Hi all,

I've just had two days at Ecobuild and, as usual, it was great, but my head is fried. I went to source MHRV. I knew that my solid walled 1925 semi wasn't really as such air tight, but I reckoned that I'd improve it over time (EWI with parge coat, intello etc) and as I did so, the MHRV would become more effective.

The reason that I'm ventilation shopping was that, as discussed on a previous thread, I've got real issues with high humidity (65-70%), mould and respiratory problems (GP thinks I now have asthma so I really need to do something about it asap). Everyone's helpful contributions made me realise just how urgent ventilation was and one or two people's positive experiences of MHRV dealing with mould, made me think that it would do the trick.

However, I spoke to an architect I really respect at the show and he pointed out that unless I'm lower than 1 airchange per hour at 50 pascals etc etc, installing MHRV is likely to overventilate the house and I'll lose more heat than i need. He thought that Postive input ventilation was bad because the humid air then has to go through the fabric of your house, with the comcommittent danger of condensation. He therefore recommended mechanical extract ventilation.

I talked to a lot of reps flogging ventilation kit. TBH, a lot of them were either lying or ignorant ("You don't need your house to be airtight for mhrv"), although I will give a special mention to the woman at Envirovent who really seemed to know her stuff and was happy to explain the concepts behind PIV. I now have an inner database of ventilation possibilities.

What I'd really appreciate is some guidance on what I should do now and whether I've understood the relative merits and demerits of various possibilities correctly. This is my summary so far...


MHRV

Pros: heat recovery
Cons: may over ventilate your house if not sufficiently airtight; hassle & expense of retrofit; expense of buying; expense of running 24/7 (although I think you'd turn off in summer)

MEV
Pros: gets rid of humid air
Cons: gets rid of heat; offends my energy conscious soul;designed to be always on (?); hassle & expense of retrofit

DEV (Demand Extract Ventilation)
Pros: gets rid of humid air, less ducting and hassle, I think. Only on when you need it. Liked the German study apparently demonstrating that was more cost effective than MHRV because it wasn't on all the time (although I haven't read the paper and if I haven't read something properly, I'm a bit sceptical)
Cons: gets rid of heat; sounds possibly overly complicated

Single Room Heat Recovery
Pros: heat recovery! no ducting hassle or expense
Cons: not convinced it works; surely must have same issues as whole house MHRV in terms of airtightness?
I found an interesting Ukrainian model, with a ceramic core, that didn't have a heat exchanger as such, but expelled the air through the core for 70s and then took air from outside through the same core for 70s. Somehow I quite liked this idea, until I saw the line that the core "moistened", as well as warmed the air. Great for continental climates that can be very dry in winter, but not so great when you're trying to get rid of the humidity.

Positive Input Ventilation
Pros: relatively inexpensive to buy; looked pretty simple to fit; used on a lot of social housing properties with condensation and damp to great effect. Liked the idea that you'd benefit a bit from the somewhat warmer air in the loft (although as you reduce air loss from the warm rooms below and increase your insulation, the air up there will be colder). I did wonder whether heating element might not be totally mad either- because when it's sunny in winter, I could "subsidise" from pv, depending on wattage; although I'm mindful that air isn't a great heating medium in this context & this would only work during the daytime, when I'm usually out.
Cons: On all the time; cold(ish) air coming in to house. I do worry about what the Architect said about moisture and the fabric of the house - where does all that damp air go? It can't just dematerialise! Will this be more of a problem as I increase air tightness or should I just give up on that?

Positive Input Ventilation with solar warming
This was a variant one of the major manufacturers do, which reminded me somewhat of the solar venti. Essentially you put a couple of solar air panels on your south facing roof; the PIV then takes from either those (when they're warm enough) and from the loft when they're not (you can also get a version with a take on the N facing roof for summer cooling; bit optimistic imho)
Pros: nice energy saving concept; no complicated ducting in house
Cons: reading between the lines, I got the impression that it might not be that effective (although Solar venti appear to get great reviews for their single room version). It's pretty pricey too - and you need sufficient and appropriately orientated roof space.

Finally....

Good old fashioned single room extraction
Pros: been used for years; quick response to humid air in kitchen/bathroom before it gets a chance to go elsewhere; only on when you need it.
Cons: I'm not sure it would help my high humidity in my bedroom (both my kitchen and bathroom are downstairs, with a room inbetween them and the stairs. I don't have enough wall in the bathroom to mount an extractor fan and would have to do something interesting with ducting and the roof (it is only a single storey though).

So....what have I misunderstood/what have I missed out/ what should I do?! I should add that I don't have trickle vents on my dg and I have no extraction in my bathroom or kitchen. Hell, I don't even have a letter box I've been so paranoid about air infiltration ;-)

Thoughts and advice very gratefully recieved; tbh, I'm freaked out by the asthma thing which I am pretty confident is because of the damp and mould so I need to decide and act.

Thanks,

Tania

Tania,
Willie.Mcleod's comment is echoing what others have pointed out about heat input on your original other thread. Have you already done that and has it helped?
Martin.

Posted By: nbisharaI found an interesting Ukrainian model, with a ceramic core, that didn't have a heat exchanger as such, but expelled the air through the core for 70s and then took air from outside through the same core for 70s. Somehow I quite liked this idea, until I saw the line that the core "moistened", as well as warmed the air. Great for continental climates that can be very dry in winter, but not so great when you're trying to get rid of the humidity.

