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Oversized boilers are bad for your bank balance & the planet




Oversized boilers are more expensive to buy and fit, more expensive to run and terrible for the environment, yet they are present in the vast majority of UK homes. According to the Energy Saving Trust, the average UK home requires just 6kW of heat on a very cold day, but we frequently fit boilers three times that size. Modern boilers are capable of 90% or more efficiencies but the unspoken truth is that due to boiler oversizing and incompatible controls this is rarely achieved.


Boilers are the heart of our homes

Problems with oversizing

How we get 90% efficiencies

The heart of our homes

We have come to see the boiler as ‘the heart of the home’ and we need it to continue beating. Like our own hearts, most of us pay little attention to the health of our boilers until they stop working. Also like heart problems, the high cost to fix the problem could usually have been prevented with a healthier lifestyle.

The health of our heating systems has quietly deteriorated following two decades of design and installation practices that have not been updated for modern boilers. In the vast majority of UK homes we find the wrong sized boiler, system corrosion and ineffective controls, all of which are akin to a poor diet that leads to health problems, premature aging and boiler seizures.

So little is understood about the damage caused by poor system health that when we find boilers in poor condition the solution is almost always just change the boiler, but this is a short-term fix.

A brief history of unhealthy heating systems

The late 1990s saw the development of the condensing boiler and 89-94% efficiencies under factory test conditions. This was a vast improvement on older boilers that could lose up to 40% of the heat they generated.

The condensing boiler was a great technological leap forward and one that should have made a huge contribution to meeting our emissions targets as well as reducing our own fuel bills. The unspoken truth is that 90%+ potential efficiencies are almost never achieved because

1) the boiler is oversized; and

2) the system (by which we mean radiators, pipework and controls) has not been designed to maximise the time the boiler spends in ‘condensing mode’, i.e. when it can recover all the previously lost heat and run at 90%+ efficiencies

How does over sizing happen?

If you own an average 3 bed house, you will most likely have a 15-24kW boiler. Combi boilers are confusing because they show capacities of 24kW – 50kW, but this is for hot water production. On the heating side, combi boilers are commonly 16-18kW and upwards.

It is somewhat astonishing to learn that the average heat requirement for the majority of properties in the UK is 6-8kW, i.e. on a very cold day in February a 6-8kW boiler can heat most homes. So why are we fitting boilers with a capacity three times in size?

Due to time pressures and now installations without a home survey, sizing a boiler is usually a ‘rule of thumb’ guestimate. As larger boilers still appear work - because we do not measure efficiency to know they are operating poorly - so overtime these estimates have unquestioningly graduated towards bigger boilers, just in case, so that customers now ask for bigger boilers, just in case.

The problems with over sizing

1) It is important to understand that all boilers operate within a range, for example 2-20kW or 8-32kW. The lower a boiler can go, the more efficiently it can operate all year round. The minimum output tends to increase in bigger models and some exceed the maximum heat requirement for the average UK home, i.e. 6-8kW. Boilers with high minimum outputs will never be as efficient as a boiler that can go down to 2kW.

2) Whilst all boilers have a high maximum output, it can be reduced down to meet the maximum heat requirement, e.g. 6-8kW, to make the boiler more efficient. But this is not always reduced or reduced low enough.

Poor system design

The other big barrier to achieving factory rated efficiencies is heating controls that do not speak the same language as the boiler. We would all be forgiven for thinking that by investing in a smart control we will benefit from much greater efficiencies. However, maximum efficiencies are only possible when the boiler is paired with a 'compensation' control, which is a separate function in smart controls.

Compensation controls adjust the boiler up and down as outside temperatures change. They make the boiler really efficient, but they must speak the same language. We have a situation in the UK whereby many smart controls and standard controls do not speak the same language as the boiler and this is not made clear. The only option for consumers it to buy the boiler manufacturer's own compensation control or buy a boiler and control that both speak the OpenTherm language.

Elsewhere, standard thermostatic radiator valves (TRVs) have some surprising implications for efficiency, or at least not maximising efficiencies, and air in the system has created a corrosion problem we have not properly addressed...


