Understanding Heat Loss Calculations

To understand how heat loss calculations work you must first understand the basics of heating a home.

A home will lose heat when the heat in property escapes to the colder air outside your home this is done through heat radiating through the fabric or the property or pushed out through gaps in the property’s fabric through ventilation and infiltration of air which replaces the warmer air in your property with colder air.

So firstly, understand the different between heat and temperature is a must for performing a heat loss calculation.

Imagine you light a candle in a living room, this candle will burn at a temperature of 1000°C which far hotter than any heating system you would use in a property but yet the candle would not be able to heat the room up. Although the candle burns at high temperature its such a small mass that it doesn’t produce enough heat to allow the room to warm up. Now if you heated a 1m x 1m radiator up to 70°C this would produce a lot more heat than the candle because of the mass of the heat allowing the room to heat up.

Heat Transfer

Rule number 1 of energy is that you cannot create or destroy energy you can only transfer it to a different state, heat is a just a form of energy.

Rule 2 is much more important when understand heat loss calculations as anything that has a high temperature will always be attracted to objects with a lower temperature, this is why the heat in your home will always try to escape to the cooler air outside.

For an example if you had 2 balls and one was 0°C and the other ball is 100°C if you where to put these 2 balls next to each other the second ball that is hotter would transfer the heat to the 1st ball until both balls where the same temperature, so after a period both balls would effectively be 50°C.

This is called the Newtons law of cooling, there are 3 main ways that you can transfer heat from one object to another and these are.

Conduction – is flow of heat energy through solid objects.

Convection – is the flow of heat energy though liquids.

Radiation – is the flow of heat energy through the air.

What are U-Values

To be able to perform heat loss calculations you need to take into consideration what U-Values mean, in basic terms the lower the U-Value the better insulation the fabric provides. The U-Value is the calculation of how quickly the heat will be conducted through the material giving you a figure of how much heat loss will escape through the object.

As an example, a typical cavity wall that has been filled will have a U-Value of around 0.56 if the cavity is not filled it will have a U-Value of 1.58.

Apex Renewable Energy Surveys have developed our own tool with all the U-values for different building materials allowing us to access the home in very fine detail meaning we would know the heat loss calculation for every individual room in the property.

Heat Loss Calculations Design Temperatures

So, when we perform heat loss calculations, we also take into consideration a few factors that do make a difference to the outcome.

Outside Temperature – We use the average low mean temperature of the location of your property, and we perform the heat loss calculations with this temperature in mind, for an example the Manchester area is typically designed with a -2.2°C in mind.

Internal Temperature by Room – So each room is marked as what type of room it is and set to the correct temperature for example a bedroom would be set to 18°C and a living room would be set to 21°C this allows us to size the radiators correctly for that room.

Building Fabric and Surface Area – We would need to input the surface area of every wall, floor, ceiling, door and window of every room while also inputting the correct U-value for every factor of that room to allow us to get the correct heat loss.

How important is it to do a correct heat loss calculation when using an air source heat pump.

The time it takes to do a heat loss calculation can vary from house to house and vary depending on the heating system you are having installed in your property. If you are having an air source heat pump installed in your property if very important that you have a full heat loss calculation performed in high detail, Air source heat pumps run at a lower flow temperature than a traditional boiler so not doing a fully detailed heat loss calculation can cause long term problems with the performance of the air source heat pump. One of the main factors that can cause problems due to in adequate heat loss calculations is radiator sizing, installing the correct radiators is one of the main key factors that will make the heat pump work efficiently.

The flow temperature of a boiler is typically 65°C where a heat pump using radiators will normally be set around 50°C flow temperature this means that the radiators need to be sized with this in mind as the radiators currently in a property will be sized for 65°C.

Apex Renewable Energy Surveys have performed over 25,000 heat loss calculations for over 100 installers across the UK and we are currently the only design only company in the UK to be accredited MCS (Microgeneration Certificate Scheme). This was a massive achievement for us at Apex as the MCS would only acknowledge installers at one point.