Heating ninety square metres

Ask a family how many bedrooms they need and they will answer for the busiest week of the year. This is entirely reasonable — the busiest week is the one where the house visibly fails if it is too small — and it produces houses that are wrong for the other fifty-one.

The clients at Corran House are two people. At Christmas they are nine. Designing for the average would have produced a house that suits nobody; designing for the peak would have produced two hundred and forty-five square metres to heat all winter for a couple.

Three volumes, three thermostats

The house is three linked timber volumes sitting on one plinth.

The centre volume holds the kitchen, the hearth and one bedroom. It is the year-round house, about ninety square metres, and it functions completely on its own — its own entrance, its own bathroom, its own heating circuit.

The two flanking volumes hold the additional bedrooms and a workroom. Each has its own circuit and can be dropped to a maintenance temperature of around twelve degrees, enough to protect fabric and services and nothing more, at a single valve.

For most of the year the clients heat ninety square metres. At Christmas they heat two hundred and forty-five.

The numbers

Splitting the servicing cost about eleven thousand pounds more than a single-zone system would have: three circuits, additional valving, a slightly larger buffer, more commissioning time.

The modelled saving, against heating the whole house to twenty degrees through a Skye winter, was around eight hundred and fifty pounds a year at the tariffs current at completion. Measured consumption in the first full year came in modestly better than the model.

That is a payback somewhere around thirteen years on the raw arithmetic, which is not spectacular. But the raw arithmetic understates it, because the alternative was not a single-zone house of the same size. The alternative the clients were seriously considering was a smaller house plus an annual argument about where everyone sleeps.

Zoning only works if the fabric is good

None of this would function in a leaky building. Zoning an unsealed house simply moves heat from the warm zones to the cold ones through the fabric and through the plan, and you end up heating everything badly.

Corran House tested at 0.42 air changes per hour at fifty pascals, which is well inside Passivhaus territory, and the party constructions between volumes are insulated as though they were external walls. That last detail is the one most often missed: if you are going to run one part of a house at twelve degrees and another at twenty, the wall between them is doing external-wall work and needs to be built accordingly.

It also needs to be detailed for the vapour conditions that arise across an eight-degree difference, which is not a condition most residential details are drawn for.

The behavioural part

The part we could not model is whether the clients would actually use it.

Zoned systems have an unhappy history of being commissioned beautifully and then left in whatever state the installer walked away from. We spent an afternoon at handover going through it, and — more usefully — put the three valves in one accessible cupboard with a printed plan on the door showing which volume each one serves.

Not an app. A laminated drawing and three clearly labelled valves. Eighteen months on, the clients report using it seasonally without thinking about it, which is the only measure of a control strategy that matters.

Where else this applies

The logic generalises to any building with a large, predictable variation in occupancy: village halls, small churches, rural schools, holiday accommodation, most community buildings.

In almost all of those, the peak defines the size and the average defines the running cost, and the two are rarely designed against each other. Splitting the plan into independently servable pieces is not an elegant move architecturally — it constrains the section and complicates the circulation. It is frequently the difference between a building the client can afford to open and one they cannot.