What are OpenTherm Boilers & Heating Systems?

Last updated: April 29, 2024

If you’re looking for the most efficient way to heat your home and water while optimising energy usage and reducing costs, OpenTherm boilers are your best bet. But what are OpenTherm boilers and heating systems? Read on to find out.

Energy used for space heating and hot water in homes contributes to around 30% of the UK’s carbon emissions.

Ensuring boilers meet the highest levels of efficiency is one of the UK government’s key objectives to help reduce emissions and lower consumer bills.

Boiler Plus standards increasingly encourage the adoption of controls that can lower flow temperatures through effective boiler output modulation to improve efficiency.

At a minimum, controls should be able to calculate how long it takes for your home to reach the desired comfort level.

They should then time the heating system’s operation to minimise the amount of work or energy required by optimizing modulation.

This requires effective communication between the boiler and the control.

However, limited interoperability caused by the different operational protocols in the heating industry prevents controls from offering the full functionality they’re designed to deliver.

OpenTherm solves such issues by featuring ‘open protocols’ that allow interoperability and better communication between controls and boilers produced by different manufacturers.

This guide explores everything you need to know about OpenTherm boilers and heating systems, including how they work, their benefits, and how they can help you save on energy costs.

OpenTherm Boilers
Image by Worcester Bosch

Key Takeaways:

  • OpenTherm allows heating controls like thermostats to communicate more effectively with your boiler.
  • OpenTherm boilers feature more temperature control and eliminate overshooting and undershooting.
  • OpenTherm boilers feature enhanced efficiency, reduced wear and tear, reduced emissions, increased comfort and advanced monitoring and control.
  • OpenTherm boilers usually feature a distinguishing logo.

Learn all about the different types of boilers, including condensing boilers, LPG boilers, oil boilers, and Opentherm boilers, and calculate boiler size in our guide.

What Are OpenTherm Boilers?

Opentherm boilers are boilers that use OpenTherm technology.

OpenTherm is an innovative communication protocol that allows heating controls like room thermostats and modulating central heating appliances like your boiler to interact more effectively.

The OpenTherm protocol sets the rules on how the boiler and thermostat communicate.

It allows bidirectional information exchange that enables precise control over the heating output based on real-time temperature readings.

OpenTherm boilers feature OpenTherm-compatible controls. These control the boiler’s water temperature and can modulate or adjust the boiler output to more accurately match actual heat demand.

This optimises energy usage, lowers heating costs, and minimises emissions.

How Do OpenTherm Boilers and Heating Systems Work?

The OpenTherm protocol is based on the idea that if appliances like boilers and controls like thermostats can communicate constantly in the same language, a much more efficient system can be achieved.

OpenTherm boilers achieve this through load compensation.

This is a process where the control monitors the difference between the actual room temperature and the temperature set on the thermostat and modulates the boiler’s output to close the gap.

It also reduces the flow temperature needed for the radiators as temperatures start getting closer to the set point temperature.

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The graph below shows an example of a set temperature point of 19°C. Once the room’s temperature nears its temperature set point, modulation control in OpenTherm boilers reduces its output to prevent the room temperature from overshooting.

OpenTherm boilers modulation control

Running at lower flow temperatures for extended periods helps improve the boiler’s efficiency.

It ensures that more of the boiler’s heat exchanger is at condensing temperatures, the boiler’s most efficient point.

How Do Basic Heating System Controls Work?

Most central heating systems in the UK are controlled with a basic on/off room thermostat.

The thermostat sends simple signals that turn the boiler on when the room temperature falls below the set point and off again when it rises above it.

However, the boilers usually operate at maximum output and the radiators remain hot even after the boiler is off. The radiators would continue transferring the heat to the room until before cooling down.

This excess heat being transferred into the room would result in an overshoot, where the room gets hotter than desired, usually by a couple of degrees.

Undershooting also happens due to a delay in getting the radiators hot enough to heat the room adequately when the boiler turns on. This can make the room uncomfortable, and people usually set the thermostat higher than necessary.

The graph below shows an example of a set temperature point of 19°C where overshooting and undershooting occur due to inaccurate control.

The process where the boiler turns on and off is called cycling, and it can stress your boiler, impact efficiency, and lead to quicker wear and tear.

Although there might be some output modulation, it’s unlikely to be optimised for efficiency since it will not adjust the flow temperature.

Benefits of OpenTherm Boilers

Below are some of the advantages of OpenTherm boilers:

Enhanced Efficiency

The significant advantage of OpenTherm is its energy efficiency, which helps to reduce fuel costs.

By operating at lower flow temperatures for extended periods, OpenTherm keeps the ideal room temperature set point constant and improves the boiler’s efficiency.

