Is my boiler the right size for this house?

Most people ask this question the wrong way round. They ask whether the boiler is big enough, having spent years being told that a bigger one is a safer bet.

In a British house with a combi in it, the far more common fault is the other direction. The boiler is too big for the heat the house actually loses, and the symptoms of that read to a homeowner like an unreliable appliance rather than a sizing mistake.

Why combis end up oversized

A combi does two jobs with one burner. It heats the house, and it heats water on demand at the tap. Those two jobs need very different amounts of power, and the hot water job needs far more.

So the size gets chosen on the hot water. Somebody wants a decent shower, the output goes up to deliver it, and the heating side inherits a burner sized for a job it only does for a few minutes a day. On a cold January night that is fine. In October it is the problem.

What it looks like from the sofa

Oversizing does not announce itself. It shows up as a pattern, and the tell is that everything is worse in mild weather than in cold weather, which is the opposite of what people expect from a struggling boiler.

• The burner lights, runs for a couple of minutes, stops, then lights again shortly afterwards, over and over. This is called short cycling.
• Radiators get hot quickly, then go lukewarm, then get hot again, so rooms never settle at a steady temperature.
• The house is comfortable enough, but running costs are not what you were told to expect, because every start is less efficient than steady running.
• In the depths of winter it behaves better, because the house is finally losing heat as fast as the boiler is making it.
• Over years, more wear on the parts that work hardest at the start of a cycle, which is where ignition faults and fan faults come from.

Before you blame the size, rule out the impostors

Short cycling has several causes and only one of them is the boiler being too big. It is worth being honest about this, because replacing a boiler that is cycling for a different reason buys you a new boiler that cycles.

The common alternatives are all restrictions on how fast water can move through the appliance, rather than anything to do with its output:

• A blocked or heavily loaded system filter, or sludge in the radiators and pipework, slowing the flow through the heat exchanger.
• A pump set too low, failing, or a pump speed changed at some point and never put back.
• Too few radiators open. Thermostatic valves all closing at once in mild weather leaves the boiler nowhere to send its heat.
• A room thermostat in the wrong place, in a hallway by the front door or above a radiator, giving the boiler a false picture.
• An automatic bypass or a system layout that was never set up properly after a previous replacement.

The rule almost nobody quotes at you

Here is the part that makes this a quote question rather than a curiosity. The Building Regulations guidance that came into effect in June 2022 changed how a boiler is supposed to be chosen, and the part that applies to a straight boiler swap is not about the maximum output at all.

For complete new system installations the guidance asks for a proper heat loss calculation for the dwelling, a recognised system sizing methodology such as the Chartered Institute of Plumbing and Heating Engineering's design guide, and a system designed to run at a 55 degree flow temperature. For a boiler swap it asks something narrower and more interesting: where a combination boiler is used, the boiler type should be selected so that it can modulate down to the typical heating load of the dwelling.

Read that again, because it is the whole article. The requirement is about how far the boiler can turn itself DOWN. A burner that cannot throttle back below what your house needs on an ordinary day has no choice but to fire and stop, fire and stop. That is not a fault. That is the appliance doing the only thing available to it.

Source: Baxi, Building Regulations for Boiler Installation; Approved Document L, Conservation of fuel and power

So what does undersized actually look like

It exists, it is just rarer, and it looks nothing like short cycling. An undersized boiler runs and runs and the house still does not reach the temperature on the thermostat, and it only does that on the coldest handful of days in the year. If your heating copes all winter and only struggles in a hard frost, that is closer to a design margin question than a fault.

On the hot water side, undersizing shows as a shower that goes cool when a tap is opened elsewhere, or as poor flow in winter when the incoming mains is colder and the boiler has more work to do to reach temperature. That is often a mains flow question rather than a boiler question.

What can be done without replacing anything

If the boiler is oversized but otherwise healthy, replacing it is not the first move. There are settings inside the appliance that change how it behaves, and they are engineer settings rather than anything a homeowner should be adjusting.

Ask your engineer, at the next service, about three things: whether the boiler can be range rated down so its maximum heating output is closer to the house's actual demand, what the flow temperature is set to and whether it can come down, and whether weather compensation or a load compensating control would suit the system. Those three between them fix a lot of what looks like an unreliable boiler.

Anything involving the gas circuit itself belongs to a Gas Safe registered engineer, always, without exception.

The question to put on your next quote

When the day comes to replace it, this is where the article earns its keep. Most quotes name a boiler and an output and stop there, and most homeowners compare the two numbers and assume the bigger one is the better buy.

Ask for three things instead, and note who answers without hesitating:

• What heat loss figure did you use for this house, and how did you arrive at it?
• What is the minimum modulated output of the boiler you are proposing, not the maximum?
• Is the size you have chosen driven by the heating or by the hot water flow rate I asked for, and what is the trade off?

Why this is worth the trouble

An installer who has done a heat loss calculation is quoting your house. An installer who has read the badge on the old boiler and matched it is quoting the last installer's guess, which may itself have been a guess about the guess before that. Houses get insulated, windows get replaced, extensions get built, and the number on the old casing stops describing the building years before anyone notices.

It is also the fairest way to compare two quotes that name two different boilers. The appliance is the most comparable line on a quote and the least likely reason two prices differ, but the sizing work behind it tells you a great deal about who did the thinking.

If you would like that thinking done properly, tell us about your property and we will arrange up to three quotes from vetted local installers who survey before they price.