Heat Pump Specification in London Renovation: Air Source, Ground Source, and the Path to Low-Carbon Heating

The heat pump has moved from a niche technology to the mainstream of prime London renovation in the space of a decade. The combination of the UK government's phase-out of gas boiler installation in new build (from 2025) and in existing homes (from 2035), the Boiler Upgrade Scheme grant (£7,500 for air source heat pumps), rising gas prices, and falling heat pump costs has made the business case for heat pump installation in a comprehensive renovation compelling.

But the transition from gas to heat pump is not a straight swap. A gas boiler works at high flow temperature (70–80°C) and can be oversized without significant efficiency penalty; a heat pump works most efficiently at low flow temperature (35–45°C) and is significantly penalised in efficiency by oversizing or by a poorly insulated building that forces high flow temperatures to maintain comfort. Specifying a heat pump correctly requires understanding the building's thermal performance, the distribution system, the hot water strategy, and the operational profile in a way that gas boiler specification never required.

How Heat Pumps Work

A heat pump moves heat rather than generating it. It extracts low-grade heat energy from the outdoor air (air source) or from the ground (ground source) and upgrades it to a useful heating temperature using a refrigerant cycle — the same thermodynamic process used in a domestic refrigerator, run in reverse.

The efficiency metric is the Coefficient of Performance (COP): the ratio of heat output to electrical energy input. A COP of 3.5 means that for every 1 kWh of electricity consumed, 3.5 kWh of heat is delivered — 1 kWh from electricity, 2.5 kWh from the environment. The Seasonal COP (SCOP) averages this efficiency across a full heating season; UK heat pumps typically achieve SCOP 2.5–4.5 depending on the system design and the building's heat emitters.

The COP is strongly dependent on the temperature difference between the heat source (outdoor air or ground temperature) and the flow temperature delivered to the heating system. The relationship is approximately: every 1°C increase in flow temperature reduces COP by approximately 2–3%. A system designed to deliver 35°C flow temperature achieves COP 4.0+; the same system forced to deliver 55°C flow temperature achieves COP 2.0–2.5. This is the fundamental reason why heat pump specification is inseparable from distribution system design.

Air Source vs Ground Source

Air Source Heat Pump (ASHP):

An ASHP extracts heat from outdoor air using an outdoor unit (a fan-driven heat exchanger, similar in appearance to an air conditioning condenser unit) connected to an indoor unit (the refrigerant-to-water heat exchanger, controls, and typically a buffer vessel and hot water cylinder).

*Advantages*: Lower installation cost (no ground works); compact outdoor unit footprint; works at air temperatures down to -15°C (modern units); can be installed on most London properties.

*Disadvantages*: Performance degrades in cold weather (the coldest days — when heating demand is highest — also produce the lowest COP); outdoor unit produces noise (fan and compressor) that must be considered in relation to neighbours and planning; outdoor unit requires a suitable location (not visible from the street in a Conservation Area without planning consent in some boroughs).

*Cost*: Supply and installation typically £8,000–£18,000 for a residential unit, before the £7,500 BUS grant (net cost after grant: approximately £500–£10,500).

*Appropriate for*: The majority of London renovation projects. The default heat pump type for a London terrace or townhouse with a suitable garden or rear elevation for the outdoor unit.

Ground Source Heat Pump (GSHP):

A GSHP extracts heat from the ground via a heat exchanger buried in horizontal trenches (requiring significant garden area — approximately 2× the floor area of the heated space) or in vertical boreholes (drilled to 80–150m depth, requiring much less surface area).

*Advantages*: Ground temperature is relatively stable year-round (10–12°C in London) compared to air, so performance does not degrade in cold weather; no outdoor unit noise; higher average SCOP (typically 3.5–5.0) than ASHP.

*Disadvantages*: Significantly higher installation cost due to ground works; horizontal collectors require a large garden (typically not available in London); vertical boreholes require specialist drilling and London geological assessment (made ground layers, contamination risk); planning consent may be required for borehole drilling.

*Cost*: Supply and installation typically £20,000–£45,000 for a vertical borehole system (2–3 boreholes for a large London house), before the £7,500 BUS grant.

*Appropriate for*: Large London properties with garden space for horizontal collectors, or properties where the premium performance and absence of outdoor unit noise justifies the higher cost.

The Building Fabric Prerequisite

The single most important factor in heat pump performance and running cost is the building's heat loss. A poorly insulated building with high infiltration requires a high heat input to maintain comfort; a well-insulated, air-tight building requires a fraction of that heat input.

Why this matters for heat pump specification:

A Victorian London terrace in unimproved condition may have a Design Heat Loss of 15–25 kW (the maximum heat input required to maintain 21°C inside at -3°C outside, the UK design temperature). The same house after solid wall insulation (internally or externally), floor insulation, window replacement, and air-tightness improvement may have a Design Heat Loss of 6–10 kW.

The heat pump must be sized to meet the Design Heat Loss — not the gas boiler it replaces, which was almost certainly grossly oversized. Installing a 16kW ASHP in a house with a 7kW design heat loss creates an oversized system that short-cycles (turns on and off repeatedly rather than running continuously), degrading efficiency and component life. A correctly sized 8–10kW ASHP running near-continuously on a cold day is far more efficient than an oversized unit cycling on and off.

