Smart or wireless thermostat for a combi boiler (UK): what fits, and what Boiler Plus requires

1. What Boiler Plus requires when a combi is installed (England)

The Government's Boiler Plus factsheet (BEIS, dated 03/01/2018) says the standards apply from April 2018 to a boiler "installed in an existing heating system anywhere in England", that is "when households in England replace an existing boiler, or purchase a boiler for the first time in an existing dwelling". Three parts matter to someone choosing a thermostat:

• The boiler: "The new minimum performance standard for domestic gas boilers in English homes will be set at 92% ErP."
• Controls on every gas and oil system: the standard makes "timers and room thermostats an explicit requirement for all gas and oil systems".
• One extra measure on a gas combi: "When a gas combination boiler is installed, an additional energy efficiency measure will be required." The factsheet lists four technologies that comply.

The 2021 review (BEIS Research Paper 2021/012) spells out the two functions. A smart control "can include either load or weather compensation, otherwise it must include both of":

• Automation, "where the device automatically controls the heating system output in response to programmed demand or occupancy detection", for example switching the heating off when the phones' GPS shows the home is empty.
• Optimisation, which "means the device calculates how long it takes the property to reach the desired comfort level (i.e., the temperature on the thermostat), and times the systems operation to minimise the amount of work it has to do": it works out when to start the boiler so the room is warm at the chosen time.

So a smart thermostat is not compulsory with a new combi: weather or load compensation, or flue gas heat recovery, satisfies the rule on its own. The factsheet adds that the smart option "must include optimisation, as defined in the 2018 amendments", so an app on its own is not enough. Everything in this section is England only.

2. Wired, wireless or smart: what each means

Wired. The thermostat is cabled to the boiler. Resideo's wiring guide shows a combination boiler with a hard-wired room thermostat on the boiler's "Room Thermostat Terminals", with or without a timer in the thermostat.

Wireless. Two parts: a thermostat you can place in the room, and a receiver box wired at the boiler. Resideo's installation guide for its T4R wireless thermostat says to "find a suitable location near to the boiler, leaving at least 30cm distance from any metal objects including metal wallboxes and boiler housing" for the receiver box, and the thermostat runs on batteries (FAQ below). Wireless says nothing about the internet: a plain wireless thermostat has no app.

Smart. Energy Saving Trust: "Smart heating controls can do everything that conventional controls do. The main difference is that smart controls are connected to the internet and offer more functionality." The Government's 2021 review puts it as "products that let consumers remotely control their home temperature via a tablet, smartphone or desktop". Smart does not by itself mean Boiler Plus compliant: the control needs automation and optimisation, or load or weather compensation (section 1).

OpenTherm. The OpenTherm Association calls it "a manufacturer-independent system of communication between modulating HVAC heating appliances and room thermostats". Drayton explains the difference from on/off control: a basic thermostat switches the boiler on and off at one fixed water temperature and "often over/under shoots the set point", while with OpenTherm control "is achieved more directly by setting the desired water temperature from the boiler". It needs a boiler that speaks it. Viessmann: "Your boiler will require a pre-installed OpenTherm interface in order to be compatible" and "Most older boilers, therefore, won't work with OpenTherm". Resideo's wiring guide: "For OpenTherm compatibility please refer to the boiler manufacturer." The OpenTherm connection is two wires powered from the boiler, not the volt-free switching terminals, so the wiring differs from an on/off thermostat.

"No subscription": what people mean

A smart thermostat is "connected to the internet" (Energy Saving Trust), so the search "smart thermostat UK no subscription" is asking whether the app, the schedule and the remote control keep working without a monthly fee. We do not rank brands on this. Before buying, read the maker's page for three things: what the free app includes and whether any feature needs a paid plan; whether an account and the maker's cloud are needed to set it up; and whether the thermostat still runs its schedule if the internet drops.

3. What to check before buying

1. Channels. Energy Saving Trust: "Single channel: for combi boilers. This only controls central heating as hot water is provided on demand." "Two channel: for system boilers controlling heating and hot water, or combi boilers controlling two heating zones"; three channels for a cylinder plus two heating zones. A combi with upstairs and downstairs zones is the one case where a combi needs more than one channel.
2. OpenTherm. Only if your boiler maker lists it for your model (section 2). Otherwise the thermostat works as an on/off control through its relay.
3. How the receiver switches the boiler. The T4R receiver box has a 5(3)A switching contact and the guide shows two wiring layouts, one for a "Basic 230V Boiler" and one for a "Combi-Boiler"; the same guide lists it as compatible with "24-230V on/off & OpenTherm appliances" and "not compatible with 230V electric heating". Your boiler's manual names its room thermostat terminals and whether they are volt-free.
4. Where the receiver goes. Near the boiler and at least 30 cm from metal, including a metal back box (section 2).
5. Who wires it. Resideo's wiring guide: "ALL WIRING SHOULD BE CARRIED OUT BY A COMPETENT INSTALLER OR ELECTRICIAN." The T4R guide: "Must be installed by a competent person."
6. The gas side. Under the Gas Safety (Installation and Use) Regulations 1998 a gas engineering business must be on the Gas Safe Register to carry out gas work legally; the register covers Great Britain, Northern Ireland, the Isle of Man and Guernsey (HSE). A thermostat swap does not touch the gas, but a boiler installation does.
7. Not the boiler's size. A thermostat changes when the boiler runs, not its output. For the kW your home needs, the combi boiler size calculator and the boiler output calculator; for each room, the radiator size guide.

4. Brands people search for

The searches around this page name Hive, Nest, tado, Honeywell Home, Salus, ESi and Drayton Wiser. This page does not rank them and has not tested any of them; the Resideo and Drayton documents in the Sources list are quoted only for how a thermostat is wired. Whatever the brand, the checks in section 3 are the same. Brand names belong to their owners.

5. System and regular boilers with a hot water cylinder

A system or regular boiler heats a cylinder, so the control has two jobs. Drayton: for a conventional boiler with a hot water cylinder a two-channel kit "is needed to control the central heating via one channel and hot water via the other", and a vented-cylinder system with fully pumped heating "may also be referred to as BiFlo, TwinZone, Y Plan, S Plan, or S Plan Plus". In Resideo's wiring guide the S Plan diagram has two two-port zone valves, one on the hot water circuit and one on the heating circuit, with a cylinder thermostat and a room thermostat; the Y Plan diagram has one mid-position valve (a three-port valve) feeding the cylinder and the radiators, again with a cylinder thermostat and a room thermostat. Both are "fully pumped system only". A single-channel combi thermostat cannot time the hot water on these systems; the control needs a hot water channel.

6. Underfloor heating

Water underfloor heating is controlled zone by zone. Warmup: "each zone is typically controlled by a zone-specific thermostat, which is installed within the heated area", with "actuators which open and close when instructed by the thermostat (via the Wiring Centre)"; "The Wiring Centre controls the electrical aspect of a UFH system". Heatmiser describes the wiring centre the same way: "the actuator, boiler and pump connections are wired from a single point". So the shopping list is one thermostat per zone plus a wiring centre with enough channels, and the boiler (combi or otherwise) is switched by the wiring centre, not by a room thermostat of its own.