MVHR ventilation in a UK flat (apartment)

Question

I'm interested in getting MVHR (mechanical ventilation heat recovery) or some similar system in a small flat (apartment) in the UK.

I see all sorts of products advertised, which seem quite varied in design and applicability, but living in a flat, in the past I've been told that it's not practical for reasons of space usage for equipment, air leakage and insulation.

However, I like to keep CO2 concentration indoors not too high, which means opening windows a lot, and I find it hard to believe I can't do better with some sort of heat exchange instead of open windows in the middle of winter. I guess they don't have that situation in mind when they judge that it's not worthwhile in a flat. I also suspect a rather small system would solve my CO2 problem, without requiring a lot of bulky equipment.

What type of system or unit available in the UK might be suitable, if any (or who could I pay to give me good advice on that)?

Answer 1

There are single room MVHR units, also called Decentralised MVHR (dMVHR). They give you a continuous supply of fresh outdoor air while retaining heat from the indoor air that they're pumping out. They can often replace an extractor fan with a single hole in the wall, but add heat exchange too. They usually operate in a trickle mode for a continuous low volume of air exchange, but some can boost if humidity goes up - for showering or cooking.

Typically they either use concentric pipes (stale warm air goes out, transfers heat to fresh cold air coming in the adjacent pipe) or a counter-flow design with a heat-absorbent material (stale warm air blown out, warming up material, switch direction, fresh cold air pulled in, takes heat from material) where the flow direction changes about once a minute.

The advantage of a dMVHR system is there's no central unit and ducting, you can just install these in holes in the walls, as many units as you need. I'm not sure how they compare with a whole-house MVHR in terms of efficiency, but note that efficiencies of ~80% are quoted: that means 20% of heat from extracted air will be lost (since they are in effect a big unobstructed wall opening), and they can cause draughts especially if the wind blows through them (check baffle arrangements, and maybe there's a way to close them when it's freezing outside?)

Some examples:

Envirovent HeatSava

Vent Axia LoCarbon HeatSave or Tempra

Kair HRV150

Answer 2

Open window. Mount some sort of insulated panel in opening, with edges and window sealed to prevent leaks. Mount HRV unit to that panel. (Not necessarily in this order.)

That seems to address your request, though I think you're being lied to by a cheap "CO2" monitor which isn't actually measuring what it claims to measure.