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What is heat loss and why is it important?

Thermal image of a house
If you’re thinking of upgrading your heating system, you’ve probably come across the phrase ‘heat loss’ and may be wondering what it is, and why it’s important when you’re looking for a new boiler. Heat loss is the gold standard in determining the correct size of boiler for your home. But despite this, a lot of heating engineers are ignoring best practice, and are falling back on the old, and frankly crude method, of simply counting radiators. And this really grinds our gears. So, in today’s blog, we want to talk about what heat loss is, and why it’s so important in heating your home.

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If you’re thinking of upgrading your heating system, you’ve probably come across the phrase ‘heat loss’. But what is it? and why is it important when you’re looking for a new boiler?

Heat loss is the gold standard in determining the correct size of boiler for your home.

Heat loss calculations are the industry standard for sizing boilers under Part L of the Energy Efficiency Regulations.

But despite this, a lot of heating engineers are ignoring best practice; and are falling back on the old, and frankly crude method, of simply counting radiators.

Instead of ensuring that your new boiler is sized to operate at its optimum efficiency; a lot of ‘engineers’ are just replacing ‘like for like’ or installing oversized boilers.

And this really grinds our gears.

So, in today’s blog, we explain everything you need to know about heat loss; and why it’s so important in heating your home. 

 What is heat loss?

A kettleIt’s easier to understand heat loss, if you start by thinking about how your central heating system works.

Your central heating system’s purpose is to produce thermal energy, otherwise known as heat. Heat for your water, and heat for your rooms.

Your boiler does this through several thermodynamic interactions; which enable it to transfer the heat generated from burning a fuel (usually gas), to water. Once the water is hot; your boiler pumps it around the pipes in your house, to supply your hot taps and your radiators.

When you turn on a hot tap, you’ll get hot water. But how does having hot water in your radiators warm your house up? Well, this is where the second rule of thermodynamics comes in.

This sounds very complicated, but just imagine you’ve made a hot cup of tea.

What will happen to your tea if you put it on a table and leave it?

That’s right, it’ll go cold.

And that’s what the second rule of thermodynamics tells us. Energy (in this case heat) spontaneously disperses from being concentrated; to becoming spread out, if it’s not stopped from doing so.

At its simplest level, heat flows in one direction: from hot towards cold. This can happen through conduction, convection, radiation, or a combination of all three.

Heat loss occurs because warm air moves towards cold

When your boiler pumps hot water into your radiators, the heat will move towards the cold air in your room. Anlthough they’re called ‘radiators’, most of the heat transferred from your radiator to the cold air, occurs through convection.

The result is that your once chilly room is now nice and warm, happy days. Or it would be, if you could hold onto this heat forever – you’ve paid for it after all!

But unfortunately, some of the heat will escape. The same science that allows us to make our rooms warm, is working against us at the same time.

Once the air in your house is warm; it naturally wants to move towards the cold air, which is now outside of your house. In other words, your warm air wants to escape the building!

Heat loss is a measure of the amount of heat escaping through your building’s fabric; from the inside to the outside. 

Different properties are built using different methods, materials, and levels of insulation; all of which affect the rate at which heat transfers through them.

The colder the outside temperature; the warmer the inside; and the worse the insulation of your building, the greater the heat loss will be.

How much heat can be lost from a building?

Whether we like it or not, all buildings lose heat.

But the amount of heat loss you will experience, depends on several factors. The age and type of property you live in will effect heat loss.

For example, if you live in a terraced house; you’ll lose the highest proportion of your heat through your floor and roof. But if you live in a flat, you’ll lose most of your heat through your external walls.

And generally, the older your building is, the more susceptible it will be to heat loss. This is because older buildings are more likely to have poor to no insulation, single glazing, and poor airtightness.

This is a particular problem in the UK. The Climate Change Committee (CCC) explain that our housing stock is: “one of the oldest and worst insulated in Europe; with 38% of homes built before 1946; and only 15% since the 1990s.”

