New lives for old chimneys

New Lives for Old Chimneys...

...or ideas for heat recovery in older houses

In olden days...

Right through the first half of the twentieth century the typical house had a fireplace in each of its principal rooms. A pair of semis or terrace houses might share a chimney stack with eight chimney pots, one for the front room, dining room and front and back bedrooms in each house. These fireplaces were open to the rooms and even without a fire thermal convection caused volumes of warm air from indoors to escape up the chimney to the outside, replaced by cold air from outside drawn in through air bricks or as draughts under doors and around leaky windows. With a fire lit to really drive convection, this loss of heat was multiplied.

When I was a boy...

Since the sixties central heating grew to be almost universal and houses were built with fewer chimneys and most new homes now have none. Older houses were retrofitted with boilers and radiators and fireplaces were blocked up. Building regulations have required increasing levels of insulation and air tightness, reducing fabric heat losses (through walls, roof, doors and windows and floors, but necessitating extract fans and trickle vents to give adequate ventilation. With more insulation fabric losses have reduced but extract fans pump warm air straight out of the home pulling cold air in to replace it, so ventilation heat loss becomes a major factor.

These new-fangled ideas...

A tiny (but increasing) number of new homes are built to the Passivhaus standard or close to it. They incorporate very high levels of insulation and air-tightness to minimise the need for heating. They still need ventilation though, but rather than throw heat away via extract fans they have mechanical ventilation with heat recovery (MVHR). A central unit, often sited in the roof space, has fans to extract moist or smelly air from the kitchen and bathroom and pull in fresh air from outside and filter it to supply to the living spaces and bedrooms. That's the MV bit. The HR bit is the clever part and is amazingly simple: a heat exchanger with a double labrynth of air channels bringing the warm extract air into close contact with the cooler fresh air supply without letting them mix.

Imagine the heat exchanger channels as two ducts sharing a thermally conductive wall. The temperatures in this explanation are based on actual measurements I took on our own MVHR system. The house is heated to 21-22C while the outside temperature is around 10C. The fans run quietly but continuously passing outgoing and incoming air in opposite directions. The extract air from the kitchen and bathrooms enters the heat exchanger at 22C while clean, fresh air arrives in the house at 20C. Outside, the supply air is drawn in at the ambient temperature of 10C while the expelled air is a little warmer at 12C. All the way through the heat exchanger the extract air is a couple of degrees warmer than the supply air but is giving up it's heat to warm the incoming air - heat recovery. Instead of throwing out 11-12C of heat just around 2C is lost and around 85% is retained. Ventilation heat loss is slashed.

Distributing air to and from all the rooms in a house from a central MVHR unit can involve quite a lot of ducting. If it is designed into a new house this need not cause any problems. Even in many existing homes - especially bungalows - it may still be easy by using the roof space. In a two or three storey house, though, routing ducts from several rooms on lower floors to a central point and then arranging main supply and exhaust ducts to the outside can be awkward. Which is where I want to come back to the chimney.

Retrofit for purpose

With a very high proportion of Britain's carbon footprint being due to domestic heating, it is essential to make homes more energy efficient. Standards are very slowly improving for new build but we build relatively few new houses and most of the nation's homes are old and leaky. Repair is less wasteful than replace so even if we could demolish and rebuild like they did in the sixties, improving the existing housing stock is a better solution - retrofit. Going beyond the usual cavity-wall and loft insulation and solar panels to wide-ranging improvements to wall, floor, door and window insulation, air-tightness and low-carbon heating is even more effective - deep retrofit.

Once insulation has been radically increased and fabric heat losses minimised, ventilation heat loss becomes more significant and throwing warm air out of extract fans seems an even dafter idea. The answer, we have established, is heat recovery, but retrofitting a whole-whose ventilation system, like insulating walls and floors, can be difficult and disruptive. But what if our house has a chimney? Around 53% of homes were built before 1965 and were invariably built with chimneys. Disused and sealed off since the central heating went in, the chimney provides an air channel connecting both floors of the house to the outside via the roof space. In many homes it would be possible to make use of it to route ventilation ducts.

Illustrated is an example where the MVHR heat exchanger and fan unit is sited in the roof space extracting air (orange) through the bathroom ceiling and via a duct from the kitchen, routed through the airing cupboard, and out to the upper part of the chimney where it is exhausted (green) to the outside having given over its heat to warm up fresh air (blue) taken from the well-ventilated roof space and fed (yellow) into the bedrooms though the ceilings and to the ground-floor living spaces via the lower part of the chimney stack. Using the the roof space, a redundant chimney and a first-floor cupboard minimises clutter in the rooms themselves an old house might be brought up to near Passivhaus standard.