Boiler Plus: the extra your combi quote has to include

Two quotes for the same combi. One of them has a line for a smart thermostat. The other does not mention a control at all. It is easy to read that as one installer adding an extra and the other keeping the price honest.

It is usually neither. Both installers are meeting the same requirement, and only one of them has written it down. There is a rule that applies to every gas combination boiler fitted in an existing home in England, almost nobody outside the trade has heard of it, and it is one of the reasons two quotes for the same appliance arrive looking different.

What the rules actually say

Approved Document L is the building regulations guidance on energy efficiency. Section 6 deals with gas-fired heating, and paragraph 6.2 says that if a gas-fired combination boiler is installed in an existing dwelling, at least one of the following energy efficiency measures, appropriate to the system, should be installed:

• Flue gas heat recovery.
• Weather compensation.
• Load compensation.
• A smart thermostat with automation and optimisation.

Source: Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, paragraph 6.2

The trade calls it Boiler Plus, after the policy that brought it in during April 2018. It applies to combis, in existing homes, in England. It is not optional and it is not a sales technique, so a quote that does not appear to include any of the four is the one worth asking about, not the one that does.

It sits on top of the things that apply to every boiler rather than instead of them. The appliance itself has to meet a minimum seasonal efficiency, and every installation needs time control, temperature control and a boiler interlock so the boiler and pump switch off when nothing is asking for heat. The combi measure is the extra on top of all that.

Source: Heating and Hotwater Industry Council, Boiler Plus additional guidance for installers

The four options, and what each one actually does

They are listed as alternatives, which makes them look interchangeable. They are not. They do different jobs, they suit different houses, and only one of them touches your hot water.

• Flue gas heat recovery. A device that takes heat out of the boiler's flue gases and uses it to warm the cold water before it reaches the boiler's hot water heat exchanger. It helps with hot water rather than heating. Some units sit above the boiler, some boilers have it built in, and a separate unit has to be one the boiler manufacturer has approved for that particular boiler.
• Weather compensation. A control that varies the flow temperature the boiler sends out based on the outside air temperature. Colder outside, hotter water in the radiators. Milder outside, cooler water, which keeps the boiler condensing, which is where its efficiency comes from. It needs an outside temperature sensor, or, as the guidance allows, local weather data pulled from the internet.
• Load compensation. The same idea, but the reference point is the temperature inside the room rather than outside the house. The control measures the gap between the room and the temperature you asked for and the boiler modulates its flow temperature to close it, using only as much gas as the gap needs.
• A smart thermostat with automation and optimisation. Two specific functions, both required. Automation means it adjusts the times and temperatures on its own, from occupancy detection or from what you have done in the past. Optimisation means it works out how long your house takes to warm up and starts the boiler at the right moment to hit your set temperature when you want it, rather than at a fixed time.

Source: Heating and Hotwater Industry Council, Boiler Plus additional guidance for installers

The trap: a smart thermostat is not automatically the answer

This is the part that catches people, and it is worth knowing before you accept anything.

Not every smart thermostat does both automation and optimisation. Plenty of well known ones let you control the heating from your phone and do very little else, and remote control on its own is not one of the two functions the guidance names. So an installer saying they will fit a smart thermostat has not, by itself, told you which of the four measures the job is meeting.

There is a second one underneath it. A common type of programmable thermostat, known in the trade as a TPI control, adjusts how often the boiler cycles rather than how hot the water it sends out is. Government guidance published alongside the policy states plainly that the description of load compensation should not be misinterpreted as including TPI controls. A TPI thermostat is a perfectly good control and it can be fitted alongside any of the four. It just is not one of them.

Source: Heating and Hotwater Industry Council, Boiler Plus additional guidance for installers, quoting the government FAQ on load compensation

The question that separates a real measure from a box on the wall

Two of the four, weather compensation and load compensation, only work if the control can change the temperature of the water leaving the boiler. That means the control and the boiler have to be able to talk to each other, either through a shared language such as OpenTherm or through the manufacturer's own system. A thermostat wired to switch the boiler on and off cannot compensate, no matter what it says on the box.

So the useful question is not what make of thermostat is going in. It is whether it is paired to the boiler so it can modulate the flow temperature, and whether the installer will set it up that way on the day rather than leave it switched off. This is genuinely common. A compensating control that nobody enabled at commissioning is an expensive room thermostat.

What this means when you are comparing two quotes

Three questions, and all three can be asked by email tonight:

• Which of the four measures does this price include? If the answer is a control, ask which of the four functions it is providing.
• Will it be paired to the boiler so it can vary the flow temperature, and will that be set up and left working at commissioning?
• If it is flue gas heat recovery, is that unit approved by the boiler manufacturer for this exact boiler?

If one quote is cheaper and cannot answer the first question, you are not comparing two prices for the same job. You are comparing a complete installation with an incomplete one, and the difference will appear later, either as an extra or as a boiler that never quite condenses.

Which one suits your house

You are allowed a preference here, and most people are never offered one. Broadly, the compensating controls pay you back through the heating, so they suit a house that is warm for a good part of the day and has radiators with a bit of surface area to give. Flue gas heat recovery pays you back through the hot water, so it suits a household that showers a lot.

Around Northampton and Milton Keynes there is one more thing worth raising, because the water here is hard. Flue gas heat recovery works on the domestic hot water side, which is the side of a combi that scales, so it is a fair question to put to your installer: how is this unit affected by our water, and what scale protection are you fitting with it? A good installer will have an answer ready.

The short version

One of those four measures has to be part of your combi installation, so it is already in the price whether or not it is on the page. The one thing worth doing is making the installer name it, because the four are not equal and the one you end up with was probably chosen for you.

If you want the same job priced by more than one installer, we pass your details to vetted Gas Safe registered firms around Northampton and Milton Keynes who fit these controls every week. Tell us about the job and there is no obligation either way.