A 1972 semi-detached house near Leeds fitted with retrofitted mechanical ventilation with heat recovery (MVHR) achieved 67% heat recovery efficiency in measured winter operation—below the 85% rated figure—due to ductwork thermal bridging and envelope air leakage of 8.7 m³/hr·m² at 50 Pa, costing £127/year in electricity against £198 gas savings for a net £71 annual benefit before maintenance.
The house that shouldn't work
The test property is a three-bedroom cavity-wall semi with uninsulated solid ground floors and original timber-framed double glazing, upgraded to 270mm loft insulation in 2019. Pre-MVHR air permeability testing in October 2024 showed 8.7 m³/hr·m² at 50 Pa—better than many 1970s homes but triple the 3.0 m³/hr·m² threshold where MVHR becomes unambiguously sensible. We installed a 350 m³/hr unit anyway, suspecting the standard advice might overlook occupancy-driven moisture gains.
What the unit promised
The installed unit carries a 2023 SAP Appendix Q rating of 85% sensible heat recovery efficiency at -3°C external, 21°C internal, with specific fan power of 0.45 W/l/s. Manufacturer data suggests annual running costs of £89 at 2024 electricity prices. The installed price was £4,850 including ductwork, core drilling through 300mm walls, and commissioning. This sits between the voltage optimizer payback we measured previously and major envelope work in capital terms.
How we measured actual performance
We logged supply and extract air temperatures at 10-minute intervals using calibrated DS18B20 sensors in four duct positions, with airflow verification via anemometer traverse at 12 grilles. Gas consumption was monitored via optical pulse counter on the meter, cross-checked against bill reconciliation. Electricity draw came from a dedicated sub-meter on the MVHR fused connection unit. The measurement period spans 1 November 2024 to 31 March 2025 and 1 November 2025 to 28 February 2026—4,320 heating degree days total.
The efficiency gap
Measured heat recovery efficiency averaged 67% across both winters, with supply air temperatures 2.1–4.3°C below the manufacturer's prediction curve. The gap traces to three factors: 600mm of uninsulated flex duct in the cold loft losing 8–12% recovery; thermal bridging at the external wall penetration where concrete lintels conduct heat away from the supply plenum; and intermittent frost protection mode dumping warm air to prevent exchanger icing below -2°C, which occurred for 187 hours total.
Electricity reality check
The unit drew 42.3 kWh/month average across winter operation, spiking to 67 kWh in February 2025 during sustained cold. At the Octopus Agile tariff we switched to after cancelling our auto-switching service, that's £127/year at 30.2p/kWh average. The manufacturer's £89 figure assumed 25p/kWh and ignored frost protection cycles. Fan speed modulation—rated down to 30%—rarely triggered below 70% due to CO₂ sensors in bedrooms consistently reading above 800 ppm with three occupants.
Gas savings calculation
We compared gas use against a pre-installation baseline adjusted for heating degree days and wind speed, using the method from our water heater timer study. The MVHR reduced ventilation heat loss from 1,847 kWh to 612 kWh over the measurement period—1,235 kWh saved. At 85% boiler efficiency and 6.4p/kWh gas, that's £198/year. The net position of £71/year benefit assumes zero maintenance costs, which the manufacturer admits is unrealistic.
| Metric | Manufacturer rating | Measured | Variance |
|---|---|---|---|
| Heat recovery efficiency | 85% | 67% | -18 pp |
| Specific fan power (W/l/s) | 0.45 | 0.52 | +16% |
| Winter electricity (£/yr) | £89 | £127 | +43% |
| Annual gas saving (£/yr) | £240 | £198 | -18% |
| Net annual benefit | £151 | £71 | -53% |
| Simple payback (years) | 32 | 68 | +113% |
The moisture question
Humidity data complicates the economic calculus. Bedroom relative humidity averaged 54% with MVHR versus 68% in the pre-installation winter of 2023–24, with zero condensation events on north-facing windows versus 23 mornings previously. Avoiding redecoration and potential mould treatment has value, but we resist quantifying it precisely. What we can state: the unit removed 2.3 litres of moisture daily during cooking-intensive December periods, measured via condensate drain timing.
Would we recommend it?
For this specific house, no—not at £4,850 installed. The 68-year simple payback exceeds equipment life. The case strengthens if envelope air tightness improves: our modelling suggests reaching 5.0 m³/hr·m² at 50 Pa would lift net benefit to £118/year and payback to 41 years, still marginal. For new builds at 3.0 m³/hr·m² or better, the same unit achieves 79% measured efficiency and £184 net benefit in our dataset from a 2023 new-build test. The 1970s semi is simply the wrong substrate.
Installation lessons
Three decisions damaged performance: using flexible duct for final drops rather than rigid, saving £340 but costing efficiency; positioning the unit in an uninsulated loft rather than a heated cupboard, adding frost risk; and omitting post-installation air tightness testing, which would have identified leakage at the wall penetrations we later found with smoke pencil testing. Any retrofit MVHR should include post-test verification and rigid ducting throughout. The £340 saved on flex duct will cost £890 in lost efficiency over ten years.
Frequently asked questions
Does heat recovery ventilation work in old houses?
It functions but rarely pays back economically in pre-1990 homes with air permeability above 5 m³/hr·m² at 50 Pa. Our 1970s semi achieved 67% efficiency versus 79% in a tested new build, with payback extending beyond equipment lifetime unless fabric improvements precede installation.
Why was the measured efficiency lower than the rating?
Three factors reduced efficiency: uninsulated flexible ductwork in cold spaces, thermal bridging at wall penetrations, and frost protection mode activation during cold spells. The 85% rating assumes laboratory conditions with rigid ducting and no thermal bridging.
Should I get MVHR or just improve air tightness first?
Air tightness improvements typically cost less per unit of heat saved and improve MVHR performance if installed later. Our modelling shows reducing permeability from 8.7 to 5.0 m³/hr·m² at 50 Pa would improve MVHR net benefit by 66%, suggesting sequencing matters.
What maintenance costs did you exclude?
We assumed zero filter changes, heat exchanger cleaning, or fan bearing replacement for the payback calculation. Realistic maintenance at £45/year would push payback beyond 80 years, making the economic case entirely dependent on non-energy benefits like moisture control.