Heat Pump Pipework Upgrade Requirements UK 2026

Heat delivered = flow rate × temperature drop across radiator × specific heat capacity of water.

Gas combi at 70C flow / 50C return (Delta-T 20): Each litre/sec of water delivers 84 kW heat.

Heat pump at 50C flow / 45C return (Delta-T 5): Each litre/sec of water delivers only 21 kW heat - 4x less than gas.

This means heat pumps need 2-4x higher water flow rates to deliver the same heat output. Result: bigger pipes (less friction loss) + harder-working pumps.

Practical implication:

• 22mm pipe carries ~16 L/min at acceptable head loss.
• 28mm pipe carries ~38 L/min - 2.4x more.
• For a typical 7-9 kW heat pump, primary loop needs 28mm. Period.

28mm pipework (was 22mm in gas systems):

• Primary flow + return: heat pump → cylinder coil + manifold start.
• Whole-house primary loop on >7 kW heat pump.
• Mandatory for heat pump install > 7 kW per MCS guidance.
• Cost premium vs 22mm: ~30% more for copper, ~15% more for plastic.

22mm pipework (was 15mm in gas systems):

• Branches feeding multiple radiators (3-5 radiators on one circuit).
• Feeds to ground-floor radiator banks.
• Upper-floor branch trunks.

15mm pipework (unchanged from gas systems):

• Individual radiator drops (final 1-2m to each radiator).
• Towel rail feeds.
• Underfloor heating manifold loops (typically 15mm or 10-12mm UFH-specific PEX).

10mm microbore pipework (LEGACY ONLY - replace):

• Common in 1970s-1990s UK central heating systems.
• Carries insufficient flow for heat pump operation - acts as flow bottleneck.
• Replace with 15mm minimum on every radiator drop.
• Adds ~GBP 1,000-2,500 to retrofit cost; non-negotiable for heat pump operation.

Most modern UK heat pumps (R290 Vaillant aroTHERM, Daikin Altherma 3, Mitsubishi Ecodan) include an internal pump rated for ~4-5m head, sufficient for typical UK 3-bed install with 28mm primary + 22mm branches.

Secondary circulator (booster pump) typically needed when:

• Total pipe run > 30m (primary + branch).
• UFH manifold + radiator circuits both present (split flow needs).
• Heat pump > 12 kW (proportionally more flow demanded).
• Hydraulic separator + low-loss-header configuration.

Pump head calc rough rule:

• Primary flow + return resistance: 0.05-0.10m per linear metre of 28mm pipe.
• + radiator + TRV + manifold resistance: 1-2m total.
• + cylinder coil resistance: 0.5-1m.
• Total typical UK 3-bed: 3-5m head. Heat pump internal pump suffices.

If install design exceeds 5m head, specify a secondary circulator (Wilo Stratos PICO 25/1-6 or similar at ~GBP 200-300) at the design stage rather than retrofitting later.

Heat pumps work best with steady continuous flow. Two design features ensure this:

1. Lockshield balancing valves (each radiator):

• One TRV on supply side, one lockshield on return side per radiator.
• Lockshield restricts flow to that radiator - achieves design Delta-T (typically 5C).
• Without balancing, hot water short-circuits to the nearest radiator + cool rooms.
• Heat pump install MUST include balancing - typically GBP 200-500 commissioning fee.

2. Auto-bypass valve (single):

• Located near heat pump or main flow manifold.
• Opens at high system pressure (TRVs all closed = backed-up flow).
• Prevents heat pump short-cycling on rapid demand changes.
• Cost: GBP 50-150 valve + install.

3. Low-loss header / volumiser tank (large installs):

• 40-50L thermal store decouples heat pump flow rate from radiator demand.
• Reduces short-cycling at low demand.
• Common on 12+ kW heat pumps; rare on 4-9 kW UK domestic units.
• Cost: GBP 300-500.

Scenario 1: Modern (post-2000) 15-22mm pipework, no microbore:

• Upgrade primary loop 22mm → 28mm (heat pump → cylinder + manifold start).
• ~10-15m new 28mm copper + fittings: GBP 400-700.
• Labour: 4-6 hours: GBP 250-400.
• New balancing valves + auto-bypass: GBP 150-300.
• Total: GBP 800-1,400.

Scenario 2: Mixed 10-15-22mm with some microbore drops:

• Primary loop 22mm → 28mm: GBP 400-700.
• Microbore replacement on 4-6 radiator drops (15mm): GBP 800-1,500.
• Branch 15mm → 22mm where needed: GBP 300-600.
• Balancing + bypass: GBP 150-300.
• Total: GBP 1,650-3,100.

Scenario 3: Full 1970s-1980s microbore system:

• Full repipe primary + branches + drops: GBP 3,000-5,500.
• Includes lifting floors / running new runs.
• Typically requires 2-3 day install.
• Total: GBP 3,000-5,500.

1. Confirm primary loop is 28mm for heat pumps >7 kW.
2. Heat-loss calculation per room (NOT 'rule of thumb') determines branch sizing.
3. Microbore replacement quoted separately with itemised radiator drops + labour.
4. Balancing commissioning included - all TRVs + lockshields set, written record.
5. Auto-bypass valve installed on flow manifold.
6. Pump head calc shown - prove the heat pump internal pump suffices OR secondary circulator quoted.
7. Magnetic system filter (MagnaClean or similar) on return - protects heat pump exchanger from system sludge.
8. Inhibitor + glycol fill spec shown - 30-40% propylene glycol for frost protection; system inhibitor X100 standard.