The fabric-first principle: Before specifying the heat pump, the building fabric should be improved as far as is practical in the renovation scope. The sequence is: 1. Insulate walls (solid wall insulation — internal or external — for a Victorian property) 2. Insulate floors (over slab or below floorboards) 3. Replace windows (double or triple glazed) 4. Improve air-tightness 5. Commission an accurate heat loss calculation (to BS EN 12831 or equivalent) 6. Size the heat pump to the resulting Design Heat Loss

This sequence is important because the heat pump size is a capital cost; right-sizing it by improving the fabric first produces a smaller (cheaper) heat pump, lower running costs, and better efficiency.

Heat Emitter Design

The heat emitter — the terminal unit that delivers heat from the water circuit to the room — determines the flow temperature required and therefore the efficiency at which the heat pump operates. The priority is to design emitters that allow the lowest possible flow temperature.

Underfloor heating (UFH): The ideal emitter for a heat pump. UFH operates at 35–45°C flow temperature, which is in the sweet spot of heat pump efficiency. A house with whole-house UFH can be heated by an ASHP at SCOP 3.5–4.5 on a full year basis. Where UFH is not possible (existing floors that cannot be lifted, listed buildings with historic floors), low-temperature radiators are the alternative.

Low-temperature radiators: Radiators sized to deliver the required heat output at 45–55°C flow temperature rather than the standard 70–80°C. A low-temperature radiator is physically larger than a conventional radiator of the same output (because it operates at a lower temperature differential with the room) — typically 50–100% larger. In a renovation where radiators are being replaced, specifying oversized low-temperature radiators (or fan-assisted low-temperature radiators, which achieve the required output in a standard-size unit) allows heat pump operation at 50–55°C flow temperature, achieving SCOP 2.5–3.5.

Existing radiators (transitional installations): Where a heat pump is being installed in a property without a full renovation (keeping existing radiators), the radiators should be assessed for their ability to deliver adequate output at 50–55°C flow temperature. Many existing radiators, particularly double-panel convectors, can deliver adequate heat at 50–55°C in a reasonably well-insulated property, even if they were originally designed for 70–80°C. The heat loss calculation and emitter assessment determines whether existing radiators are adequate or must be replaced.

Domestic Hot Water

A heat pump heats water to the distribution temperature (35–55°C) efficiently; it can also heat domestic hot water (DHW), but DHW storage cylinders must be heated to 60°C to prevent Legionella growth — a temperature at which heat pump efficiency is significantly reduced (COP approximately 2.0–2.5).

The standard approach: - Heat pump heats a well-insulated hot water cylinder (typically 200–300 litres for a 4–5 person household) to 55°C via an indirect coil — the primary hot water temperature most heat pumps achieve efficiently - Once per week (or as required by risk assessment), the cylinder is boosted to 60°C by an immersion heater for Legionella prevention (a "pasteurisation cycle") — a short, low-energy event - Some heat pumps can reach 60°C at reduced efficiency if the manufacturer specifies this; this eliminates the need for the immersion heater boost

The hot water cylinder must be correctly sized and well-insulated (a heat loss of less than 1.5 kWh/day from a 250-litre cylinder is the target). Undersized cylinders run out of hot water during peak demand; oversized cylinders require more energy to maintain temperature. A specialist heat pump installer will size the cylinder to the property's occupancy and demand profile.

Planning and Installation Considerations in London

Permitted Development for ASHP: An air source heat pump is permitted development (no planning application required) if it meets the conditions of Class G, Schedule 2, Part 14 of the GPDO: - One unit per dwelling only - Minimum 1m from the property boundary - Not on a wall or roof facing a highway (including a footway or bridleway) - Not in a Conservation Area on a wall or roof visible from a highway - Noise level at 1m from a neighbour's window: no more than 42 dB(A) - Must comply with the MCS planning standard

In a Conservation Area, an ASHP outdoor unit located in the rear garden (not visible from the highway) is generally PD. An ASHP on the front elevation is not PD in a Conservation Area.

MCS certification: To qualify for the Boiler Upgrade Scheme grant, the heat pump installation must be by an MCS-certified installer and the system must be designed and installed to MCS 020 (heat pump planning and installation standard). The MCS certification requirement includes an accurate heat loss calculation, correctly sized emitters, and a commissioned system with the MCS Microgeneration Certification Mark.

Running Costs

The running cost of a heat pump relative to gas depends on: - The electricity-to-gas price ratio (currently approximately 3:1 in the UK — electricity costs approximately 3× more per kWh than gas) - The SCOP of the heat pump (a SCOP of 3.0 means the effective cost per unit of heat is equal to gas at a 3:1 price ratio; SCOP above 3.0 means heat pump is cheaper to run than gas)

A well-specified ASHP with UFH distribution in a well-insulated London house typically achieves SCOP 3.2–4.0 — making it cheaper to run than gas at current prices, with a further advantage as gas prices increase and electricity prices decrease (driven by growing renewable generation).

Budget Summary

These figures exclude the M&E design, the UFH or low-temperature radiator installation, and any electrical supply upgrades required for the heat pump circuit. A comprehensive heating system replacement (heat pump, UFH, hot water cylinder, controls) in a 250m² London townhouse: total installed cost approximately £30,000–£60,000 before grant.