Heat pumps in an energy strategy: what officers check

The three things a planning officer and their energy consultee actually test in a heat pump strategy are whether the seasonal performance figure matches the flow temperature the emitters are sized for, whether the external unit has a location that works acoustically and visually, and whether the noise assessment uses the right assessment positions. Everything else in a typical energy statement is accepted or lightly queried.

Get those three right and the strategy passes. Quote a seasonal coefficient of performance taken from a manufacturer's headline figure at 35°C flow while the drawings show standard radiators, and the consultee will say the carbon saving is overstated, because it is.

Where a heat pump sits in the energy hierarchy

Most English authorities assess energy strategies against a hierarchy of demand reduction first, then efficient supply, then renewable or low-carbon supply, then reporting. In London that is the Be Lean, Be Clean, Be Green and Be Seen sequence in the London Plan, and the carbon saving is reported at each stage against a notional baseline.

Heat pumps now carry most of the supply-side saving on all but the largest schemes, partly because the emission factors used in current compliance calculations treat grid electricity far more favourably than they once did. Where a scheme follows the London hierarchy, agree with the authority whether the heat pump is reported at the clean or the green stage before you build the table, because a strategy that reports it in the wrong place reads as though a stage has been skipped.

On sites inside or adjacent to a heat network priority area, expect a question about connection before any individual plant is accepted, and expect to show the comparison. Where connection is not currently viable, the safeguarding question becomes live, and the practical measures are set out in safeguarding a future heat network connection.

Seasonal performance and how it is evidenced

The seasonal coefficient of performance is the figure that converts a heat demand into an electricity demand, so the whole carbon calculation hangs on it. What a reviewer looks for:

• The flow temperature the figure relates to, stated explicitly. A performance figure at 35°C and the same unit at 55°C are different numbers, and the difference is substantial.
• The source of the figure: a product database entry, a manufacturer declaration against the relevant test standard, or an assumption. Name it.
• Consistency with the compliance calculation, so that the figure in the energy statement is the figure the software used rather than a more flattering one from a datasheet.
• Treatment of domestic hot water separately from space heating, because hot water is produced at a higher temperature and at a lower efficiency, and a single blended figure hides that.
• Defrost cycles, auxiliary immersion use and distribution losses, which a headline figure excludes.

Where the design is early and the unit is not selected, use a conservative figure and say that it is conservative. A strategy built on an optimistic assumption has to be revisited at discharge of condition stage, in front of an officer who now has the as-built evidence.

Flow temperatures and emitter sizing

This is where energy statements and architectural drawings most often disagree. A heat pump running at 35°C needs emitter surface area far greater than a gas boiler system at 70°C, which in practice means underfloor heating or radiators two or three times the size.

The consequences for the drawings are real:

1. Radiator sizes change room layouts, particularly under windows in bedrooms and in bathrooms.
2. Underfloor heating changes floor build-ups, which changes floor-to-floor heights and sometimes the overall building height.
3. Hot water needs a cylinder, typically in the range of 180 to 250 litres for a family house, in a location with access for maintenance and with the airing cupboard space the original plan may not have.
4. Pipework sizes increase at lower temperature differentials, which matters in risers on flatted schemes.

If the energy statement assumes 35°C and the drawings show conventional radiators, the officer is entitled to conclude that either the carbon saving or the drawings are wrong. Reconcile them before submission, and say in the statement which flow temperature the scheme is designed around.

Plant location, space and acoustics

External units need airflow, clearance for maintenance, protection from recirculation of their own discharge air, and a position where the sound they make is not pointed at a bedroom window. Those requirements compete with the ones the design team cares about, which are that the unit should not be visible from the street and should not occupy usable garden.

On a flatted scheme the options are roof-mounted plant, a plant enclosure at ground level, or individual units on balconies, and each has a different acoustic and visual consequence. Roof-mounted plant needs a screen that is itself a design element, and the screen changes the roofline the elevations show. Balcony-mounted units are the worst acoustically because they sit beside habitable rooms, their own and the neighbours'.

Show the unit on the elevations, not only on a services layout. An external unit that appears for the first time at building control stage, on an elevation the planning permission did not show, is a departure from the approved drawings.

Refrigerant choice has a reporting dimension too, and on schemes where whole life carbon is assessed it is worth settling early, as refrigerant choice and global warming potential explains.

Noise conditions and the permitted development route

For a heat pump on a house, Class G of Schedule 2, Part 14 to the General Permitted Development Order 2015 permits installation subject to conditions covering the size of the unit, its siting and compliance with the microgeneration certification scheme's planning standard for noise. Those conditions have been amended more than once, so check the current text of the Order rather than a summary, because the limits on volume, position relative to a boundary and the number of units permitted have all moved.

The certification scheme's planning standard works to a limit at a defined assessment position, taken at a window of a habitable room of a neighbouring property, using the unit's declared sound power level and the distance and screening between. The figure in use is 42 dB(A), and the assessment is a calculation rather than a measurement. Check the current version of the standard before relying on the figure.

Where the installation is outside the permitted development route, or the scheme is a flatted or commercial one, the assessment is normally a BS 4142 exercise instead: the rating level of the plant against the measured background sound level at the nearest noise-sensitive receptor, with a correction for acoustic features such as tonality or intermittency, and with the context considered. A rating level around 5 dB above background indicates an adverse impact and a rating level at or below background indicates a low impact, with everything in between a matter of judgement and context.

Two practical points. Measure the background at the right time, which for heat pumps usually means night-time when the unit may be running and background levels are lowest. And do not design to the limit, because the measured outcome after installation on a site with hard surfaces and reflective facades is rarely the modelled one.

Interaction with the fabric specification

A heat pump and a poor fabric specification is the worst combination available. Lower heat loss means a smaller unit, a lower flow temperature, a better seasonal performance and less noise, and all four of those help the planning case. The fabric work costs less than the plant upsizing it avoids.

The junction details matter as much as the U-values, because the thermal bridging allowance in the calculation has to be buildable, which is the point made in thermal bridging detail at planning stage. Officers do not usually interrogate psi values. Their energy consultee sometimes does.

Overheating pulls the other way on glazing, so run that assessment alongside rather than afterwards.

Evidence at discharge of condition stage

A typical energy condition requires the development to be carried out in accordance with the approved strategy and evidence to be submitted before occupation. What to assemble:

• The as-built compliance calculation and its output report, matched to the plot or block.
• The selected unit's datasheet with the declared performance at the design flow temperature.
• The commissioning record, including the flow temperature actually set.
• The noise calculation or assessment as installed, with the assessment positions marked on a plan.
• Photographs of the installed unit in position, with any acoustic screen.

Where the installed unit differs from the one assumed, say so and show the recalculation rather than hoping the discrepancy goes unnoticed. A short covering note comparing approved against as-built, prepared alongside the Energy Statement, closes conditions faster than a bundle of certificates with no narrative.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

To see which energy and environmental documents a scheme needs, enter your postcode on PlanScope and work through the guided questions. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.