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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)?

Steve Melnikoff
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Croad Langshan
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    usually the regular leakyness of your house is plenty to vent out the CO2 and bring in O2. no additional venting required. – ratchet freak Oct 09 '23 at 15:29
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    Needs a lot of ducting & an exchanger, which is best placed in loft space. If you're not on the top floor I'd forget it. CO₂ buildup is really not a real concern, it's one for the 'worriers', unlike CO. – Tetsujin Oct 09 '23 at 16:13
  • Window mounted heat recovery ventilators exist, but I have no experience with them. – negacao Oct 09 '23 at 16:17
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    @ratchetfreak "usually the regular leakyness of your house is plenty to vent out the CO2 and bring in O2. no additional venting required." - I have measured CO2 levels in my flat, and they're higher than I would like. Also, I find myself more awake having opened windows. – Croad Langshan Oct 10 '23 at 14:31
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    @Tetsujin "CO₂ buildup is really not a real concern, it's one for the 'worriers', unlike CO". This question is about reducing CO2 practically, I think a debate about its effects belongs elsewhere – Croad Langshan Oct 10 '23 at 14:32
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    There *is no* practical problem to be solved, because CO₂ buildup is not an issue unless you live in a sealed box… it's like worrying about enough oxygen getting in the house - & you need a whole lot more of that. What are your measured CO₂ levels, that would cause concern? – Tetsujin Oct 10 '23 at 14:36
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    @Tetsujin CO2 levels sometimes hit 1000s of PPM, before going off the scale of my CO2 monitor. Understanding of this has changed in recent years, and there is published evidence of even low CO2 levels affect health and cognition https://www.nature.com/articles/s41893-019-0323-1 . You may well be right and it all turns out to be nonsense - I just don't think that debate belongs here. – Croad Langshan Oct 10 '23 at 15:04
  • @Tetsujin Re "needs a lot of ducting" Just as an "existence proof" that there are products that *claim* to do something useful without that (no claim from me that they work! :-) ): https://www.vent-axia.com/range/lo-carbon-tempraselv (that's an extractor, but powered extraction should bring in more air from elsewhere). It does seem to me that something like this *should* be able to do better than an open window, and my suspicion is that the usual advice is based on assumptions that don't apply in my case. Perhaps somebody has studies with proper calculations to prove me wrong. – Croad Langshan Oct 10 '23 at 15:12
  • A friend of mine used to blow up buildings for a living - no, not with explosives;) He was a professor in computational fluid dynamics & one of his courses they would encase houses in enormous plastic bags & see how much they leak. Anything built more than a couple of decades ago is going to leak far more than ever needs an air exchanger [other than a bathroom fan, predominantly for moisture]. Any extractor fan will reduce the internal pressure, allowing air to be pulled in through all the other leaky bits of the building. – Tetsujin Oct 10 '23 at 15:19
  • Your posh ventaxia there is not a simple extractor, it's a heat pump, stopping extracted air removing heat from the building. It just removes 'air'. The 'carbon' mentioned is carbon footprint. Don't throw away heat & save electricity/the planet. – Tetsujin Oct 10 '23 at 15:20
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    @Tetsujin Ventaxia: I didn't mean to claim it doesn't do heat recovery (rather, that it does do that). Re "Any extractor fan will ... air to be pulled in through all the other leaky bits": yes (that's what I meant by "powered extraction should bring in more air from elsewhere"). "The 'carbon' mentioned is carbon footprint.": yes, that's my understanding too. Re your claim (as I understood it) that mechanical ventilation and/or MVHR is not needed in houses: I hear you, I don't agree, and I'm not sure there's much more I should said about that in SE comments here, so I will stop there. – Croad Langshan Oct 10 '23 at 15:52

2 Answers2

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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 HeatSava

Vent Axia LoCarbon HeatSave or Tempra Vent Axia LoCarbon HeatSave

Kair HRV150 Kair HRV150

user1908704
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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.

keshlam
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    It seems this answer doesn't address the question: what type of unit (and why, of course)? Re my CO2 monitor: It's an NDIR sensor (should be robust I think) and I've verified the calibration myself. – Croad Langshan Oct 12 '23 at 17:08
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    What kind of unit is a shopping question, out of scope. What can you mount on your new virtual wall? How does the advertised air exchange rate compare with your volume and how far you want to bring levels down? – keshlam Oct 12 '23 at 17:28
  • By "type of unit", I'm referring to the design of the unit (design in abstract terms: I don't mean a blueprint), not a brand or product SKU or similar. Does that help? By "virtual wall", you mean your suggested panel in a window opening, right? Unfortunately there are some reasons it would be difficult to go that route - I imagine instead knocking a hole through a wall (but the focus of the question isn't on the hole-knocking part). The constraints I know about for what I could mount on a wall include: mustn't be too "obtrusive" on the outside, and indoor noise level should be "low". – Croad Langshan Oct 12 '23 at 17:42
  • If you are in a rental unit, knocking holes in wall won't happen without landlord permission. Hence my suggestion to find a way to mount in existing opening, window. Design: Off the shelf self-contained HRVU, commercially available, no design involved beyond deciding if it will mount to your "wall" and whether it needs your needs. – keshlam Oct 12 '23 at 18:05
  • I'm not in a rented flat. In my situation, using windows would entail problems getting permission (and I don't want to lose a window). Again, the question is about what sort of MVHR designs are appropriate. – Croad Langshan Oct 12 '23 at 19:38
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    I'm not sure what you are asking them. What do you mean by "designs"? Basic MVHR is a simple concept; there isn't much meaningful variation, so what's appropriate is strictly your needed specifications and your mounting details – keshlam Oct 12 '23 at 22:15
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    As an example: there are many units that are "through-the-wall" single-room units. Some of those reverse direction every minute or so (and some don't, I assume). Others are ducted. I think for flats likely there is more variation here than for houses, because it's not popular in flats at present. – Croad Langshan Oct 16 '23 at 13:17
  • Not sure why the selection criteria would be different in this case from any other case. Figure out how much air exchange you want, get a unit which meets that spec. – keshlam Oct 16 '23 at 13:27