What's the best boiler for a 3-bed house?

For a typical three bedroom house with one bathroom, a 28 to 30kW combi is the usual answer, and a straight swap in the same position is the cheapest version of that job. If you have two bathrooms used at the same time, a system boiler with a cylinder is generally the better call.

Why 28 to 30kW

In a combi, the headline output is about hot water rather than heating. A three bedroom house needs perhaps 12 to 15kW to heat, but a decent shower needs far more instantaneous power than that.

A 30kW combi delivers roughly 12 litres a minute, which comfortably runs one good shower. A 24kW model delivers around 10, which works but is noticeably weaker if someone runs a tap at the same time.

Going above 35kW in a three bed house is usually money wasted, unless your mains flow rate is unusually strong and you genuinely run two showers at once.

One bathroom or two?

This is the question that changes the answer, more than bedrooms do.

• One bathroom, one or two occupants: 24 to 28kW combi is plenty.
• One bathroom, family of four: 28 to 30kW combi.
• Two bathrooms, used at different times: 32 to 35kW combi, if mains flow supports it.
• Two bathrooms, used simultaneously: system boiler with a cylinder. A combi will disappoint here regardless of output.

Check your mains flow rate first

A combi cannot deliver more hot water than your mains supplies. If your incoming flow is 12 litres a minute, a 35kW boiler will not give you more than 12, and you will have paid several hundred pounds for output you cannot use.

Any installer quoting a combi should measure this. It takes two minutes with a flow cup on a cold tap. If nobody measures it, that is a reasonable reason to prefer a different quote.

What about a system boiler in a three bed house?

Worth considering if you already have a cylinder, if you have two bathrooms, or if your mains flow rate is poor.

Keeping an existing system rather than converting to a combi is both cheaper and less disruptive, because the cylinder stays where it is and the pipework does not need reworking. You keep reliable simultaneous hot water. You give up the airing cupboard.

The usual reason people convert is space. It is a legitimate reason, but it is worth being clear that you are buying a cupboard, not better hot water.

How many radiators can a 30kW combi run?

Comfortably more than a three bedroom house has. Heating demand for a typical three bed is in the region of 12 to 15kW, so a 30kW combi has ample capacity for ten to twelve radiators.

The output figure on a combi is about hot water delivery, not radiators. This is the single most common misunderstanding in boiler sizing, and it is why people are sold larger boilers than they need on the basis of house size alone.

What to check before you commit

• Mains flow rate measured, not assumed. Under 10 litres a minute changes the recommendation.
• Whether the gas supply pipe needs upgrading to 22mm. Common on older properties and a frequent mid-job surprise.
• Whether a system flush is included, and which kind.
• The warranty length and exactly what keeps it valid.
• Whether the quoted boiler physically fits where the old one was, including the flue route.

What drives the price

A straight combi swap in the same position is the cheapest version of this job. Moving the boiler within the room adds to it, because the gas, flue and condensate runs all move with it. Converting from a conventional system with a cylinder is the most expensive by some way. Get three quotes in your own postcode rather than trusting a national average: regional labour rates move the total more than most people expect.

If you are replacing something genuinely old, Energy Saving Trust currently puts the annual fuel saving at around £420 in Great Britain, on the basis of a G-rated gas boiler with no heating controls being replaced by an A-rated boiler with a full set of controls. A three bed house that already has working controls will see less than that, because part of the headline is the controls rather than the boiler.

Source: Energy Saving Trust, How to improve your EPC rating