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Sizing the radiators on your air source heat pump installation is one of the most important aspects to having an air source heat pump installed. Whether you are an installer or a consumer it’s very important to understand why the radiators are an important part of your heating system. Installing an air source heat pump with badly designed radiators is a recipe for disaster as the heating system will not be efficient this will dramatically increase the running costs.
Flow Temperature of Heating Systems
Sizing radiators for a boiler system is a simple process as the boiler can handle flow temperatures of up to 70°C so you have a lot of room for error as the boiler can get to a temperature even if an undersized radiator is installed in the room. This is where the complication of having an air source heat pump comes into play. Typically, an air source heat pump will have flow temperatures running at 55°C or less so this means that most radiators in current properties are not suitable for an air source heat pump. The reason heat pumps run at lower temperature is to make them more efficient which is also true for boilers but most systems for boilers where not designed to run at lower temperatures, so the radiators are not designed this way.
The New Heating Regulations
Next year in 2025 the building regulations in the UK are changing stating that all new heating systems installed will have to be designed for at a maximum temperature of 55°C this includes gas boilers. This is to allow the switch to air source heat pumps easier in the future. The changes to part L of the future home standards will help the transition to air source heat pump in the future with these key changes.
Key changes to all new homes being built.
New wet central heating systems are to be designed to a maximum flow temperature of 55°C.
The appliance providing the heat must be sized for the correct heat load of the property.
Exposed pipework must be insulated.
Thermostatic Radiator Valves must be installed on all radiators except for the room where the thermostat is placed.
Mandatory airtightness test for all homes.
Key changes to existing properties
New or Replacement wet central heating systems should be sized for the maximum flow temperature of 55°C when possible. If the property does not allow for a system to be sixed this way, then the lowest temperature that is possible must be used.
The new appliance providing the heat must be sized for the correct heat load of the property.
Commissioning and system preparation has been upgraded and is now part of the formal regulations made in the BS7593 standard.
Heat Pump Radiator Design
So, after Apex Renewable Energy Surveys have performed a full technical heat pump survey, we will then move on to designing the system. Once the full heat loss calculation has been done the property in question, we will then design the radiators that are need.
We take into consideration a lot of factors when picking the correct radiator for the room here is just a few things we consider.
Room available for the radiator as we may need to use taller radiators if we don’t have the width for the radiator of vice versa.
The flow temperature of the designed system.
The rooms heat loss.
The available pipe runs.
As you can see there is a lot to consider on something as simple as changing a radiator this is why it’s best to leave designs to the experts.
The Size of Radiators Needed With an Air Source Heat Pump
If you look online for the size of radiators you will need for an air source heat pump you will see numbers thrown around like they need to be double the size. We will explain to you why this is technically true but is often misunderstood.
If you had a living room that was 5m x 5m room and this room needed a radiator that was a single panel radiator 1m wide by 700mm high set at 70°C. This radiator wouldn’t be able to handle the room if it was currently set to a 50°C flow rate so we need to increase the size of the radiator by roughly 2.4 times the size. This doesn’t mean we need to install a radiator that is 2.4m wide and 1.68m high. We would install a radiator that had 2 panels and then make the radiator 1.4m wide and 700mm high. So, the radiator would have only been made 400mm longer but could still heat the room, all we need to do is increase the surface area using a double radiator would only increase the thickness of the radiator by 25mm but would effectively double the size of the radiator.
Please understand the above information is for understanding the sizing of radiators and does not replace the need for heat loss calculation.
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