The traditional answer is simple… when you feel cold.
But some of the most efficient buildings and heating systems don’t wait until occupants reach for a jumper. In fact, many larger buildings and weather-compensated heating systems begin adding small amounts of heat during September or even August to help maintain the heat already stored within the building.
That might sound wasteful, but it is often doing the opposite.
Buildings Are Thermal Batteries
Throughout spring and summer, a building absorbs heat from the sun and surrounding air. Walls, floors and furniture gradually store this energy, much like a battery stores electricity.
As autumn arrives, that stored heat slowly begins to leak away. At first, the losses are small and largely unnoticed. Eventually, however, the building cools enough that occupants start to feel uncomfortable.
For many people, this is the point at which the heating is switched on.
The problem is that the building’s thermal battery is already partly empty and so requires more energy to refill it.
The Cost of Waiting
Most people think heating only warms the air. In reality, a large proportion of the energy is used to warm the building itself.
If a property is allowed to cool significantly, the heating system must warm not only the air but also the walls, floors, ceilings and contents of the building. In heavyweight buildings such as churches, stone cottages and other traditional buildings, this process can take days rather than hours.
Autumn also introduces another challenge: moisture.
As temperatures fall, the air across Britain often becomes cooler and wetter. Internal surfaces become colder, humidity levels rise and condensation becomes more likely. Traditional buildings can also absorb moisture from the environment into the fabric as rainfall increases and drying conditions reduce.
This means that by the time occupants feel cold, the heating system may be trying have two additional tasks.
- Re-heating the building fabric
- Evaporating the additional moisture that’s been accumulated
Before a damp building can warm up efficiently, some of the heat supplied by the heating system is used to dry surfaces and evaporate moisture from the building fabric. Damp materials also tend to feel colder and conduct heat more readily than dry ones, making the building less comfortable and more expensive to heat.
The Smarter Approach
Many modern weather-compensated heating systems work differently.
Rather than waiting for occupants to become uncomfortable, they monitor outdoor temperatures and gradually add small amounts of heat as autumn progresses. The aim is to slow the building’s heat loss, keep surfaces above condensation temperatures and prevent the thermal battery becoming depleted in the first place.
The result is often:
- More stable internal temperatures
- Improved comfort
- Reduced condensation risk
- Drier building fabric
- Lower peak heating demand
- Less energy spent reheating the building
The heating may run for more days of the year, but often at a much lower output.
An ever increasing number of buildings are heated using Air Source Heat Pumps, they make great use of this early heating as the warmer weather means they can have very high efficiencies and often electricity tariffs can be cheaper in the autumn before the depths of winter when the demand for natural gas greatly increases.
So When Should the Heating Turn On?
There isn’t a specific date. The real question isn’t:
“Am I cold yet?”
Instead, it might be:
“How quickly is my building running out of it’s summer heat?”
For a very lightweight modern home, waiting until October may make little difference. For a large church, solid-wall property or traditional stone building, maintaining a small amount of background heat can often be more effective than allowing the building to cool down and then trying to reheat it later.
A building doesn’t suddenly become cold in October.
It spends September quietly running out of stored heat from the summer.
How Woohoo Can Help
At Woohoo, we help building owners understand how their buildings actually behave, rather than relying on rules of thumb.
By looking at thermal mass, insulation, ventilation, moisture and heating controls, we can identify how a building stores heat, how quickly it loses it and whether the current heating strategy is working with the building rather than against it.
Sometimes the solution isn’t turning the heating on earlier or later.
It’s understanding how the building stores heat, manages moisture and stays comfortable throughout the year.