Domestic gas boiler control developments28 May 2021

A Dutch association of boiler makers promotes a brand-agnostic communication standard between boilers and their controllers that reflects more efficient control methods then are currently in common use in the UK

The advent of smart home appliances such as Nest, which connect light switches and thermostats to an internet-enabled gateway, conceal an embarrassing truth: domestic heating controls, almost all of which switch the boiler on or off depending on a thermostat’s reading of room temperature and set temperature, have barely changed in a generation.

A more sophisticated control system can offer energy savings of 10-15%, claims Huite Jan Hak, technical manager of the OpenTherm association. “The UK is perhaps 5-10 years behind the Continent,” he observes, though adds that within the last few years it has made big strides.

For example, in 2018, the UK government published its Boiler Plus regulation, which among other requirements states that new combi boiler installations must have at least one of four new energy-saving measures: load compensation (modifying boiler flow based on the indoor temperature); weather compensation (modifying boiler control based on outdoor temperature); smart control (a system that optimises the boiler configuration); or a passive flue gas heat recovery device. Also, recent building control guidance requires that new-builds contain at least two heating zones, to prevent wasting heat where it isn’t needed.

What these requirements ask of a heating system is a smarter controller. Rather than merely sending a heating call, there is a modulated signal: the controller also has access to trend information about how quickly rooms have heated up in the past, so modifies its request. It could also optimise the length of time of boiler firing; for example, from five minutes on/ five minutes off, to longer cycles at lower temperature for greater overall efficiency.

With smarter systems come more complex communications. Usually, suppliers that go to the trouble of developing more sophisticated digital communication systems make them proprietary. That means that customers are not free to mix and match boilers and controllers; they are locked in to their original vendor.


Twenty-five years ago, Honeywell realised that this danger would apply to a new communication protocol it had devised. Instead of developing it further, the Dutch operation ‘sold’ it for one guilder (£1) to a new industrial association, OpenTherm, dedicated to a vendor-agnostic communications platform between controller and boiler, domestic hot water, cooling, solar panel or ventilator.

Today, not only has the protocol been picked up UK heating brands including Ideal Boilers, InterGas, Honeywell Resideo, Danfoss, SALUS, ESi and Schneider Electric, but also it supports advanced heating controls that save energy, and in so doing meet the terms of Boiler Plus and building control.

The system is based on communication over a twisted wire pair, and requires a few parameters, depending on application. For boilers, those include thermostatic control of the boiler, temperature settings and remote reset. For domestic hot water, that includes on/off control of the ‘comfort function’ of the combi boiler. For cooling and solar systems, that involves not only the setpoints, but also control of heat load. For ventilation, it includes setpoint adjustment, exhaust control and heat recovery.

On top of those are some 200 other IDs for different types of data, which manufacturer members are free to use if they wish. “If they have a feature-rich product, then other parties can build rich features in the controls,” says Hak. Because the standard is vendor-agnostic, customers can mix and match to suit.

OpenTherm relies on connecting devices directly; it’s a point-to-point system. That suits serial control architectures such as controller-boiler or hybrid controller-heat pump-boiler. And it does allow, incidentally, remote diagnostics and remote access, if a gateway connected to the internet via other protocols has been installed on the boiler or air conditioning compressor (as diagrammed above).

What OpenTherm cannot do is provide building automation functions of the sort normally based on control buses, says the technical manager. That parallel-processing many-tasks-at-once control architecture is more suited to the complex needs of large commercial and industrial premises, he admits. But one exception is boiler cascading in smaller commercial buildings. In such systems, a number of smaller boilers are daisy-chained together to provide 100-300kW of heating capacity. OpenTherm-compliant controllers are often installed for boiler control, but not for room control. In such cases, the OpenTherm network makes up just a part of a bigger system that has other means of overall control.

Summarising, Hak states: “It’s a low-level, low-cost system which gives a lot of functionality but is also restricted in its complexity. But it’s not restricted in its applications.”

As of last year, there were 50 million OpenTherm-enabled products installed; 20m boilers, 20m controllers and 10m HVAC appliances. Manufacturers have to be members of the association to have access to the OpenTherm protocol – there are more than 80 – and members are also in charge of updating the standard. (Rather than having to wait to install a new central heating system to receive the benefit of new features, a new technology trend, Hak adds, is over-the-air updates for WiFi-connected controllers.)

He observes: “Customers are already used to that in smartphones, so updating is moving into more traditional applications, including HVAC. Of course you need to consider that there are all kinds of certifications and safety checks. An update is not so easy as phone firmware; safety needs to be guaranteed to prevent fire or explosion.”


OpenTherm is in the midst of a three-year UK rollout, which Hak admits has not gone completely smoothly. Normally, setting up a connection between controller and boiler using the OpenTherm protocol is as simple as plugging in the cables. But some UK installers discovered incompatibilities between boilers and controllers.

Hak blames the issues on misinterpretation of the protocol, adding that relatively few systems were affected, and that all of the problems either have been resolved, or will be with installation of a new thermostat. He says: “How we have addressed it in the UK is to gather as much information as we can, contact manufacturers and resolve the issues.”

Another development that will help smooth over the troubles is a testing tool launched late last year that is claimed to almost completely automate checks for OpenTherm compliance, rather than sending them to athird-party testing house (though OpenTherm will still double-check the results).

A recent trend is demand from Asia-based manufacturers of traditional air conditioning systems that want to enter EU markets, but which need to provide familiar controllers, Hak adds. “There is huge demand.”


One of the ways to achieve Boiler Plus compliance is with load compensation. Huike Jan Hak explains how it works. “Load compensation has to do with systems that are over-dimensioned for their application. A combi boiler is dimensioned in energy terms for domestic hot water rather than central heating. A 30kW combi needs its power for hot water; for central heating it only needs 4, 8 or 11kW, depending on the house and its insulation. Compensation tries to efficiently run over-dimensioned systems in the most energy-restrictive way. It uses modulating control and some kind of TPI – time proportional control – of the boiler itself to request the lowest possible load of the boiler. If it’s not low enough, it will add a time factor, where the controller decides how much time it should go on and off for efficiency.”

William Dalrymple

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