General: Passive v MVHR

31 to 60 of 60

- CommentAuthorDamonHD
- CommentTimeJul 18th 2012
That's more than our entire house! B^> (Still, maybe I could put it in the hall and rely on moisture diffusing in from kitchen and living room even with doors mainly shut...)

I don't suppose that there are small units are there? Good for 20m^2?

Rgds

Damon
- CommentAuthorViking House
- CommentTimeJul 18th 2012
How many occupants?
- CommentAuthorDamonHD
- CommentTimeJul 18th 2012
4, each in the majority of every 24h typically.

Rgds

Damon
- CommentAuthortimberframing
- CommentTimeJul 18th 2012
Hi VH

How does the FiWi cope with bathroom/kitchen moisture?
Where do you site the FiWi units in relation to these rooms?
Are extract fans still needed to comply with UK Bldg. Regs?

Rgds

Phil
- CommentAuthordjh
- CommentTimeJul 18th 2012
Isn't 97% efficiency better than the claimed efficiency of the 'best' products on the SAP Q and PHI lists? Must be made of unobtainium
- CommentAuthorJoiner
- CommentTimeJul 18th 2012
"unobtainium" -
- CommentAuthorskyewright
- CommentTimeJul 18th 2012
Posted By: Viking HouseHere's some pics of the latest Breathing Window

Does that image show the "inside" view of the unit or the outside, or maybe it's the same both sides?
I think probably inside, but I could easily be wrong?
- CommentAuthorfostertom
- CommentTimeJul 18th 2012
Posted By: djhIsn't 97% efficiency better than the claimed efficiency of the 'best' products on the SAP Q and PHI lists?
Sounds ambitious - but this unpowered design is going to be different, for better or worse, than all the fan-powered ones. With v low air resistance and enormous transfer surface ...?
- CommentAuthorViking House
- CommentTimeJul 18th 2012 edited
Hi Phil, We use Odour Buster toilets http://www.viking-house.co.uk/downloads/Odourbuster%20Toilet.pdf and Charcoal filters in kitchens.
- CommentAuthortimberframing
- CommentTimeJul 18th 2012
VH

Looks a good solution, I had the same idea about ten years ago, always thought it made sense to extract odours at source..........well as close as possible! I should have worked up a design and taken out a patent! Thanks for the link.
- CommentAuthorSteamyTea
- CommentTimeJul 19th 2012
You have the toilet in the kitchen
- CommentAuthorSprocket
- CommentTimeJul 20th 2012 edited
"97%" at heat exchange is OK but not particularly meaningful by itself.
To be useful in real situations it has to be able to recover latent heat such as condensation as well as recovering heat from dry temperature differences.
Many designs are not so good at that.
The horrible looking plastic counterflow exchangers in recent MVHR are theoretically pretty good at that because their efficiency is not significantly reduced by moisture. But it is not easy to compare numbers because there are no standard measurement conditions (are there?).
The low efficiency metal crossflow exchangers common in industry are also pretty good at that (if they drain well) on account of being made of metal (conducts well) and the fact that their comparatively low efficiency figures are quoted for dry heat transfer to make it easier to compare them. The domestic exchangers we see are not limited by anything as clumsy as a common reference requirement (ie. industrial standards).
- CommentAuthorTomski
- CommentTimeJan 21st 2013
Some (late) feedback on Ventive (www.ventive.co.uk) regarding the passive stack v. heat recovery:

95% of the time buoyancy is the main driving force of current Ventive range. It generates between 8m3 and 20m3 of airflow per unit (depending on the difference between internal and external temperature). The heat is recovered from the outgoing air at the highest point in the stack with about two feet of cooled stale air above it slowing the flow down and about 3m of vertical stack of warm air (and the whole room) driving it up. The driving force outweighs the resisting force by a fair margin and the room is ventilated.

