When installing an air source heat pump, it has traditionally been standard practice to install a new 'heat pump ready’ hot water cylinder – but in many instances it’s not the most practical solution as it presents homeowners with an additional cost that can sometimes prevent them from going green.

At Igloo Works, our mission is to make air source heat pumps more accessible, which is why we discuss several scenarios in this article under which you can keep your existing cylinder while showing you the pros and cons of doing so.

The first thing to consider when looking at whether it is best to replace your hot water cylinder with a ‘heat pump ready’ one is the size of your existing cylinder coil. If your existing cylinder has a coil area of 2.5m² or more, it is already ‘heat pump ready’ and you do not need to replace it.

If the area of your cylinder coil is smaller than 2.5m², or if it’s not possible to tell how large the cylinder coil is, we recommend you read on. 


Just like a boiler, an air source heat pump is responsible for supplying hot water to the central heating as well as the cylinder. It’s important to note, though, that a heat pump can only do one thing at a time; it either heats your property or it produces hot water for your cylinder. So, in order to reduce the amount of time it takes the heat pump to generate hot water for the cylinder and allow it to go back to heating the property, the immersion element of the cylinder will kick in for longer periods.

The time the immersion is used to heat your hot water is the key difference between using your existing cylinder and using a ‘heat pump ready’ one. Hot water cylinders that are ‘heat pump ready’ have larger cylinder coils and therefore the time the immersion is used for is less, whereas your existing cylinder may have a smaller cylinder coil and therefore the immersion will need to be used for longer.

  • Heat pump + ‘heat pump ready’ cylinder = immersion used for shorter periods 

  • Heat pump + your existing cylinder = immersion used for longer periods

The Pros and Cons

The impact of keeping your existing hot water cylinder is a potentially higher running cost as it costs more to heat your hot water using the immersion than the heat pump, but on the positive side you would see an upfront saving of £2,000-£2,700 as you are no longer buying a new hot water cylinder.

Every house is different, and we will work with you to understand your existing setup to work out what the potential additional running costs and savings could be for you, while also providing you with a projection of the payback period for a new cylinder. 


If you opt for keeping your current hot water cylinder, you will save between £2,000 and £2,700 on the installation costs but you will potentially face higher running costs as hot water is not generated as efficiently as it would be with a ‘heat pump ready’ cylinder since the immersion is used to heat water for longer.

In turn, if you choose a new ‘heat pump ready’ cylinder for your air source heat pump, your installation costs will be higher, but your running costs will be lower as hot water is generated more efficiently due to a shorter usage of the immersion.

Lastly, if your boiler cylinder has a large enough heat exchanger coil, keeping it in place is the way forward as it’s likely that there won’t be any increase to running costs and no impact on comfort levels as hot water will be generated efficiently already.

Switching to a low-carbon air source heat pump to heat your home is typically the single biggest action you can take to reduce your home's carbon footprint. It can slash it by up to 65% and help you make significant savings on our heating bill.¹

1Subject to system design and usage.