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Heat Pump Pipe Sizing Service for Installers
Turn system duty and pipe layout into a section-by-section pipe schedule.
- Last updated: September 2026
- Heat Loss Hub · For Installers
Heat Loss Hub can calculate the required flow through each part of a heat-pump system and turn that into clear pipe-size decisions.
The service can assess:
- Heat pump primary pipework
- Main distribution
- Zone branches
- Terminal runs
- Existing pipework
- Microbore sections
You can outsource just the pipe-sizing stage without handing over the complete hydraulic design.
KEY TAKEAWAY
You provide the system duty and layout. We calculate what each section actually needs.
GET IN TOUCH
Send us the system duty and pipe layout and we’ll return a clear, section-by-section pipe schedule.
Pipe Sizing Only, if That Is All You Need
Not every installer needs a complete outsourced hydraulic design. You may already have the heat pump selected, room-by-room heat-loss calculations, an emitter schedule, a proposed system layout, your own pump and control strategy and your own final design process.
You may simply need the pipework checked properly. Heat Loss Hub can calculate the flow requirement and pipe size for the defined sections, then hand the project back to your team. This can suit:
- MCS installers with their own design process
- Internal designers needing extra calculation capacity
- Installers checking existing pipework
- Businesses dealing with retrofit constraints
- Companies experiencing temporary design backlog
You do not need to outsource the whole hydraulic design to outsource the pipe calculations.
What Heat-Pump Pipe Sizing Actually Needs
Pipe sizing should be based on what the section of pipe needs to carry. The main inputs are:
- Heat duty
- Design temperature difference
- Required water flow
- Pipe length
- Pipe material
- Internal diameter
- Velocity
- Pressure loss
The important point is that the pipework should be calculated around the heat-pump system rather than copied from the heating system it replaces.
KEY TAKEAWAY
Pipework should be sized for the required flow, not copied from the boiler it replaces.
Explore: Heat Pump Pipework & Hydraulics
One pipe size does not necessarily suit the entire heating system. Different parts of the circuit carry different loads.
Primary Pipework
This may include the section between the heat pump and plant room, the heat pump and hydraulic separation, or the heat pump and main distribution connection. This section often carries the full system flow.
Main Distribution
As the system divides, the main branches carry only the load served downstream. That means the required flow may reduce.
Terminal Branches
The final branches to individual radiators or small groups of emitters may carry much smaller loads. A correctly designed system may therefore use several pipe sizes rather than one diameter throughout.
KEY TAKEAWAY
Required flow falls as the system divides, so pipe size may reduce downstream.
Existing Pipework Assessment
A heat-pump installation does not automatically mean replacing all existing heating pipework. The useful question is:
Can this section carry the required flow within acceptable hydraulic limits?
For each section, the result can be classified as:
Retain
The existing pipework is suitable for the required duty and flow.
Upgrade
The existing pipe is too restrictive or otherwise unsuitable.
Partial Upgrade
Only part of the run needs increasing.
Review
More information is required before a reliable decision can be made.
KEY TAKEAWAY
The answer is not always “replace everything”. It is “calculate the sections that matter”. This is particularly useful on retrofit projects where replacing all distribution pipework may be disruptive and unnecessary.
Required Flow Rate by Section
Every pipe section serves a particular heating duty. The required flow should therefore be linked to the load it carries. For example, the primary pipe may carry the full heat-pump output, a ground-floor branch may carry only the ground-floor load, and a bedroom branch may carry the load of one or two emitters.
The pipe schedule should make that relationship clear. This avoids blanket rules such as “all heat pumps need 28 mm pipe.” Sometimes they do. Sometimes they do not.
KEY TAKEAWAY
The answer depends on the actual required flow and circuit conditions.
Velocity Checks
Pipe size affects water velocity. If the bore is too small for the required flow, velocity can become unnecessarily high. That can contribute to higher resistance, increased noise, greater pump requirement and poorer hydraulic performance.
A pipe-sizing calculation should therefore consider more than whether water can physically pass through the pipe.
KEY TAKEAWAY
The section needs to carry the required flow sensibly, not just physically.
Pressure-Loss Checks
Pipe length matters. A short section of small-bore pipe may perform acceptably. The same diameter over a long run may create much more resistance. Pressure loss can also be affected by bends, tees, valves, fittings, pipe material, internal bore and flow rate.
Where relevant, these should be considered as part of the calculation.
