Combi, System or Heat-Only: Which Boiler Suits an Edinburgh Tenement Flat?

Most boiler advice online is written for a three-bedroom semi with a loft, a garage and a garden. That is not what Marchmont, Bruntsfield, Gorgie or Leith are made of. In a tenement flat the choice between a combi, a system boiler and a heat-only boiler is decided by things a generic guide never mentions.

What the three types actually do

A combi heats water on demand and stores nothing. A system boiler heats a cylinder that stores hot water and delivers it at mains pressure. A heat-only, or conventional, boiler works with a cylinder and a cold water tank, usually in the loft or a boxed press, and relies on gravity for pressure. Most Edinburgh flats built with wet central heating started life on the third arrangement.

The flue usually decides it first

Before anything else, ask where the flue can go. A terminal has to sit in a legal position on an external wall, clear of windows, doors, boundaries and anything above it. On a tenement rear elevation with a stair window and neighbouring flats close by, the options narrow very quickly. That constraint frequently decides where the boiler can live, and the boiler position then constrains the type. It is why a proper survey beats a phone quote every time.

The cupboard question

The other driver is space. A cylinder cupboard in a two-bedroom flat is a real amount of storage, and getting it back is often the whole motivation for a change. A combi frees the cupboard and clears the header tank. But the flat has to be able to support it, which brings us to the mains.

Why we test the mains before recommending a combi

A combi delivers hot water at whatever the incoming mains can give it, at a fixed flow rate. If the supply into the flat is poor — and in older tenements with long shared runs and original pipework it sometimes is — you end up with your cupboard back and a worse shower than you had. We test pressure and flow before recommending a conversion, because that measurement is the difference between a good decision and an expensive regret.

When stored hot water is still the right answer

If the flat has two bathrooms, or a family that all shower in the same half hour, stored hot water wins. A system boiler with a properly sized unvented cylinder gives mains-pressure hot water at every outlet without a flow rate to share out, and it does not care how many taps are open. It costs more to install and it needs a home for the cylinder, but for the right flat it is the better system by a distance. Our hot water cylinders page explains how vented and unvented compare.

And the practical one: getting it up the stair

A fourth-floor flat with no lift means the old boiler comes down and the new one goes up by hand. It is not a reason to choose one type over another, but it is a real part of the job and it belongs in an honest quote rather than in a surprise.