Pipes & Heat in Old Cambridge

A reading site on water and heating in older Cambridge and South Cambridgeshire homes. Not a plumbing or heating firm.

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Boilers and heat pumps ยท In brief

The boiler controls a new combi has to come with

Under the boiler controls rules, a new gas combi in an existing home needs one of four extras: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation.

Most useful for: Householders and landlords replacing a gas combi in an existing home

Compiled from the sources at the end. Updated .

Small white square smart thermostat with an e-ink temperature display on a painted wall.
A small smart thermostat with a temperature display on a painted wall.

The four options come from para 6.2 of Approved Document L. The rule does not rank them, and it is worded for gas combis, so ask the engineer what applies if you are fitting a different boiler. The 2021 text with 2023 amendments is current until the 2026 version takes effect on 24 March 2027, when paragraph numbers can change.

What each option means

  • Flue gas heat recovery: a unit that recovers heat from the flue gases. It is fitted with the boiler and is engineer work.
  • Weather compensation: adjusts the boiler's flow temperature by reference to the temperature outside.
  • Load compensation: adjusts the flow temperature by reference to the room.
  • Smart thermostat: the wording is "with automation and optimisation", so a plain timer or a basic app switch may not count. Ask the engineer to confirm in writing.

Separately, para 5.25 requires a boiler interlock for work on a system with a boiler, which in general terms means the controls stop the boiler firing when nothing is calling for heat. Older systems can lack this, so raise it when the boiler is replaced.

Who fits them

Controls that touch the boiler are for the Gas Safe registered engineer doing the job, and wiring is for them or an electrician. Under Part P, a new circuit, a consumer unit replacement, or an addition or alteration in a special location around a bath or shower is notifiable.

In a listed building, new controls and any wiring can raise a consent question. South Cambridgeshire's 2009 guidance answers "maybe" on services and asks for care in rooms with historic features, so ask the conservation officer first. See radiators and pipework in listed buildings. Lower flow temperatures also tie in with radiator size.

Sources, read 4 November 2026: Approved Document L, Volume 1, 2021 with 2023 amendments; Approved Document L publication page, GOV.UK; Approved Document P, 2013; South Cambridgeshire Listed Buildings SPD, 2009.