home. They’re ideal for allergy sufferers that don’t want dust pushed around by a hot air / forced air heating system.

Explaining a Radiant Heat System

This infographic can be referred to throughout our explanation of a radiant heat system.

How Does Radiant Heat Work

It shows a radiant floor heating system with tubes running beneath the flooring in each room. But the same basic mechanics are used for radiant heating systems that employ baseboard or other radiators.

When useful, words are defined to provide clarity and detail.

Step 1: The Thermostat Controls Temperature for the Zone

Notice that there are two thermostats in this “home.” That means there are two zones, explained further in the definition below.

OK. Let’s say the temperature in your Room 1 is 68F and you think 72F sounds cozier. You turn up the thermostat to 72. The thermostat signals the control panel that Zone 1 needs a little more heat.

Zone: Room or adjoining rooms served by a single pump or control valve. Zones can be heated separately when controlled by separate thermostats.

Step 2: The Control Panel Starts the Boiler

When one of the zones requires more heat, the Control Panel signals the boiler to fire up. But it also controls zoned radiant heating, which is discussed in later steps.

Step 3: The Boiler Fires & Communicates with the Control Panel

Two important steps happen here.

The first is that the boiler begins to heat.

Most boilers are fueled by gas, either natural gas (NG) or propane (LP for liquid propane), but oil-fired boilers and those with electric elements are available too.

And where wood is readily available and inexpensive, outdoor wood boilers are quite popular for heating and providing hot water.

Secondly, as the water in the boiler becomes hot enough to provide heat, the boiler signals the Control Panel.

Low-temperature Limit: The temperature at which the water is hot enough to provide adequate heat. Another way of putting it is that the low-temp limit is the lowest temperature to do the job.

Step 4: The Control Panel Sends Hot Water to the Right Zone or Zones

This is where we discuss zoned heating with a radiant heating system. Zoned operation is designed in one of two ways.

Small systems have one pump. Each zone is controlled by a zone control valve. The circulation pump starts, and the valve for Zone 1, in our example, opens. This allows the circulation of water into the pipes, radiators or tubes in the zone.

Large systems often have a separate pump for each zone. So, for example, if Zone 1 needs heat, the Control Panel starts the Zone 1 pump and opens its valve to allow for circulation.

Supply Water: This is the name given to the water leaving the boiler and heading to the room or zone to deliver its heat.

Step 5: The Floor or Radiators Get Warm

In large and small systems, the pump is used to circulate water through the tubing or the pipes and radiators.

Hot water is pushed into the pipe (see the red pipes) while the cool water (gray and blue pipes) is pushed back to the boiler to be heated and made ready for circulation.

The heat from the water radiates up through the flooring and into the air of the room. This is gentle heat without the noise or blowing air (and potential blowing dust) of a forced air system where air is being circulated with the use of a powerful blower fan.

You might notice in the picture that Zone 1 on the left splits to feed hot water to two separate sections of tubing beneath the flooring. This ensures that the floor is heated more evenly than if one system covered the whole room.

The water in a single pipe system, such as on the right in Zone 2, might cool off before it got to the far side of the room. Both the floor and eventually the air in the room or zone would experience a temperature imbalance.

By the way, if you’re considering both a forced air furnace system and a boiler system, this guide is full of useful information: Furnaces vs Boilers: Which One is the Better Heating Solution?

As you’ll see, each has its pros and cons. The guide includes cost information too.

Step 6: Cool Water Cycles Back to the Boiler

We use the term “cool water” for comparison to the hot water leaving the boiler.

But in truth, if the radiant heating system is sized properly, water will still be quite warm when it heads back to the boiler. While each radiant heat system is designed with unique specifications, most heat water to 140-160F. It returns to the boiler at about 80-100F.

Boilers don’t boil? That’s right. While still called boilers, most residential models do not heat the water to boiling. They are safer and more efficient when heating water to a temperature short of boiling.

Return Water: This is the technical term for the water being pushed back to the boiler to be heated again.

Additional Boiler & Radiant Heat System Resources

We’ve produced a wide range of pages on boiler systems. These additional guides might prove useful to your research on How a Radiant Heat System Works.

Best Residential Boilers for Radiant Floor Heat 2020

How Boilers Compare to Furnaces and Heat Pumps

Outdoor Wood Furnace/Boiler Cost, Reviews & Buying Guide

We’ve also reviewed the top boiler manufacturers including Laars, Peerless, Weil and many others. Those reviews and others can be found by browsing the results on this search result page.

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