Gas Safe Registration (ACS) Central Heating Boilers and Water Heaters (CENWAT) Syllabus

What is the minimum clearance of a fanned flue terminal from an internal or external corner of a building?

$300\ \mathrm{mm}$ from an internal or external corner.

State the minimum distance a fanned-draught flue terminal must be from the ground, a balcony, or a flat roof.

$300\ \mathrm{mm}$ above ground, roof or balcony level.

What is the minimum horizontal clearance between two fanned-draught flue terminals facing each other across an opposing wall/boundary?

$1200\ \mathrm{mm}$ between terminals facing each other (e.g. opposite walls).

Why must a condensate pipe run be kept as short as possible internally and adequately sized/insulated when run externally?

To prevent freezing in cold weather, which blocks the trap and causes boiler lock-out; external runs should be in $32\ \mathrm{mm}$ pipe minimum, kept short, insulated, and ideally terminate to an internal soil/waste point.

What is the minimum recommended internal diameter for an external condensate pipe to reduce the risk of freezing?

$32\ \mathrm{mm}$ (1¼ inch) minimum outside diameter for external runs, versus $22\ \mathrm{mm}$ acceptable for short internal runs.

List the acceptable termination points for boiler condensate disposal in order of preference.

1) Internal soil and vent stack; 2) internal kitchen/bathroom waste pipe (above trap); 3) external soil stack; 4) external rainwater/combined drain; 5) purpose-made soakaway as a last resort.

What is the purpose of the condensate trap fitted inside a condensing boiler?

It provides a water seal preventing flue products escaping into the room/drain while allowing acidic condensate to drain away; it must be primed (filled) before commissioning.

Outline the key sequence of a boiler commissioning procedure under the manufacturer's instructions.

1) Visual and tightness (gas soundness) check; 2) check gas/electrical supply; 3) fill and vent system, prime condensate trap; 4) check inlet working pressure; 5) light boiler and check gas rate/burner pressure; 6) perform flue gas analysis (combustion ratio); 7) check controls, safety devices and flue integrity; 8) complete benchmark/commissioning record and instruct user.

During commissioning, what is the acceptable CO/CO2 ratio for a correctly burning condensing boiler measured with a flue gas analyser?

A $\mathrm{CO/CO_2}$ ratio not exceeding $0.004$ (i.e. $\le 0.004$); typical good combustion is around $0.001$ to $0.003$.

What standing (no-flow) inlet gas working pressure should be confirmed at the boiler for natural gas during commissioning?

A working (running) inlet pressure of $20\ \mathrm{mbar}$ (nominal) for natural gas, within the appliance manufacturer's stated tolerance, measured with the boiler at full rate.

What is the maximum permissible pressure drop across the whole installation from meter to appliance under working conditions for natural gas?

A maximum of $1\ \mathrm{mbar}$ pressure drop between the meter outlet (ECV/governor) and the appliance inlet under full flow conditions.

During system filling, why must the heating system be flushed before adding inhibitor?

To remove installation debris, flux, swarf and grease that cause blockages, pump wear, noise and corrosion; BS 7593 requires cleaning/flushing before the corrosion inhibitor is dosed.

Which standard governs the treatment of water in domestic heating systems, and what does it require?

BS 7593 — it requires cleaning/flushing of the system, dosing with a corrosion inhibitor, and periodic checking (typically annually) and re-dosing of inhibitor concentration.

What is the purpose of a corrosion inhibitor in a sealed heating system?

To prevent internal corrosion of steel/aluminium components, suppress hydrogen gassing and the formation of magnetite (black sludge), and protect against scale, thereby maintaining efficiency and component life.

State the typical cold-fill pressure and the normal operating pressure range for a sealed domestic heating system.

Cold-fill pressure of about $1.0$ to $1.5\ \mathrm{bar}$, rising to roughly $1.5$ to $2.0\ \mathrm{bar}$ when hot; the pressure relief valve typically lifts at $3\ \mathrm{bar}$.

What is the function and typical lift pressure of the pressure (temperature) relief valve on a sealed system boiler?

It protects against over-pressure by discharging to a safe visible point; it typically operates at $3\ \mathrm{bar}$ (the maximum safe working pressure of the system).

What is the role of the expansion vessel in a sealed heating system and how is it charged?

It accommodates the increase in water volume as the system heats up, limiting pressure rise; it contains a nitrogen/air charge (typically pre-charged to about $0.75$ to $1.0\ \mathrm{bar}$) behind a flexible diaphragm.

What is a multipoint instantaneous water heater and how does it differ from a single point heater?

A multipoint heats domestic water on demand and supplies several outlets (e.g. bath, basin, sink) from one unit; a single point supplies only one outlet (one tap), usually sited at that point of use.

Why does an instantaneous water heater's delivered flow rate fall as incoming cold water temperature drops in winter?

Because output is limited by heat input: a fixed kW must raise water from a lower starting temperature to the set temperature, so to achieve the same temperature rise the flow rate must be reduced. Higher required $\Delta T$ means lower litres/min.

State the formula linking heat input, flow rate and temperature rise for an instantaneous water heater.

$$Q = \dot{m} \, c \, \Delta T$$ where $Q$ is power (kW), $\dot{m}$ is mass flow rate (kg/s), $c \approx 4.186\ \mathrm{kJ/(kg\cdot K)}$ for water, and $\Delta T$ is the temperature rise in K. Roughly, $\text{kW} = \frac{\text{litres/min} \times \Delta T}{14.3}$.

