Ventilating a flat when you cannot fit a vent

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No vent means scheduled ventilation, not continuous ventilation

Ventilation guidance divides the job in two. Background ventilation is a small, permanent flow that runs whether anyone is thinking about it, normally through trickle vents or a continuously running extract unit. Purge ventilation is a large flow for a short time, normally through an opened window, used to clear a specific load such as a shower, a pan of boiling water or a night of two people breathing in a closed bedroom. A flat with no permanent opening has lost the first category entirely. Its only background flow is adventitious: the gaps around a door, a letterbox, an unsealed floor void, a disused chimney. Those are uncontrolled, they shrink every time anything gets draughtproofed, and they shrink most on the still, cold nights when moisture load is highest.

Two consequences follow. The first is that your routine is now load bearing, because nothing happens unless somebody does it. The second is that the places where moisture accumulates stop being obvious: a wardrobe against an outside wall, the back of a bed headboard, the underside of a windowsill. Water vapour arrives wherever people are and condenses wherever the surface is coldest, and those are rarely the same room.

Carbon monoxide is not carbon dioxide, and the difference decides your monitor

This trips up more purchases than any other confusion in the category, and it is worth being blunt. Carbon dioxide is the gas people exhale, it is harmless at indoor concentrations and it is an excellent proxy for how much fresh air a room is getting. Carbon monoxide is a product of incomplete combustion, it is acutely poisonous, and it has nothing to do with ventilation rate. A monitor that reads one does not read the other.

So check the bullet list, not the category name. The Amazon Smart Air Quality Monitor states that it tracks five factors: particulate matter at 2.5 micrometres, volatile organic compounds, carbon monoxide, humidity and temperature. Carbon dioxide is not among them. That is a capable instrument for particles and for combustion safety, and it is colour coded and will push an alert to a phone, but it cannot perform the decay test described below because it cannot see the gas the test uses. The two pack of the same monitor states the same five factors per unit, which makes it useful for watching two rooms at once and no more useful for a ventilation rate. Neither listing gives a detection range or an accuracy figure for any of the five factors, so those are Not stated in the listing bullets. If you need to know how many air changes a room gets, you need an instrument whose bullets say carbon dioxide, and why does my carbon dioxide monitor spike when people enter shows what that measurement looks like minute to minute.

Reading an air change rate off a decay curve

Here is the whole method. Let the room build up carbon dioxide with people in it, note the reading, then remove the source: everyone leaves, or at minimum nobody comes in. Open whatever you are testing. Note the reading again after a measured interval. Because the gas now has only one way out, the excess above outdoor concentration decays exponentially, and the air change rate per hour is the natural logarithm of the starting excess divided by the finishing excess, divided by the elapsed time in hours.

Use 420 parts per million as the outdoor baseline unless your monitor tells you otherwise. Worked examples, all with that baseline.

Run that test three times: with everything shut, with your current routine, and with the best arrangement you can manage. The three numbers tell you how much of the problem is the building and how much is the habit. A bottom row result of a quarter of an air change per hour is a flat that cannot dilute anything, and no amount of wiping windows will fix it. A top row result shows that the fabric is capable of plenty of ventilation when two openings are available, which turns the problem into one of scheduling. Airflow at a given opening can be measured directly as well, and how to measure air velocity in a vent covers that route.

How deep a single open window reaches

One window on one wall is the hardest case in ventilation design, because the same opening has to serve as both inlet and outlet. Air enters low and leaves high, driven by the small temperature difference across the opening, and the circulation it sets up reaches only a limited distance into the room. The usual design rule of thumb is that single sided ventilation serves a depth of about two to two and a half times the floor to ceiling height, so a room with a ceiling at 2.4 metres is reasonably served to roughly five or six metres back from the window and poorly served beyond that. Openings on two different walls change the mechanism entirely, because wind pressure difference now drives a through flow, and the same rule of thumb stretches to about five times the ceiling height.

That arithmetic explains a lot of flats. A long single aspect living room with a kitchen at the dark end will never ventilate its far end from the window, however long it is left open. The fix is a path rather than a bigger opening: prop the internal door, open a window at the other end of the hall, and let the flat work as one volume instead of several sealed boxes. The decay test will confirm it in twenty minutes.

Trickle paths that do not need a core drill

• Undercut or vent internal doors so the extract fan you already have can draw from the rooms that need it. An extract point with no makeup air path moves very little.
• Use the existing wet room fan deliberately, running it with the bathroom door open after a shower rather than only during one.
• Slot vents cut into an existing window frame avoid touching the wall, which is often the part a lease or a listing protects.
• Open a chimney that has been blocked rather than removed, with a cap and a guard, if the flue is sound and unused.
• Keep furniture off outside walls so that the air that is moving can reach the surfaces that get cold.
• If a fan can be fitted somewhere, a controller can run it against a measurement rather than a clock. The Inkbird ICC-510 carbon dioxide controller states dual outputs that drive gas equipment and a ventilation system together against a carbon dioxide setpoint, with a light sensor and a 100 to 240 volt supply. It is sold for grow rooms and its control logic is built around raising carbon dioxide for plants, so treat it as an example of setpoint control rather than as an occupant ventilation product.

A ventilation routine you can actually keep

Measure first, then design the habit around what the measurement showed. A purge that reached three air changes per hour in twenty minutes is worth doing twice a day; one that reached a quarter of an air change is not worth doing at all and tells you the conversation needs to be with a freeholder. Put the monitor in the room that gets coldest rather than the one you sit in, watch the overnight trend rather than the instantaneous number, and re run the decay test after any change so you know whether it helped. And keep combustion safety separate from all of it, because the gas that matters there is a different gas on a different sensor with a different service life, as covered in when to replace a gas detector and how many gas detectors do you need at home.

Frequently Asked Questions