Mucka will work out the ventilation free area an appliance needs and put it on the certificate next to the ventilation check, so the number you passed or failed it against is on the record rather than in your head.
You need the Gas Calculators skill installed. It sits alongside Gas & Oil Certs.
What it does, and what it does not
This is the honest bit, and it is worth reading before you rely on it.
It covers, to BS 5440-2:2023:
- Open-flued appliances in a room — 5 cm² of free area for every kW of net input above 7 kW.
- Room-sealed appliances in a room — which need no purpose-provided vent for combustion.
It does not cover yet:
- Flueless appliances — cookers, hobs, instantaneous water heaters. These are sized off the room volume as well as the appliance.
- Decorative fuel effect fires.
- Anything in a compartment — a cupboard or enclosure built round the appliance. A compartment needs high and low vents, and that is true even for a room-sealed boiler that would need nothing on an open wall.
Those three are listed in the calculator and marked Coming soon. Tap one and it tells you so, rather than quietly giving you the open-flued answer. That is deliberate: a ventilation figure that is too small is the kind of mistake that matters, so we would rather say we do not know it yet. Size those from BS 5440-2 and put the figure on the record by hand.
Where to find it
- Calculators in the menu, under Trade, then pick Ventilation off the board.
- From a certificate. On any appliance, tap Work it out next to Ventilation free area, under the ventilation check.
- Ask Mucka. "How much ventilation does a 24 kilowatt open-flued boiler need?"
- With no signal. The offline screen's Calcs button.
Doing it
- Pick the appliance — open-flued, room-sealed, flueless or decorative.
- Say whether it is in a room or in a compartment.
- For an open-flued appliance, put in the total net heat input of the open-flued appliances in that room. Net, not gross.
The answer appears as you type. For a 20 kW net open-flued appliance that is 65 cm² — 20 kW less the 7 kW that adventitious air covers, times 5.
You will also see the figure before the air allowance. That is there so you can see what the allowance did, not because you should use it: the 7 kW deduction and the 35 cm² deduction are the same allowance said two ways, and taking both off is the classic way to end up 35 cm² short.
Two things it will always tell you
The manufacturer's instructions win. If they ask for more ventilation than the standard, theirs is the figure. For ventilation that happens often, so it is not a disclaimer.
An open-flued figure is not the whole story if there is a cooker in the room too. A room with both needs the flueless requirement working out as well, and that part is not built yet.
Putting it on the certificate
Tap Work it out next to Ventilation free area on the appliance, then Use the figure. If the answer is that none is required, it records that as none required rather than as a blank — a blank means nobody said, and none required means somebody checked.
The figure prints on the certificate PDF under the appliance table.
An existing vent should never be blocked up on the strength of a "none required" answer.