Part 6

Ventilation, Chimneys & ACH

Ventilation, Chimneys & Air Changes Per Hour

So far, we’ve talked a lot about heat escaping through solid stuff — walls, floors, windows, roofs.

Now it’s time to look at what happens when your heat escapes because of moving air. We're talking about:

  • Ventilation (the stuff you actually want)

  • Infiltration (the stuff you didn’t ask for)

  • Chimneys, flues, and open appliances (the heat-stealing culprits)

  • And of course, air changes per hour (ACH), the key to calculating it all

Your Home Is Not Airtight (And That’s a Good Thing)

Every building needs fresh air. Without it, we’d get damp, mould, and a stuffy, unhealthy environment. So a bit of air change is essential — but from a heating perspective, every bit of air that leaves the house has to be replaced with cold air from outside… and that air needs to be heated up.

So we walk a fine line: enough air to keep things healthy, not so much that your boiler or heat pump runs a marathon every day.

Ventilation vs Infiltration: What's the Difference?

Ventilation = intentional

  • Extractor fans

  • Trickle vents

  • MVHR systems

  • Opening windows

Infiltration = unintentional

  • Draughts

  • Gaps under doors

  • Leaky floorboards

  • Chimneys and flues that aren’t sealed properly

We include both when calculating ventilation heat loss — and we express the total as air changes per hour (ACH).

Fun fact: More than 80% of UK homes are heated with gas — one of the highest rates in Europe.

What Is ACH, and Why Does It Matter?

ACH = how many times the volume of air in a room is replaced in one hour.

If your lounge has a volume of 50 m³ and an ACH of 1.0, it means the entire air content is replaced every hour — and all that incoming air needs heating to 21°C (or whatever your setpoint is).

We use this number in a key formula to calculate how much heat is lost through air movement:

Ventilation Heat Loss (W) = 0.33 × V × N × (Tin – Tout)

Where:
V = Room volume (m³)
N = Air changes per hour (ACH)
Tin = Indoor temp (°C)
Tout = Outdoor temp (°C)

Example: Calculating Heat Loss from Ventilation

Let’s say your kitchen has a volume of 40 m³, an indoor temp of 21°C, and we expect 1.5 air changes per hour (due to extraction, gaps, etc). Outdoor temp: –3°C.

Plug into the formula:

0.33 × 40 × 1.5 × (21 – (–3))
= 0.33 × 40 × 1.5 × 24
= 475.2 W

That’s just for air movement — the fabric heat loss gets added on top.

Typical ACH Rates (Rule of Thumb)

A chart comparing different building types and their typical air changes per hour, from passive house to open chimneys.

Note: These are approximations, and the final number should reflect things like extractor fans, window trickle vents, fireplace openings, etc.

Chimneys, Flues & Open Appliances

This is where things get a bit sneaky.

If your home has an open chimney, unused flue, open-flame gas fire, or an open combustion appliance, they can act like heat chimneys — sucking warm air out and pulling cold air in.

Some key points:

  • An open chimney can add +0.5 to +1.0 ACH (depending on size and condition)

  • A cooker hood venting directly outside can pull air even when switched off if there’s no backdraft damper

  • Old gas fires or back boilers often rely on air from the room and flue it outdoors — again, contributing to air movement

  • Solid fuel stoves with open air intakes can affect the overall infiltration rate

Rule of thumb: if the house has a flue or chimney that isn’t properly capped or sealed, it’s leaking heat — and we need to allow for it in the ACH value.

Intermittent Heating Makes It Worse

If your system only heats for short bursts, you get a stop-start effect:

  • Room warms up

  • Air expands and escapes through gaps

  • System stops

  • Cold air seeps back in

  • System starts again

That yo-yo cycle increases heat loss and energy use. It’s another reason why low-temperature, continuously-modulated heating (like a modern condensing boiler or heat pump) works better — it keeps the thermal envelope steady and the ACH impact lower over time.

What to Remember

  • Ventilation + infiltration = total air change

  • The more air that comes in, the more energy we need to heat it

  • We express this in ACH, then calculate the watts lost using the ventilation heat loss formula

  • Chimneys, flues, extractor fans and gaps = sneaky heat thieves

  • Intermittent heating increases losses from air exchange