How To Retrofit Radiant Heating in a Bathroom

Nobody likes standing on a cold floor with bare feet, especially on an icy winter morning. Adding radiant floor heating can transform that chilly space into a cozy retreat. In this guide, we’ll explain how to retrofit radiant heating in your bathroom. We’ll also share insights from Ask This Old House’s Richard Trethewey, who shares the installation process in the video above. 

How Radiant Floor Heating Works

Radiant floor heating warms your bathroom from the ground up. This heating system uses a network of pipes or electric cables that sit beneath the floor surface. They warm the floor directly, and the warmth then rises to heat the rest of the room. This heating method is highly efficient because it doesn’t rely on circulating air to distribute warmth.

Benefits of Radiant Floor Heating in Bathrooms

Radiant floor heating offers several advantages, including:

• Consistent warmth: Radiant floor heating eliminates cold spots by distributing heat across the floor evenly.
• Energy efficiency: This heating method operates at lower temperatures than forced-air systems, so you may even experience lower energy costs.
• Improved air quality: You may breathe easier because this system doesn’t circulate dust or allergens like forced-air heat does.
• Increased comfort: Your feet will be toasty when they stand on the warm floors
• Silent operation: Radiant floor heating doesn’t use noisy fans or vents.

Types of Radiant Floor Heating Systems

There are two main radiant floor heating systems available:

• Hydronic systems: Hydronic systems circulate hot water through pipes under the floor. They connect to your existing boiler or water heater, using a heat exchanger to distribute warmth. These systems are especially efficient in larger spaces and can be more cost-effective over time.
• Electric systems: Electric systems use cables or mats installed under the floor’s surface. They connect to your home’s electrical system and are controlled by a thermostat. Installation is generally easier than hydronic systems, making them a good choice for smaller areas like bathrooms.

For this project, we’ll focus on installing a hydronic system that taps into an existing boiler or water heater, as Trethewey demonstrates in the video segment.

Planning Your Radiant Heating Retrofit

Before you start, here are a few steps to help the installation go smoothly. This will help you determine whether you can retrofit the heating system in your existing space.

Assessing Your Existing Bathroom

Take a look at your bathroom’s current layout and flooring. You can install radiant heating under many floor types, including tile, stone, and some engineered wood products.
Check if adding the new system will change your floor height—you might need to adjust doors or fixtures.

Gathering Your Materials and Tools

To install a radiant floor heating system, you’ll need:

• Check valves
• Compression connections
• Expansion tank
• Heat exchanger with pumps
• Insulation
• Manifold
• Oscillating tool
• PEX cutter
• PEX tubing
• Power drill
• Pressure relief valves
• Push-to-connect fittings
• Rubber mallet
• Screws
• T-fittings (solderless)
• Transfer plates

Radiant Floor Installation Process

Now that you’re prepped, follow these steps for a successful retrofit:

Clearing the Space

Remove any existing floor coverings and inspect the subfloor for damage. Clean the area thoroughly and make any repairs to the subfloor. Then, identify and mark the locations of joists and any obstacles beneath the floor.

Mounting the Heat Exchanger and Manifold

In the video, Trethewey begins by mounting the heat exchanger and manifold to the wall near the water heater. Placing these items close together allows the heat exchanger to run as effectively as possible without the water cooling too much. Here’s how to do it:

1. Secure the heat exchanger to the wall using screws.
2. Mount the manifold nearby and make sure there’s easy access for future maintenance.
3. Connect the pump and manifold using a solderless press machine for strong, reliable connections.

Installing Transfer Plates and Tubing

Now, carefully trim and knock out the blocking between the joists using the oscillating tool and a hammer. You don’t need to remove the entire blocking board—just remove the top 2 inches or so, leaving enough clearance for the plate and the heating tubing. Then, follow these steps:

1. Install the transfer plates in sets of two rows per joist bay. This allows the tubes to track down the joist bay and loop back before moving to the next bay so that there’s efficient heat distribution.
2. Drill holes in the joist bays to pass the PEX tubing through from bay to bay. Aim to drill these holes close enough to the subfloor to keep them out of the way but low enough that the tubing is safe from nails and screwdriving from the floor above in case you change the flooring in the future.
3. Starting from the section farthest from the manifold and working your way back, weave the PEX tubing through the joist bays.
4. Use a rubber mallet to secure the tubing into the transfer plates.

Connecting the System

Once you’ve installed those components, connect the system. Follow these steps so that the system is safe and operates properly:

1. Reinstall any insulation that you removed.
2. Connect the new radiant system at the manifold to operate the system. Use the compression fittings included with the manifold to attach the PEX piping. If there are additional zones not being used, you can use a small length of pipe known as a jumper to close the loop.
3. Connect the new system back to the water heater using the solderless connection tool.
4. Install shut-off valves to make future maintenance easier.

Finishing Steps

Check that all your connections are secure and insulated. Test the system for leaks and proper operation before you replace the floor covering.

We recommend having your system inspected by a local plumbing inspector or contractor to guarantee that all the proper safety devices are installed, including pressure relief valves, fill valves, expansion tanks, and other devices not mentioned above.

Additional Considerations for Radiant Flooring

While radiant floor heating offers many benefits, there are some factors to consider before installation.

Cost Analysis 

The cost depends on the size of your bathroom and the type of system you choose. On average, installing radiant floor heating can cost around $4,128*. Hydronic systems might cost more upfront but can save money over time.

*Cost information is based on 2024 data from Angi.

Maintenance Requirements

Radiant floor heating systems don’t usually require much maintenance. However, it’s a good idea to have a professional check the system periodically to make sure everything’s running smoothly.

Potential Challenges

There are some potential challenges to consider:

• Floor height: Adding radiant heating can increase the floor height, which can affect your doors, cabinetry, or fixtures.
• Initial cost: While radiant heating can be more energy-efficient, the initial installation cost may be higher compared to traditional heating systems.
• Complexity: Retrofitting radiant heating requires careful planning and may require significant labor, especially if you have to make modifications to the existing system’s structure.

We recommend you evaluate your space and determine your budget before retrofitting radiant heating in your bathroom. If the project feels a bit overwhelming, a professional installer can help.