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Heat Pump Cylinder Sizing & Selection Service for Installers
Size the cylinder for the household, the heat pump and the available space.
- Last updated: September 2026
- Heat Loss Hub · For Installers
A heat-pump cylinder should be selected around more than litres alone.
Heat Loss Hub can assess the domestic hot-water requirement, check the selected heat pump, review the available cylinder space and specify a suitable cylinder for the project. The service can cover:
- Domestic hot-water demand
- Required storage volume
- Coil suitability
- Recovery performance
- Heat-pump compatibility
- Existing cylinder assessment
- Physical fit
- Proposed cylinder selection
KEY TAKEAWAY
Cylinder sizing is a demand-and-recovery calculation, not a litres-per-bedroom guess.
GET IN TOUCH
Send us your household demand and plant-space details and we’ll return a justified cylinder selection.
Cylinder Sizing Only, if That Is All You Need
You do not have to outsource the complete heat-pump design. You may already have room-by-room heat-loss calculations, heat-pump selection, emitter design, pipe sizing, hydraulic design and your own MCS workflow.
You may simply need the domestic hot-water side of the project checked and the cylinder selected properly. Heat Loss Hub can complete that stage and return the specification to your team. This can suit:
- MCS installers with their own design process
- Internal designers needing extra support
- Installers converting combi systems to stored hot water
- Retrofit projects with limited plant space
- Higher-demand properties
- Projects where the existing cylinder may or may not be suitable
You do not need to outsource the whole heat-pump design to outsource the cylinder selection.
What Cylinder Sizing Actually Needs to Decide
A useful cylinder selection should answer:
- How much hot water does the household need?
- How much stored volume is appropriate?
- How quickly can the selected heat pump recover it?
- Is the cylinder heat exchanger suitable?
- Will the cylinder physically fit?
- Can the existing cylinder be retained?
- Is the proposed cylinder compatible with the selected heat pump?
The objective is to select a cylinder that works thermally, hydraulically and physically.
Hot-Water Demand
The starting point is the household demand. Useful information can include:
- Number of occupants
- Number of bedrooms
- Number of bathrooms
- Baths
- Showers
- High-flow outlets
- Simultaneous use
- Customer hot-water habits where relevant
Two properties with the same number of bedrooms can have very different hot-water requirements. For example, a four-bedroom house with one standard shower is not the same design problem as a four-bedroom house with two bathrooms, a large bath, high-flow showers and heavy simultaneous use.
KEY TAKEAWAY
Two homes with the same number of bedrooms can have very different hot-water demand. The selection should reflect the actual project rather than relying on a simple rule of thumb.
Storage Volume
Cylinder volume matters, but volume alone does not determine whether the system will perform well. The required storage depends on household demand, peak usage, storage temperature, usable mixed-water volume, heat-pump recovery capability and customer expectations.
A larger cylinder stores more energy. A smaller cylinder may still work well if it can recover quickly enough. The correct balance depends on the project.
KEY TAKEAWAY
The correct cylinder size is the amount of stored energy the household needs, not just the biggest vessel that fits.
Recovery Performance
A cylinder must do more than store hot water. It also needs to recover at an acceptable rate. Recovery depends on factors such as heat-pump output during DHW operation, primary water temperature, coil performance, cylinder design, incoming cold-water temperature and stored-water target temperature.
This means two cylinders with the same nominal capacity may not perform the same way behind the same heat pump.
KEY TAKEAWAY
A cylinder is not correctly selected just because it stores enough water. It also needs to recover at an acceptable rate.
Coil and Heat-Exchanger Suitability
The cylinder coil is critical. A cylinder may have enough storage capacity but still be a poor match for a heat pump if its heat exchanger cannot transfer enough energy at the available primary temperature.
Heat pumps generally operate at lower primary temperatures than traditional boilers. That means the cylinder needs a heat exchanger that can perform effectively under those conditions. The selection should therefore consider coil area, manufacturer heat-transfer data, primary temperature, heat-pump output and recovery requirement.
KEY TAKEAWAY
Cylinder volume can be correct while the coil is still wrong for the heat pump. This is one of the main reasons why standard boiler-cylinder rules should not simply be transferred across to heat-pump design.
Existing Cylinder: Retain, Replace or Review?