Heat loss figures from the Energy Saving Trust

According to the Energy Savings Trust, the following estimates indicate the proportionate heat loss from a badly insulated house:

25% of your heat can be lost through the roof;

35% through outside walls;

25% through doors and windows;

15% can be lost through ground floors.

This also helps to illustrate why bigger houses don’t necessarily need bigger boilers to heat them. It’s not about the size of the house, that matters, it’s their heat loss.

To illustrate this, imagine that there are two neighbouring houses of the same physical size. But one house is very old with single glazed windows, and no insulation; and the other house is a modern new-build with double glazing and lots of insulation.

The first draughty old house will lose its heat very quickly; and will therefore need much more power from a boiler to keep it warm. Conversely, the second house will lose its heat much more slowly; and will therefore need less power from a boiler to keep it warm.

What factors increase heat loss?

Here are just a few of the factors that increase heat loss in your home:

Poor insulation increases heat loss

Since heat always moves towards cold; to keep your home warmer for longer, you need to prevent the heat from escaping through conduction, convection, and radiation.

 Insulating your home means that materials with low U-values (more on these later) are used to slow down the transfer of heat; from inside your home (where you want it) to outside.

So, insulation is a sort of buffer zone in between your warm home and the cold outdoors. Buildings with poor (or no) insulation will lose their heat much faster than buildings with insulation.

This is easily explained if you think about a hot mug of tea again. If you leave your mug on the table for an hour; the heat will have moved towards the cold air in the room; and you’re left with a cold brew.

Now imagine you had put your hot tea in a vacuum flask before you left it for an hour. When you come back this time, it’s still hot, because you insulated it.

The lid on your flask has reduced heat loss by convection; the vacuum in your flask reduces heat loss by conduction; and the metal foil around your flask reflects infrared radiation back to the liquid.

Insulating your roof and your walls; is as close as we can get to putting your home in a vacuum flask. This will save energy, and money on your energy bills.

Air leakage increases heat loss

Air leakage is a way of describing uncontrolled draughts or ventilation. There’s little point in taking lots of time insulating your roof and walls; if you have big gaping holes around your windows. All your lovely warm air will pour straight out. Even with insulation, if your home leaks heat, then its efficiency will plummet.

Of course, it’s important that our homes can circulate fresh air, and this is a requirement of the Building Regulations.

But controlled ventilation is one thing, and uncontrolled air leakage is quite another. Too much leakage, and you’ll be losing your warm air faster than you should. This results in colder rooms and escalating heating bills.

Check out our related blog: How to reduce heat loss at home, for tips on how to minimise your heat loss.

Why does heat loss matter?

A calculator next to a thermal image.Calculating the heat loss of your home is vital in choosing the correct size of boiler for two main reasons.

Firstly, working out your heat loss tells you how much energy is needed to keep your home warm. This enables you to choose a boiler which can supply the right amount of power to meet your needs, because the system output must be greater than the heat loss.

Undersizing, or oversizing your boiler, will be a costly mistake. Too small, and your boiler may not be able to keep your home warm enough; or give you sufficient hot water. Too big, and you’ll be wasting money on the initial purchase; escalating energy bills; and you might encounter boiler cycling, which will shorten your boiler’s lifespan.

The second reason that heat loss is important when it comes to heating; is that it’s the industry standard for sizing boilers under Part L of the Energy Efficiency Regulations.

How we measure heat loss: U-Values

It’s easy to see why heat loss will be greater for houses with poor insulation and air leakage; when you look at U-values.

U-values are sometimes referred to as heat transfer coefficients, or thermal transmittances. They are used to measure how effective elements of your building’s fabric are at insulating against heat loss.

U-values are measured in Watts per square metre. They tell you how much energy is lost for every 1°C difference between the two sides of each material used in your building.

