Five Minute Guide to... Me­chan­i­cal Ven­ti­la­tion with Heat Re­cov­ery

five-minute guide to Sus­tain­abil­ity ex­pert Tim Pullen ex­plains the pros, cons and costs — and why a MVHR sys­tem is es­sen­tial to an air­tight home

Homebuilding & Renovating - - Contents -

Sus­tain­abil­ity ex­pert Tim Pullen ex­plains the pros, cons and costs — and why a me­chan­i­cal ven­ti­la­tion with heat re­cov­ery (MVHR) sys­tem is es­sen­tial in an air­tight home

Ven­ti­la­tion is nec­es­sary for a healthy liv­ing en­vi­ron­ment and a me­chan­i­cal ven­ti­la­tion sys­tem may be the an­swer — with or with­out the heat re­cov­ery bit. Sell­ers of such sys­tems – the good ones, at least – will tell us that we need an ‘air­tight’ house for the sys­tem to be ef­fec­tive. But ‘air­tight­ness’ is not an ab­so­lute. It is mea­sured on a slid­ing scale and there are points on that scale where a me­chan­i­cal ven­ti­la­tion with heat re­cov­ery (MVHR) sys­tem will work and points where it will not. An ef­fec­tive ven­ti­la­tion sys­tem will be part of an over­all de­sign and it can be ar­gued that it is an es­sen­tial box to be ticked in the de­sign of an en­ergy-ef­fi­cient house.

How Does MVHR Work?

There will be an air-han­dling unit, nor­mally in the at­tic or in a plant room, and duct­ing to each room. Some of that duct­ing will draw out stale air and other duct­ing will re­place it with fresh air. The heat re­cov­ery bit is where the warm, stale air is passed through a heat ex­changer in the air han­dling unit and the heat ex­tracted is then used to warm the in­com­ing fresh air.

The duct­ing will tend to be 100mm to 150mm in di­am­e­ter, de­pend­ing on the size of the house. The warmed in­com­ing air also needs to be in­su­lated, to make sure the heat gets to the right place, which could in­crease the di­am­e­ter to 250mm. Clearly duct­ing can’t be put just any­where and needs to be prop­erly de­signed in.

Retrofitting to older prop­er­ties is pos­si­ble but there are some prob­lems: hid­ing the duct­ing within the fab­ric of the build­ing is usu­ally prob­lem­atic and older prop­er­ties tend to be less air­tight. Retrofitting is also more ex­pen­sive than in­stalling a MVHR sys­tem in a new build, so it could well be that the cost of over­com­ing the aes­thetic prob­lems and get­ting the build­ing suf­fi­ciently air­tight out­weighs the po­ten­tial ben­e­fit.

The Im­por­tance of Air­tight­ness

Part L1A of the Build­ing Reg­u­la­tions (which con­cerns it­self with the con­ser­va­tion of fuel and power in new builds) re­quires a max­i­mum air­tight­ness of 10m3/hr/ m2 at a pres­sure of 50 Pas­cals (Pa) in new homes. Good prac­tice in the UK is said to be 7m3/hr/m2 and the Scot­tish Build­ing Reg­u­la­tions re­quire that as the max­i­mum. (See page 208 for more on achiev­ing com­pli­ance with Part L of the Build­ing Regs).

Pas­sivhaus is said to be the best en­ergy ef­fi­ciency build­ing stan­dard cur­rently avail­able and re­quires an air­tight­ness of 0.8m3/hr/m2 at 50Pa. There is no ques­tion that air­tight­ness is a key fac­tor in achiev­ing ther­mally ef­fi­cient houses. Air­tight­ness stops heat es­cap­ing from the house and stops cold air en­ter­ing (in the form of draughts).

MVHR works by ex­tract­ing air from a room and re­plac­ing it with a sim­i­lar vol­ume of warmed air. If we imag­ine a sit­u­a­tion where the room is very leaky, the ex­trac­tion will draw in cold, out­side air, out­weigh­ing the value of the warmed air be­ing in­tro­duced. So clearly there is a level of air­tight­ness at which MVHR will start to be ef­fec­tive. That is gen­er­ally ac­cepted to be 5m3/hr/m2. The more air­tight the house, the more ef­fec­tive the MVHR sys­tem will be, and again it is gen­er­ally ac­cepted that 3m3/hr/m2 level is nec­es­sary to see real ben­e­fits. It has to be said that most MVHR sys­tem sup­pli­ers do not ad­ver­tise this, as achiev­ing that level of air­tight­ness, even in a new build, is nei­ther cheap nor easy.

