Mixed Deck — All CENWAT Topics Flashcards

What is the purpose of a flue draught diverter on an open-flued appliance?

Answer: To prevent down-draughts from extinguishing the pilot or burner and to dilute flue gases with room air

A draught diverter prevents excessive or reversed draught from affecting combustion; it mixes room air with flue gases to reduce corrosive condensation and protect the heat exchanger.

What does LTHW stand for in heating system terminology?

Answer: Low Temperature Hot Water

LTHW (Low Temperature Hot Water) refers to heating systems operating at flow temperatures typically below 90°C, which is standard for domestic central heating.

What information must be recorded on the Building Regulations notification when installing a replacement boiler?

Answer: The installer Gas Safe registration, boiler details, controls fitted, and commissioning data

When notifying Building Control of a replacement boiler installation via a Competent Persons Scheme such as Gas Safe Register, the notification must include the installer Gas Safe registration number, boiler make, model, serial number, efficiency rating, type and details of controls fitted, and commissioning data including flow and return temperatures and gas rate readings.

What is the correct procedure for pressure testing a new central heating system before commissioning?

Answer: Pressurise the system to 1.5 times the maximum working pressure for a minimum test duration, checking for pressure drop

A new central heating system should be pressure tested at 1.5 times the maximum working pressure, or a minimum of 3 bar for sealed systems, for a defined test period. The system should hold pressure with no detectable drop and all joints should be visually inspected.

In an S-plan central heating system, which valve configuration is used?

Answer: Two 2-port motorised valves

An S-plan system uses two separate 2-port motorised zone valves, one controlling the heating circuit and one controlling the hot water circuit. This allows completely independent control of each circuit, unlike the Y-plan which uses a single 3-port valve.

In a Y-plan system, what happens when both the room thermostat and cylinder thermostat are calling for heat?

Answer: The 3-port valve moves to the mid-position allowing flow to both circuits

In a Y-plan system, when both thermostats call for heat simultaneously, the 3-port mid-position valve moves to its mid-position, allowing water to flow to both the heating and hot water circuits at the same time. This is the normal operating condition during cold weather.

What instrument is used to perform a gas tightness test?

Answer: A manometer (U-gauge or digital manometer)

A manometer (either a traditional U-tube water gauge or a digital manometer) is used to measure and monitor gas pressure in the pipework during a tightness test, detecting any pressure drop that indicates a leak.

A Honeywell V4073A valve is used in which system plan, and what type of valve is it?

Answer: Y-plan; 3-port mid-position valve

The Honeywell V4073A is a 3-port mid-position valve, the standard component in a Y-plan fully pumped central heating system.

What colour is the flame on a correctly burning gas boiler burner?

Answer: Predominantly blue with a small yellow tip

A correctly burning gas flame is predominantly blue, indicating complete combustion. A yellow or orange flame indicates incomplete combustion and possible carbon monoxide production.

What is the minimum flue gas temperature that a flue must be designed to withstand on a condensing boiler?

Answer: The condensing boiler flue must withstand flue gas temperatures specified by the manufacturer, typically up to 120°C

Condensing boiler flues operate at much lower temperatures than non-condensing boilers (typically 50–120°C) but must still be rated for the maximum temperature specified by the appliance manufacturer.

What is the maximum CO level in a flue gas (as measured at the flue terminal) that is generally considered acceptable for a gas boiler?

Answer: CO:CO2 ratio below 0.004 (or as specified by the manufacturer's commissioning data)

The CO:CO2 ratio method is used to assess combustion quality; a ratio above 0.004 may indicate incomplete combustion and requires investigation. Absolute CO limits vary by manufacturer.

What does it indicate if a flue gas analyser shows oxygen readings significantly higher than expected, for example above 9%?

Answer: There is excess air entering the combustion chamber or a flue leak

Significantly elevated O2 readings in the flue gas indicate excess air in the combustion process. This can be caused by a cracked heat exchanger, a poorly sealed flue connection, incorrect burner pressure, or damaged seals. Excess air reduces efficiency and can mask dangerous CO levels by diluting them.

What is a room-sealed (balanced flue) boiler?

Answer: A boiler that draws combustion air from outside and discharges flue gases externally, sealed from the room

A room-sealed boiler draws all combustion air directly from outside and discharges flue gases externally, completely isolated from the indoor air; no room ventilation is required for combustion.

What is the function of the end-of-travel (auxiliary) microswitch in a motorised zone valve?

Answer: To prove the valve is fully open and complete the circuit to fire the boiler

When the valve reaches fully open, the end-of-travel microswitch closes, completing the low-voltage circuit that energises the boiler, ensuring the boiler only fires with an open valve.

How does an optimum start controller differ from a standard heating programmer?

Answer: It uses indoor and outdoor temperature sensors to calculate the latest start time needed to reach comfort temperature by the set time

An optimum start controller learns from internal and external temperatures to calculate how early the boiler must start to achieve the desired room temperature by the programmed comfort period.

How often should the temperature and pressure relief valve on an unvented cylinder be manually tested by the householder?

Answer: Every 6 months

Manufacturers typically recommend that the householder manually operates the temperature and pressure relief valve every 6 months to ensure it has not seized or become blocked with limescale. The full annual service and inspection should be carried out by a qualified person.

What is a single-point water heater?

Answer: A small instantaneous heater designed to serve only one outlet, such as a single washbasin

A single-point water heater is a small instantaneous appliance installed directly at a single outlet (e.g., a washbasin), providing hot water at the point of use.

What action should be taken if CO is detected in a habitable room during a gas safety inspection?

Answer: Treat as an immediately dangerous situation: evacuate, ventilate, isolate the gas supply and investigate the source before re-entering

CO in a habitable space is a life-threatening emergency; the property must be evacuated, the gas supply isolated, the property ventilated, and the source of CO identified and made safe before re-occupancy.

What is a condensate pipe on a condensing boiler?

Answer: A pipe that drains acidic condensate produced by flue gas cooling from the boiler to a drain

Condensing boilers produce acidic condensate (approximately pH 3–4) as a by-product of recovering latent heat; this must be drained safely via a condensate pipe to an approved drain.

What is the minimum free area of permanent ventilation required for an open-flued gas appliance of 10 kW input rating?

Answer: As specified in BS 5440-2, typically 5 cm² per kW for a rated input above 7 kW (so approximately 15 cm² plus 5 cm² per kW above 7 kW)

BS 5440-2 specifies ventilation requirements for open-flued appliances; the formula gives the minimum permanent vent area based on the appliance heat input to ensure adequate combustion air.