An existing cylinder does not automatically need replacing. But it should be assessed properly. The review may consider storage volume, coil size and suitability, cylinder condition, insulation, immersion provision, existing connections, physical location, available manufacturer data and heat-pump compatibility. Each project can then be classified as:
Retain
The existing cylinder is suitable for the required demand and heat-pump operation.
Replace
The cylinder is unsuitable because of volume, coil performance, condition or other limitations.
Review
There is not enough reliable information to make a confident decision.
KEY TAKEAWAY
A cylinder should be assessed as a heat exchanger as well as a storage vessel.
Large Baths and High-Demand Properties
Standard domestic assumptions do not suit every property. Additional consideration may be needed where there are large baths, multiple bathrooms, high-flow showers, body jets, frequent simultaneous use, higher-than-normal occupancy or heavy morning or evening demand.
In these situations, peak demand may be more important than simple household size. A five-person house with one standard shower is not the same hot-water design as a five-person house with two large baths and high-flow outlets.
KEY TAKEAWAY
The cylinder should be selected around how the property is actually expected to use hot water.
Heat-Pump Selection Affects Cylinder Selection
The same cylinder can perform differently behind different heat pumps. The selected heat pump may affect available DHW output, maximum primary temperature, recovery rate, reheat strategy and manufacturer compatibility.
KEY TAKEAWAY
The cylinder and heat pump should be checked as a pair. A cylinder should not be selected in isolation and then assumed to work equally well with any heat pump.
What Happens if the Ideal Cylinder Will Not Fit?
The technically ideal cylinder still has to work in the real property. Common constraints include narrow airing cupboards, low ceilings, restricted loft access, small utility rooms, tight plant spaces and difficult access routes.
Possible alternatives may include a taller and narrower cylinder, a shorter and wider cylinder, a different manufacturer, a different location, or a revised storage/recovery balance.
KEY TAKEAWAY
The technically ideal cylinder still has to fit through the door and into the plant space. This is why cylinder selection should include physical fit as well as thermal performance.
Available Space and Access
Useful plant-space information can include maximum height, maximum diameter, cupboard width, door width, access route, service clearances, pipework access, maintenance space and immersion access.
A cylinder that technically fits into the cupboard may still be unsuitable if it cannot be installed, connections cannot be accessed, the immersion cannot be serviced or future maintenance is impossible.
The full installation space should therefore be considered.
Immersion and Backup Strategy
Many cylinders include an immersion heater. Depending on the project, this may provide emergency backup, supplementary heating, sterilisation support where applicable, or temporary hot water if the heat pump is unavailable.
The immersion arrangement should be considered alongside the cylinder selection, particularly where the customer has specific resilience or backup expectations.
What You Need to Send Us
Typical project inputs include:
- Number of occupants
- Number of bedrooms
- Number of bathrooms
- Baths
- Showers
- High-flow outlets
- Any unusual hot-water demand
- Selected heat pump
- Heat-pump DHW performance data where available
- Available cylinder space
- Existing cylinder details
- Preferred cylinder manufacturer where relevant
- Customer preferences
- Photographs
- Cupboard or plant-space dimensions
If the existing cylinder is being assessed, useful information can include make and model, storage capacity, coil details, age, immersion arrangement, and photos of the cylinder and surrounding pipework.
Explore: What Information Does a Heat Pump Designer Need From an Installer?
What You Receive
The exact output depends on the agreed scope. A cylinder selection may include design DHW demand, proposed cylinder capacity, recommended cylinder or model, coil suitability, heat-pump compatibility, recovery basis, existing cylinder retain/replace/review decision, space-fit notes, immersion or backup notes, design assumptions and revision reference.
The output should explain why the cylinder is suitable, not just state its capacity.
The aim is to give the installer a selection that can be understood, checked and carried into the wider system design.
Example Cylinder Selection Summary
A finished summary may look something like this:
| Item | Design Result |
|---|---|
| Occupancy basis | 4 people |
| Bedrooms | 3 |
| Hot-water demand | Calculated from project demand |
| Proposed cylinder | 250 L |
| Coil suitability | Suitable for selected heat pump |
| Recovery basis | Based on selected heat-pump DHW output |
| Existing cylinder | Replace |
| Space fit | Confirmed |
| Notes | Large bath / higher peak demand |
The exact format can vary, but the important thing is that the selection is justified.
What Is Not Included in Cylinder-Sizing-Only?
