What a boiler replacement actually involves

Sizing, which almost everyone gets wrong

The standard approach is to read the output off the old boiler and fit something similar. It is quick, it is what most quotes are based on, and it is usually wrong – because the old one was very likely oversized when it went in, and the house has probably had insulation, glazing or draught-proofing since.

An oversized boiler does not heat the house faster in any useful sense. It short-cycles – fires, satisfies demand, shuts down, cools, fires again – spending its life in the least efficient part of its modulation range, and frequently running at return temperatures too high to condense, which is the specific thing you paid extra for.

On a combi, hot water demand usually sets the output because instantaneous heating requires far more power than radiators do. On a system boiler with a cylinder, the space heating load usually sets it. Either way it comes from a calculation, and the calculation takes an hour.

The gas supply check nobody does

A modern boiler needs a specified gas rate at a specified pressure. Older or long pipe runs cannot always deliver it, particularly when the new appliance has a higher output than the one it replaces.

The check is straightforward – a pressure gauge at the appliance with everything running – and it should be part of every survey. Skipping it produces a boiler that never reaches its rated output, locks out under load, and gets blamed for being a poor appliance.

Where the pipe is inadequate, the answer is upsizing the run, or occasionally a meter upgrade through the gas transporter. Either way it belongs on the quote rather than being discovered on the day. See gas supply and pipe sizing.

Flue and condensate

Flue

Terminal positions are governed by minimum clearances to openings, boundaries, adjacent surfaces and other terminals. Those clearances frequently constrain where the boiler can go, rather than the other way round, and it is far better to establish that at survey than after the old appliance is out.

Plume is also a consideration. A condensing boiler produces a visible plume of water vapour, and it goes somewhere – across a path, a window, or a neighbour’s fence, unless somebody thinks about it. Plume management kits exist and they are a design decision rather than an afterthought.

Condensate

Condensing boilers produce acidic condensate that must drain away, and where that pipe goes is the single most common cause of trouble in the years after an installation.

• Internal termination wherever the building allows it. A condensate pipe that never goes outside cannot freeze. This should be the default and it is frequently not.
• Where it must go outside: the larger pipe size, the shortest possible run, proper insulation, a generous continuous fall, and a sensible termination.
• Not into a soakaway unless it is a purpose-made one to the manufacturer’s requirements.

See frozen condensate pipes – the problem and the fixes are identical on gas.

System cleaning, and why it is not an upsell

Every manufacturer requires the heating system to be cleaned to BS 7593 and dosed with inhibitor before a new appliance goes on it, and most now require a magnetic filter as well.

The reason is straightforward. A modern condensing heat exchanger has very narrow waterways, because narrow waterways transfer heat well. That makes them exactly the wrong shape for tolerating the magnetite that circulates in a dirty system. A new heat exchanger connected to an unclean system is a new heat exchanger with a short life.

Manufacturers decline warranty claims on heat exchangers fouled by system debris, and they are entitled to. A quote that omits system cleaning has not saved you money – it has transferred a risk to you without mentioning it.

Whether that clean needs to be a full power flush or a chemical clean depends on how dirty the system actually is – see power flush, chemical flush, or neither.

Controls

Boiler Plus requirements mean a replacement gas boiler in England must have time and temperature control, and for a combi one additional efficiency measure from a defined list.

The options are not equivalent. Weather or load compensation lets the boiler run at the lowest flow temperature that will heat the house, which is exactly where a condensing boiler is most efficient. Other measures are legitimate and deliver less.

Ask what is being fitted to meet the requirement and why. And ask whether the control actually modulates the boiler or simply switches it on and off, because that distinction determines whether it saves you anything – see heating controls, TRVs and smart thermostats.

The day itself, and what you should be left with

1. Isolate and drainGas, electrics and water. The system comes down.
2. Old appliance outAnd taken away for disposal – confirm this is included.
3. System cleanChemical or power flush as assessed, before the new appliance is connected.
4. New boiler fittedWith flue and condensate installed to the manufacturer’s instructions.
5. Gas and electrical workIncluding any pipe upgrade and the tightness test.
6. Filter and inhibitorMagnetic filter fitted, inhibitor dosed to the system volume.
7. Controls fitted and configuredNot left on factory defaults.
8. CommissionGas rate and combustion checked with an analyser, readings recorded, flow temperature set sensibly.
9. Balance the systemThe step most commonly skipped, and the one that determines whether the far rooms get warm.
10. HandoverIn person, with the paperwork.

You should end up holding: a Building Regulations compliance certificate, the manufacturer’s instructions, a completed benchmark record with gas rate and combustion readings, warranty registration done rather than left for you, and a record of the system clean and the inhibitor used.