What's the best boiler for a 4-bed house?

Short answer: if your four bedroom house has one bathroom, a high output combi is usually still the right call. If it has two bathrooms that get used at the same time, or an en-suite plus a family bathroom and teenagers in the house, a system boiler with a cylinder will make you happier than any combi will, whatever the kW figure on the box says.

Bedrooms are not really what decides it. This article is about the things that do, and about the two site checks that should happen before anyone recommends a boiler to you.

Why the bedroom count is the wrong measure

Bedrooms tell an installer roughly how much heat the house loses, and heat loss is the easy part. A four bedroom semi might need somewhere in the region of 15 to 20kW to heat, which almost any modern boiler can do standing on its head.

What actually strains a boiler is hot water, and hot water demand tracks bathrooms and habits, not bedrooms. Four bedrooms with one bathroom and two occupants is a gentle job. Four bedrooms with a family bathroom, an en-suite and four people trying to leave the house at eight in the morning is a completely different problem, and it is the one that gets solved badly.

So the first question is not what size boiler. It is how many hot taps or showers might realistically run at once, and whether anyone would be irritated if the pressure dropped when they did.

The mains supply check that decides everything

A combi cannot deliver more water than your mains brings into the house. That sentence is the whole of combi sizing, and it is why a 40kW combi on a weak supply is a waste of money.

Someone quoting you should measure the incoming cold flow rate at the kitchen tap. It takes two minutes with a jug and a stopwatch, and you can do it yourself before anyone visits. Under about 10 litres a minute, a big combi has nothing to work with. Twelve to fifteen is comfortable for one strong shower. To get two decent showers running simultaneously off a combi you need a genuinely strong supply and a boiler built for it, and plenty of houses simply do not have the supply.

The same check matters if you are going the cylinder route, because an unvented cylinder has its own minimum. Manufacturers typically specify around 1.5 bar working pressure and about 20 litres a minute at the incoming main before an unvented system will perform as intended. If your supply is below that, the honest options are a vented cylinder, a pumped system, or accepting the limitation, and it is much better to hear that before the boiler is ordered than after.

Source: UK Cylinders, indirect unvented cylinder installation guidance

If a combi is the answer: what comes with it

High output combis are heavier on gas than the boiler they replace, and that has a consequence people are rarely warned about. The gas supply pipe has to be sized so that the pressure drop between the meter and the appliance stays within limits at full demand, with every other gas appliance in the house running. BS 6891 sets that out for domestic pipework, and the working figure engineers design to is a maximum drop of 1 millibar from meter to appliance.

In practice that means a lot of older houses need the run from the meter upgraded, commonly to 22mm, sometimes larger where the run is long or full of bends. Four bedroom properties tend to have long runs, because the meter is by the front door and the boiler is in a utility or a loft at the other end of the house. This is the single most common mid-job surprise on a large combi swap, and it is entirely predictable if someone checks properly at survey.

Two other things travel with a big combi. The condensate has to drain somewhere frost-safe, which is a real design question when the boiler ends up in a cold loft or garage. And the flue route has to work from the new position, including the clearances from windows, doors and the boundary.

Source: BS 6891, low pressure gas installation pipework

If a system boiler is the answer: sizing the cylinder, not the boiler

Once there is a cylinder in the picture, the boiler output stops being about showers and goes back to being about heating the house, which is why system boilers in four bedroom homes are often smaller than the combi that would have replaced them. The number that decides whether you are happy is the cylinder size.

The Hot Water Association guidance is to allow roughly 35 to 45 litres of stored hot water per person, which puts a typical four or five person household in the region of a 200 to 250 litre cylinder. The better rule, and the one worth insisting on, is to size the cylinder to the house rather than to the people currently in it. A couple living in a four bedroom, two bathroom house should still get a cylinder that suits the property, because the cupboard will not get any bigger when the household does.

An unvented cylinder over 15 litres is notifiable work and has to be installed, commissioned and serviced by someone holding the G3 qualification, on top of being Gas Safe registered for the boiler. It is a fair question to ask, and a competent firm will not mind being asked.

• Ask what cylinder capacity is being quoted, in litres, not just the boiler model.
• Ask whether it is vented or unvented, and who on the team holds G3.
• Ask where the expansion vessel and the discharge pipe from the safety valve are going.
• Ask what the reheat time is, because that is what governs the second shower.

Source: Hot Water Association, sizing a hot water cylinder; Approved Document G, unvented hot water storage

The case for leaving a regular system alone

Plenty of four bedroom houses around Northampton and Milton Keynes still have a regular boiler, a cold tank in the loft and a cylinder in the airing cupboard. The reflex is to rip it all out and fit a combi. Sometimes that is right. Often it is not.

If the house has two bathrooms and the cylinder is doing its job, replacing the boiler alone is a smaller, cheaper, less disruptive job that keeps the thing you actually want, which is reliable hot water at more than one tap. Converting to a combi buys you loft space and an airing cupboard. It does not buy you better hot water in a house of that size, and quite often it takes some away.

The honest version of the advice is this. If you are converting, be clear you are buying storage space and simplicity, and price the loss of simultaneous hot water into that decision. If you are not bothered about the cupboard, keep the system type and spend the difference on a better boiler and a proper flush.

What we see locally

Two patterns come up repeatedly in the four bedroom stock around here. The first is the extended semi or detached with a loft conversion and a shower room added upstairs. The heating was designed for the original house, the new shower room went in later, and nobody revisited the hot water. That house needs the hot water rethinking, not just a like-for-like boiler.

The second is the newer estate house that came with a system boiler and a modest cylinder from the developer. Those cylinders are frequently sized to the minimum that satisfied the specification rather than to the way a family actually lives. When a household on one of those complains about hot water, the boiler is usually not the problem.

In both cases the useful thing an installer can do is spend twenty minutes measuring and counting rather than five minutes quoting a model number. If a quote arrives without anyone having measured your flow rate or counted your radiators, treat it as a guess.

What to have straight before you commit

• Your measured mains flow rate in litres per minute, taken at the kitchen cold tap.
• How many bathrooms and showers, and whether two ever run at once.
• Whether the gas run from the meter needs upgrading, and whether that is in the quote or excluded.
• If a cylinder is proposed: its capacity in litres, vented or unvented, and who holds G3.
• Where the boiler is going, and whether the flue and condensate routes work from there.
• What kind of system clean is included, and the warranty length with what keeps it valid.

The short version

One bathroom in a four bed house, and a decent mains supply: a high output combi is usually the sensible choice, with the gas run checked properly. Two bathrooms in real simultaneous use: a system boiler with a cylinder sized to the property will serve you better than any combi, and the cylinder is the number that matters. Weak mains: that constrains both routes, and it needs measuring before anyone recommends anything.

Whatever the answer turns out to be, the quality of the survey predicts the quality of the outcome far better than the badge on the boiler does.

If you want quotes from Gas Safe registered installers working around Northampton and Milton Keynes who will measure before they recommend, we can arrange them.