Choosing a boiler: A beginner’s guide

With a market full of different boiler types and models, how do you choose a system that’s going to be right for your home? Sustainable energy expert David Hilton looks at the options…

Boilers have been at the heart of our home central heating systems for many years, and despite recent proposals to ban them from being installed in new builds from 2025, for now, they are still the mainstay. Manufacturers are hard at work continuing to develop technologies to produce more energy efficient boilers — I looked at the innovations that could be set to revolutionise the industry in last month’s issue of this magazine.

But, if you’re building a new home, extending or renovating and want or need to install a new boiler, how do you navigate the market to decide which system is right for you? To get more of an idea, here’s a look at the different types of heating and hot water systems available on the market (for gas, LPG and oil), what properties they’re suited to and how they work.

Combination (‘combi’) boiler

usually suited to: Smaller properties with one bathroom or where one point of use will be used at a time. how it works: Combi boilers are the most straightforward boilers and usually do not have a hot water cylinder. This heating system accounts for more than half of all boilers sold in the UK each year.

The heat exchanger is a key part of the boiler — and there are two parts to a combi boiler heat exchanger: one that supplies heat to the central heating system and a second part that usually provides instantaneous heat for hot water, typically lifting the temperature of the water passing through it by 35°C at a given flow rate.

If, for example, it’s a cold day and the water starts at 5°C, then it will come out at 40°C, but if the weather is warmer and the water goes in at 12°C, then it will come out at 47°C. The power of the boiler will determine the flow rate that produces this 35°C temperature increase; a 24kw boiler will have a flow rate of around 11 litres per minute, while a 30kw boiler will have a flow rate of around 14 litres per minute.to get hotter water the flow rate will need to be slowed down.you will also find that if you open a hot tap elsewhere in the property then the hot water flow will be shared, meaning that anyone in a shower is likely to experience the water getting cooler.

It might be tempting to install a bigger combi boiler in order to increase the flow rate to the shower, but if you do then you need to be absolutely sure that it can modulate down enough to also match the heat load of the property.this is the amount of heat needed to keep a space at a certain temperature and is usually calculated by the heating engineer on a room-by-room basis.

Many combi boilers now have the option to add a flue gas heat recovery (FGHR) unit, which is a little box that sits on top of the boiler and collects additional heat from the flue gasses that it then uses to preheat the water going into the boiler,

‘‘ Modern boilers have the ability to modulate. This means that they can adjust their heat output to suit different heat loads

thus increasing the flow temperature and efficiency of the boiler. This can increase the flow temperature by as much as 20°C, depending on how long the boiler has been running. This is the maximum boost that can be achieved this way, as putting water into a combi boiler that is hotter than around 29°C is potentially unsafe.

Regular boiler

usually suited to: Older buildings or in geographic areas where the main water pressure is low or erratic. how it works: Regular boilers have a hot water storage cylinder (that can usually hold up to around 300 litres) and cold water feed tank that is usually in the loft. They are gravity fed, so they do not have any great pressure. Additional pumps would be required if an increase in pressure were needed for a shower or tap.

System boiler

usually suited to: Larger homes with more than one bathroom. how it works: System boilers use a hot water cylinder together with a high pressure water supply. As there is no gravity tank or open vented tank in the loft the safety risks are greater and are covered by Part G of the Building Regulations. High pressure systems must therefore be installed by appropriately qualified installers. Water boils at 100°C at atmospheric pressure, but at 2 bars, water will boil at around 120°C. If there are no safety devices in place and water is allowed to heat beyond 100°C, the cylinder could explode when the hot tap is opened and the pressure drops.

As a general rule of thumb you would expect to size the hot water storage capacity of the cylinder at around 40 to 50 litres of hot water per person per day. The number of people is calculated as the number of bedrooms plus one. Therefore, for a four-bed home we would expect a hot water cylinder of between 200 to 250 litres depending on the stored temperature. If you store the water at a lower temperature than

60oc you may need to increase the storage capacity as well as also making sure you have an automatic periodic sanitation cycle to mitigate Legionella. A competent heating engineer will carry out the relevant checks.

Storage combi boiler

usually suited to: Larger family homes with more than two bathrooms. Storage combi boilers are especially suited to properties with a high demand for hot water that lack space for an open vent or system boiler. how it works: Storage combi boilers combine the benefits of combi boilers and system boilers discussed above.they are traditionally floor-mounted combi boilers with a small hot water store built in.the boiler manufacturer Viessmann offers wallmounted storage combi boilers that have a built-in storage capacity of between 46 and 170 litres.

According to the manufacturer, the storage combi boiler eliminates the need for a separate domestic hot water cylinder.

The appliances efficiently regenerate hot water — basically the hot water in the store adds heat to cold water that passes through the heat exchanger.this means that you can effectively have more heat in the water than would be available purely from the calorific value of the gas.

Let’s talk energy efficiency

The law affecting boilers changed in April 2007 and all boilers on the market since then have very high efficiencies (typically over 90% — older boilers had efficiencies of around 70% to 80%) and are all condensing type boilers.

Older boilers also produced flue gas temperatures in excess of 150°C. As boilers become more efficient they are able to extract more and more heat from the flue gasses. Modern boilers, with highly efficient heat exchangers, have flue gas temperatures of only 50°C, as much more heat is absorbed into the water. Hot air has the ability to hold more water vapour and as it cools down this vapour

condenses and becomes water, hence why modern, efficient boilers are referred to as ‘condensing’ boilers.

Modern boilers also have the ability to modulate. This means that they can adjust their heat output to suit different heat loads. If the boiler controls detect that the room or water temperature is nearly met then it will lower the flame temperature to put less heat into the heating system.

Better quality boilers will have the ability to modulate more than some of the cheaper ones, giving a much better heat range and avoiding short, inefficient startstop cycles. For example, some boilers can now modulate down to less than 2kw and also have software that allows the room thermostat to fully modulate the boiler rather than just turning it off and turning it on. Many manufacturers have their own control systems available as open source software, such as Opentherm; the latter has been adopted by many boiler manufacturers, control manufacturers and some smart home system manufacturers and allows the systems to ‘talk’ to each other to optimise their modulation and provide holistic functionality.

Most gas boilers are now wall-mounted units and are balanced flue appliances. This means that they have two pipes in the flue, one within the other. The outer part of the flue draws air in from outside for combustion, ensuring that no oxygen is depleted from the home. Meanwhile, the inner pipe expels the products of combustion, ensuring that they don’t find their way in to the home.