Gas Safe Registration (ACS) Gas Fires and Wall Heaters (HTR1 / Fire Categories) Syllabus

Describe the flue flow (draught) test for a gas fire.

With the fire not yet running, a smoke pellet/match is used in the fireplace opening (closure plate fitted) to confirm the flue draws all the smoke up and discharges it at the terminal, proving the flue is clear and has adequate updraught before commissioning.

Describe the spillage test procedure for an open-flued gas fire.

Run the fire on full rate (allow it to warm up, typically $\approx 5$ min), close all doors/windows, operate extract fans on full, then hold a smoke match around the edge of the appliance/draught diverter. Smoke must be drawn into the flue; if it spills out, the appliance fails.

Why are extract fans and other appliances run during a spillage test?

To recreate the worst-case condition: fans and other open-flued appliances create negative pressure (depressurise the room), which can pull POC back out of the flue (spillage). The test must prove the fire clears safely even under these adverse conditions.

What is 'spillage' and why is it dangerous?

Spillage is the escape of products of combustion (including carbon monoxide) from the appliance into the room instead of up the flue. It is dangerous because it can cause CO poisoning and indicates inadequate flue draught or ventilation.

How is a coal-effect fuel bed correctly laid on a DFE/ILFE fire?

Lay ceramic coals/pebbles strictly per the manufacturer's diagram — correct number, size, position and spacing, never overlapping the burner ports or flame-sensing device. Incorrect layout causes flame impingement, sooting and dangerous carbon monoxide production.

What is the danger of incorrectly positioning the ceramic coals on a gas fire?

Coals over or too close to the burner ports cause flame impingement and incomplete combustion, leading to sooting (yellow/sooty flames) and elevated carbon monoxide — a serious CO hazard. Always follow the manufacturer's fuel-bed layout exactly.

What flame appearance indicates correct vs incorrect combustion on a gas fire?

Correct: stable, even flames with the design colour (DFEs intentionally show yellow flames but should not soot). Incorrect/dangerous: lifting, floating, sooty/streaky yellow flames on appliances designed to burn cleanly, indicating incomplete combustion and CO production.

What is the general ventilation rule for open-flued (Type B) gas fires regarding heat input?

Open-flued appliances may need purpose-provided ventilation when input exceeds $7\,\text{kW}$ (net). The free air requirement is $5\,\text{cm}^2$ per kW of input above $7\,\text{kW}$ net.

State the air vent free-area formula for an open-flued appliance above the threshold.

$$A = 5 \times (Q_{net} - 7)\ \text{cm}^2$$ where $Q_{net}$ is the net heat input in kW. Below $7\,\text{kW}$ net, adventitious ventilation is generally assumed adequate (subject to manufacturer's instructions).

Calculate the required vent free area for an open-flued gas fire with a net input of $15\,\text{kW}$.

$$A = 5 \times (15 - 7) = 5 \times 8 = 40\,\text{cm}^2$$

What is the ventilation requirement for a flueless gas fire/heater?

Because all POC enter the room, flueless appliances require both purpose-provided ventilation and (usually) an openable window. Typical requirement: $100\,\text{cm}^2$ permanent vent plus an openable window, with sizing depending on appliance input and room volume per the standard.

Why do room-sealed (Type C) appliances normally need no purpose-provided combustion ventilation?

Because they take combustion air directly from outside and discharge POC to outside through the sealed balanced flue, the combustion circuit is independent of room air, so no room vent is required for combustion (only for cooling if specified by the maker).

What does ODS stand for and what is its function?

Oxygen Depletion System (also called Atmosphere Sensing Device, ASD). It monitors the oxygen level in the room air feeding the flame and shuts off the gas supply before the atmosphere becomes dangerously oxygen-deficient (and CO-rich).

Explain how an ODS/ASD physically operates to shut off the gas.

An ODS uses a specially aimed pilot flame heating a thermocouple. As room oxygen falls, the flame lifts/destabilises off the thermocouple, the thermocouple cools, the millivolt signal collapses and the gas valve closes (flame-supervision shut-off) — preventing CO build-up.

At roughly what oxygen / CO2 condition is an ODS designed to shut down a flueless appliance?