KEY TAKEAWAY
A pipe can carry the flow and still be a poor choice once circuit resistance is considered. This is why pipe sizing should not be based on diameter alone.
Which Section Is Actually Causing the Problem?
The smallest pipe is not automatically the most restrictive part of the system. For example, a short 10 mm terminal branch serving a small bedroom radiator may be acceptable. At the same time, a much longer 22 mm primary run carrying the entire system flow may be the section that needs upgrading.
The important question is not what is the smallest pipe? It is which section cannot carry its required flow within acceptable hydraulic limits?
KEY TAKEAWAY
The critical section is the one that cannot carry the required flow within acceptable hydraulic limits. A good pipe schedule should make that section obvious.
Microbore: Retain or Upgrade?
Microbore should not automatically be condemned. It should be assessed. For each relevant section, consider heat duty, required flow, pipe length, internal diameter, velocity and pressure loss.
Some short terminal runs may be perfectly usable. Other long or heavily loaded sections may need replacing.
KEY TAKEAWAY
Microbore should be calculated, not automatically condemned. That gives the installer a much more useful answer than a blanket recommendation to repipe the entire property.
Explore: Microbore Pipework and Heat Pumps
Partial Upgrade Strategy
One of the most useful outcomes of a pipe-sizing assessment can be identifying exactly where upgrades are required. For example, the result may show upgrade heat pump to plant-room primary, retain main distribution, upgrade one long first-floor branch, and retain short terminal microbore runs.
That can reduce disruption, installation time, material cost, unnecessary floor lifting and customer inconvenience.
KEY TAKEAWAY
A targeted upgrade can be better than replacing an entire heating system unnecessarily. The objective is to fix the hydraulic constraint, not replace pipework for the sake of it.
Pipe Material and Internal Diameter
Nominal pipe size does not always tell the whole story. Different pipe systems can have different internal bores — for example copper, multilayer, plastic and barrier pipe.
Two products described by a similar nominal size may not provide identical hydraulic performance. The calculation should therefore consider the actual pipe system being used where that information is available.
This is especially relevant where plastic pipe inserts reduce bore, multilayer pipework is proposed, or existing pipe material differs across the property.
What You Need to Send Us
Typical project inputs include:
- System heat duty
- Selected heat pump or required heat output
- Design ΔT
- Proposed system layout
- Approximate pipe lengths
- Existing pipe sizes
- Pipe material
- Branch duties
- Zone loads
- Emitter loads
- Relevant valves and fittings where known
- Photos, sketches or drawings
The information does not need to arrive as a perfect CAD drawing. A clear sketch showing pipe route, approximate lengths, sizes, branches and loads can often provide the basis for the calculation.
Where something critical is missing, we raise a query before relying on an unsupported assumption.
Explore: Room-by-Room Heat Loss Calculations for MCS Installers
What You Receive
The aim is to provide a practical schedule the installer can work from. For each defined pipe section, this may include section reference, heat duty, required design flow, approximate length, existing pipe size, proposed pipe size, pipe material where relevant, velocity, pressure-loss basis, retain/upgrade/review status and notes.
The wider schedule can also identify the critical section, key assumptions, areas requiring further information, revision number and current issue date.
The output should tell the installer what size goes where and why.
Example Pipe-Sizing Schedule
A finished schedule may look something like this:
| Pipe Section | Duty | Design Flow | Existing Size | Proposed Size | Status |
|---|---|---|---|---|---|
| Heat pump to plant | 9.0 kW | 0.43 L/s | 22 mm | 28 mm | Upgrade |
| Main heating branch | 6.2 kW | 0.30 L/s | 22 mm | 22 mm | Retain |
| Bedroom branch | 1.1 kW | 0.05 L/s | 10 mm | 10 mm | Retain |
| Rear extension branch | 2.4 kW | 0.11 L/s | 15 mm | 22 mm | Upgrade |
A more detailed version can also include velocity, pressure loss, length, pipe material and notes on fittings or restrictions.
The purpose is to turn hydraulic calculations into clear installation decisions.
What if the System ΔT Changes?
The required water flow depends on the temperature difference across the system. If the design ΔT changes, the required flow can change too. That means the pipe schedule may need to be reviewed.
For example, a system designed around one flow/return condition may require different flow rates if the final operating strategy changes. That can affect primary pipe size, branch pipe sizes, velocity, pressure loss and pump requirement.
