How do I know what size radiator I need with a Heat Pump?

How do I know what size radiator I need with a Heat Pump?

Posted on April 21, 2022

Last week we talked about Delta T ratings and how they are used to ensure you’re choosing the right radiator with the required output for your home and heating system. 

It all depends on the desired room temperature (usually about 20 degrees C) and the flow temperature of your heating system – The Delta T rating is the difference between the two. 

Flow temperatures for modern condensing gas boilers are usually between 60 and 70 degrees C. This means that radiators with a lower output can still work effectively to heat the room. 

Air Pumps however tend to have a flow temperature much lower than boilers, usually around 45 degrees C. This means you would need a radiator with a higher output in order to heat the room effectively. 

If I replace my existing boiler with a Heat Pump, can I keep my radiators?

You will need to carry out some calculations to determine whether you can keep your existing radiators with a newly installed Heat Pump. 

When working out the radiator size you require, you need to consider:

  • The dimensions of the room
  • The materials used in the wall, ceiling and floor and what their u-values are (this is how well insulated they are)
  • Which part of the house the room is
  • Which direction the room faces

This information is then used to size the radiators to match the required output for the room to be heated. 

For Gas boiler heating systems with a typical flow rate of 60-70C, small output radiators can be used to achieve the desired temperature of the room as they match the heat input. 

In order to work out radiator sizing, you need to know the Delta T rating, which as we said above, is the difference between the required room temperature and the system output temperature. 

So for a Gas boiler, a flow rate of 70C and a room temperature of 20C would give a Delta T rating of 50. 

Heat Pumps operate at a much lower flow temperature. A flow rate of 45C and a room temperature of 20C would give a Delta T rating of 25C. 

Can I work out what size radiator I need myself?

As you can imagine, working out what size radiator you need for each room in your home that you want heated is quite an involved set of calculations with lots of different variables to consider and take into account. 

For this reason, we would recommend hiring a professional, qualified Heating Engineer to come and carry out a survey of your home. 

Experienced Heating Engineers know instinctively what size will work and will be able to expertly estimate what will be needed. 

Tempted to just oversize the radiator and let the thermostatic valves do the hard work for you? With Gas heating systems, it’s possible that this could work. This is because boilers are designed to start and stop on a regular basis. 

Heat Pumps however are designed to run continuously, which means you need to be a bit more careful with your radiator sizing if you want your Heat Pump heating system to work effectively and efficiently. What’s the point in installing a Heat Pump if you’re going to miss out on the benefits by using the wrong size radiators?

So what size radiator would I need with a Heat Pump? 

This isn’t actually an easy, straightforward answer that we can just give you unfortunately. 

Firstly, each manufacturer has their own output for a specific size of radiator. This means that you would need to calculate the required heat input to the room and then size a radiator to suit that application. 

A general rule of thumb would be to say 80C onto the radiator and 60C off the radiator, giving an average temperature on the radiator of 70C with a room temperature of 20C. This gives a Delta T rating of 50C. 

Then, if we look at a radiator that gives out 1000 watts at these figures, when we change the conditions to be flow 45C onto the radiator and 40C off the radiator as for a Heat Pump system, then the heat output of the radiator drops to be about 350 watts. 

Therefore you would need to do a room by room heat loss calculation and then size the radiator or radiators to suit the room requirements. Feeling a bit overwhelmed? That’s okay! DIY sizing of radiators can be really tricky. But that’s what we’re here for isn’t it!

Our expert heating engineers have the knowledge and experience to work out exactly what size radiators you need in each room of your home – Whether that’s running with a gas boiler or a heat pump. 

Upgrade your heating system with Aura Heating

If you are in the market for a Heat Pump, new radiators, a new central heating system or a new boiler, then get in touch with our friendly team today. 

Our heating engineers are heating experts, and we can help make sure you are making the right decisions for heating your home in the most efficient and cost effective way. 

That includes all those calculations to make sure every room has the right size radiator with the output you need to meet your heating needs.

Give us a call today and we’ll be more than happy to help.