A 24 kW instantaneous heater raises water by $35\,^{\circ}\text{C}$. Approximately what flow rate (litres/min) can it deliver?

Using $\text{l/min} \approx \frac{\text{kW} \times 14.3}{\Delta T} = \frac{24 \times 14.3}{35} \approx 9.8\ \mathrm{l/min}$.

For an open-flued single point instantaneous water heater in a room, what is the basic permanent ventilation requirement?

Permanent air supply for combustion: for open-flued appliances above $7\ \mathrm{kW}$ net, $5\ \mathrm{cm^{2}}$ of free air per kW for the amount of input above $7\ \mathrm{kW}$ (per BS 5440-2).

Why must instantaneous (open-flued) water heaters never be installed in a bathroom unless room-sealed?

Open-flued appliances draw combustion air from and can spill products into the room; in a bath/shower room this risks carbon monoxide poisoning. Only room-sealed (Type C) appliances are permitted in bathrooms (and they must not be within reach of a bath/shower).

What is a storage water heater and what is its main advantage over an instantaneous heater?

It heats and stores a volume of hot water in an insulated vessel; the advantage is a high peak delivery rate (large draw-offs) and simultaneous multi-outlet supply, independent of instantaneous heat input limits.

What thermostat setting is typically used for stored domestic hot water, and why this temperature?

Around $60$ to $65\,^{\circ}\text{C}$ stored: hot enough to control Legionella bacteria, while limiting scale formation and scald risk; delivery to outlets is often blended down to $\le 43$ to $45\,^{\circ}\text{C}$.

What is Legionella and at what stored water temperature is it controlled/killed?

Legionella pneumophila is a waterborne bacterium causing Legionnaires' disease. It proliferates between about $20$ and $45\,^{\circ}\text{C}$, survives dormant below $20\,^{\circ}\text{C}$, and is killed rapidly above $60\,^{\circ}\text{C}$ — hence storage at $\ge 60\,^{\circ}\text{C}$.

To minimise Legionella risk, what minimum temperatures should hot water be stored at and distributed to taps?

Store at $\ge 60\,^{\circ}\text{C}$ and distribute so that water reaches the tap at $\ge 50\,^{\circ}\text{C}$ within one minute; cold water should be kept below $20\,^{\circ}\text{C}$.

What device is used to prevent scalding at the outlet while still storing water hot enough to control Legionella?

A thermostatic mixing valve (TMV) blends stored hot water with cold to deliver a safe outlet temperature (typically $\le 43\,^{\circ}\text{C}$ for baths/showers) while storage remains at $\ge 60\,^{\circ}\text{C}$.

Outline the key steps of a routine annual boiler service.

Visual inspection and check controls/operation; gas tightness test; check flue and ventilation/clearances; combustion (flue gas) analysis before and after; remove and inspect/clean burner, heat exchanger, condensate trap and ignition/sensing electrodes; check seals, fan, expansion vessel charge and PRV; check gas rate/burner pressure; reassemble, re-test combustion, and record on service record.

During servicing, what must be done to the condensate trap, and why?

It should be removed, cleaned of debris/sludge and re-primed (refilled with water) to maintain the flue gas seal and ensure free condensate drainage; a blocked trap causes lock-out and potential flue product spillage.

Name three common combi boiler faults and a likely diagnostic cause for each.

1) No hot water but heating works — failed diverter valve or flow turbine/sensor; 2) Pressure dropping — leak or failed expansion vessel/PRV passing; 3) Ignition lock-out — faulty ignition/sensing electrode, blocked condensate trap, gas supply/flue fault, or PCB fault.

A boiler keeps locking out on overheat with the heating very hot then cold (kettling/short cycling). What are the likely causes?

Restricted primary flow: scale/sludge in the heat exchanger, a stuck/failed pump, a blocked filter, air in the system, or a faulty overheat/flow thermistor — reducing circulation so the heat exchanger overheats.

What is the function of an overheat thermostat (high-limit/ECO stat) on a boiler?

It is a non-self-resetting (or manual-reset) safety device that cuts the gas supply if the primary water temperature exceeds a safe limit (e.g. around $90$ to $100\,^{\circ}\text{C}$), protecting against boiling/overheat if the control thermostat fails.

What safety role does the air-pressure switch (or fan speed feedback) perform on a fan-assisted boiler?

It proves correct fan operation/air flow before allowing ignition; if airflow/flue is obstructed or the fan fails, it prevents the burner from firing, guarding against incomplete combustion and CO production.

What is the purpose of a flue gas (combustion) analyser test and what two readings are central to it?

It verifies safe and efficient combustion by measuring carbon monoxide (CO) and carbon dioxide ($\mathrm{CO_2}$); the key output is the CO/CO2 ratio (should be $\le 0.004$) confirming complete combustion.

What controls combine to form a typical fully-pumped heating and hot water control system (S-plan / Y-plan)?

Programmer/timer, room thermostat, cylinder thermostat, and motorised zone valve(s) — S-plan uses two 2-port valves (separate heating and hot water zones); Y-plan uses one 3-port mid-position valve to share the boiler between heating and hot water.