MVHR ducts and heat pump pipes: sealing the penetrations low-carbon homes depend on

The Future Homes Standard makes heat pumps the default for new homes in England from March 2027, and the tighter a home gets, the more likely it is to need mechanical ventilation with heat recovery (MVHR). Both are good news for energy use. Both also put large-diameter services through the airtight layer, and both rely on that layer to work.

An MVHR system can only recover heat from air that passes through the unit. Air leaking in around a badly sealed duct bypasses the heat exchanger completely. Get the duct penetrations wrong and you undo part of what you paid for.

MVHR ducts

A typical MVHR install has two ducts through the envelope, intake and exhaust, usually 125 to 160mm across and often insulated. Supply and extract ducts may also cross the airtight layer at ceilings, depending on where the unit and the airtight line sit.

• Expanding foam is not enough. Ducts carry vibration from the fan, and foam cracks. A duct penetration needs a flexible seal: a sized grommet, or a liquid grommet where the duct is oval or angled.
• Seal the duct, not just the insulation. On insulated ducts, the airtight seal has to reach the duct itself, or the insulation jacket has to be sealed to it at the airtight line. Otherwise air runs straight along inside the jacket.
• Keep the external ducts short and the penetrations close to the unit. Fewer and shorter cold ducts inside the envelope means less condensation risk and fewer crossings of the airtight layer.
• Ceiling penetrations. Where the airtight membrane is on the underside of a ceiling, every supply and extract valve that goes through it is a penetration. Use a service void below the membrane where you can, so the valves never cross it.

Heat pump pipework

An air source heat pump outside means flow and return pipes, a condensate line if it is a split system, and power and control cables, all coming through the external wall.

• Plan one entry point. Bring everything through at one location, ideally in a sleeve, so there is one detail to get right instead of four.
• Allow for movement. Heating pipes expand and contract every cycle. A flexible grommet copes; rigid mortar or filler does not.
• Seal both ends of any sleeve or duct. If the pipes run inside a sleeve or spiral conduit, air can travel along the inside of it from the outdoor unit straight into the house. Sealing the wall perfectly does not help if the sleeve is open at the other end.
• Weatherproof the outside face separately. The airtight seal belongs on the warm side. The external face needs its own weather seal, with the pipe falling slightly to the outside.

Condensate, PV and the extras

Heat pumps and solar bring small extras: condensate drains, PV cable from the roof, battery and inverter cabling, EV charger feeds. Each is small, but each is still a hole. Small single pipes and cables suit a small grommet. Groups of cables are better sealed with a liquid grommet such as The GOAT, which forms round the bundle, as shown in the key-shaped grommet.

Retrofitting a heat pump

A heat pump install is one of the best chances to improve airtightness in an existing home. The installer is making new penetrations and disconnecting old pipework anyway. Agree with them to:

1. fit grommets to new pipework before it is connected;
2. seal the old boiler flue and any redundant holes; and
3. leave nothing sealed only with foam.

Our retrofit penetrations guide covers pipes that cannot be disconnected.

Flues are different

A boiler flue, stove flue or other hot exhaust needs a detail rated for its temperature, and the flue manufacturer's clearance and sealing instructions come first. A standard airtight grommet is not a flue seal, and it is not a fire stop.

The airtight layer around it

A duct seal is only as good as the layer it joins. On masonry and timber frame, that layer is often Passive Purple, with the grommet flange bedded into it. See how to fit a grommet into a liquid membrane.

Our own airtight pipe grommet, TED (Tight Entry Detail), arrives in December 2026, with sizes up to large ventilation ducts. Talk to us about your MVHR or heat pump details now, and start with airtight pipe grommets explained if you are new to the subject.