Mechanical ventilation with heat recovery (MVHR) and a split do different jobs. The split cools or heats the rooms it serves. The MVHR supplies and extracts air for the house. Running both means the cooling fights the ventilation rate, especially on a hot day when the supply air is warm. Do not switch the MVHR off, block terminals, or tape vents so the air conditioner looks stronger. Ask the ventilation designer how summer bypass and the cooling load sit together. The refrigerant work is still an F-gas install.
Cooled room air meeting a warm supply
MVHR brings outdoor air in and takes stale air out, with a heat exchanger that tempers the supply in winter. A split recirculates the air already in the room and adds or removes heat. When both run, some of the cooled air is extracted, and the supply air arrives at whatever temperature the MVHR is delivering.
That is a real load, not a fault in the split. The survey should include the ventilation rate if the designer can provide it, instead of sizing the room as if it were sealed. Leave MVHR running as designed. Keep cooling setpoints realistic on the hottest afternoons.
Bypass, the airtight hole and the supply jet
Many MVHR units can bypass the heat exchanger in summer so they do not recover unwanted heat. That is still ventilation. It does not remove the sun load from a glazed room. A split can do that cooling. The bypass decision belongs with the MVHR commissioning, not with a piece of tape over a ceiling valve.
If the house is very airtight, which is why the MVHR is there, opening windows at random while the split runs also upsets the balance. Agree with the designer when windows are for purge and when the house should stay closed.
A wall hole for refrigerant pipes is a hole in the airtight layer. It needs sealing to the standard the ventilation system was designed around, or the MVHR spends its life making up leakage. The MVHR installer and the air conditioning installer should know about each other's routes. Controls stay separate unless someone competent has designed a link. Do not put the split's sensor in the MVHR supply jet, or the unit will chase that air and short-cycle. Outdoor unit rules in England are on the Planning Portal.
Frequently asked questions
Should I turn MVHR off when the air conditioner is on?
No. MVHR is the ventilation. The split is heating and cooling. Leave the ventilation as the designer set it.
Can I tape the ceiling valves to keep the cool air in?
No. Those terminals are required for the ventilation to work. Taping them up is not a cooling method.
Will MVHR cool the house if I add a bypass?
Bypass stops the exchanger recovering heat. It does not give you the capacity of a split on a sunny room. Treat cooling as its own survey.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.