Heat pumps and MVHR in a new build: what you need to know

A heat pump and a mechanical ventilation system are two of the best things you can put in a new house, and two of the easiest to get wrong. Neither is complicated. Both depend on decisions made long before the installer arrives, and most of those decisions are about the house rather than the kit.

This isn't a guide to brands or models. It's what needs thinking about so that whichever you choose works properly.

Build it tight, then ventilate it

New houses are built to be far more airtight than older ones, and they're air tested before building control will sign them off. That's good for heating bills, but the fresh air that used to leak in through gaps now has to be designed.

There are two broad ways to do that. One is trickle vents in the windows and extract fans in the kitchen and bathrooms. The other is mechanical ventilation with heat recovery, or MVHR. An MVHR unit draws stale air out of the kitchen, bathrooms and utility, supplies fresh air to the living rooms and bedrooms, and passes the two airstreams through a heat exchanger so the outgoing warmth heats the incoming air without the two mixing.

MVHR only earns its place in a house that is genuinely airtight. In a leaky house, air goes round the system rather than through it. That means the airtight layer has to be designed, and then protected through the whole build. Every cable, pipe and duct that passes through it has to be sealed, and the trades who make most of those holes arrive after the shell is finished.

The air test doesn't really measure the walls. It measures every hole that every trade made in them afterwards.

Design the heating and ventilation together, and early

The heat pump should be sized from a room-by-room heat loss calculation, not a rule of thumb. That calculation depends on the insulation, the glazing and the airtightness, so it can't be done properly until those are settled, and it should be revisited if they change.

MVHR needs duct routes. Ducts are bulky, they have to run between the unit and every room, and they have to cross the structure somewhere. If nobody plans those routes before the floor structure is designed, they end up boxed in below ceilings or fighting with steelwork and drainage. Open-web joists, which let ducts and pipes pass through the floor zone, are one common answer, but they have to be chosen at design stage.

Your energy assessor's calculations for building control will make assumptions about the heating, the ventilation and the air test result. What's built has to match them, so changes on site need to go back to the assessor.

Overheating is part of this as well. New homes have to meet the overheating requirements in the building regulations, which deal with glazing, orientation and how heat is got out of the house in summer. MVHR helps with fresh air, but it isn't air conditioning, and it won't cool a house with too much unshaded glass.

The energy rules for new homes have been tightening, and there are further changes in progress. Check what applies to your project with your energy assessor and building control when you apply, rather than relying on what a neighbour's house needed.

Heat pumps and underfloor heating

Heat pumps work most efficiently when they're producing water at a lower temperature than a gas boiler would. That's why they pair so well with underfloor heating, which warms a large surface gently, or with radiators sized larger than you'd fit with a boiler.

A few things follow from that:

• The screed that the underfloor pipes sit in has to dry properly before floor finishes go down. It's a stage in the programme, not a gap in it.
• Floor finishes affect the output. Thick carpet and underlay, or thick timber boards, hold the heat back, so choose finishes with the heating designer rather than afterwards.
• Controls work differently. Heat pumps generally prefer to run steadily, and the control strategy should be set by the designer, not carried over from a boiler.
• Hot water usually comes from a cylinder rather than a combi, so the cylinder needs a home.

An air source unit needs an outside position with free airflow, away from bedroom windows and neighbours. A ground source system needs boreholes or trenches, which have to be dug before the landscaping, not after it.

Room for the plant, and a proper handover

Allow space for the hot water cylinder, the heat pump controls, the underfloor manifolds and the MVHR unit, ideally together in a plant room or utility where they can be reached. An MVHR unit in a cold loft is possible, but the ducts need insulating and someone has to be able to get to it to change the filters.

Check the electrical supply early, too. A heat pump, an induction hob and an electric car charger together can need more capacity than a standard connection provides, and upgrades go through the local network operator.

At the end, both systems have to be commissioned. The ventilation is balanced and its airflows measured, the heat pump is set up and tested, and the results go to building control. Then someone needs to show you how it all works.

If you're planning a new home, send us the drawings or the plot, and you'll get a price for the build with the heating and ventilation in it from the person who would run the job. The order of work is set out in from empty plot to keys, and the build rates are on our cost guide. More on how we build is on our new build homes in Cheshire page.