An ODS shuts the appliance down before the room atmosphere becomes hazardous — typically when the carbon dioxide concentration rises to about $1.5\%$ (corresponding to oxygen depletion from $\approx 20.9\%$ down towards $\approx 18\%$), well before CO becomes dangerous.

On which appliance types is an ODS mandatory?

On flueless space-heating appliances (flueless gas fires/heaters), because all combustion products discharge into the room. An ODS is the primary safety device protecting occupants against oxygen depletion and CO build-up.

Can an ODS be repaired or interchanged with a different part?

No. The ODS pilot/injector is a matched safety-critical assembly; it must never be modified, drilled, cleaned with abrasives or substituted with a non-original part. Only the manufacturer's exact replacement may be fitted, or the appliance is condemned.

What is a 'balanced flue' and how does it self-compensate for wind?

A balanced flue is a concentric/twin terminal where the air inlet and POC outlet sit in the same pressure zone outside. Wind pressure acts equally on inlet and outlet, so the pressure difference across the appliance stays balanced and combustion is unaffected.

Why must DFE fires never be fitted to a precast (Class 2) flue?

DFEs have low efficiency and high POC volume needing a large updraught; a small precast flue cannot remove the products fast enough, causing spillage and CO build-up. DFEs require a full Class 1 chimney.

What is a 'catalytic' heater's main combustion characteristic compared with a conventional flueless radiant fire?

Catalytic heaters burn gas flamelessly (or with very low flame) on a catalyst at lower temperature, producing very low NOx/CO and mostly radiant heat, whereas a conventional flueless radiant fire burns with an open flame onto a radiant matrix at higher temperature.

What checks confirm a chimney is 'suitable' (the four key flue properties)?

It must be of correct class/size for the appliance, clear (unobstructed/swept), sound (no leakage between flue and dwelling), and correctly terminated above roof level — all confirmed by visual inspection plus flue flow and spillage tests.

What is the minimum height a flue/chimney terminal should generally project above the roof, and why?

It must terminate in the correct external zone clear of pressure/turbulence (e.g. suitable height above the ridge/roof per the standard) so wind cannot cause down-draught. Correct termination ensures reliable updraught and prevents POC re-entering the dwelling.

Define 'net' vs 'gross' heat input as used when sizing fire ventilation.

Gross heat input uses the gross calorific value (includes latent heat of water vapour in POC); net uses the net calorific value (excludes that latent heat). UK ventilation calculations use the net input; net $\approx$ gross $\times 0.9$ for natural gas.

Convert a gross heat input of $10\,\text{kW}$ (natural gas) to net for ventilation purposes.

$$Q_{net} \approx 0.9 \times Q_{gross} = 0.9 \times 10 = 9\,\text{kW (net)}$$

What non-combustible clearance requirement applies to the surround and shelf above a gas fire?

Combustible materials (e.g. a mantel/shelf or fascia) must be kept the manufacturer's specified minimum distance from the appliance (commonly the shelf must clear the fire by a stated dimension, e.g. $\geq 50\,\text{mm}$ projection rules). Always follow the manufacturer's clearances to combustibles.

What is the purpose of the catalyst/ODS combination in modern flueless fires regarding CO?

The catalyst (or refined burner) ensures very clean combustion to minimise CO, while the ODS guarantees shutdown before oxygen depletion makes combustion incomplete — together keeping room CO within safe limits despite the POC entering the room.

If a gas fire fails its spillage test, what are the first things to investigate?

Check for a blocked/obstructed flue, missing or leaking closure plate, inadequate room ventilation, incorrect fuel-bed/coal layout, wrong appliance for the flue class, terminal in a high-pressure/down-draught zone, and operation of extract fans depressurising the room.

After any spillage test that initially passes with fans, what should you do to confirm a stable result?

Re-run/extend the test (e.g. let the fire run a further period such as $10$–$15$ minutes and repeat) to confirm spillage does not occur once the flue is fully warmed, and record the result. A momentary clearance is not acceptable — it must clear consistently.

Summarise the safety hierarchy: which flue type needs the most room ventilation and which the least?

Most ventilation: flueless (Type A) — all POC enter the room, needs vent plus openable window and an ODS. Intermediate: open-flued (Type B) — needs vent above $7\,\text{kW}$ net. Least: room-sealed (Type C) — normally no combustion ventilation required.