Installing a heat pump does not necessarily mean that your existing hot water cylinder has to be replaced.
However, an existing cylinder needs to be assessed carefully because heat pumps operate differently from gas boilers, and the way heat is transferred into the hot water cylinder is particularly important.
At Thermodynamics South Yorkshire, we don't automatically replace existing cylinders as part of a heat pump installation.
We assess the cylinder, its heat exchanger, the required hot water performance and the proposed system design to determine whether it can be retained or whether replacement is necessary.
In some situations, an existing cylinder can be retained and adapted for heat pump operation, potentially saving the homeowner a significant amount of money.
Why Does a Heat Pump Need a Different Type of Hot Water Cylinder?
A gas boiler can produce water at a relatively high temperature very quickly. Heat pumps generally operate at lower temperatures and transfer heat into the hot water cylinder more gradually.
This means that the cylinder needs to be capable of transferring heat efficiently from the heating system into the domestic hot water.
One of the important components is the internal coil within the cylinder.
A conventional unvented cylinder may have a coil designed around the higher temperatures produced by a gas boiler.
Depending on its design and surface area, that coil may not provide sufficient heat-transfer capacity for a heat pump.
This is why simply looking at the age or condition of a cylinder isn't enough to determine whether it can be retained.
The question is:
Can the existing cylinder safely and effectively transfer heat from the proposed heat pump system into the domestic hot water?
Does an Existing Unvented Cylinder Automatically Need Replacing?
An existing unvented cylinder may be perfectly serviceable and relatively new, but that doesn't necessarily mean it is compatible with a heat pump.
Equally, an older cylinder isn't automatically unsuitable.
The cylinder needs to be assessed as part of the overall system design.
We would consider factors including:
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Cylinder manufacturer and model
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Cylinder capacity
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Internal coil design
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Coil surface area
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Heat-transfer capability
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Condition and age
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Hot water requirements
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Heat pump output
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Proposed system temperatures
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Existing controls and valves
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Available installation space
Only after assessing these factors can we determine whether the cylinder should be retained, adapted or replaced.
What If My Existing Cylinder Isn't Heat Pump Compatible?
1. The existing cylinder can be retained
If the cylinder has an appropriate heat exchanger and is suitable for the proposed system, there may be no need to replace it.
This is the simplest outcome and can reduce both installation cost and disruption.
2. The existing cylinder can potentially be adapted
In some installations, an existing unvented cylinder can be used with a plate-loading system.
This is a different approach to simply relying on the cylinder's existing internal coil.
A plate heat exchanger is used to transfer heat from the heat pump circuit into the domestic hot water system.
This can allow a suitable existing cylinder to be retained while providing the heat-transfer characteristics required by the heat pump.
3. The cylinder needs to be replaced
If the existing cylinder cannot provide the required heat-transfer performance, is unsuitable for the proposed system, or does not meet the property's hot water requirements, replacement may be the correct solution.
A dedicated heat-pump-compatible cylinder can then be selected as part of the overall system design.
A Real Example: Retaining an Existing Cylinder in Chesterfield
We recently installed a heat pump system in a 1950s semi-detached property in Chesterfield.
The property was 86 m² with a calculated heat loss of 4.07 kW.
The customer already had a relatively new 125-litre Telford Tempest unvented hot water cylinder.
Rather than automatically replacing it, we assessed whether it could be incorporated into the new heat pump system.
The existing cylinder used a conventional coil which was not designed specifically for heat pump operation.
Instead of replacing the cylinder, we installed a plate-loading kit.
This allowed us to bypass the existing cylinder coil and transfer heat into the cylinder through a plate heat exchanger.
The existing cylinder itself was retained, along with its existing controls, sensors and valves.
The system was then paired with a 5 kW Samsung EHS Mono R290 heat pump.
The result
The customer was able to retain their existing hot water cylinder.
The installation therefore avoided the cost and disruption associated with replacing the cylinder.
In this particular installation, retaining the existing cylinder saved approximately £500 compared with installing a new heat-pump-compatible cylinder.
