I've been doing a little bit of reading, including on here, about the in-wall MHRV units that are gradually becoming more available for a couple of hundred quid.

It seems they are really intended for bathrooms and kitchens as an alternative to conventional extract fans. But it looks like some people have used them with the aim of dealing with an area extending beyond that.

My question, basically, is:

Is there any real-world data or even theory that lets me judge or understand how great an area/volume they are going to have an effect on?

At first glance it seems that because, unlike ducted systems, they can't have the intake and extract at different locations, they can only have quite a local effect. A certain proportion of the air they pull in must end up going out again before it's circulated very widely. I assume they are designed to push the air in a different direction from where they are pulling it from.

I realise it may be a bit more complex than just where a body of air physically moves to though. I have a half baked understanding that humidity will tend to even itself out within a space but I'm not clear to what extent (if at all?) the moisture can "move" from a humid body of air to an adjacent dry body, without the air itself circulating.

At a pragmatic level, the question is: will one of these in-wall units have any chance of having a meaningful impact on humidity or CO2 levels at a point, say, 10m away?

I'd be installing it in an open plan living space containing a kitchen area. It's highly insulated and pretty airtight, with the complication of being connected by an open stairwell to a not-so airtight lower storey. Almost all of the time it's unsurprisingly considerably warmer in the upstairs area, and my feeling is that humidity from cooking sits in this warm air upstairs and has little impact downstairs. At present I deal with it by opening windows. This is pretty effective but obviously involves dumping a load of heat away.

In a way, it's more general air quality I'd like to address with the MHRV. Humidity is actually easily dumped out the windows as described, and if I've been cooking, if anything it's already too warm so a bit of cool air from outside is not unwelcome. But I do sometimes feel the need to open the windows a bit at other times of day because it feels stuffy. In that scenario, during the winter I'll notice a temperature drop that's not so welcome.

Is a heat exchanger unit going to have a noticeable impact if it's 8-10m away from where people are sitting?

Don't whisper - sounds like we all need to know

I think most of us know them don't we? Didn't want to break the rules. Lunos are an example. Fresh-R does not co-operate but is ductless.

For Lunos, correct, they co-ordinate airflow. One blows, the other sucks, then they swap and so on. They retain energy in a ceramic lump.

Posted By: gyrogear

Posted By: lineweightI'd be installing it in an open plan living space containing a kitchen area. It's highly insulated and pretty airtight, with the complication of being connected by an open stairwell to a not-so airtight lower storey. Almost all of the time it's unsurprisingly considerably warmer in the upstairs area, and my feeling is that humidity from cooking sits in this warm air upstairs and has little impact downstairs. At present I deal with it by opening windows. This is pretty effective but obviously involves dumping a load of heat away.

In a way, it's more general air quality I'd like to address with the MHRV. Humidity is actually easily dumped out the windows as described, and if I've been cooking, if anything it's already too warm so a bit of cool air from outside is not unwelcome. But I do sometimes feel the need to open the windows a bit at other times of day because it feels stuffy. In that scenario, during the winter I'll notice a temperature drop that's not so welcome

As far as I can see, you are suffering from stack effect: this could be addressed firstly by sealing the basement area to prevent (or limit) inward air-rush.

A basement ceiling fan could also be used to pull down warmer air from upstairs...

gg

No, thermally it works fine. Warmer upstairs (living) and cooler downstairs (bedrooms) is how it's supposed to work. I don't want to pull warm humid air downstairs. I want to keep the heat upstairs but get rid of humidity and stale air.

Posted By: lineweightI see the Fresh-R is a development of something I saw on here a few years back but hadn't yet been made available commercially

Fresh-R is indeed the commercial development of Viking House's WiFi units which yes got a lot of coverage here a few years back.

Posted By: lineweightthis "equalisation of gases" principle. Is it a principle that's generally accepted/understood? Any reading up I can do?

I was a promulgator of this notion but am not so sure now, having corresponded with BRE boffin and with Fresh-R tech - but not conclusive. I failed to find much info online about it so would be pleased to hear of sources, whether confirm or deny.

I too have wondered if the FreshR works then why duct a typical MVHR unit to each room e.g would the following work - one extract in each of the bathroom and kitchen and one input somewhere between the two.

However for the test done by RobL,

Posted By: RobLThen just open the nearest window to where you would place the single room mvhr, and see if the air is still fresh enough for you! It's only valid to do this if there's no through draft through the window - ie. no other windows open, airtight house, pref. warm outside so no stack effect.

