Heating and hot-water timing, boiler interlock, bypass flow and the model-specific pump, preheat and anti-cycling functions that can operate between demands.
Heating controls fall into two groups: operating controls that set the day-to-day demand and temperature, and safety controls that supervise or limit safe operation. The boiler interlock ties the operating controls together so the boiler fires only on a real demand.
Operating controls
- Programmer (time clock): selects the periods when heating and hot-water operation is allowed. It sets the schedule, not the target temperature.
- Room thermostat: senses the air temperature in a reference room, sited away from draughts and heat sources, and switches the heating demand on and off to hold the set temperature.
- Cylinder thermostat: monitors stored-water temperature. At its setpoint it ends the call for cylinder heating, contributing to the boiler interlock.
- Boiler thermostat: sets the temperature of the water the boiler heats.
- TRV: throttles the flow through its own radiator by the temperature around it, giving room-by-room control. It is usually not fitted in the room with the room thermostat, so the two do not conflict.
- System bypass: with a fixed-speed pump, TRVs or zone valves that close down can starve the boiler of flow, so a bypass keeps a minimum circuit open. The Greenstar 8000 Life instructions ask for one that gives at least a 3 metre circuit when it opens; a boiler with a modulating pump may not need one.
Safety controls
Some safety devices trip at a limit; others must prove a safe condition during each normal starting sequence. Their behaviour depends on the appliance design.
- Overheat (high-limit) thermostat: stops the burner above a safe temperature. Loss of circulation as well as failure of normal temperature control can cause overheating.
- Air pressure switch, where fitted for fan proving: the required pressure difference must be established before ignition can proceed. Fan or air/flue-path faults can prevent that signal. A switch that has made does not mean every joint is sound or that nothing is obstructed; other models prove air flow differently.
- Pressure relief valve on a sealed system, typically set to 3 bar: releases water if the pressure rises too high.
Boiler interlock
Interlock removes the normal firing demand when the heating and stored-hot-water controls are all satisfied, so the boiler cannot burn fuel just to keep itself and the pipework warm. It does not cut all power: pump overrun may continue, and frost protection and combi preheat act separately. Closing ordinary TRVs may not end the boiler demand.
Several functions work outside the normal demand. Of these, only frost protection and combi preheat can light the burner when no heating or hot-water control is calling.
- Frost protection fires the boiler, even with the programmer off, when the water inside it gets cold enough to risk freezing, around 5 °C on typical models. It guards the boiler itself; a frost thermostat paired with a pipe thermostat guards pipework in unheated spaces.
- Combi preheat fires the boiler now and then with no tap open, keeping it warm so hot water reaches the tap sooner. On some combis, eco mode turns it off.
- Pump overrun runs only the pump, for a set time after the demand ends, to clear residual heat.
- Anti-cycling makes the boiler wait a set time after the burner stops before it fires again for heating. It delays firing; it never creates a demand.
- Pump anti-seize runs the pump briefly after a long idle spell, daily on some models, and may also move the diverter valve, so neither sticks.
Examples from specific boiler controls
The Ideal Logic+ Combi2 C24/C30/C35 (October 2023) briefly exercises its pump after 24 hours without a system demand. A warm boiler doing this after a long idle spell is different from one circulating continuously or firing without a valid demand. Pump overrun follows recent operation; it does not explain a fresh exercise run after a whole idle day.
Its Timed preheat learns when domestic hot water was used during the previous week, then schedules preheating around those periods. The hot-water temperature dial sets a temperature, while the heating programmer schedules room heat; neither supplies that learned pattern of tap use.
The same instructions require some radiators with twin lockshield valves to leave a heating load of at least 10% of the boiler's minimum output available. Size that load from heat output, not radiator count or maximum boiler output. Where zone valves could stop all system flow, the instructions also require a bypass.
On the ecoTEC plus VU/VUW ..6/5-5 covered by Vaillant's 2021 manual, the burner anti-cycling time applies only to heating. Opening a hot tap can still bring on domestic hot-water operation, and that operation does not reset or otherwise affect the heating delay. A waiting period for heating is therefore different from a fault lockout.
What Approved Document L 2021 asks for when heating work is done
These points concern the 2021 edition, incorporating the 2023 amendments, for work in England.
- Combi boilers in existing homes need at least one extra measure fitted with them: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation (paragraph 6.2). An ordinary programmable thermostat is not one of them.
- Room controls. In a new dwelling, and whenever a boiler is replaced, each heated room gets thermostatic control. A zone can stand in for rooms in a single-storey open-plan home whose living area exceeds 70% of the floor area, or for two adjacent rooms with similar functions and heating requirements, such as a kitchen and a utility room. Being used together does not mean two rooms have similar heating needs. A room thermostat with TRVs on the emitters outside its room meets the standard. The guidance also identifies cases where individual room controls may not be technically feasible, such as a dwelling with very low heat demand.
- Circuits. A new dwelling of 150 m² or more needs at least two independently controlled heating circuits.
- Interlock and timing. Work on a system that includes a boiler brings in a boiler interlock. Heating and stored hot water each get their own time control, unless an existing system already runs them from one controller. A replacement cylinder needs an electronic temperature control, such as a cylinder thermostat.
- Design. Primary circuits are fully pumped where the heat source allows. A newly installed or fully replaced wet system is sized to meet the heating needs at a maximum flow temperature of 55 °C or lower, where feasible.
Exam cues
- Boiler firing in a hard frost with the programmer off: suspect frost protection before an interlock fault.
- Frost protection and preheat ruled out, and the boiler still fires after every control is satisfied: a wiring error or stuck contact is keeping a demand.