Summary
Adveco looks at the classifications and critical requirements for flue systems when replacing gas-fired water heaters on commercial premises across the UK.
Adveco looks at the classifications and critical requirements for flue systems used when replacing commercial gas-fired water heaters in the UK.
Flueing and ventilation are inextricably linked. For every single unit of gas burned, a burner requires 10 units of air for complete combustion. This air must be supplied reliably. Starving the appliance of air leads to incomplete combustion, burner ‘hunting’, flue failure, and the high risk of carbon monoxide poisoning because of spilling. Spilling (or flue gas spillage) occurs when the products of combustion – Carbon Monoxide (CO), Nitrogen Oxides (NOx), and water vapour – instead of exiting through the flue terminal, enter the room where the appliance is installed. This happens because the pressure inside the flue or appliance is higher than the pressure in the room, and there is a path of least resistance (such as a draft diverter, a hole in the pipe, or a dormant appliance in a shared header). Such scenarios can lead to oxygen depletion and potentially fatal poisoning. On older atmospheric appliances, this is checked via a ‘Spillage Test ‘ using a smoke match. Modern fan-assisted units often rely on pressure switches to shut down the unit if a blockage (which causes spillage) is detected.
The primary difference between flue types is where the combustion air is sourced. Flue Systems in the UK are subsequently classified by this air source and exhaust location. Type A, which is flueless (e.g., Cooker), will source combustion air from the room and exhaust into the room. Type B, open flue, which sources air from the room and exhausts out of the building, and Type C, room sealed, which sources air from outside and exhausts out of the building. These latter two systems will be encountered in plant room scenarios for commercial gas water heating.
Type B: Open Flue Systems
Open flue systems are categorised as either B11 or B23. B11, or Atmospheric / Open Flue, relies on the ‘buoyancy’ of rising hot gases, so do not feature a fan. B11 is mostly based on vertical chimney systems, requiring negative draught for flueing. Installations are typified by galvanised steel flue with a non-continuous spiral and unsealed joints.
B11, which is non-condensing, provides a low efficiency approach. Critically, as there is no fan to force the gases out if the chimney is cold or blocked, there will be a high risk of spillage.
As a result, new systems will opt for B23, a forced/induced draught open flue. This system of flueing uses a fan to push or pull gases. Efficiency can be high when correctly implemented, and it supports flue runs that can be horizontal or vertical, with continuous sections with sealed joints. For condensing appliances, the flue will be either stainless steel or polypropylene (PP).
Because the flue is under positive pressure, a hole or leak would spill exhaust into the room. As a result, permanent room ventilation is mandatory for both combustion air and cooling.
Type C: Room Sealed Systems
In room sealed systems (Type C), air is drawn in from outside via a dedicated air pipe and flue gases are forced out by a fan. This high-efficiency/condensing approach also requires minimal ventilation. None is needed for combustion air, though some may be required for plant room cooling or DSEAR compliance. Since the flue is positive pressure, and the air intake is negative pressure, the Type C flue is the safest configuration available. Even if spillage occurs within the appliance casing, it is contained and typically drawn back into the burner.
A comparison of Open Flue vs. Room Sealed Systems
Feature | Open Flue (B23) | Room Sealed (C) |
Air Source | Indoor air (often dirty/dusty) | Clean outdoor air |
Comfort | Can cause cold draughts in the room | No draughts |
Reliability | Air vents are often taped over by users | Highly reliable; no vent risk |
Installation | Requires large louvres in walls | Compact and flexible |
Plant Room Safety | Risk of spill if the flue is damaged | Higher safety; isolated from the room |
Component Life | Internal parts get dirty (dust/lint) | Internal parts stay clean |
With gas installation no longer supported in new build projects, except under very specific conditions, moving forward, flueing will be a process of replacement or refurbishment of existing structures. Installers should always replace the flue when replacing a gas water heating appliance. For Atmospheric (B11) systems, the existing flue will not be suitable for positive pressure or condensing. It will leak condensate and exhaust, so it must be changed. Old B23 systems are likely to be aluminium and aged 7+ years. As Aluminium corrodes in the presence of acidic condensate, it too must be changed. Room sealed systems should also see the flue changed as the flue is likely aluminium or an incompatible plastic brand. Due to seal tolerances, it is not recommended to mix brands.
Flue systems will also have a maximum safe run distance determined by fan power, stated in the manual as Total Equivalent Length (TEL). However, the maximum length will be impacted by the resistance caused by the addition of a bend to the flue run. Every 45° or 90° bend adds a specific ‘equivalent length’ (e.g., a 90° bend might equal 1.5m of straight pipe). As a result, the flue run total length needs to be calculated as a sum of straight pipe + sum of bend equivalents. This must not exceed the manufacturer’s TEL limit.
Common Header Flue Systems
Typically used for high-efficiency condensing boilers, often with fan-assisted extraction, a common header flue system (or common/communal flue) is a shared venting system used in commercial buildings. This approach, which allows for multiple gas-fired appliances to vent into a single shared duct. This is a popular option as it minimises the number of wall or roof penetrations.
Positive-pressure common headers do pose a higher risk of ‘recirculation,’ where if one heater is off, the fan from the other heater can push exhaust back down into the dormant unit, causing it to spill into the plant room through the air intake or burner. For this reason, appliances must have Non-Return Valves (NRV). Water heaters that do not have internal NRVs cannot be used in a positive-pressure common header. As a result, Negative Pressure Common Headers have become mandatory for water heaters lacking NRVs to prevent spilling. The header must be sized so that even with all fans running, the chimney maintains a negative pressure (suction). In general, such systems will require a rise to run ratio of 2:1 or 3:1. If an external chimney fan is used, it must be interlocked via the appliance blocking contacts.
When it comes to replacing ageing, inefficient gas-fired appliances with modern condensing water heaters, there are a few key takeaways. Ensure flue material is either Polypropylene (PP) or S701/High-Grade Stainless Steel, and concentric flue is preferred for Type C systems to minimise building penetrations. Consideration must be given to condensate management and the plume. These can be addressed by the flue being installed with a fallback to the appliance (min 3° or 50mm per metre) and siting terminals away from windows and air intakes to avoid nuisance. And one simple thing which can often be overlooked is to use only manufacturer-approved silicone for seal lubrication, as petroleum-based products will destroy the EPDM seals.