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The health implications for our heating systems

1) High temperatures

For the boiler to operate at 90% factory stated efficiencies the temperature of the central heating water as it returns to the boiler - i.e. after it has travelled around all the radiators and been used - must 54 degrees C or less. If the boiler is oversized, for example outputting at 18kW when the radiators only total 6kW of capacity, then the radiators cannot dissipate all of the heat and the water temperature back to the boiler can be much greater than 54 degrees C.

2) Palpitations and premature ageing

Oversized boilers ‘cycle’ on and off excessively, i.e. they heat up too quickly, run for short blasts and then turn off because they are too hot. Each fire up is something close to full capacity which dumps a huge amount of heat into the system and overheats the radiators. Like the clutch of a car stuck in town centre traffic, the stop-start cycles will use more fuel and put more stress on the components than when cruising along a motorway. The same principle applies to boilers. A boiler is at its most efficient when it can run for long periods at a lower output. A cycling boiler will increase wear and tear and shorten the lifespan of the boiler.

3) Pipe clots

We frequently hear about ‘sludge’ in our central heating pipes and radiators. Sludge refers to the build-up of corroded metal particles that eventually cause blockages and prevents the hot water from circulating. Corrosion occurs when air gets trapped in the system and cannot escape. For example when we ‘top up the pressure’ on our boilers we introduce air. Older systems naturally released air however combi boilers require a closed system and the air has nowhere to go. As the sludge builds up so the boiler has to work harder to heat the home which in turn increases fuel bills and accelerates wear and tear.

4) Circulation problems

Balancing for most of us means simply that the flow of water around the system is such that every radiator gets hot. This was fine for older boilers. For modern boilers to be 90% efficient it is paramount that the radiators flow - by which we mean how the water is distributed around the system - is set up to prevent overheating in radiators. Thermostatic radiator valves (TRVs) help us control and stablise temperatures in rooms we use a lot and reduce temperatures in little used rooms. However, as some rooms switch off, the flow of water around the system becomes unbalanced and remaining radiators overheat. This results in the temperature of the water as it returns to the boiler again becoming too high.

5) Lazy brain

Most of us know that some form of heating control will improve the efficiency of our heating system. The popularity of smart controls has driven sales in this sector with high expectations around energy saving. But once again things are not quite as they should be. All modern boilers are what are known as 'modulating'. In simple terms this means they can vary their output up and down (say between 3kW and 18kW) according to the outside temperature. When boilers can reduce their output they use less gas and do not ‘cycle’ on and off. However a boiler can only reduce its output when paired with a compatible control and so this rarely happens.

Pioneering change

There is a small group of trainers, manufacturers and industry professionals pioneering these changes (see contributors' section below), but it will take a national effort to make them mainstream and everyone has a part to play.

As consumers, we need to be prepared to engage just a little more with our heating systems to know they are right. If we can push for bigger boilers - just in case - then we can push for smaller/better modulating boilers - to achieve better efficiencies. It is also necessary to do a little more research - which is why we are here of course - to get the best modulating boilers. The most suitable, best-performing boilers are not always the most expensive. Some entry-level priced boilers have these attributes. A properly fitted entry-level boiler can be just as reliable as a high-end model.

Installers also have to change sizing and installation practices that they have been using for decades, but old habits are hard to break and peer pressure can be a barrier. Training in modern techniques is very limited, but a few courses are out there for those with a desire to work better and smarter.

It is really within the manufacturer’s gift to pioneer the changes. By utilising the marketing and training facilities they already have in place, they can educate and inform us how to get 90% efficiencies out of their wonderful boilers. They also need to ignore so called ‘market demands’ for bigger boilers – as the market is getting it very wrong – and provide us with smaller boilers as standard.

Without this, all of the progress made by boiler manufacturers to evolve the efficiencies of their boilers would have in vain and we may as well return to fitting non-condensing boilers. Let’s face it, they were cheaper to produce, cheaper to buy and with less that could wrong. On balance we may find that we are worse off using poorly installed condensing boilers than properly installed non-condensing boilers and that makes a travesty of our industry.


Author: Jo Alsop, founder of The Heating Hub

Contributors to this blog and jointly pushing for change in our industry:

Rob Berridge, Heat Engineer

Kimbo, Heating Academy Northampton

Neil Bunning, IMI Hydronics,

Nathan Gambling, Betateach

Richard Burrows, The Intergas Shop,

Paul Hull, The Commercia Group,

Jo Alsop

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