Water Heaters

How Big of a Water Heater Do I Need for Radiant Heat?

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There is no such thing as a water heater sizing chart. Every contractor and company will give you a different answer to that question because there are just too many variables involved in the proper sizing of your tankless water heating system.

We do our best to simplify things by breaking down all the different components and considerations that go into properly sizing your radiant heat system.

Radiant heat is an advanced on-demand heating system that works by heating the floor first, then the air on top of it. The fact that radiant heat heats up the floor means you need to have a minimum temperature in order for it to work. A general rule of thumb for sizing your water heater is 1 gallon per square foot of heated space.

How Does a Radiant Heating System Work?

A radiant heating system is a closed-loop piping system. The main components of a radiant heat system include a boiler, heater, and manifolds, as seen below.

Water from the boiler is sent to the manifold, which distributes it back to each room via tubing on the floor or on top of insulation on the ceiling. In some instances, a return manifold is used to send water back towards the boiler for continuous heating.

When a room’s temperature reaches the thermostat setting, the system shuts off and blows cold air through the tubing until it needs to be turned on again. A thermocouple or fiber-optic sensor is often installed in each room that tracks radiant floor and air temperature.

Radiant heat works by heating objects in a room, not the air itself. This means you need to have a floor with an adequate level of insulation before any radiant system can be installed. We recommend using our Insulation Installation Calculator when deciding what type of insulation is right for your project.

Once your building is ready for radiant installation, you will need to decide if you want your radiant heat system installed under the floor or on top of the ceiling. There are major pros and cons to both styles that should be carefully considered before making a final decision.

How to Size Your Radiant Heat?

Sizing your radiant heat system is no easy task. It often requires consulting with a professional in order to get the most accurate estimate.

There are so many factors that go into heating an entire building, which can include insulation levels, room-to-room temperature variance, natural sunlight, etc.

Measure your room:

The first step to properly sizing your water heater and radiant heating system is measuring the square footage of your heated areas. You will need to know this value in order to figure out how many gallons per minute (GPM) you’ll need for your system.

Scale the length of the room by the width to find out how many square feet you have. If your room is long and narrow, it will be harder for the system to distribute hot water evenly throughout the entire space so keep this in mind during installation.

Once you have your total square footage, multiply that number by 1.5 (or 2 if you want a high level of heat) to determine your GPM (gallons per minute).

For example, A room that is 200 square feet will require a minimum of 300 gallons per minute.

Calculate the size of the heater you will be needing:

Every room requires 10 watts of heating power per foot. So in order to determine the size of your heater, you will need to use this formula:

(The GPM you determined in step 1) x 10 = wattage required.

For example, if your room requires 300 gallons per minute, it would require 3000 watts to heat correctly. To convert this into watts per hour (WPH), simply divide by 60 to find out that you’ll need a 50-gallon water heater.

1 Gallon per square foot is good for low-level heating. We recommend using 2 gallons per square foot if you want adequate heat coverage in your home.

Calculating the BTU:

The next step is calculating how many BTUs you’ll need. The equation, as seen below, calculates the amount of BTUs needed based on the GPM and square footage.

In our example above, a room that is 200 square feet would require a minimum of 3000 BTUs to heat properly. This can change if your insulation levels are low or you want an extremely high level of comfort from your radiant heat system.

Another way to calculate this number is by using the equation below. Simply select your preferred insulation levels and desired temperature, then plug in your square footage and GPM to determine how many BTUs you’ll need to heat your room(s).

How to install a radiant heating system?

Cut the floor open and install heating elements:

The first step to installing a radiant system is finding an area for your boiler. If you’re installing under-floor tubing, the system will need to be near an accessible water source and electrical outlet (for powering the pump). You’ll also want to find a good location for the boiler. If you’re installing on top of the ceiling, you’ll need to decide where your radiator units will go (generally in a centralized location). A double boiler system allows for one unit to heat water and send it underground while using another unit to heat water and send it into the house.

Install Heat-Transfer Fluid:

After laying out your tubing, you will need to install the heat-transfer fluid. First, use a grinder to smooth the edges of your heating elements in order to ensure that they are flush with each other.

This way, when you pour in the heat-transfer fluid it won’t get stuck between the heating tubes or pads. Heat-transfer fluid also comes in two forms: water-based or oil-based. The choice of which to use is up to you, but for safety reasons, we recommend a water-based heat transfer fluid.

Once all your tubes are installed and cleaned, pour the fluid into each section until it’s filled to about one inch shy of the top (be sure not to overfill).

When your heat-transfer fluid is installed and your tubing is filled, you’re ready to install the elements. Check out this step-by-step guide for more specific information on how to do this.

New Types of Underfloor Heating Solutions:

Due to ease of use and the level of comfort radiant heating provides, it’s no surprise that underfloor systems are becoming extremely popular amongst homeowners.

To make sure you have all the tools necessary for a successful system installation, we recommend using a pro who has experience laying systems in homes with high levels of foot traffic or in homes where natural stone tiles are being used throughout the entire house.

If you want an aesthetically pleasing look without compromising comfort or cost, then underfloor heating is definitely the right choice for you.


In conclusion, if you have a room that’s big enough to require a higher wattage system, then the best choice for you might be a standard electric water heater. You can choose a unit with either 50 or 80 gallons in capacity. Be sure to check your room requirements for specific information about wattage and GPM capacities.

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