An MVHR (Mechanical Ventilation Heat Recovery) unit is a magic box that recovers heat as it leaves the house.
And what is MVHR for?
It’s for recovering heat from air that leaves your house, which can reduce both your bills and energy use – in the right circumstances.
Normally when you turn the extractor on in your bathrooms or kitchen the heat is lost. A correctly designed and installed MVHR unit can recover “up to 98%” of this heat and return it to the other rooms in your house. (A meaningless statistic from the manufacturers sales pitch!)
Refurbishing a house? OK so problem solved; fit some ducts and a MVHR unit and your energy bills will be a fraction of what they were. Obviously this is not so or everyone would be doing it! For a MVHR to work efficiently you need an “air tightness of below 3m3/hr per m2”. See airtightness testing for an explanation.
This figure of 3 m3/hr per m2 – I have no scientific backing as to why 3 is the magic number and can’t remember where I got the number but it is oft repeated. Logically it makes sense not to fit an air pump into a leaky house: it will draw cold air into rooms where it is extracting air and blow warm air out of rooms where it is ventilating.
Airtightness of 3 m3/hr per m2 is fairly air tight standard to reach – to give you an idea of what a air factor of 3 m3/hr per m2 means; look at it this way.
New build houses have to be below a maximum of 10 m3/hr per m2 to meet building control and 10 m3/hr per m2 is only acceptable if other extra energy saving measures are included in the build. If there are no extras to standard building regs. then 7 m3/hr per m2 has to be achieved.
Building Regulations also require a “mechanical ventilation” if your house has an air tightness below 5 m3/hr per m2 to prevent condensation and mould.
My friend who worked on new-build estate houses said they once got a house to test at 5.5 m3/hr per m2 but mostly they get a result of around 7 and have to go round with expanding foam, mastic and sprays and fill leaks to achieve the building regulations standard.
When I did the refurbishment of my sister’s house at 89 Culford Road – a no compromise energy efficient refurbishment – on the initial test we had a reading of 5.5 m3/hr per m2 which was to say the least disappointing! A quick wander round the house and we found that a “tilt or turn” window had tilted open because it had not been locked shut, closing this window reduced the air tightness factor to 3.4, m3/hr per m2 then an unsealed 110 mm soil pipe under a pile of insulation was remembered and once that was sealed it dropped to under 1.5. m3/hr per m2 Some further work and we achieved 1.12 m3/hr per m2 not below “the iconic 1” but bloody good for a first-time work on this sort of project. But look at those figures again.
- Closing a 1100×500 tilt and turn window, in effect closing an approx 1600 cms sq. hole
reduced the leakage by 2.1m3/hr per m2
- then sealing a 100mm diameter pipe in effect closing an approx 95 cms sq hole
reduced it by a further 1.9 m3/hr per m2
The hockey stick shaped graph is getting very steep as you try and get down below 1! We figured that we were looking for the equivalent of an 20mm diameter hole by the end – an approx 3 cm2 hole – to get below 1. (Some time later having a beer with the electrician I think he admitted to drilling a 15mm hole through the air-tightness line for his wire for the bedroom central light!)
(A big aside – why did we want to get below 1? Well if we had got below 1 we would have been close enough to a “PassivHaus” equivalent for refurbishment refurbishment, the EnerPHit standard that, because our insulation levels and a MVHR unit recovering heat we would hardly have needed to heat the house – just by living in the house it would have been heated!)
What’s the point of you reading all this? Well if you are thinking of fitting a full MVHR unit and want to have open fires then forget it, the cat will have to live outside as well unless someone comes up with an airlock cat flap and your letter box is going to be a problem too!
Let’s head to the real world and look at what is MHVR ‘s likelihood of success. If you are doing a complete strip out and refurbishment and think from the outset about fitting a full MVHR system then it may well be an economic proposition. Once you start getting a house air tight below 5 m3/hr per m2 you will require mechanical ventilation or you will get mould growing in corners of rooms so you might as well go the extra mile and get below 3 m3/hr per m2 and fit a MVHR unit. Insulation gets you a better bang for your buck to start with but there comes a time when increasing insulation starts to hit the laws of diminishing returns and reducing air leakage kicks in.
So if you decide to fit a MVHR system think about it from the very beginning. When you start drawing up your plans and elevations you need a continuous surface line around the whole house marking the airtightness line. Mark this line on all drawings in RED! Through this line any joist, door, window, pipe, cable, duct, vent, overflow or expansion pipe that has to pass through causes a penetration and has to be sealed. You will need special traps for condensate and expansion pipes for your boiler, hot water tank etc. Your windows and doors will need airtight seals.
What is MHVR like to live with?
This all seems like a lot of bother and more trouble than it is worth but if you are with it from the beginning and are always aware of the problems you can design out problems and get good results for little extra cost!
What no one tells you is the collateral advantages of an MVHR system. You have filtered air so you breath less of the grime of general city pollution or the dust of country living, you have less housework to do, there is less dusting and you have no annoying fans that come on and whirr away in bathrooms when you turn the light on in the middle of the night.
My sister’s house went from an annual energy bill of around £1200 a year (2008 prices) to £109 a year in 2010. The estimate of the energy saving works for the build was £50,000 (of a total rebuild cost of over £250,000). So it would take 50 years to recoup the expenditure at 2010 prices – at today’s prices it is down to below 35 years and I doubt that energy cost are going to drop.
Budget MVHR units
Single room application MVHR units are now coming on the market. Costing (2014 prices) around £800 they are not a efficient as a top-end full-system MVHR unit only getting “up to 85%” efficiency (and tend to be a bit noisier than top of the range MVHR units) but they are quick and easy to install. Fitted to replace a bathroom extractor fans they save some of the heat from your expensively-heated steam from your shower being blown straight out of the house. (You can even get heat recovery units that you can plumb into the bath/shower-waste water systems but I have never fitted one and have no knowledge as to whether they are worth fitting.)
So what is MVHR, and why should I consider it?
In short, it’s a way of keeping the heat inside your house as air leaves. It can reduce heating bills and energy use drastically, it delivers filtered air to everyone who is in the house but it only works efficiently in airtight buildings and so need to be designed into the works from the start.