This guarantees the boiler continues condensing for a longer duration. Studies show that modulation control on OpenTherm boilers can enhance efficiency by up to 14%.

Reduced Wear and Tear

Your boiler faces less strain since there is no regular cycling or turning on and off.

The boiler will operate at lower outputs and condensing “mode” for longer periods of time, resulting in lesser damage to components.

The boiler can have a longer lifespan and run at peak efficiency for many years.

Some OpenTherm heating controls display boiler states and a number of problem codes right on the thermostat’s screen, giving the installer more information.

Reduced Emissions

OpenTherm boilers contribute to sustainability efforts by reducing energy consumption and carbon emissions.

By optimizing heat output based on demand, OpenTherm boilers minimize energy waste and operate more efficiently, resulting in lower greenhouse gas emissions and a smaller carbon footprint.

Studies show that you can reduce carbon emissions by up to 14% by using a modulating condensing boiler.

Comfort and Precise Temperature Control

OpenTherm boilers offer greater comfort and consistency in indoor temperature control and you don’t have to worry about the undershooting and overshooting present in basic on/off control systems.

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The ability to modulate heat output allows for more precise regulation of room temperature, minimizing temperature fluctuations and providing a more comfortable living environment.

Whether a chilly winter evening or a mild spring day, OpenTherm boilers can adapt to varying heating demands, ensuring you can always enjoy optimal comfort levels throughout the home.

Open Standard Design

OpenTherm has an open standard, meaning it can be used by any manufacturer without restrictions.

This provides increased flexibility and choice, allowing you to choose from a wide range of products compatible with OpenTherm technology.

Advanced Monitoring and Control

OpenTherm boilers leverage smart technologies to enhance monitoring and control.

They’re compatible with home automation systems and modern smart thermostats that provide real-time temperature data and adjustable heating schedules.

You can remotely control and monitor the heating system through online platforms and smartphone apps.

It also allows you to adjust settings, schedule heating cycles, and monitor energy usage from anywhere, providing greater convenience and peace of mind.

How Do You Know If You’re Using OpenTherm Boilers?

The fastest and most efficient method to learn is to check your boiler for an OpenTherm logo. It’s probably not an OpenTherm boiler if you can’t see one.

To be sure, it’s worth consulting the instructions manual or giving the manufacturer a quick call.

How old is your boiler? Should you consider a boiler repair or start looking at the latest boiler prices? It’s no secret that boiler efficiency deteriorates over time.

What OpenTherm Boilers Are Available?

Opentherm is fully compliant with the boilers listed below. These include companies like Baxi, Main, Ferroli, Ideal, and Viessmann, which implement this technology.

However, not all boiler manufacturers have made their Opentherm boilers compatible with third-party Opentherm controls.

The controls provided are the manufacturer’s own controls, so they should function just fine.

ManufacturerBaxiFerroliIdealMainViessmann
OpenTherm Ranges100 Combi 200 Combi 400 Combi 600 Combi 800 CombiiBoiler Combi i29Logic Combi + C24, C30, C35 Logic Plus VogueEco Compact Combi 25kW, 30kWVitodens 100

The OpenTherm logo serves as a quick way to recognize its goods.

The logo ensures that there will be at least a minimal amount of communication between the room thermostat and the central heating system.

The extra features the manufacturer built will determine how much involvement there is.

The room thermostat and central heating unit terminal clips are close to where you may find the logo.

Third-Party OpenTherm Controls

You can also install certain third-party controls that can still communicate intelligently with the boiler thanks to the open standards of the OpenTherm language.

To be fairly obvious, you should refer to boiler controls created by companies other than boiler manufacturers as third-party controls, such as Honeywell Evohome or Google Nest.

The boilers indicated in the table above can all be operated by any of the third-party controls listed below, but this is not necessarily the case in reality.

Not all Opentherm controllers from third parties work perfectly with Opentherm boilers.

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Check with the boiler manufacturer to ensure the third-party control you want to use is compatible with their boilers.

Two of the seven third-party controls are common programmable room thermostats, and five are internet controllers.

Want to learn how to use your boiler better? Read our complete guide on boiler flow temperature, how to optimise your boiler settings and find out how much gas a boiler uses here.

They are all in compliance with Boiler Plus since they all include load compensation controls (Opentherm).