Are they Noisy?

This is largely a cost is­sue. Flex­i­ble duct­ing is cheaper but has a rip­pled in­ter­nal sur­face. That increases re­sis­tance to air pres­sure and thereby cre­ates noise — not a lot, but enough to no­tice. Rigid duct­ing is more ex­pen­sive but has a smooth in­ter­nal sur­face. Even th­ese will not be silent but the noise level is so low as to be in­audi­ble.

“The more air­tight the house, the more ef­fec­tive the MVHR sys­tem will be”

Are MVHR Sys­tems Ef­fi­cient?

Fig­ures of 80% to 90% will be bandied about, which sounds im­pres­sive but what those fig­ures ac­tu­ally mean is more dif­fi­cult to es­tab­lish. The im­pli­ca­tion is that 80% or 90% of the heat in the ex­tracted air is trans­ferred to the fresh, in­com­ing air. What it does not tell us is how much heat is be­ing ex­tracted. Typ­i­cally (but not al­ways) the heat­ing sys­tem de­sign for a new build will ig­nore the heat re­cov­ered by the MVHR sys­tem, mainly be­cause the oc­cu­pier has the op­tion to turn it off. The heat­ing sys­tem has to be de­signed to meet the whole heat­ing de­mand, with the MVHR con­tri­bu­tion of­ten taken as a bonus.

How Much Does an MVHR Sys­tem Cost?

The price varies with the house. For a stan­dard four bed­room house the sys­tem it­self is likely to cost up­wards of £3,000. But the in­stal­la­tion cost could eas­ily dou­ble that, de­pend­ing on the room lay­out and the dif­fi­culty of in­stalling the duct­ing. The sav­ings in terms of the heat­ing bill will have a di­rect re­la­tion­ship to the air­tight­ness of the build­ing. At 10m3/hr/m2 (Build­ing Regs stan­dard), the sys­tem sim­ply will not work. At 5m3/hr/m2 it starts to work but heat­ing bill sav­ings are likely to be less than 5% per year. At 3m3/hr/m2, sav­ings could get to 20%.

Are There Any Al­ter­na­tives?

Trickle vents and ex­tract fans are es­sen­tially the base­line stan­dard to com­ply with Build­ing Reg­u­la­tions. They are also a cause of draughts. Con­tin­u­ous ex­tract ven­ti­la­tion pro­vides con­tin­u­ous, very low level ex­trac­tion from kitchens and bath­rooms. This needs rel­a­tively poor air­tight­ness to work ef­fec­tively. Pos­i­tive in­put ven­ti­la­tion is a sin­gle unit which typ­i­cally sits in the loft — fresh air is pumped into the house, forc­ing stale air out of the gaps and cracks (that should be sealed to get a rea­son­able level of air­tight­ness). Pas­sive stack ven­ti­la­tion uses the same prin­ci­ple as a chim­ney to draw out stale air, and trickle vents, or sim­i­lar, to al­low in fresh air. Pro­pri­etary sys­tems can also come with heat re­cov­ery. How­ever, this is de­pen­dent on the wind to work well.


Ther­mal in­su­la­tion stan­dards are only in­creas­ing, along with the air­tight­ness needed to achieve those stan­dards. And with that comes the need for bet­ter con­trolled ven­ti­la­tion that can’t pos­si­bly be achieved with trickle vents and ex­tract fans. Adding heat re­cov­ery to that sys­tem is still an op­tion rather than a manda­tory re­quire­ment at the mo­ment, but whether that re­mains true as we move closer to Pas­sivhaus as a stan­dard, is an open ques­tion.

Tim Pullen Tim is Home­build­ing & Ren­o­vat­ing’s ex­pert in sus­tain­able build­ing and en­ergy ef­fi­ciency. He is the au­thor of Sim­ply Sus­tain­able Homes.

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