An Overview of Heat Loss Calculations

What You Will Fine in This Overview of Heat Loss Calculations

Apex Renewable Energy Surveys will outline all the basics of performing a heat loss calculation so you can see how much work and effort must go into designing an air source heat pump. With our overview of heat loss calculations, you will get a better understanding of how heat loss calculations work.

What is a Heat loss

A heat loss determines how much heat escapes a sealed envelope such as a property. This happens through different fabrics and surfaces such as walls, doors, windows and roofs. This happens due to ventilation, as the temperature difference inside a property is different to the outside ambient temperature. Some parts of a property may have weaker point, where heat can escape easier, such as windows. If a room is heavily glazed then there is a strong possibility this will be colder than the rest of the property. This can be seen in a conservatory. The polycarb roof and volume of windows means these rooms can be very cold during winter.

Versatility of Heat Loss Calcs

Heat loss calculations are versatile in a sense that it can be used for multiple purposes. This doesn’t necessarily mean it has to be used for decarbonisation. This could be used to size radiators heated by a boiler for example. Although, more commonly used for renewable energy systems such as ASHP and GSHP. 

Importance of a Heat Loss Calculation

Heat loss calculations are crucial to make sure a heating system is tailored for the property to ensure efficiency. If the heat loss is calculated wrong, it can cause a variety of problems. One issue which can arise is the heating system being over sized. This can be a common problem with Air source heat pumps when not calculated correctly. This will lead to the pump short cycling, which means the pump will shut off when the desired temperature is achieved. This will increase bills, reduce life span. Also this can cause noise issues, as the pump will be constantly switching on and off. If the Pump is undersized, it will provide insufficient heating to the property. The pump will also be operating continuously as it will never reach its desired temperature.

Site Survey

A heat loss calculation should start with a full site survey. Floor plans and ceiling heights are needed to get the volume of space required that needs heating. The survey should also outline the fabric and insulation of the property. Previous regulations made EPC’s mandatory to ensure government funding, such as the BUS (Boiler Upgrade Scheme) voucher. Pipe sizes and material are important to know so that velocity calculations can be performed to ensure the hot water is pumped around the system without losing too much heat. Existing radiator sizes will tell us whether we need to upgrade the sizes due to heat pumps being a lower temperature heating system. New-build houses tend to have adequate radiators and minimal changes to be made, but not all properties are the same. Photos and or videos are good to visualise wall space for radiator upgrades and pump/cylinder locations. All this information recorded on site, helps keep a log if any changes happen to the property such as increasing insulation measures or if the design is to be done off site. Other information which should be outlined on the survey should help flag up potential issues when installing the system. This includes information such as mains water access or if there's a one pipe system.

U-values and Insulation Measures

U-values will determine how well the property holds in heat, the lower the u-value the lower the thermal transmittance. The higher the u-value the higher the transmittance, meaning heat will escape easier leading to bigger radiators and increase the pump size. By adding extra insulation to your property, this will reduce the wattage required to heat it, potentially leading to smaller radiators and a smaller pump. Common insulation measures include EWI (external wall insulation), IWI (internal wall insulation), filling cavities, and adding or increasing loft insulation. These are often performed on Eco 4 jobs, which is another government scheme helping more properties become suitable for heat pumps.

Pipe Sizes & Pipe Runs

Pipe sizes determine how well hot water is pushed around the system. Smaller diameter pipes such as 8mm will restrict the flow, whereas 15mm pipe will allow higher velocity thus getting water around the system quicker. This means the system will retain more heat and the heat pump will require less power to get back up to temperature. Another variable such as the material of the pipe can affect the velocity as the roughness is different, which will affect friction. It is important to consider the distance of the pipe runs and how many bends will be in the system. The further the water has to travel the more the temperature will drop. Depending on the number of bends in the system, will again cause more issues with restricting the flow. Wider pipes does not necessarily mean better results, as water travelling round the system too quick can also cause issues. Smaller pipe diameters will often work well in smaller properties as there is less distance for the water to travel, meaning it won't be pushed round too quick. Large houses will often need wider diameters so the temperature drop off is less. To find the correct size and pipe material, a calculation must be performed. It is advised velocities should not exceed 3 m/s, with the target range being 0.5 m/s to 2 m/s (dependant on property).

