Plant room ventilation for gas boilers: why the grilles must stay open

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Plant room ventilation for a gas boiler does two jobs: it supplies the air the burners need to burn gas completely, and it carries heat away so the room, the boilers and their controls stay within safe temperatures. On open flued commercial boilers, which take their combustion air from the plant room itself, the high and low level grilles or louvres are part of the gas installation's safety design, not an optional extra. Ideal's Evomax 2 commercial boiler manual says "All vents must be permanent with no means of closing, and positioned to avoid accidental obstruction by blocking or flooding", and HSE's advice to employers is to check that rooms with gas appliances have adequate ventilation and "do not block air inlets to prevent draughts". This guide explains why the grilles must stay open, which standards apply, why vents tend to get blocked in winter, and what site staff, engineers and duty holders should each do.

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Commercial heating tips and advice · 27 October 2026 · Fast Commercial Boiler Help, Gas Safe 985448

Two jobs: combustion air and cooling

Combustion air. A gas burner needs a steady supply of fresh air. Hamworthy's installation guide for its Upton floor standing commercial boilers says "In all cases there must be provision for an adequate supply of air for both combustion and general ventilation, in addition to that required for any other appliance." For industrial and commercial gas plant, HSE says the equipment "should be designed, operated and maintained to make sure dangerous levels of carbon monoxide (CO) are not produced. It should not be used in poorly ventilated spaces. There should be enough ventilation to remove combustion products." Starve a burner of air and combustion suffers; our guide to carbon monoxide, ventilation and flues in commercial kitchens explains why that matters for people working nearby.

Cooling. Boilers, pumps, pipework and control panels all give off heat. The same Hamworthy guide sets maximum boiler house temperatures that the ventilation air must achieve: 25°C at floor level (or 100 mm above it), 32°C at mid level (1.5 m above the floor) and 40°C at ceiling level (or 100 mm below it). Where a boiler sits in a cupboard or compartment, it says permanent high and low level ventilation "is required which must communicate direct to outside, for cooling purposes." Ideal's Evomax 2 manual gives the same 25°C, 32°C and 40°C limits for compartment installations.

Why plant room ventilation matters for a gas boiler

Whether a plant room needs purpose-provided ventilation, and how much, depends on the type of flue, the total heat input of the appliances in the room, and whether the boilers are in a large room or a tight compartment. That is a design decision for a Gas Safe registered engineer, but it helps facilities teams to know the basics.

Open flued boilers draw their combustion air from the room. Hamworthy says its Upton boilers are suitable for open flue (type B23P) installation, "drawing combustion air from the plant room", and that "The combustion air for the boiler is drawn through the rear of the boiler. The air inlets must be kept open and free from blockage at all times." Block the plant room grilles and you block the boiler's air supply.
Room sealed boilers take combustion air through their own flue system, but a plant room full of them still gets hot. For a single Evomax 2 of up to 70 kW, Ideal says "If installed as a room sealed appliance in a room or internal space, then no purpose provided ventilation is required." For room sealed installations of 70 kW and over, though, the same manual says "A minimum of 2cm2 free area per kW of net heat input at both high and low level is required for boiler rooms." So "room sealed" does not automatically mean "no vents needed" in a commercial plant room.
Mechanical ventilation. Some plant rooms use fans instead of, or as well as, natural grilles. Hamworthy says mechanical ventilation "must be interlocked with the boilers to prevent operation in the event of ventilation fan failure", that "Mechanical extract ventilation with natural inlet ventilation MUST NOT be used", and that an automatic control should cut off the gas supply to the boiler if air flow fails in either the inlet or extract fans. HSE's employer guidance makes the same general point for commercial gas plant: "Make sure the gas supply is interlocked with the ventilation of the appliance." It is the same principle as the gas interlocks used in commercial kitchens.

