Combi or system boiler for a Sheffield home?

When a boiler needs replacing, the big choice is usually between a combi and a system boiler. Both are good. The right one depends on the size of your household, how many bathrooms you have, and something Sheffield homes vary on a lot: how much water your mains supply can deliver.

How each one works

A combi boiler heats water for the radiators and heats tap water on demand. There is no hot water cylinder and no tank in the loft. Turn on a hot tap and the boiler fires up and heats the water as it passes through.

A system boiler heats water for the radiators and heats a separate hot water cylinder. The cylinder stores hot water ready to use. Most new system boiler setups use an unvented cylinder, fed straight from the mains, so there is no tank in the loft.

There is a third option, a regular boiler, which works with a vented cylinder and cold water tank in the loft. It suits some older homes where you want to keep the existing layout.

Side by side

	Combi	System with unvented cylinder
Space	Boiler only, no cylinder or loft tank	Boiler plus a cylinder, usually in a cupboard
Hot water	On demand, as long as the boiler runs	Stored, so it can run out if a lot is used
Two showers at once	Usually weak unless flow is very good	Usually fine, if mains flow is good
Depends on mains flow	Very much	Very much, for unvented
Best for	One bathroom, smaller households	Two or more bathrooms, larger households
Backup if boiler fails	None	Immersion heater in the cylinder

Why mains flow matters so much in Sheffield

Both a combi and an unvented cylinder rely on the mains supply. A combi can only heat as much water as comes in. An unvented cylinder can only deliver its hot water at the pressure and flow the mains gives it.

In Sheffield, that varies by street. Yorkshire Water’s water pressure guide explains that homes closer to the height of the storage tank get lower pressure, and that it must maintain at least seven metres of static head. On the Crookes ridge, at the top of Walkley or in parts of Stannington, flow can be lower than you expect. Old lead supply pipes, which are still common in pre 1970 homes, can choke the flow further.

Before recommending either boiler, a good installer measures the flow at the kitchen cold tap and checks the pressure. If the flow is weak, a combi will give a disappointing shower and an unvented cylinder may not be allowed or may not perform. In that case, a regular boiler with a vented cylinder and pumped shower, or a new, larger supply pipe, may be the better answer.

Our Crookes area page covers pressure on that ridge in more detail.

Typical Sheffield homes

Two or three bedroom terrace, one bathroom

Most suit a combi, as long as flow is decent. It frees up the airing cupboard and the loft, and suits how most couples and small families use hot water.

Student house with five or six people

This is where a combi often struggles. Five people showering between 8am and 9am means one shower at a time, with each person waiting. A system boiler with a good sized unvented cylinder lets two showers run at once and stores enough for a busy morning, if the mains can feed it. Landlords in Broomhill, Crookes and Walkley often find this solves complaints about lukewarm showers.

Large Victorian villa, two or three bathrooms

Nether Edge and Broomhill villas often suit a system boiler with an unvented cylinder, or a regular boiler if the existing tanks and pipework are worth keeping. Long pipe runs to a top floor bathroom mean a combi would take a long time to deliver hot water there.

Interwar or post war semi, one or two bathrooms

Depends on the household. A family with one bathroom often suits a combi. Add an en suite and a busy morning, and a system boiler starts to make more sense.

Water hardness

In west Sheffield, where Yorkshire Water reports very soft water, scale in a combi’s heat exchanger is rarely a problem. In Handsworth and Rotherham, where the water is moderately hard to hard, a combi benefits from a scale reducer on the cold feed. Our post on Sheffield water hardness has the figures.

Rules that apply either way

Any new gas boiler must be fitted by a Gas Safe registered engineer, who notifies the work so you receive a Building Regulations compliance certificate. New boilers in England must be condensing models with time and temperature controls. An unvented cylinder must also be fitted by someone competent in unvented systems.

Getting the choice right

The best way to decide is a survey that measures flow, counts bathrooms and asks how you use hot water. Our new boiler page explains how our quote works. Tell us how many people live in the house and how many bathrooms you have, and we will tell you which way we would lean before we visit.