From EnviroWiki
Why would I want mechanical ventilation with heat recovery?
So, you've draught-proofed your windows, doors and floor. That's great for keeping precious heat energy within your house during winter but it's not so great from the perspective of - you know - breathing.
To be fair, even a thoroughly draught-proofed room wont be completely sealed, enough air will move through small holes in the room's envelope to mean you shouldn't suffocate. But it could easily get stuffy. And the relative humidity could increase to risky levels in bathrooms and kitchens during normal use and in other rooms if you're drying clothes or having a party. You could open the window to get some fresh air but then you lose some of your valuable heat to the outside.
So how do you get fresh air without wasting heat during winter? The answer is to use a ventilation system which extracts the heat content from the outgoing air and transfers that heat to an incoming stream of fresh air. So you get fresh air without wasting your heat.
Single-room Mechanical Ventilation with Heat Recovery
The easiest way to add mechanical ventilation with heat recovery to an existing house is to fit single-room units in the rooms which require it. These units look a little like a standard bathroom extractor fan.
Comparison of some of the single room MVHR units on the market
| | |||||
| Heat recovery | Power consumption | Noise | Extract performance | Price & suppliers | |
| Vent-Axia Lo-Carbon HR25 Solo Plus | up to 84% | 1.9 Watts | 16.8 dB(A) @3m | 12m3 per hour | £200-£250
|
| Silavent Energex | 15-40 Watts | ||||
| Airflow HRV150 | up to 80% | 9.3 Watts | 21dBa @ 3m | £303.15 | |
| Kiltox / Kair KHRV150 | up to 86% | 9 Watts | 21dBa | £288 inc VAT from I-Sells | |
| Johnson and Starley Aeroplus | 60% | 8 Watts | |||
| VentAxia Lo-Carbon Tempra | 70% | 3.4 Watts | 20dBa | £123 (not available until July 2010) | |
Trickle mode and boost mode
Most single room MVHR units have two fan speeds. The lower speed is designed to run constantly and should be fairly quiet. The higher speed can be triggered by a variety of mechanisms, depending on the design of the MVHR unit (e.g. a pull-cord, a humidity sensor or a movement sensor).
Potential issues to consider
- The room needs to be thoroughly draught-proofed for MVHR to work effectively
- Some MVHR units are not sufficiently powerful to function in high humidity environments like bathrooms; check the specifications
- Some MVHR units won't work effectively if the external section is exposed to strong wind
- If the external temperature drops below freezing then condensate can freeze inside the MVHR unit. This shouldn't damage or disrupt the MVHR unit but it may make it considerably noisier.
- Make sure the unit you're considering is long enough to span the width of your wall, especially if you've added insulation to the wall. Some models have a "long" variety.
- Make sure the unit you select will be quiet enough to not cause annoyance (especially important in bedrooms and living rooms).
Installation
These units can be installed by a competent DIYer.
The outline of the installation process is:
- Check the manufacturer's guidelines!
- Drill a circular hole through the wall (most units need a 152mm (6”) diameter hole). You can hire a 240v Light Duty Diamond Driller Kit from HSS for £45 for a day. Don't use a standard drill, you need one with a clutch so it doesn't kick back too hard when the drill bit gets stuck (the HSS drill has this). Obviously, make sure you're not going to cut through any pipes, wires or structural timbers within the wall. Wear all the recommended protective clothing. Seriously. Wear ALL the recommended protective clothing. Getting a chip of brick in your eye isn't amusing. The hole usually needs to slope downwards a little bit going from inside to outside to ensure the condensate drips out but check the manufacturer's guide.
- Install the MVHR unit in the hole
- Connect the MVHR unit to the mains via a fused switch (a legal requirement)
- Tidy up the external brickwork (which may require scaffolding if it's high up)
Disclaimer
Stuff breaks; people get hurt; bad things happen. EnviroWiki takes no responsibility for these unfortunate occurrences. Whilst the community tries its hardest to make these practical guides as good as possible, full responsibility (including ensuring safety and compliance with building regulations) lies with you. Use at your own risk. But you knew that already, right?
Why is adequate ventilation important?
Condensation
There are several sources of water vapour within the home: breathing mammals, plants, drying clothes etc. Warm air holds more moisture than cold air, so the water vapour will condense on cold surfaces (e.g. uninsulated external walls). Damp walls are susceptible to mould. Damp woodwork is susceptible to rot and animal attack.
Ventilation is important to keep the internal humidity as low as possible, hence significantly decreasing condensation problems.
Removing toxic vapours and smells
Several products around the home slowly leak toxic vapours.
Health benefits of good ventilation
A well ventilated home isn't just more comfortable, there is also evidence that it's genuinely healthier.
The general conclusions from the WHO report on a technical meeting on quantifying disease from inadequate housing (150 page PDF) seems to be: high relative humidity increases damp and mould. MVHR can, in most circumstances, reduce relative humidity. There is some evidence that installing MVHR can reduce the prevalence of ill health.
Whole-House Mechanical Ventilation with Heat Recovery
If you're building a new house or if you're doing a whole-house refurbishment then you might consider a whole-house MVHR system where a single heat exchanger services the whole building and is connected to several rooms by insulated pipes. This requires careful planning and also requires the entire house to be as air-tight as possible which remarkably tricky to achieve, especially in a refurbishment. Whole house systems start at about £650+pipes, and often a great deal more.
But don't MVHR units require electricity to run? I thought we were supposed to be reducing electricity consumption, not buying more gadgets
Yes, an MVHR unit requires mains power to run the fan. A single-room unit might draw 3 Watts while in trickle mode. Compare this to the heat output of a standard radiator: about 1,500 Watts; or to the output of a standard boiler of around 24,000 Watts.
So if your MVHR unit allows you to hold on to even a small fraction of your boiler's output then it has paid for itself in terms of energy efficiency.