Third Party manufacturers that will work with all Opentherm boilersDraytonNestTadoHoneywellHoneywellHoneywellSalus
ModelWiser (Kit 1, 2 and 3)Thermostat ETado Smart thermostat (with extension kit)Lyric T6R and T6R-HWT4R, TRMEvohomeRT520 + RT520 RF
Base unit/thermostat – suitable combi boilers£135.00£199.00£200.00£242.00£127.00£249.99£70.00
Additional costs for separate hot water controlNAIncludedIncludedIncludedNoincludedNo
Combined costs for heating and hot water£149.99 Kit 2 – heating and hot water control£199.00May require an extension kit – £89.99£242.00NA249.99NA
Warranty/guarantee1 year2 years2 years1 year1 year1 year5 years
Heating controlsYesYesYesYesYesYesYes
Hot water time/boostKit 2 modelYesYes (possible extension kit)YesNoYesNo
Weather compensation, load compensation or smart automation and optimisationLoad compensation and smart automation and optimisationLoad compensation and smart automation and optimisationLoad compensation w/Opentherm boilers and Smart automation and optimisationWeather and load compensationLoad compensation and weather compensation (T4M)Load compensationLoad compensation
Works with Smart TRVsYes – Drayton Wiser Smart TRVYes – EnergenieYes – Tado Smart TRVsNoNoYesNo
TPI softwareNoYesYes‘Fuzzy logic’‘Fuzzy logic’YesYes
OpenThermNoYesYesYesYesYesYes
Boiler Plus compliant for combis?YesYesYesYesYesYesYes
Boiler Plus compliant for heat only and system?YesyesYesYesYesYesYes
Heating zone capabilities1 (Kit 2 gives 2 channels – 1 heating and 1 hot water)3 zones with pipework adjustment or more with Energenie smart TRVs1 heating zone1 heating zone1 heating zoneMultiple with additional pipework or via Smart TRVs1 heating zone
Combi, heat only, and systemYes – via IFTTTYesYesYesNoYes – via IFTTTNo
compatible boiler typesCombi, heat only, and systemCombi, heat only, and systemCombi, heat only and systemCombi, heat only, and systemWeather compensation, load compensation, or smart automation and optimisationCombi, heat only and systemCombi, heat only and system

Alternatives to OpenTherm

However, that does not mean you cannot install an advanced control on other boiler types or on older boilers.

Remember that only combi boilers must be fitted with an advanced control under Boiler Plus.

Check out our video on how to vet boiler brands/models in the UK:

A brief overview of additional choices for older and non-combi boilers is provided below.

There are two alternatives available to customers installing a new combi boiler:

  • The most effective operation of the boiler is achieved by optimisation controls rather than compensatory controls, which change the gas valve. All boilers can use them to increase their efficiency. There are numerous smart controllers from third parties that offer optimization features and are compatible with the overwhelming majority of boilers.
  • Six boiler manufacturers create their internet heating controls that are Boiler Plus compliant and can be used with some of their boiler models. Numerous sophisticated controls are available from boiler manufacturers with the appropriate type in their line.

What Is Boiler Modulation?

Modulation refers to the ability of the boiler to turn down its output.

Boilers are at their most efficient when they only use the energy required to meet the demands of a property.

Domestic hot water demand usually requires more energy than space heating demands, especially in combination boilers.

Boilers address such differences by modulating their output to reflect the space heating demands of your home and do as little work as possible.

For example, if you install a 30kW combination boiler in your home but only require 10kW of heat for the next hour to maintain temperatures, the boiler can modulate its output to 1/3 of its total output to only use the 10kW required.

What Are Boiler Plus Standards?

The UK introduced Boiler Plus standards in 2018 through an amendment of the statutory guidance for Building Regulations.

The standards aim to ensure all England households have a reasonable level of control and choice over their heating so they can achieve efficiency and comfort.

The standards required that all types of gas boilers installed should meet the following requirements:

  • Have a minimum efficiency standard of 92%.
  • The installation of a boiler interlock
  • If not already present and working, you must install temperature and time controls simultaneously with the new boiler.

It also required the installation of an additional measure for energy efficiency with combination boilers. Such energy-saving technologies can include:

  • Weather compensation
  • Load compensation
  • Smart controls with optimisation and automation.

Final Thoughts on OpenTherm Boilers

OpenTherm boilers are an excellent choice for anyone looking to reduce energy bills, increase comfort levels and reduce their carbon footprint. If you don’t already have a boiler and controls that can communicate through OpenTherm, ensure you look out for the logo the next time you’re replacing your boiler.

Sources and References

  • https://assets.publishing.service.gov.uk/media/63e25c96e90e076266ed429c/Improving_boiler_standards_and_efficiency_consultation.pdf
  • https://www.semanticscholar.org/paper/Control-Schemes-for-OpenTherm-Boilers%3A-Energy-and-O’hara-Smith/378860adcc4e3899f96468afe23d534ec87d8204