Single Room MVHR

[Crofter]

Bit of a double question here.

Does anybody have experience of those single room heat recovery units, e.g. the HR25 from Vent Axia? I've read Damon HD's comments about it on GBF, and you see them come up on eBay from time to time for about £100 or so.

 

What I'm toying with is getting one of these units, ostensibly to install in the bathroom of my existing house, which has no extractor fan at present and a single glazed window which is perpetually running with condensation. However my new build is at the vapour barrier stage and I could 'borrow' the unit to temporarily install and help tackle condensation within the shell. At only 43m2 it is not far outside the rated capacity of one of these units. Once I am ready to install the proper multi-room ducted system, I can whip out the single room unit and put it in my bathroom (OK, put it in a box in the loft for a few months till I get round to it!).

 

A final thought: would it be stupid to kit out the new house with two or three of these little units? It is very tempting indeed, as it saves running ducts everywhere and would work out much cheaper. It would require one in the kitchen/lounge, one in the shower room, and one in the bedroom. Mostly worried about noise, aesthetics, and any performance issues if they try to 'fight' each other.

[TerryE]

I am sure that a couple of the other guys have hard experience of these, as I can recall a couple of strong recommendations (from @ProDaveor @Stones ??)

 

However as an alternative to one of these for a build under development, have you considered just using a standard budget dehumidifier?  We have used one to great effect in drying out excess moisture after plastering,

[Stones]

It was something I was considering for the house we are currently renting, but we took the easy way out and went for a small dehumdifier.  The only issue I can see with individual units is getting them all balanced with each other as presumably, they will be extracting / supplying at different rates? 

 

I would certainly agree with @TerryE that a dehumdifier would be the simplest and easiest way to dry the build out inside.

[gravelld]

Another ductless option is Lunos, but more expensive I think.

 

Not sure if this helps when Googling but these units are often called SRHRV.

[Crofter]

Thanks all.

My main concern is about the building sitting at the vapour barrier stage for a while and it becoming an uncomfortable environment. I may well find that simply leaving a window cracked open will provide sufficient ventilation, of course.

The attraction of the single room unit over a dehumidifier is that I would have a use for it afterwards. And it would be interesting to get my hands on one and thereby be able to make an informed decisin about multiple SRHRV vs centralised MVHR.

 

I presume the central unit wins out in noise and over efficacy, but as I mentioned on another thread nobody is making decent high efficiency ones for houses the size of mine.

[TerryE]

4 minutes ago, Crofter said:

The attraction of the single room unit over a dehumidifier is that I would have a use for it afterwards.

 

Anyone who has to dry clothes in winter would make good use of a dehumidifier!!

[Onoff]

Does a single room MHVR have to be mounted on a wall or can it be via a ceiling vent?

 

Cheers 

[Crofter]

Can't say definitively, but all the ones I've looked at show through-wall mounting, with a straight duct run. There may be condensate drain issues if you were to mount the unit in the ceiling with a bend in the duct.

[Crofter]

Any thoughts on this chappie?

http://www.ebay.co.uk/itm/Single-Room-Heat-Recovery-Kit-Extractor-Fan-Extract-Supply-Fresh-Air-HRU-HRV100-/322424498584?hash=item4b11ff6998:g:thIAAOSwVcFXO4hT

[ProDave]

19 minutes ago, Crofter said:

Any thoughts on this chappie?

http://www.ebay.co.uk/itm/Single-Room-Heat-Recovery-Kit-Extractor-Fan-Extract-Supply-Fresh-Air-HRU-HRV100-/322424498584?hash=item4b11ff6998:g:thIAAOSwVcFXO4hT

I have read the description. 

 

It is a SINGLE fan that runs 60 seconds in one direction, then 60 seconds in the other. While it is extracting, it heats up the heat accumulator. Then when drawing air in, hopefully the heat stored in the accumulator warms the incoming air.

 

Not convinced how that will work in practice, but in order to alternately supply then extract air on a 1 minute cycle, must rely on the house being leaky enough to absorb that waxing and waning air flow.

 

Start running 2 in different rooms, and I can see it getting messy.

 

I would avoid.

[A_L]

It claims to have an aluminium heat exchanger. Other units of this type have a ceramic heat exchanger so that heat transferred at a particular location along the length of the heat exchanger does not move along it to cooler areas too readily. This allows a temperature gradient along the heat exchanger which allows you to heat the incoming air. So given the high thermal conductivity of aluminium I would want to know a lot more about how it is supposed to work.

More expensive versions of this idea often have pairs of ventilators which communicate with each other and are set to 'blow' and 'suck' at the same time and then swap over, maintaining balance

It is a generally good idea but more expensive than centralised MVHR for a whole house

[Crofter]

Yes I should probably update to say that I've got a system already for the new house, so this single room thingy would just be for the bathroom in my current house.

 

I am also toying with the slightly hairbrained concept of fitting one to my boat, but not sure exactly where and how to mount it- I think the 'outside' bit would need to be in the cockpit, for protection.

[Crofter]

So some more thoughts on this- the daft boat-mounted MVHR idea, that is.

 

How much free space might the outside part of a unit require? I'm struggling to think of somewhere to mount it that is well enough protected, and wondering about putting it in a cockpit locker. This would place the inside component right at the galley which is good.

I obviously can't have it venting in and out of a cockpit locker- but I could build an enclosure around it, and have a grille. Not sure if these units need to see a nice free flow of air though, and that this might be too restrictive?