Diagram showing high U-valuesThe higher the U-value of an element of a building’s fabric; the more quickly heat can transmit through it; and so the worse it performs as an insulator.

A draughty old house with no insulation and single glazing, will have a high U-value, indicating poor thermal performance. Whereas the new build with double glazing and plenty of insulation, will have a low U-value, indicating good thermal performance.

Diagram showing low u-values.So generally, the better (i.e. lower) the U-value of a building’s fabric; the less energy is required to maintain comfortable conditions inside the building. And the less output you will need from a boiler.

And this is why heat loss matters for getting a new boiler…




Why are heat loss calculations vital for installing new boilers?

Worcester Bosch Greenstar 8000 next to a ruler and a question mark.Heat loss is the gold standard in determining the correct size of boiler for your home.

Completing a whole house heat loss calculation is the industry standard for sizing boilers; under Part L of the Energy Efficiency Regulations. These regulations provide guidance on the conservation of fuel and power under The Building Regulations Act 2010.

Performing heat loss calculations ensures that boilers are not oversized, and are working to their maximum efficiency, thereby saving energy.

Saving energy will not only save you money on your fuel bills; but it’s also helping the planet by reducing your carbon emissions.

Under the Paris Agreement 2015, the UK has a legal commitment to reach net zero carbon emissions by 2050. To achieve this, heating plays its part.

The Energy Saving Trust says that we need to reduce our heating emissions to 138 kg CO2 per household. This is a reduction of 95%. Increasing the use of renewable energies is the main way of tackling this issue.

Nonetheless, carbon emissions are also reduced by making our current central heating systems more efficient.

What are heat loss calculations?

Here’s the science bit… Heat loss calculations determine your property’s heat transfer coefficient. They look at the thermal transmittance or ‘U-values’ of all the external fabric of your building; and its volume and average ventilation rate.

The altitude and exposure of your home will also affect this.

The calculation provides the total space heating energy flow rate in watts; divided by the temperature difference between the inside and outside air.

Alt attribute: Diagram of a house with heat being lost, next to a clock and a calculator.

Or, in other words, a heat loss calculation works out how quickly heat can escape out of the fabric of your home; whether from conduction; convection; or radiation; on the coldest day of the year.

This allows you to choose the optimum size of boiler for your needs. It avoids wasting energy and money whilst being comfortably warm.

How do The Heating People perform heat loss calculations?

The Heating People always perform a whole house heat loss calculation to determine the correct size boiler for your home.

Our whole house heat loss calculation is included as standard as part of your quotation for a boiler replacement.

It’s conducted as part of your survey, and only takes about ten minutes on site. We use heat engineer software. This enables us to size a gas or an oil boiler.

For more detailed calculations for renewable energy systems; or if your house is borderline between two boiler sizes; we can complete a room-by-room cross calculation for a more accurate result.

During a whole house heat loss calculation, we will:

  • measure the outside of your building;
  • record what the fabric of your building is made from;
  • count the number of skylights you have;
  • look at how many stories your home has;
  • measure the height of the stories in your home;
  • record the location of your home; and
  • factor in the altitude.

Our standard heat loss calculation is based on an indoor temperature of 21°C. The outdoor temperature varies depending on the altitude and location of your home. In Merseyside and most of Cheshire the outdoor temperature for our calculations would be -2.2°C.

The room-by-room method of heat loss calculations is much more detailed and is a separate chargeable service. It takes up to two hours on site, and is then completed away from site on a desktop computer.

With room-by-room calculations, we start with the same basic information as for the whole house calculations. But we then measure each room, window, door and skylight individually. This allows us to consider small variations in building fabric; for example, where some walls have been insulated, but others haven’t.

So, if you’re thinking of investing in your next boiler, contact The Heating People today.

Useful Links:

Conservation of fuel and power: Approved Document L

The Energy Saving Trust: Domestic sizing method

Energy Saving Trust: Reducing heat loss