Which standards apply: IGEM/UP/10, BS 6644 and BS 5440

IGEM/UP/10 Edition 4 (Installation of flued gas appliances in industrial and commercial premises) is the main industry standard. IGEM says it applies to gas fired open flued and room sealed appliances including hot water boilers of net heat input exceeding 70 kW, steam boilers, thermal fluid heaters, storage water heaters over 70 kW, warm air heaters and radiant heaters, and that it contains details of legislation, location of appliances, pipework, "Ventilation", chimneys, gas and fire detection, commissioning and handover, and servicing and maintenance. Its scope note adds that it applies where a smaller appliance shares a plant room with others "where the total net heat input exceeds 70 kW".

BS 6644 also covers hot water boilers: IGEM notes that boilers "of gross heat input not exceeding 2 MW (1.8 MW net) are also covered in BS 6644". Ideal's manual tells installers of boilers of 70 kW and over that "Detail reference should be made to BS. 6644 and IGEM/UP/10 for inputs between 70kW and 1.8MW (net)." For single appliances of 70 kW net or less, IGEM says BS 5440 applies to flues and ventilation.

Two further points from IGEM matter to building owners. First, the standard "applies to new installations and it is not the intention to apply any requirements retrospectively", but if existing plant is being upgraded it recommends following its intent "as far as reasonably practicable", and "A risk assessment is required whenever changes are proposed for any plant". Second, IGEM/UP/10 and BS 6644 allow deviation from the prescribed ventilation requirements in some cases, for example for summer heating loads, but IGEM says "any calculations, methods of assessment and conclusions are recorded and retained with the plant records", and a system design verification notice (SDVN) is required to be displayed. If your plant room has one of those notices, leave it in place and keep the paperwork with your heating maintenance records.

What the manufacturers' figures look like

To show why grilles are sized and placed with care, here are the natural ventilation figures one manufacturer publishes for open flued installations. They are for engineers designing to BS 6644 and IGEM/UP/10, not a method for anyone else to size or alter vents.

Hamworthy Upton, open flue, natural ventilation direct to outside air	Low level (inlet)	High level (outlet)
Boiler house	4 cm² per kW of total rated input (net)	2 cm² per kW of total rated input (net)
Cupboard or compartment	10 cm² per kW of total rated input (net)	5 cm² per kW of total rated input (net)

Ideal publishes the same per-kW figures for open flued Evomax 2 installations of 70 kW and over. Position matters too. Hamworthy says the boiler room "must have, or be provided with, permanent air vents directly to the outside air, at high level and at low level", that low level grilles "should be located within 1 metre of the floor for Natural Gas and within 250mm of the floor for LPG", and that high level grilles "should be positioned within 15% of the boiler room height from the ceiling". It also says air vents "must not be sited in any position where they are likely to be easily blocked or flooded or in any position adjacent to an extraction system which is carrying flammable vapour."

The point for facilities teams is simple: every grille in a gas plant room is there for a reason, its size has been worked out against the heat input of the plant, and covering even part of it reduces the free area the design relies on.

Why vents get blocked in winter

Plant room ventilation openings are, by design, permanent holes to the outside, so in cold weather people are tempted to do something about them. The usual ways they end up obstructed are:

Covering grilles to stop draughts or cold. Cardboard, boards, tape, foam or bags pushed into louvres. HSE's advice is direct: "do not block air inlets to prevent draughts, and do not obstruct flues and chimneys."
Trying to keep frost out. Frost protection belongs in the controls and the plant design, set by your engineer, not in closing the air supply. Our guide to frost protection for commercial heating plant covers what to agree with your engineer instead.
Storage. Pallets, chairs, cleaning stock, bins or spare equipment stacked against a low level grille inside or outside. Ideal's Evomax 2 user guide says "If the boiler is installed in a compartment then the compartment MUST NOT be used for storage purposes", "Do not store objects around or on the boiler, and keep access clear at all times" and "Do not obstruct ventilation ducts, condensate pipework, grilles or openings in the boiler room, room space or compartment that the appliance is installed in". Hamworthy's Upton user instructions say "Do not block or obstruct ventilation grilles."
Leaves, litter and snow building up against outside louvres, and planting or bins placed in front of them.
Building works. Hoardings, sheeting, new partitions or sealed doors can cut off a vent path, and dust is a problem in its own right. Hamworthy says "The air supply should be free from contamination such as building dust and insulation fibres from lagging", and that the boilers "should not be fired whilst building work is being undertaken" to avoid unnecessary cleaning and servicing.
Closable or replaced grilles. A permanent vent swapped for one that can be shut, or a louvre replaced with a smaller one. Ideal's "no means of closing" requirement exists for exactly this reason.