[Jeremy Harris]

What's the primary objective for fitting this to the boat?

 

My experience over the years has been that there are three main problems inside boats.  They get too hot in summer, too cold in winter and are more or less constantly damp inside.  You may not have the first problem, being where you are!

 

Condensation has always been the most annoying thing on every boat with a cabin I've owned, and is even the most annoying thing on the open inland waterways boat, with canopy and sleeping "tent" I currently own.  The cold I could always cope with, although I will admit to a few nights on board were a bit of warmth wouldn;t have gone amiss.  Overheating, especially when cooking or brewing tea invariably seems to go hand in hand with condensation.

 

So, what are the priorities?

 

-  Less heat loss?

 

-  Less condensation?

 

-  Less over heating?

 

It may be that an MVHR type system could help, but it could also be that you might be better looking at other approaches.  A friend (who has the luxury of shore power where he's moored) runs a small dehumidifier in the boat when it's alongside, and that makes a very big difference, with the effect lasting for several days at sea afterwards.  My last yacht had two solar powered ventilation fans fitted in the coachroof, one right forward, over the loo compartment, the other over the galley area.  They worked pretty well at keeping the boat dry-ish inside when it was moored up, but weren't at all effective at sea.

 

I had the use of a boat with a small charcoal stove for heating once, and that was brilliant for both keeping the boat warm and dry, as the charcoal didn't introduce moisture the way a gas heater does.

 

When it comes right down to basics, the very best modification I ever made to my old yacht was to build an old-fashioned dorade box on the foredeck, fitted with a removable, and rotatable, ventilation "elbow" (it was modified 160mm drain pipe bend..........).  This was unplugged and a blanking plug fitted when the boat was moored up, but when out sailing the vent could be rotated to blow fresh air in, whilst the dorade box kept even pretty heavy water out.

[Crofter]

The primary purpose would be to address condensation whilst not losing too much heat.

My thinking is that a small MVHR unit acting as the primary means of ventilation would shift a decent amount of air, but without lowering the cabin temperature right down to the same as outside. Am I right in thinking that by maintaining a higher cabin temperature, an MVHR system would shift more water vapour than a normal extractor fan?

I do have a rudimentary heating system, which is diesel fuelled with its own flue. This makes a modest difference to the cabin humidity by raising the temperature, but is not enough to overcome the condensation produced by cooking or breathing, unless the ambient temperature is already quite mild.

Overheating is, as you say, not likely to be an issue where I am!

 

I am toying with wiring the MVHR up in two ways- manual boost for when the boat is occupied, and on/off via a VSR otherwise, so that is acts as a dump load for the solar panel. That might be more hassle that it's worth actually because in winter I will have barely any excess power, and that is when I would need it the most. And it might be that for an unoccupied boat, where cabin temperature is irrelevant, it's best just to rely on the dorades and the forehatch which can be left slightly ajar.

 

Shore power would be a massive game changer, but a marina berth would involve moving the boat either hundreds of miles or a ferry ride away, and would put up my annual costs by about five-fold

[ProDave]

Assuming you are talking of a domestic unit, I guess you are planning an inverter to power it from the boat's battery?

 

Condensation is a major problem in my boat when at home for the winter.  In summer my cabin and bilge is bone dry, so I now it does not leak from above or below. But leave it over the winter unatended and when I look in in March there can be 2 inches of water in the bilge, that can only be condensation.  The annoying thing, my dehumidifier shuts down if you  try and run it when it's too cold, so that's no use. So it's a regime of regularly drying out the bilge in winter.

[SteamyTea]

may be worth someone that is living in a caravan on site to try it out.

[Crofter]

There are 24v and 12v units out there, so it would be either directly wired in, or powered using a DC-DC converter.

[Jeremy Harris]

If you can get a DC operated one that's cheap enough to try, then it might be worth a go.  Having good ventilation whilst reducing heat loss should help reduce the condensation.  You may even be able to build one.  @SteamyTea built a heat exchanger a while ago that worked fairly well, and 12 or 24 V DC fans are cheap and easy to get hold of (computer cooling fans are often 12V).  The advantage of a DIY unit (apart from cost saving) is that you could shape it to fit in an awkward space, something that might be useful for a boat, where space is always at a premium.

 

You'll need to arrange a condensate drain, I'm sure, as the chances are that the heat exchanger will end up operating well below dew point a fair bit of the time, so if you can position it high enough in the boat to have a gravity drain to an existing seacock, then that may be useful.

[ProDave]

Perhaps I need to look at forced ventilation while my boat is at home in winter storage. I could quite easily make a spare replacement for one of the wash boards and fit a fan in that to extract air, the extracted air would be made up by air drawn in through the mushroom vent. Perhaps maintaining a through flow of air all winter might stop my condensation problem?

[Jeremy Harris]

51 minutes ago, ProDave said:

Perhaps I need to look at forced ventilation while my boat is at home in winter storage. I could quite easily make a spare replacement for one of the wash boards and fit a fan in that to extract air, the extracted air would be made up by air drawn in through the mushroom vent. Perhaps maintaining a through flow of air all winter might stop my condensation problem?

 

The two small solar powered fan vents I fitted to my boat worked pretty well when she was moored or ashore, and not in use.  They weren't horrendously expensive, and were easy enough to fit.  The only snag is that they won't work if you keep your boat under a cover when out of the water.  You could probably rig up a cheap solar panel and fan to work as a winter ventilation system.