Wood burners and MVHR

Can you have a wood burner with an MVHR system?

You can, but the stove must be room-sealed with a direct external air supply, so it takes its combustion air from outside rather than from the room. That keeps it working safely whatever the MVHR is doing to the air pressure in the house. An ordinary open-flued stove and MVHR together in an airtight home is not a safe combination and should not be installed.

A room-sealed, or direct air supply, stove has a duct that feeds outside air straight into its firebox and a tight door seal, so its operation does not depend on the pressure in the room. That matters because MVHR, and the airtightness of the building, both change room pressure. A conventional stove that draws air from the room can be starved of air or have its flue gases pulled back into the room, which is a carbon monoxide risk. This is why in an airtight home with MVHR the room-sealed type is the only sensible choice, and many stoves are now sold with an external air kit for exactly this reason.

Why is an open-flued stove a problem with MVHR?

An open-flued stove relies on the room supplying its combustion air and on the chimney drawing steadily. MVHR and a tight building can lower or disturb the pressure in the room enough to weaken that draw, or to pull combustion gases back down the flue into the home. That backflow, called spillage, can put carbon monoxide into the living space, which is why the combination is treated as a hazard.

The chimney of an open-flued appliance works on a small pressure difference. If a mechanical ventilation system, an extractor elsewhere, or simply a very airtight envelope reduces the pressure in the room, the flue can stop drawing properly or even reverse. Combustion products, including carbon monoxide, then enter the room instead of leaving up the chimney. Because MVHR runs continuously and can boost, the risk is present day to day, not just occasionally. The way round it is to remove the room from the equation entirely with a room-sealed, direct air supply stove.

What does Approved Document J require for a stove in an airtight home?

Approved Document J requires that any combustion appliance has an adequate, permanent supply of air for safe burning and for the flue to work. In airtight homes, at or below 5.0 cubic metres per hour per square metre air permeability, it calls for additional permanent ventilation provision for open-flued appliances, because there is less natural air leakage to rely on. A room-sealed stove with its own external air supply is the cleaner way to meet this.

Approved Document J, the building regulations guidance for combustion appliances, sets the air supply, the flue and the safety provisions for solid fuel, gas and oil appliances. Its central requirement is adequate combustion air. The 2022 edition added guidance specifically so that appliances can keep working safely in the more airtight homes now being built, and it recognises the 5.0 cubic metres per hour per square metre air permeability threshold below which extra provision is needed for open-flued appliances. In practice, on an airtight home with MVHR, meeting Approved Document J means a room-sealed appliance with a dedicated external air duct, sized and installed by a competent person rather than to a single rule of thumb.

Sources: GOV.UK

Do I need a carbon monoxide alarm with a wood burner?

Yes. Approved Document J requires a carbon monoxide alarm to be fitted whenever a fixed solid fuel appliance, such as a wood burner, is installed. This applies regardless of MVHR, and it applies even to a room-sealed stove. The alarm should be sited in the same room as the appliance, following its instructions and the standard for domestic carbon monoxide alarms.

The 2022 amendments to Approved Document J, in force from 1 October 2022, require a carbon monoxide alarm on the installation of fixed combustion appliances burning solid fuel, oil, or gas, with cooking-only gas appliances the main exception. For a wood burner that means an alarm is not optional. It is a cheap and essential backstop, and it does not replace correct installation and a room-sealed appliance; it sits alongside them. A registered installer will fit and position the alarm as part of the job.

Sources: GOV.UK

How is a stove tested to work safely with MVHR?

At commissioning the installer runs a spillage test, checking that the flue clears combustion gases with the MVHR set to full extraction, its worst case for pulling the room down in pressure. It is best done with the stove and the MVHR commissioned together. A HETAS registered installer, or building control, signs off that the appliance is safe and compliant.

The recognised check is a spillage, or smoke, test to BS 8303, carried out with every extract in the home, including the MVHR at full boost, running at once. If the flue still clears cleanly under that worst case, the appliance is safe in normal use. Coordinating the stove installer and the MVHR commissioning engineer avoids the situation where each signs off their own kit and no one tests them together. In the UK a HETAS registered installer can self certify the work under building regulations, or building control can inspect it; either way the stove should not be used until it has passed.