Oversized boilers are bad for your bank balance & the planet
HOW TO USE THIS GUIDE
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...
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 traveled 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.
A prescription for achieving over 90% efficiencies from our heating systems
A number of small, cost-effective changes to the configuration can be made to most heating systems and/or boilers that will eradicate the majority of health problems and without necessarily having to change the boiler. The following changes require a competent engineer to attend.
1) Correctly size the boiler
The size of your boiler can only be truly determined by an accurate heat loss calculation. A heat loss calculation will work out how much heat is required to maintain an internal temperature when it is -3 degrees C outside. Heat loss calculations can be long-winded, however software is available for the attending engineer to make the calculations quickly and accurately.
2) Get a low minimum output boiler
It is worth noting here that smaller boilers, i.e. those that show a lower maximum output for example 12kW, are not necessarily the most efficient. The figure to look at is the not maximum output but the minimum output.
As set out above, all modern boilers modulate between a range of outputs. A 12kW might operate between 4-12kW. However a more sophisticated 30kW boiler might operation between 3-30kW. In this instance it would be better to install the 30kW boiler, because once it is range rated down it can operate between the lower output - 3kW - and your maximum heat requirement, say 8kW. Whereas the 12kW boiler can only operate between 4-8kW.
In spring and summer it is imperative that the boiler can operate as low as possible to prevent overheating the radiators in these months. To find out which boilers offer the lowest output in the market, try our Guide to the best boilers.
3) Adjust the maximum output down
Nearly all modern boilers can be adjusted down to match the output of the property, this is called ‘range rating’ and must be undertaken by a competent gas engineer. However, the move towards bigger boilers means that it cannot always be lowered enough, for example 6-8kW. There is not a lot of guidance in the industry for what this should be and a more widely recognised standard is definitely needed for consumers and installers.
A note on hot water
Hot water usually needs more kW of power than heating and this is the case for combi, system and heat only models. A 6-8kW boiler will be quite slow to heat a conventional hot water cylinder, therefore it will likely be the case that at least 12kW of power is needed to heat hot water, upto 30kW for big hot water demand households.
Therefore the key to achieve high heating efficiencies AND great hot water, is to find boilers that provide a high output to hot water and a low minimum output to heating.
Three great examples are 1) The Worcester 8000 - 30-50kW to hot water and right down to 3kW of heating, 2) Viessmann 200 - 25kW to hot water and 1.9kW to heating and 3) Intergas Xtreme, upto 30kW for hot water and 3.6kW to heating. For more see our Guide to the best boilers.
4) Remove air from the system
For more than a decade the standard solution to corrosion/sludge problems is to fit a magnetic filter and powerflush the system, but we are treating the problem not the cause. Fitting a device know ‘deaerator’ will remove the air as the water circulates will prevent corrosion and remove the need for chemical treatments, which is a huge environmental win.
5) Balance the radiators
Self-balancing thermostatic radiator valves prevent radiators from overheating and help the boiler operate at factory tested 89-94% efficiencies for much longer. The good news is these TRVs cost only a fraction more than standard TRVs and less than half that of smart TRVs.
6) Fit modulating/compensation controls
Due to boiler/control communication issues that I won’t go into, consumers have just two choices to fit a control that can adjust the boiler’s output. 1) fit the ‘load compensation’ control available from the boiler’s manufacturer; 2) Fit a boiler and control that both state they are OpenTherm compatible. All other forms of control are not as effective. For more try our Guide to OpenTherm or Guide to advanced heating controls.
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 www.heat-engineer.com
Kimbo, Heating Academy Northampton www.heatingacademynorthampton.co.uk
Neil Bunning, IMI Hydronics, www.imi-hydronic.com
Nathan Gambling, Betateach www.betateach.co.uk
Richard Burrows, The Intergas Shop, theintergasshop.co.uk
Paul Hull, The Commercia Group, www.thecommerciagroup.co.uk