Cylinder sizing is one stage of the wider heat-pump design. Unless separately agreed, it does not automatically include:
- Room-by-room heat-loss calculation
- Heat-pump model selection
- Radiator sizing
- Pipe sizing
- Full hydraulic schematic
- Electrical design
- DNO application
- Complete MCS design pack
You can retain those stages in-house. Or Heat Loss Hub can continue into the wider design if required.
Explore: Outsourced MCS Heat Pump Design
Revisions and Design Changes
The cylinder selection depends on the information it was based on. If the project changes, the selection may also need to change. Examples include occupancy changes, a different heat pump selected, a large bath added, an additional shower added, cylinder location changed, available space reduced, customer wants faster recovery, or a different storage strategy agreed.
KEY TAKEAWAY
If the demand, heat pump or available space changes, the cylinder selection may need to change with it. Revised selections should be clearly identifiable so the installer knows which issue is current.
One Project, Occasional Support or Regular Volume
You can use the service as much or as little as required.
One Project
A difficult retrofit needs a cylinder selection checked properly.
Occasional Support
Your team normally completes cylinder sizing internally but needs help on specific jobs.
Design-Team Overflow
Several projects need DHW design at the same time.
Regular Support
Cylinder sizing becomes a routine outsourced stage within your design process.
You remain in control of the wider project.
White-Label Cylinder Selection
Cylinder selection can also sit inside a white-label workflow. Depending on the agreed arrangement, Heat Loss Hub completes the technical work, your business remains customer-facing, your customer remains your customer, the selection can sit within your wider design pack and branding can follow the agreed delivery model.
This can add technical capacity without changing the customer journey.
Cylinder Selection Quality Checklist
A useful cylinder selection should let you answer:
- What is the design DHW demand?
- What storage volume is required?
- Why was that volume selected?
- Is the coil suitable?
- What recovery basis was used?
- Is the selected heat pump compatible?
- Will the cylinder physically fit?
- Can the existing cylinder stay?
- What assumptions were made?
- Which revision is current?
If those questions cannot be answered, the selection may not be complete enough.
Frequently Asked Questions
Yes. If the rest of the heat-pump design is already established, Heat Loss Hub can complete the DHW cylinder-sizing and selection stage on its own.
The selection should consider household demand, occupancy, baths and showers, required storage, heat-pump capability, coil performance, recovery expectations and available space. It should not be based on cylinder litres alone.
There is no single litre figure that suits every property. The correct volume depends on the actual hot-water demand and how quickly the selected heat pump can recover the cylinder.
Yes. Both can be relevant when establishing the design hot-water demand. Actual known occupancy and unusual demand should also be considered where available.
Yes. The available DHW heating capacity can influence recovery performance and therefore the overall cylinder selection.
That depends on cylinder volume, heat-pump DHW output, coil performance, primary temperature, incoming cold-water temperature and target storage temperature. Recovery should be considered as part of the project rather than assumed from cylinder size.
Heat pumps generally supply lower primary temperatures than traditional boilers. A larger or more effective heat exchanger can therefore be required to transfer sufficient heat into the stored water.
Potentially. The existing cylinder needs to be checked for volume, coil suitability, condition, insulation and heat-pump compatibility. If the technical information is unavailable, further review may be required.
You need a cylinder whose heat exchanger and overall specification are suitable for the selected heat pump and operating conditions. The label alone is less important than the actual technical suitability.
Alternative cylinder shapes, manufacturers or locations can be considered. The storage and recovery strategy may also need to be reviewed around the available space.
These can significantly increase peak hot-water demand and should be included in the cylinder selection.
Not automatically. The cylinder can be sized and selected as a standalone service. Hydraulic integration can be provided separately if required. Explore Heat Pump Hydraulic Design Service.
Yes. A white-label delivery arrangement can be agreed where required.
Need the Cylinder Specified Without Outsourcing the Whole Heat-Pump Design?
Send us the household demand, selected heat pump and available plant-space information. We assess the required storage, coil suitability, recovery performance and physical fit, then return a justified cylinder selection.
SUBMIT A PROJECT
Send us your household demand and plant-space details and we’ll return a justified cylinder selection.
Explore: Outsourced MCS Heat Pump Design
Explore: Heat Pump Hydraulic Design Service
Explore: Heat Pump Design for Installers