KEY TAKEAWAY
Change the design ΔT and the pipe calculation may change with it. This is why the design basis should be recorded clearly.
What Is Not Included in Pipe-Sizing-Only?
Pipe sizing is one part of hydraulic design. Unless separately agreed, the pipe-sizing service does not automatically include:
- Pump selection
- Pump duty calculation
- Buffer sizing
- Volumiser sizing
- Hydraulic separation strategy
- Low-loss header design
- Bypass design
- Control strategy
- Full hydraulic schematic
- Complete system design
You can keep those stages in-house. Or Heat Loss Hub can continue into full hydraulic design if required.
Explore: Heat Pump Hydraulic Design Service
Revisions and Design Changes
The pipe schedule is based on the design information supplied. If that information changes, the calculations may also change. Examples include a different heat pump selected, design load changes, flow temperature strategy changes, ΔT changes, emitter schedule changes, additional zone added, pipe route altered, longer run introduced or different pipe material selected.
Where the change affects the duty, flow or resistance, the schedule should be reviewed.
KEY TAKEAWAY
If the duty, route or design condition changes, the pipe schedule may need recalculating. Each revised schedule should be clearly identifiable so the installer knows which issue is current.
One Project, Batch Work or Ongoing Support
You can use the service as much or as little as required.
One Project
A single job has difficult existing pipework or an uncertain primary route.
Occasional Support
Your team normally completes pipe sizing internally but needs help on more complex systems.
Batch Work
Several projects are ready and you want the pipe schedules completed together.
Ongoing Support
Pipe sizing becomes a regular outsourced stage in your design process.
You remain in control of the wider hydraulic design.
White-Label Pipe Schedules
Pipe sizing can also sit inside a white-label workflow. Depending on the agreed arrangement, Heat Loss Hub completes the calculations, your business remains customer-facing, your customer remains your customer, the schedule can sit within your wider design pack and branding can follow the agreed delivery model.
This can provide extra technical capacity without changing the way your customer experiences your business.
Pipe-Sizing Quality Checklist
A useful pipe schedule should allow you to answer:
- What duty does each section carry?
- What flow is required?
- What pipe size is installed?
- What pipe size is proposed?
- What material is being assessed?
- What is the velocity?
- What is the pressure-loss basis?
- Which sections can stay?
- Which sections need upgrading?
- Which section is hydraulically critical?
- What assumptions were made?
- Which revision is current?
If those questions cannot be answered, the schedule may not be detailed enough to install from.
Frequently Asked Questions
Yes. If the rest of the system design is already established, Heat Loss Hub can complete the pipe calculations and return the schedule to your team.
The calculation considers the heat duty, design ΔT, required flow, pipe bore, length, velocity and pressure loss. The exact design basis depends on the system.
Heat pumps commonly operate with a smaller temperature difference between flow and return than traditional boiler systems. That can increase the water flow required to transfer the same amount of heat. The actual pipe size still needs to be calculated for the project.
Sometimes. Sometimes not. It depends on required heat duty, design flow, pipe length, pipe material, pressure loss and system arrangement. The heat-pump model alone does not answer the question.
There is no universal kW threshold that automatically means 28 mm is required. It depends on the required flow and hydraulic conditions of the actual pipe run.
Potentially. A 15 mm branch serving a modest load may be suitable. The required flow, length and resistance need checking.
Potentially. Some short microbore terminal runs can work. Longer or heavily loaded sections may be unsuitable. The correct approach is to calculate the actual circuit.
Yes. Longer pipe runs create more resistance than shorter runs of the same diameter at the same flow.
Yes. Bends, valves, tees and other fittings can add resistance and may need to be considered in the hydraulic assessment.
Pressure-loss checks can be included within the pipe-sizing assessment where required by the agreed scope.
Yes. Existing sections can be assessed and classified as retain, upgrade or review.
The required flow may change, which can affect pipe sizing, velocity and pressure loss. The schedule should be reviewed where the change is material.
Not automatically. Pump selection belongs to the wider hydraulic design unless it has been specifically included in the agreed scope.
Need the Pipe Sizes Without Outsourcing the Whole Hydraulic Design?
Send us the system duty and pipe layout. We calculate the required flow through each section, assess the existing pipework and return a clear retain-or-upgrade schedule.
SUBMIT A PROJECT
Send us the system duty and pipe layout and we’ll return a clear, section-by-section pipe schedule.
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