As for the Heat Recovery ventive uses completely different heat exchanger than MVHRs, it is linear and helps turbulent airflow through the unit (MVHR is all laminar) therefore maximising the heat transfer. The 'up to 97%' results are not only lab tested but also in situ tasted over a number of months. The two systems currently on sale ( S and S+) fit both existing properties (with chimneys) and new build (without chimneys).
- CommentAuthorRobinB
- CommentTimeJan 21st 2013
Your company's product sounds interesting Tomski. I'm interested in alternatives to MVHR but I can't help feeling that, if temperature difference is the main driver, when the difference is small the ventilation risks being too low or conversely when it's large you'll be drawing in too much cool air when you're least likely to want it.
- CommentAuthorEd Davies
- CommentTimeJan 21st 2013
Something to bear in mind when thinking about these passive stack systems is that humid air is less dense than dry air, even at the same temperature ('cause two widdle H atoms in an H₂O molecule are lighter than one hulking-great O atom in an O₂ or an N atom in an N₂ molecule). So you could, in principle, have some passive stack ventilation even if the indoor and outdoor temperatures are the same and there's no wind blowing.
- CommentAuthorSteamyTea
- CommentTimeJan 21st 2013
Is a molecule of water more or less dense than a molecule of nitrogen or oxygen, I never really understood chemistry (was taught by a Born Again Christian that thought the Plymouth Brethren side of my family were a bit weak, so switched off for most of it).
- CommentAuthorjms452
- CommentTimeJan 22nd 2013
Gaseous molecules all take up about the same space so you just add up the mass of the constituent atoms:

H20 = 1*2+16
O2 = 16*2
N2 = 14*2

So water vapour is about 18/28 the density of dry air.
- CommentAuthorSteamyTea
- CommentTimeJan 22nd 2013
Right
That seems simple enough
- CommentAuthorRobinB
- CommentTimeJan 22nd 2013
Posted By: Ed Davieshumid air is less dense than dry air, even at the same temperature ('cause two widdle H atoms in an H₂O molecule are lighter than one hulking-great O atom in an O₂ or an N atom in an N₂ molecule). So you could, in principle, have some passive stack ventilation even if the indoor and outdoor temperatures are the same and there's no wind blowing.

Thanks Ed, That really helps Ventive's case and,to my mind, makes a more persuasive argument than temperature difference alone.
- CommentAuthorEd Davies
- CommentTimeJan 22nd 2013
Posted By: SteamyTeaI never really understood chemistry

Funnily enough, neither did I. I got a plausible O Level but didn't carry on with it at A Level at least in part because I couldn't get my head round stuff like this. Also a bit too much rote learning for my taste. My chemistry teachers were pretty good but somehow it just didn't click for me the way physics did. What I understand about it I've picked up more recently reading about meteorology (interests in gliding then AGW) and building science.
- CommentAuthordjh
- CommentTimeJan 22nd 2013
Posted By: TomskiAs for the Heat Recovery ventive uses completely different heat exchanger than MVHRs, it is linear and helps turbulent airflow through the unit (MVHR is all laminar) therefore maximising the heat transfer. The 'up to 97%' results are not only lab tested but also in situ tasted over a number of months.

Do you know if the performance results are available?

If the type of heat exchanger used by Ventive offers better results than those used in MVHR systems, which seems to be what you're saying, why wouldn't the MVHR manufacturers have switched to the improved type?
- CommentAuthorLondonEcoHouse
- CommentTimeJan 28th 2013
Hi all,

We are actually having the Ventive system installed into our new build so can report back with any feedback once installed and complete. We have been impressed with what we have seen so far so will see how it goes.

Cheers
Peter
- CommentAuthorfostertom
- CommentTimeJan 28th 2013
Posted By: Ed Davieshumid air is less dense than dry air, even at the same temperature ... you could, in principle, have some passive stack ventilation even if the indoor and outdoor temperatures are the same and there's no wind blowing.
Very interesting! Why didn't we know that, in all our various discussions involving buoyancy? Seems a vital factor.

Tho that's the difference between pure dry air and pure water vapour. I wonder what would be the buoyancy difference between dry air, and air of same temp but with a typical %age of water vapour in it?

What temp difference in dry air wd be necessary to achieve that same buoyancy?
- CommentAuthordjh
- CommentTimeJan 28th 2013
Posted By: Ed DaviesSomething to bear in mind when thinking about these passive stack systems is that humid air is less dense than dry air, even at the same temperature .... So you could, in principle, have some passive stack ventilation even if the indoor and outdoor temperatures are the same and there's no wind blowing.

