Hydrogen boilers: how they work

As home heating accounts for a large percentage of your home’s carbon footprint, the government is looking at alternative ways you can heat your home that are more environmentally friendly. Currently, it offers grants to those who heat their homes using renewable energy or appliances such as heat pumps and it plans to ban petrol and diesel vehicles by 2035.

But something else that the government is looking into is hydrogen as a replacement for natural gas. Hydrogen is a low-carbon alternative that will have a positive impact on the CO2 emissions currently being produced in the UK. It’s a much cleaner gas that can be manufactured from water using electricity. When this electricity is generated from renewable sources, the hydrogen is 100 percent carbon-free.

While heat pumps are a great alternative to gas boilers that run using electricity, it’s nice to have choices, and a hydrogen boiler is another option for domestic heating. Below, we reveal what a hydrogen boiler is, as well as how it works (with a handy animation).

What is a hydrogen boiler?

A hydrogen boiler does what it says - it is a boiler that burns hydrogen as opposed to natural gas (or methane). These appliances work in a very similar way to natural gas boilers, by burning the gas via combustion, which in turn creates hot flue gases that can be used to heat water. This hot water can be stored for later use and pumped around your radiators to warm up your home.

As hydrogen is a thinner gas than methane, some parts of the boiler may differ slightly, including the flame detector and the burner (which we look at in more detail further down the page) but they work in much the same way as your current boiler.

Hydrogen boilers shouldn’t be confused with boilers that use hydrogen fuel cells to work or with hydrogen-ready boilers, as these are different types of systems.

Fuel cells require hydrogen in order to create electricity. Instead of relying on combustion to create heat, it’s a chemical reaction that can generate both heat and electricity. Fuel cells, therefore, are generally combined heat and power (CHP) systems or MCHP, as the heat can be used to warm the home and the electricity to power devices. This makes these kinds of systems super efficient. 

What is a hydrogen-ready boiler?

In contrast to a boiler that uses hydrogen fuel cells, a hydrogen-ready boiler is in theory an appliance that has the ability to burn both natural gas and be converted to adapt to hydrogen. This means that if it is decided after 2026 that the UK’s gas network is going to switch from methane to hydrogen, you won’t need to replace your gas boiler, just have a conversion kit added. The viability of this approach is still under early research and development; however it is being evaluated as one of the many methods of supporting overall decarbonisation. Solutions are being looked at to also see how currently qualified heating engineers may be able to support converting products which have been developed with this in mind.

If hydrogen is selected to play a role in the gas network, it’s likely that it will be phased in, rather than switching from all natural gas to all hydrogen. The HyDeploy programme run at Keele University tested the effects of using a blend of hydrogen and natural gas. With 20 percent hydrogen and 80 percent natural gas, no alterations would need to be made to the majority of UK boilers, so this change could come in almost instantly if Government decided that hydrogen was to play a role in decarbonising heating.  

When will hydrogen boilers be available?

Currently, there are no 100 per cent hydrogen boilers available, as this technology is yet to be developed, however a few of the large boiler manufacturers have created prototypes.

How do hydrogen boilers work?

1: Oxygen and hydrogen enter the boiler

Firstly, oxygen and hydrogen enter the main boiler unit. The hydrogen will come from the main gas supply when this is an option and the oxygen will come from the air. The gas inlet contains a valve that controls how much of each gas can enter the boiler. This is important as the amount of gas can impact the size of the flame.

2: The gases are mixed and burned

The hydrogen and oxygen gases are mixed together before being ignited in a catalytic burner. Hydrogen is more flammable than natural gas and so the burners on hydrogen boilers are specially designed so that the flames can be limited if too much gas is flowing through. A flame detection component is also required in a hydrogen-only boiler as hydrogen flames are invisible.

3: The hot gases enter the heat exchanger

Hot flue gases created by combustion enter the heat exchanger. The heat exchanger is made up of a series of pipes, which the hot gas travels through. Surrounding these pipes is cold water. As the hot gas moves through the pipes, it heats the surrounding water that can then be used in your heating system or supplied to your hot taps.

4: Byproducts exit the system

The only byproduct of burning hydrogen and oxygen is water, and so a condensate is required for this water to escape the system. The hot flue gases (hydrogen and oxygen) can also exit the system via the flue along with some nitrous oxide (NOx) emissions as a result of hydrogen combustion.

The hydrogen boiler continues to pull in more hydrogen and oxygen so that this whole process can be repeated again and again

Will gas boilers be replaced by hydrogen?

With the government’s ban on gas boilers in new build properties from 2025, heat pumps and hydrogen boilers could be the way forward. But currently, there isn’t a working boiler that can use 100 percent hydrogen. There are two show homes that are being built in Gateshead that have gas appliances purely powered by hydrogen, including hobs, cookers, fires and boilers. These homes have partially been built as a test but also to show people that hydrogen will be a viable option in the future to improve carbon emissions in the UK.