commercialhvacuk
Get a Quote

Can MVHR cool a building?

Updated 23 October 2026 · SEO Dons Editorial

What does MVHR do on a warm day?

A heat exchanger passes heat from the warmer air stream to the colder one, so its effect depends on which is warmer. CIBSE Journal's MVHR module, written about dwellings but the same physics, says a thermal bypass operates when heat recovery is not advantageous. It adds that in the warmer season, if the incoming air is warmer than the extract air, recovery takes place and the extract air cools the incoming air. Cooling with outdoor air tends to be limited to early evening and morning, when outdoor temperatures are below the room's.

The ecodesign regulation requires every non-residential bidirectional ventilation unit to have a heat recovery system with a thermal by-pass facility. Annex I defines that as any arrangement that circumvents the heat exchanger or controls its performance without necessarily needing a physical airflow bypass, for example a summer box, rotor speed control or control of air flow. For a thermal wheel, CIBSE Journal's non-residential module says the bypass is achieved by stopping the rotation.

WeatherWhat the exchanger would doBypass
Cold dayWarms the incoming air with extract heatClosed
Cool morning or night, room warmWould warm the cooler incoming air, undoing the coolingOpen
Hot afternoon, outdoors warmer than roomPre-cools the incoming air using the cooler extract airClosed

The bypass adds no cooling capacity. It only stops the exchanger wasting what cool outdoor air can offer, and a CIBSE Journal trial in a house, with recovery running but cooling switched off, saw extract temperatures reach 30°C.

How much cooling can ventilation air carry?

Sensible cooling in kilowatts is airflow in cubic metres per second, times about 1.21, times the gap between room and supply temperature. The 1.21 comes from the air constants in CIBSE Journal's heat recovery module (specific heat 1.005 kJ per kg K, specific volume 0.83).

For example, take a 200 square metre office for 20 people. At the 10 litres per second per person that HSE says many guides recommend, ventilation is 200 litres per second. Assume a 24°C room, and peak summer heat gains of 40 watts per square metre (people, IT, lighting and sun), which is 8 kW. These are placeholders for a real load calculation.

Supply air temperatureCooling carried at 200 l/sShare of the 8 kWAirflow needed to carry all 8 kW
20°C0.97 kW12%1,650 l/s (83 per person)
18°C1.45 kW18%1,100 l/s (55 per person)
16°C1.94 kW24%830 l/s (41 per person)
14°C2.42 kW30%660 l/s (33 per person)
12°C2.91 kW36%550 l/s (28 per person)

At 16°C the ventilation air carries about a quarter of the load, and carrying all of it would take about four times the ventilation airflow, with matching fans, ducts and filters. Colder supply air helps, but HSE says workers should not be exposed to uncomfortable draughts, and CIBSE Journal notes a bypass modulates to avoid draught or condensation. A heat pump trial it reports brought supply air to about 15°C.

The same arithmetic shows what recovery does on a hot day. Untreated 28°C air into the 24°C room adds 0.97 kW. A heat exchanger at 73%, the ecodesign minimum for most exchanger types, pre-cools it to about 25.1°C and cuts that to 0.26 kW. A coil then takes the air from 25.1°C to 16°C, a 2.2 kW duty against 2.9 kW without recovery. These figures are sensible heat only; dehumidification is extra. In a low-gain building at 20 W per square metre (4 kW), the same 1.94 kW would be nearly half the load, so the answer depends on the gains.

How is cooling added to an MVHR unit?

OptionHow it worksLimit
Bypass and night purgeCooler outdoor air passes the exchanger unwarmedNeeds cool nights; no help on a warm night
Chilled-water coilA reversible or cooling coil after the exchanger, fed from a chillerNeeds chilled-water pipework; duty is capped by airflow
Refrigerant (DX) coilRefrigerant coil in the unit, linked to an outdoor unit or a variable refrigerant flow systemBrings refrigerant duties (see the F-gas guide)
Integrated heat pump moduleHeat pump built into the unit heats or cools the supply airCIBSE Journal: heat pump size and outdoor airflow set the maximum output

CIBSE Journal's non-residential module lists reversible coil options for chilling and heating plus heat recovery, and says that where a heat pump is used the refrigerant section is normally supervised by the heat pump unit, with the MVHR coil control acting as a slave. The Energy Technology List adds that some heat recovery ventilation units can also cut air conditioning energy by removing heat from the incoming air.

Night purge works best with exposed thermal mass: CIBSE Journal's Simon Sainsbury Centre case study uses a night purge to cool the structure through the night. Research on care homes found night ventilation became less effective as night-time temperatures rose.

How are the controls arranged?

CIBSE Journal's heat pump-augmented MVHR module, for a dwelling-scale unit, shows a sequence worth asking any designer to match.

  1. Cool weather: the exchanger recovers heat and the coil heats only if needed.
  2. Mild weather: recovery reduces or shuts off, and a bypass brings in cooler air.
  3. Outdoors warmer than inside: the exchanger reduces the incoming air temperature towards the indoor one.
  4. Still too warm: the coil cools, potentially also dehumidifying.

Ask for the guards too: a dew-point limit (CIBSE Journal says a modulating bypass keeps some recovery to hold supply air above the indoor dew point), a supply-temperature floor, and a lock-out so heating and cooling coils never fight. The sequence of operation should be written down and tested at commissioning.

When is MVHR with cooling a sensible choice?

  • Low gains. Shaded, well-insulated rooms with modest equipment loads, where the ventilation air can carry a large share of the load.
  • Refurbishments. Where ducts or single-room units are going in anyway; see decentralised MVHR. A unit moving 80 litres per second carries only about 0.8 kW at an 8 K difference.
  • Schools. CIBSE Journal's summary of BB101 says mechanical ventilation should not be the sole means of summertime ventilation in occupied spaces, with windows or vents of about 3 to 5% of the floor area.
  • Not suited. High-gain spaces such as dense IT or glazed south-facing floors, where conventional cooling carries the load and ventilation is sized for air quality.

Whether the heat exchanger pays back is a separate sum, set out in the payback guide, and the wider system types are on the commercial ventilation and MVHR page. You can send an enquiry through the form on this site.

Sources

Common questions

Does the summer bypass make the air cooler?

No. It stops the heat exchanger from warming cooler outdoor air with warm extract air, so the air arrives at outdoor temperature. That helps only when outdoor air is below the room temperature, such as mornings, evenings and nights. On a hot afternoon the bypass stays closed and the exchanger pre-cools the incoming air instead.

Can one unit heat and cool the supply air?

Yes, where a heat pump or reversible coil is built in. CIBSE Journal describes a dwelling-scale unit that does this with an air-to-air heat pump, bringing the supply air to about 15 degrees in its trial. The output is limited by the heat pump size and the airflow, so check the capacity at the real airflow.

Does a cooling coil also dry the air?

It can. CIBSE Journal says a coil in cooling mode provides cooling and, potentially, dehumidification. Dehumidification adds to the coil duty and produces condensate, which the sensible-heat figures above do not include.

Does night purge still work in hotter summers?

Less well as nights warm up. A CIBSE Journal care-home study found night ventilation became less effective as night-time temperatures rose, and in some cases increased overheating. A design that relies on night cooling should be tested against warmer weather files.

Ask for commercial HVAC quotes

SEO Dons Ltd publishes this site and replies to every enquiry by email. It does not install, service or maintain HVAC.

Get a quote
Get a quote