I can't help thinking that a single open window is not the same as one fan pushing and one fan pulling.

Posted By: RobLI went through a similar thought process to you - ie. would a large single room heat exchanger do?
To answer it, I got a CO2 monitor from ebay, around £80(display only)-£150(logging). The CO2 monitor goes in your bedroom or lounge - the place you need fresh air. Then just open the nearest window to where you would place the single room mvhr, and see if the air is still fresh enough for you! It's only valid to do this if there's no through draft through the window - ie. no other windows open, airtight house, pref. warm outside so no stack effect.
The results for me were ok-ish - I got 920ppm CO2 in the bedroom, with a single window fully open ~7m away. I wasn't convinced a single room unit would be as good as an open window, and we'd had noisy experience of them before, so we went with the whole house job - which is excellent.
You don't need the CO2 monitor, but it's a lot more accurate to look at the numbers than attempting to guess air quality.

Doing some monitoring experiments myself would certainly be interesting.

Would also be interesting to see to what extent my subjective impression of "stuffiness" correlated to objectively low measures of CO2.

Unfortunately we are just now exiting the period of the year where heat-exchange ventilation would be most beneficial and which would allow me to make the most representative simulations!

A little bit further research has brought up this

http://www.blumartin.de/ueberstroemer-fuer-wohnraumlueftung-freeair-plus/

(german only)

Single-unit system that seems to operate with a kind of "cascade" strategy with air being shuffled between rooms by small in-wall fans. But can also be used as a single room unit without ducting...need to find out more.

Vent-Axia have something called HR300. Their other single-room heat recovery units are all described as suitable for kitchens/bathrooms/utility rooms. Suggesting they are designed for relatively small rooms. On a reseller website there's a reference to the HR300 being suitable for up to 276 sq m of space. No detail of how this is achieved without ducting though.

Xpelair have something called Muro, which appears similar. Again, no specific information on what size of room it's effective in, or how far from the unit one can expect effectiveness.

Posted By: lineweightSingle-unit system that seems to operate with a kind of "cascade" strategy with air being shuffled between rooms by small in-wall fans

Fresh-R similar

Most "normal" MVHR systems have a outlet or a inlet in a room and therefore don't work if you have tight fitting doors that are closed. Without closed tight fitting doors you get a lot of noise transmission......

It is not hard to make "sound proofed" are vents to install between rooms, but most people don't use them.

Posted By: ringiMost "normal" MVHR systems have a outlet or a inlet in a room and therefore don't work if you have tight fitting doors that are closed. Without closed tight fitting doors you get a lot of noise transmission......

It is not hard to make "sound proofed" are vents to install between rooms, but most people don't use them.

The Blumartin system has, potentially, vents directly between rooms - not quite the same as both those rooms having door undercuts to a hallway, as far as sound transfer issues are concerned.

But I think the inter-room vents are designed to be soundproofed to some extent.

Posted By: lineweightA little bit further research has brought up this

http://www.blumartin.de/ueberstroemer-fuer-wohnraumlueftung-freeair-plus/

(german only)

Single-unit system that seems to operate with a kind of "cascade" strategy with air being shuffled between rooms by small in-wall fans. But can also be used as a single room unit without ducting...need to find out more.

I didn't read the text but looking at the picture it appears to use a fan in the wall to move air one way between rooms but also uses a door undercut to move air the other way.

I can see the attraction of ductless systems like this for retrofit in some cases.

Posted By: gravelldThanks for the reference to bluMartin, another one to add to the list. Hopefully this causes competition and drives prices down.

What I've noticed is that the prices for these systems seem to increase capex to the point the price is the same as getting an MVHR system installed. I guess you get the benefit of less mess.

I'm in the process of getting an initial price using Fresh-R or Blumartin. As far as I can tell the Fresh-R unit is priced around £1800. The Xpelair and Vent Axia things I mentioned above are more around the £500 mark. I'm going to look a bit more into them too to understand better what extra/different you're actually getting with the more expensive options.

Posted By: lineweighthttp://www.blumartin.de/ueberstroemer-fuer-wohnraumlueftung-freeair-plus/

(german only)

type it in as
translate:http://www.blumartin.de/ueberstroemer-fuer-wohnraumlueftung-freeair-plus/