Our commercial plant room safety checklist includes a ventilation line for regular walk-rounds.

What site staff should do (and not do)

Ventilation is part of the gas installation, so designing, sizing, moving or altering it is work for a Gas Safe registered engineer qualified for commercial work. Site and facilities staff have a simpler job: keep the vents clear and report anything wrong.

Keep every grille and louvre clear, inside and outside the plant room. Move stored items away, and pick up loose litter or leaves lying against an outside grille where it is safe to do so.
Never cover, tape, board, bag or close a vent, even for a short time, and never store anything against one. Do not use the plant room or a boiler compartment as a store.
Do not unscrew, cut, resize, replace or move grilles, mesh or ventilation fans yourselves, and do not bypass or reset a ventilation interlock other than by the normal user control you have been trained on.
Report promptly to your engineer or facilities manager if you find a grille that is blocked, damaged, covered or has been replaced; a ventilation fan that is not running; a plant room that is unusually hot; sooting or staining around a boiler or flue; repeated boiler lockouts; or a carbon monoxide alarm sounding.
Tell your engineer about changes before they happen: building works near the plant room, new extract fans in adjoining spaces, new partitions or doors, or extra plant going into the room. Changes like these can affect the air supply the design depends on.
If you smell gas, follow HSE's advice: "turn off the supply and immediately phone" the National Gas Emergency Service on 0800 111 999 for natural gas, or your LPG supplier for LPG.

What engineers check

The law puts a specific duty on the engineer. Regulation 26(9) of the Gas Safety (Installation and Use) Regulations 1998 says that where a person performs work on a gas appliance "he shall immediately thereafter examine" a list of things that includes "the effectiveness of any flue" and "the supply of combustion air". Manufacturers build the same checks into commissioning and servicing. Hamworthy's pre-commissioning checks for the Upton include "Ensure that ventilation and air supply to plant room is correct", and its servicing section says a Gas Safe engineer registered for non-domestic gas appliances should check that "the flue, its support and terminal, the ventilation to the boiler house" and other items "are in a serviceable and working condition and still comply with the relevant standards and codes of practice". Hamworthy recommends regular servicing "at least annually". Our guide to what a commercial boiler service includes shows where ventilation fits in a full service.

If an engineer finds that an appliance cannot be used without danger, for example because its air supply is inadequate, regulation 34(3) requires them to take all reasonably practicable steps to inform the responsible person for the premises. Regulation 34(1) then says the responsible person "shall not use a gas appliance or permit a gas appliance to be used if at any time he knows or has reason to suspect that it cannot be used without constituting a danger to any person." In practice that can mean boilers are left off until the ventilation is put right, so it pays to fix a blocked or undersized vent before the engineer arrives rather than after.

Who does what

Task	Who
Keep grilles, louvres and the space in front of them clear; keep the plant room free of storage; report blocked or damaged vents, fans not running, excessive heat or lockouts	Trained site or facilities staff
Assess, size, design, alter or reposition plant room ventilation; check the supply of combustion air and flue effectiveness after work on an appliance; commission and service the boilers	Gas Safe registered engineer qualified for commercial work
Keep the flue, appliances and pipework maintained in a safe condition, use competent engineers, act on their findings and keep records (including any SDVN and design paperwork)	The employer or self-employed person in control of the premises (the duty holder)

Duty holder responsibilities

Regulation 35 of the Gas Safety (Installation and Use) Regulations 1998 requires every employer or self-employed person to ensure that "any gas appliance, installation pipework or flue installed at any place of work under his control is maintained in a safe condition so as to prevent risk of injury to any person." HSE's employer guidance turns that into practical steps: "Use a Gas Safe registered engineer or competent person to install, maintain or repair your gas fittings", "Ensure that your gas pipework, appliances and flues are regularly maintained", and "Check that all rooms with gas appliances have adequate ventilation". It also says "Do not use any appliance you know or suspect is unsafe."