Design Conditions & MCS standards

The design conditions for a heat loss calc can vary depending on the location where the property is situated. When determining the ambient temperature, the area must be considered. Take the UK as an example, the further south you travel, the warmer the average climate will be annually. This factor will affect the Scop (Seasonal Coefficient of Performance) of a heating system. An average heat pump in an ambient temperature of around -3 degrees celsius and a flow temperature of 50 degrees celsius, will have a Scop of 3.5. This means the system is 350% efficient, using 1kW of electricity and producing 3.5kW of heat as an example. If the ambient temperature was to increase and the flow rate temperature was to decrease the Scop would increase or visa versa. A governing body called MCS ensures that designs are compliant by following certain design conditions such as average room temperatures within a property. For instance A Living Room will be designed at 21 degrees celsius due to this being most habitable. Bathrooms will be designed at 22 degrees celsius due to the volume of hot water being used. Rooms designed at a lower temperature of 18 degrees celsius are Bedrooms, Water Closets and Hallways. Another factor to consider is the air changes of a room, which is how fast the volume of air is replaced per hour. This can be worked out by multiplying the cubic feet by 60 (minutes) and dividing this by the volume of the room.

Heating Source and Cylinder Locations

The heating source and cylinder locations need to be cited close to each other for reasons mentioned in the pipe runs section. If they are at opposite ends of the property then temperature loss in the system will occur. The cylinder is advised to be housed in the sealed envelope of the house and still requires insulating, which can be done by lagging the pipes around the cylinder. If a property has a heat pump located outside the property then there are constraints to consider. The pump needs to have a 2 metre clearance in front of the fans. Also, the location can't be enclosed as the pump can create a microclimate which will significantly reduce the efficiency. MCS also requires the pump to be installed 1 metre away from a neighbouring property. A sound calculation must be performed to ensure the pump location complies with MCS guidelines. This takes into account the pump's decibels, reflective surfaces around the pump, visibility of the pump from assessment point (closest neighbours habitable room) and metres from the pump to the assessment point.

Radiator Sizing

The radiators are sized using the heat loss of a room, the wattage of the radiator at delta T (which depends on the flow temperature of the system) and a correction factor (which also varies depending on the delta t. Lower temperature systems such as heat pumps will need bigger radiators than a gas boiler which has a higher temperature flow rate. If a property was to switch from a gas boiler to a heat pump then it is a strong possibility that the radiator will need upgrading. 

EPE

The EPE (energy performance estimate) is a figure of how much money the heating system will cost. This takes into account the unit prices of the old and new system, which means current bills are required to ensure the estimate is as accurate as it can be. This also uses the space heating figure from an EPC, which is something that can be obtained during the site survey. The EPE is a good way to visualise if the new heating system is worth getting, as it shows the consumer how much money they could save.

DNO

The DNO (distribution network operator) process is the application for connecting the new heating system to the grid. The country is split up into regions for specific network operators, for example Yorkshire are supplied by Northern Power Grid. This process is necessary because the new system will demand more AMP from the main fuse. Some properties only have 60 AMP fuses and will likely need a fuse upgrade to ensure the demand doesn't exceed 60 AMP. Applications can differ depending on the size of the system. In terms of heat pumps, the smaller sizes such as 5kW-11kW tend to be connect and notify. However, larger pumps such as 12kW-16kW need to be accepted before it’s installed. These can vary and the information on specific pumps can be found on ENA register online.

Compliance Certificate

By providing customers with MCS compliant certificates, means they can be assured their system has been designed to MCS standards. This will have all the information on which MCS require the design to include such as, flow temperature, external ambient temperature and room temperatures. There are also other benefits which come with having an MCS compliance certificate with an installer. Utility companies such as Octopus offer exclusive tariffs for heat pumps, which have lower unit rates of off-peak times. This means the user can set their heat pump to be active in these periods, significantly reducing there energy bills.