Wouldn't blowing incoming air over a moist warm-to-tepid ceramic core be a recipe for spreading mould?

<blockquote><cite>Posted By: Ed Davies</cite><blockquote><cite>Posted By: nbishara</cite>I found an interesting Ukrainian model, with a ceramic core, that didn't have a heat exchanger as such, but expelled the air through the core for 70s and then took air from outside through the same core for 70s. Somehow I quite liked this idea, until I saw the line that the core "moistened", as well as warmed the air. Great for continental climates that can be very dry in winter, but not so great when you're trying to get rid of the humidity.</blockquote>

Wouldn't blowing incoming air over a moist warm-to-tepid ceramic core be a recipe for spreading mould?</blockquote>

Yeah, I had that thought, too; shame, quite liked the idea for a while.

Loved steamy tea's matchbox - yep, I just need to get to the point where I can accept the non existence of it (& cheap effective heat pumps, Ewi, triple glazing etc etc ;-) and whip out the visa to get a reasonable compromise

And Doc, absolutely agree; I've been under heating and I have tried to remedy that & been running the dehumidifer a lot more. You've made me realise that it didn't help recently that my ch wasn't working for over a week, despite my best efforts to get it fixed. I will Ewi in the future, but it's fairly obvious that I need to address ventilation now (and as I've yet to find an Ewi installer I'd trust, I think I need to do the ventilation first)

One option is the passive (or active) ventilation with controlled vents that Aereco do. This is actually quite clever and looks almost like a low-energy system that could work. Humidity-controlled vents that are entirely passive, then a constant-pressure fan so it stops if the vents are all closed. I guess that's MEV, but 'clever MEV'. Ah, I see that's what you called 'Demand controlled ventilation'. Didn't seem overcomplicated to me. No idea if it works better than HRV or not. 'It depends', probably.

Why will MHRV over-ventilate if your house is not tight enough? If you have a model with separately controllable input and output speeds you should be able to allow for quite a lot of 'excess' air. The catch is that you lose exchanger efficiency because some fraction of the air isn't coming in that way, but I don't see how you get 'too much' ventilation (unless airtightness is terrible)

I don't think there's any one correct answer, a lot of it depends on the shape of your house.

Having tried out various other solutions, I believe a couple of the bigger eco retrofit organisations now routinely use one or two single room MVHR units (I have some personal experience of them too, and they really do work), e.g. one in the bathroom, and one in the kitchen,

Maybe consider the Envirovent Retrovent, or (if you have deeper pockets) Viking House's unit. Both are intelligent designs with high efficiency exchangers, and which respond to the conditions where they are installed, and use little energy (and produce little noise) when lots of ventilation isn't needed.

If you haven't done-so already, get some cheap (£10 or less) humidity meters and scatter them around the place. If you're not sure about the single room MVHR, then just fit one to start off with, and see how much difference it makes.

BTW, I've also fitted a couple of whole house MVHRs too, but think I'd still recommend the single room MVHRs in most situations.

I am interested in this thread as I just bought a 1950s ex council house concrete-block cavity wall, rendered (gable end mock stone face on the concrete blocks) which no one wanted to buy probably because of patches of mould on the walls owing to condensation. (Double-glazed, no active ventilation in kitchen or bathroom). I am now thinking of installing single roome HRV systems to address the problem but need to know more about the detailed way they work. I was a bit concerned that the units may recover heat when it may not actually be needed. Sorry I am hazy on detail, and need to do more research, but I gather the units are factory set to recover heat when the outside ambient temperature is a certain level and it sounded to me quite high. I am the sort of person that has to have the bedroom window partly open even if it is minus six outside. Any pearls of wisdom to help me consider this approach would be helpful. It also seems like most of the houses in this area have a similar problem, as occupants don't understand the issues of moisture and condensation. My next door neighbour has a horrendous problem with mould growing indoors. I am hoping I may be able to show people round here what they need to do.

Posted By: suegreenbuildingI am the sort of person that has to have the bedroom window partly open even if it is minus six outside.

This is probably because the internal air quality is poor due to insufficient ventilation & this is the only way you can stop it feeling "stuffy". If you get the ventilation sorted then you probably won't need to open the windows as much & you can just enjoy the warmer temperatures. That said, there are a number of MVHR units which have a programmable summer bypass, but I've not seen it on a single room unit.

David

Is it not possible for a controller for MVHR to only switch it on if the air is "bad" (CO2/RH) so if you open a window or simply by using the house, doors open etc the MVHR does not come on. This is surely "green" as it does not waste power running the unit when its not required?

Posted By: joe90Is it not possible for a controller for MVHR to only switch it on if the air is "bad" (CO2/RH) so if you open a window or simply by using the house, doors open etc the MVHR does not come on. This is surely "green" as it does not waste power running the unit when its not required?

Ventaxia do the following:

http://www.vent-axia.com/range/sentinel-totus-demand-energy-recovery-ventilation

It has heat recovery like a MHRV unit, but the vents only open when required - i.e. occupancy of a bathroom/toilet on the extract or the CO2/humidity levels of a bedroom on the supply.

I have an old leaky house that I'm slowly sealing up, but it will never be airtight enough for MHRV, so this seems to be the ideal solution for us.

Ouch! I've just seen the price!