MVHR's Are they worth it

Lots of people are telling me I will regret not installing an MVHR unit as part of my house refurbishment.

I need to make a call on it now before everything starts to get closed up as theer are easy routes to almost all of the house.

It hadn't really been a consideration despite installing them in all our new builds. Are they worth it an will they bring any benefit to my life.

Yes, its hard to quantify the difference fresh air can make unless you work outdoors.

I've lived in two houses with MVHR now. The first was a new build, fitted as part of initial construction, and such were we so impressed it with it we retrofitted one to our second house. Needless to say a retrofit is a lot of hard work, particularly if you don't want boxing in and compromised design choices but as I was building an extension anyway I had a lot more ready access to key parts of the house that I wouldn't normally have (and hopefully won't have again for a while!) so it was either then or never.

The benefits we find are numerous, including in no particular order:

- Constant supply of fresh air, but without the heat loss from opening windows during the cooler months
- Controllable supply of fresh air that isn't at the mercy of the wind like trickle vents are
- Practically silent operation unlike most localised extractor fan setups
- Faster drying of clothes indoors due to humidity levels being kept low and under active control
- Ventilation of all parts of the house rather than primarily where local mechanical extraction is located
- Elimination of buildup of poor quality air that might otherwise not be quite bad enough to make occupants do something about it
- No need for trickle vents and extractor outlets (othan the MVHR's intake and exhaust of course)

I've likely missed some others out but you get the gist.

My strong recommendation is make the most of the opportunity, if that's what you've got, to seriously consider fitting it. If money is a consideration then you might want to just install the ducting (75mm OD Radial ducting is very straightforward to to install) now and then add the unit itself later. Speaking of the unit, keep an eye on eBay as new units are often sold off relatively cheap for a variety of reasons - overstock from developers, wrongly specified units etc - and so there's some real savings to be made.

What's a MVHR - I want one!

Mottie said:

MVHR

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Mechanical Ventilation with Heat Recovery

Wickes used to sell a basic version of this in the 1980's. They were quite innovative as DIY shops go at that time, I'm sure they used to sell an "extension in a box" - I think it was a steel floor grid and some kind of early SIPs type panels.

I agree totally with MJN's comments above. We have a Vent Axia Sentinel Kinetic in our house built 10 years ago. (I'm not a salesman for Vent Axia and know of other, possibly better, machines!)

People worry about the electricity consumed by the HRV unit, so I put a meter on it for a year and recorded 214kWh per year consumption - about £25, running at about 50%.
The energy saved by not allowing heat to escape in an uncontrolled fashion through open windows - "we need fresh air in here........" is probably much more than that.

An HRV system is not meant to be a blast of air (we cannot feel draughts from it) so it won't remove the smoke from that burnt turkey at Christmas....... It is, however, more than trickle ventilation and does keep the moisture down in kitchen and bathrooms. Towels don't need a heated towelrail.

We wash and reuse the filters about 3 times per year - it says when it wants a filter change.

Nothing in the world is perfect and one problem we have had was smells being brought into the house after the fields around us were sprayed with slurry. I cobbled up a carbon filter which solved that problem. I put together a webpage for a few friends with my experience of our HRV system: http://hrv.yolasite.com/

Despite the makers' claims, it is not silent. If, like me, you are annoyed by noise put the unit out of the way, eg in the attic.

Yes, I'd echo the power consumption aspect - I've got power monitoring on ours and it too has cost £23 for the previous 12 months.

One thing I forgot to mention is the summer bypass feature that removes the heat exchanger during the summer months thus still allowing filtered fresh air into the house. If the outside air is hotter than inside then it'll put the exchanger back in circuit as it works both ways i.e. heat will be transferred from the incoming air to the exhausted air thus you can see a couple of degree drop in supply air temperature. Don't get me wrong though; it's not air conditioning (the temperature might sound good but as we're talking very low volumes of air which has a low heat capacity it can only do so much) but it does at least provide some air movement on what can otherwise often be very still, stuffy, summer nights.

Another thing I forgot to add is that it seems to cut down on the amount of dust - presumably because of the constant air exchange through filters, and changing the dirty filters certainly backs up that theory. The dust it leaves behind seems so much 'lighter' too - less dirt, more fluff - and I've never quite worked out why that appears to be the case as if anything I'd expect it to be the other way round. I've heard other MVHR owners say similar things.

And, yes, the units aren't silent although ours is in the loft above our bedroom and it cannot be heard - perhaps when on boost but there's noise from the increased air flow rate anywhen and so it all just blends in and either way is definitely not an issue and we're pretty fussy about that sort of thing.

cdbe said:

Wickes used to sell a basic version of this in the 1980's. They were quite innovative as DIY shops go at that time, I'm sure they used to sell an "extension in a box" - I think it was a steel floor grid and some kind of early SIPs type panels.

Click to expand...

Quite right. I bought and installed one of those in the late 1980's in the loft of my first insulated new starter home with electric heating. It was actually manufactured by a company called Rega but sold as a kit by Wickes. Careful design was required to ensure that air travelled through the whole property from inlet grille to outlet grille without missing areas out. This was tough for me because the areas I could get the duct to easily weren't the best places to have the grilles located.

Ideally the grilles are positioned diagonally opposite the the doors of room in which they are located so that the airflow covers the whole room, and the doors require a gap at the bottom, or a grille in to allow the air to pass through without resistance. Too much resistance and the air can start finding an easier route in and out of the building instead of going through the heat exchanger.

In the summer I fitted a switch to turn off the intake fan when I didn't want heat recovery and it just operated as an extraction system.

I liked it, but when Mrs Stem moved in she insisted on opening windows all of the time which allowed the air to take the easiest route in/out of the house without going through the heat exchanger, rending it useless for heat recovery. Because of that I've never installed one since. So my tips would be:

1. Careful siting of the grilles is critical to get maximum coverage and avoid 'short circuits'

2. The property needs to be as airtight as possible (no windows open) to force all of the air to enter and leave through the heat exchanger instead of taking easier routes. Otherwise it's just an extractor system and heat recovery is lost.

3. They are not suitable for homes with open fires / chimneys, for the same reason as 2 above. I believe enclosed stoves / wood burners whilst not ideal are better than an open chimney, but beware some stoves and some gas appliances require external air vents which would effect the air balance and efficiency of the heat exchanger

4. Other extractors such as a cooker hood / chimney with their own fan and exterior outlets also stop the heat recovery process working. Normally one of the MVHR extraction grills would be positioned close to cooking areas, and if required a cooker hood with a carbon filter and air recirculation used instead of one that vents outside.

EDIT
Looks like Rega are still in business, their website can be found here. And there's a good description / diagram that shows the process.

They also offer a test to make sure a home is suitably air tight for MVHR. I just had to seal everything up as best I could. Here's a link to the air test.