For most commercial sites that means three things: a regular boiler service and commercial gas safety inspection that includes the plant room ventilation; a simple site rule that grilles are never covered and the plant room is not a store; and a route for staff to report problems quickly. If you plan maintenance using BESA's SFG20 or CIBSE Guide M, add a ventilation walk-round to your routine site checks alongside the engineer's visits. Our planned preventive maintenance page explains how we schedule that work, and our guide on checking a Gas Safe engineer's commercial qualifications covers who should be doing it.

Concerned about your plant room ventilation?

If a grille has been covered or replaced, the plant room runs hot, boilers keep locking out, or you are adding plant to an existing room, call us on 0800 0996 007 or WhatsApp 07947 578332. A Gas Safe registered commercial engineer can assess the ventilation against the appliances installed. See our plant room repair and commercial boiler servicing pages for the work we do.

Safety note

Under the Gas Safety (Installation and Use) Regulations 1998, gas work must be carried out by a Gas Safe registered engineer, and we apply that rule on every site. Don't attempt DIY work on gas appliances, burners, flues or safety devices. On pressure systems, trained site staff should stick to visual and gauge checks, log the readings and report anything abnormal. Never adjust, cap, plug or test-lift a relief valve yourself.

Talk to a Gas Safe commercial engineer

Fast Commercial Boiler Help is a Gas Safe registered (985448) commercial heating business based at 4 Market Hill, Clare, Sudbury, Suffolk CO10 8NN. Our team of over 10 local engineers covers East Anglia and East London, working on commercial boilers, plant rooms, water heaters, and warm air, radiant and unit heaters. We work on commercial premises only.

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Frequently asked questions

Why must plant room ventilation grilles stay open?

The grilles supply the air gas burners need for complete combustion and carry away heat so the room stays within safe temperatures. Open flued boilers take their combustion air from the plant room, so covering a grille cuts their air supply. Ideal's Evomax 2 manual says all vents must be permanent with no means of closing, and HSE tells employers not to block air inlets to prevent draughts.

Can we close plant room vents in winter to stop frost or draughts?

No. HSE's advice to employers is not to block air inlets to prevent draughts, and manufacturers require permanent vents with no means of closing. Frost protection should be handled by the heating controls and plant design set up by your engineer, not by closing the air supply. If the plant room is too cold or draughty, ask a Gas Safe registered engineer to assess it.

Do room sealed boilers need plant room ventilation?

Often, yes. Room sealed boilers take combustion air through their own flue, but a plant room still needs cooling. Ideal's Evomax 2 manual says a single room sealed boiler up to 70 kW in a room needs no purpose provided ventilation, but for room sealed installations of 70 kW and over it requires a minimum of 2 cm² free area per kW of net heat input at both high and low level in boiler rooms. Your engineer will confirm what your installation needs.

What standard covers plant room ventilation for commercial gas boilers?

IGEM/UP/10 Edition 4 covers flued gas appliances in industrial and commercial premises, including hot water boilers over 70 kW net heat input, and includes ventilation. BS 6644 also covers hot water boilers up to 1.8 MW net, and BS 5440 applies to single appliances of 70 kW net or less. Manufacturers' installation manuals give their own requirements, which the engineer follows alongside the standards.

Who checks plant room ventilation, and how often?

A Gas Safe registered engineer qualified for commercial work. Regulation 26(9) of the Gas Safety (Installation and Use) Regulations 1998 requires the engineer to examine the supply of combustion air immediately after working on a gas appliance, and manufacturers such as Hamworthy include the plant room ventilation in servicing, which they recommend at least annually. Between visits, site staff should keep vents clear and report problems, and the duty holder must keep the installation maintained in a safe condition under regulation 35.

Sources and further reading