How Do You Know If Your Cylinder Can Be Kept?
The best way to find out is to have the existing cylinder assessed as part of the heat pump design process.
We would normally want to establish:
Cylinder make and model
The manufacturer's information can tell us a lot about the cylinder's design and capabilities.
Cylinder capacity
A 125-litre cylinder may be perfectly adequate for some households, while larger households may require greater hot water storage capacity.
The correct size depends on the property's hot water requirements rather than simply the number printed on the cylinder.
Coil or heat-transfer arrangement
The existing heat exchanger is one of the most important considerations.
A cylinder designed specifically for heat pump operation will generally have a heat exchanger designed to transfer heat effectively at heat pump operating temperatures.
Heat pump output and design
The cylinder cannot be assessed in isolation.
The proposed heat pump, system temperatures and hot water requirements all need to be considered together.
Existing controls
The existing cylinder controls and components also need to be checked to establish what can be retained and what may need to be modified.
Does a New Heat Pump Mean I Need a New Cylinder?
Not necessarily.
A heat pump installation should be designed as a complete heating and hot water system, rather than treating every existing component as something that automatically needs replacing.
For example, one property might require:
New heat pump + existing radiators + existing pipework + existing cylinder
Another might require:
New heat pump + upgraded radiators + existing pipework + new cylinder
Another could require significant changes to several parts of the system.
The correct solution depends on the individual property.
Why We Don't Automatically Replace Existing Components
Replacing components unnecessarily adds cost and disruption to a heat pump installation.
Our approach is to assess what is already there and establish whether it can perform the required function under the new system design.
That means looking at:
Heat loss → Radiator output → Pipework → Hot water → Heat pump → Controls
Each component needs to work as part of the complete system.
If an existing component is suitable, there may be no reason to replace it.
If it isn't suitable, we'll explain why it needs to be changed.
What About Older Cylinders?
Age alone doesn't determine whether a cylinder can be retained.
However, an older cylinder may be more likely to have limitations relating to:
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Heat exchanger design
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Condition
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Insulation
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Capacity
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Controls
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Availability of replacement components
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Compatibility with the proposed system
There can also be good reasons to replace an older cylinder even if it could technically be adapted.
The decision should therefore consider both technical suitability and overall value.
Can a Plate-Loading Kit Be Used With Any Cylinder?
No.
A plate-loading arrangement is not a universal solution for every existing cylinder.
The cylinder, its construction, capacity, connections and the proposed heating and hot water system all need to be considered.
It is also important that the system is designed and installed correctly, particularly where an unvented hot water cylinder is involved.
For this reason, we recommend assessing the existing cylinder before assuming that it can either be retained or adapted.
Our Approach
At Thermodynamics South Yorkshire, we design heat pump systems around the individual property.
That means we don't automatically specify new radiators, new pipework or a new hot water cylinder simply because a property is changing from a gas boiler to a heat pump.
We carry out the necessary calculations and assessments to establish what the property actually requires.
Sometimes that means replacing components.
Sometimes it means adapting existing equipment.
And sometimes, as our Chesterfield case study demonstrates, the best solution is to keep what is already there.
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Want to know whether your Hot Water Cylinder is suitable?
The best way to establish whether your existing hot water cylinder can work with a heat pump is through a proper heat-loss calculation and system design.
At Thermodynamics South Yorkshire, we assess the heat loss of each room and the existing cylinder before determining what needs to be changed.
The result may be that you need a new cylinder. It may also be that you don't.
The important thing is to know before the installation begins.
Heat Pump Installations with Thermodynamics
At Thermodynamics, we specialise in air source heat pump installations and support homeowners through every stage of the Boiler Upgrade Scheme in Sheffield, Doncaster, Worksop, Retford, Chesterfield and Gainsborough. From assessing your home’s suitability and system design to managing the BUS grant application and completing a high-quality installation, we handle the entire process.
If you’re considering upgrading your boiler and want to take advantage of the £7,500 BUS grant, get in touch with Thermodynamics today for expert advice and a personalised quote.