When the indoor and outdoor temperatures are the same, what are typical values for the respective RH? I'd have thought there wouldn't be a huge difference most of the time, though it depends where you are. And clearly, it won't work in muggy weather.
- CommentAuthorEd Davies
- CommentTimeJan 28th 2013
Just twisted some Python code I wrote ages ago to work this out. Columns are temperature (°C) and densities of
dry (RH=0) and saturated (RH=100%) air (kg/m³).
```
35 1.14569300213 1.12149530569
30 1.16458947256 1.14603441317
25 1.18411973371 1.17003953224
20 1.20431621561 1.19374890101
15 1.22521359919 1.2173745476
10 1.24684901503 1.24110493211
5 1.26926226355 1.26510764672
0 1.29249605933 1.28953213628
-5 1.31659630284 1.31451240309
-10 1.34161238308 1.34016966209
-15 1.36759751543 1.36661491711
-20 1.39460911952 1.393951434
-25 1.42270924283 1.42227709299
```
Water vapour doesn't make much difference at low temperatures but at higher ones it's equivalent to another couple of degrees.

Compared with the densities for dry air given here my numbers are slightly higher for some reason,

http://en.wikipedia.org/wiki/Density_of_air
- CommentAuthorfostertom
- CommentTimeJan 28th 2013
Posted By: djhtypical values for the respective RH? I'd have thought there wouldn't be a huge difference
I'd a thought it was about abs water content, not RH? i.e. density of water molecules.
- CommentAuthordjh
- CommentTimeJan 29th 2013
Posted By: fostertom
Posted By: djhtypical values for the respective RH? I'd have thought there wouldn't be a huge difference
I'd a thought it was about abs water content, not RH? i.e. density of water molecules.

Indeed that's how the physics of the flow work, but at the same temperature all the humidity measures are equivalent and RH is the familiar one.

My point is that when the inside and outside temperatures are the same, the humidities tend to be similar as well. In a dry region, there can be less humidity outside so you will get some humidity-driven buoyant flow, but in a typical green and pleasant and/or maritime region you won't get so much. And when its raining, you won't get any.
- CommentAuthorEd Davies
- CommentTimeJan 29th 2013
Posted By: djhMy point is that when the inside and outside temperatures are the same, the humidities tend to be similar as well.

Yes. From the point of view of condensation control, though, at that time ventilation wouldn't do you any good anyway. Other gases than water vapour are important as well, of course.

The case where water vapour buoyancy¹ could be helpful for passive ventilation would be a warm summer's day where the outside air is fairly dry but you're carefully shading the windows to avoid the inside getting much warmer. The inside air would be at a similar temperature but might well be much more humid.

¹ Americans pronounce “buoy” as "boo-ee"; do they pronounce “buoyancy” as “boo-ee-ancy”?
- CommentAuthordjh
- CommentTimeJan 29th 2013
Posted By: Ed Davies¹ Americans pronounce “buoy” as "boo-ee"; do they pronounce “buoyancy” as “boo-ee-ancy”?

No, they pronounce it pee-eff-dee

red-right-returning IALA region B don'cha jus lurv murricans
- CommentAuthorTimSmall
- CommentTimeJan 29th 2013 edited
I've been looking at some of the stuff on one passive-stack provider's web sites with some scepticism - lots of comparing apples with oranges, and exaggeration of the costs of alternative systems etc.

My experience of my current flat's stack ventilation (via the wood burner's chimney) is that extract is about 5 times faster with a wind speed of 20 mph than it is with a wind speed of <5 mph. As I live on the South coast, 20 mph winds are pretty common. Whilst I accept that passive stack vents could be designed to reduced this effect (although maybe they aren't as their literature explicitly mentions the use of venturi to promote extraction), and humidity controlled extract ducts would help a little too, I still find it hard to believe that a passive stack system wouldn't horrifically over-ventilate in windy conditions.

Anyone seen any numbers, or have any relevant real-world experience?

Currently, I'm leaning towards MVHR with some decent control software (plus CO2 and humidity sensors to inform the fan speed variation which the software does).

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