Edge Technical Director, Will Soden, assesses the climate credentials of a familiar technology.

Consider the combi boiler; you probably have one at home.  Well over a million are sold every year in the UK, and over 90% of all boiler sales are of the combi type; so you’re in good company.  It’s very likely a condensing one, and, unless it’s from the cheaper end of the market, nowadays fairly reliable.  It has the well-known drawbacks: if something breaks, there’s no heating or hot water; and it can only run one shower at once, despite what the manufacturer tells you.  Thanks to its design and electronics, it’s also pretty efficient, or at least, you think it is.  But it is really?  My contention is that actually the modern combi boiler is a resource-wasting, climate-damaging chimera, often sold to householders by unscrupulous installers after making a quick buck, and foisted on new builds by greedy developers whose bottom line is ever so slightly yet unacceptably affected by the loss of chargeable square footage taken up by a cylinder cupboard.

Here’s why: human nature.  Human nature dictates that we almost instantly become bored of waiting for the hot water to come through when we’re washing our hands, so we settle for the lukewarm water in the pipes – which may well be there from when the last person made the same decision.  Human nature dictates that we want massive shower heads, high flow showers and designer mixer taps in our bathrooms and kitchens, with which we mix the hot water the combi has just heated up with cold water, to make it bearable on our hands and bodies.  Human nature dictates that we use the comfort setting on our combi (where the boiler keeps the insides hot) so we don’t have to wait so long; what we don’t appreciate is that thanks to the balanced flue, these components are effectively outside the house.

Human nature means that we waste energy and resources (gas, electricity and water) in short cycling our combi boilers, not to mention the increased wear and tear.  Every time the hot tap is run for a few seconds, or when the boiler needs to heat itself up because the insides are too cold, the boiler fires up – and then stops.  This is a short cycle, and it happens millions of times a day across the UK: fans and pumps spin up, solenoids clunk, electrodes fire and a mixture of burnt and unburnt methane gas is ejected into the environment – with a fair amount of carbon monoxide too; short cycling is the absolute enemy of efficiency.  Now add into the mix the combi boiler arms race.  Manufacturers are scrambling to give us more and more kW output to try and counter the fundamental problem with the combi, which they themselves recognise but don’t like to talk about: namely, that they’re rubbish at doing hot water for a family home.  Certain manufacturers will now sell you a 50kW combi – still with 15mm pipework – that’ll do a claimed 18l/min at 40°C rise, which is still only marginally more than one large shower!  Imagine this beast short cycling away all day in a house with young children and busy adults.  The situation is laughable.

And not only this; worse still, the installation of a combi boiler completely precludes any meaningful integration of renewable heating technology, such as solar thermal or excess power from solar PV panels / wind turbine, because there is no cylinder with a second coil (thermal) or an immersion heater (excess power from microgeneration); it makes the jump to renewable technology that much more difficult simply because there is no place for the cylinder in the modern new build or newly remodelled house, and a cylinder is vital for any heat pump or biomass installation; it dictates the installation of a noisy, expensive to run electric shower for those dark moments when it breaks down and people still have to wash.

All this can be avoided with a cylinder, a decent proprietary control system and heavily insulated hot water pipework.  It’s time to stop installing these wasteful machines, and return to the more reliable, more efficient and more adaptable system that is a boiler and cylinder pair.

Next: the great myth about underfloor heating.