MVHR with extract only mode

[MikeGrahamT21]

Following on from my thoughts about smoke in area in winter, and MVHR possibly being unviable, I’ve had another think. Obviously putting this in we would do away with normal extractor fan in bathroom. Our house will never be truly airtight being a renovation that I’m doing in bits, but hoping to get it pretty decent.

 

as well as summer bypass mode, does anyone know of an MVHR unit which does summer bypass, full hr and extract only, which I appreciate won’t recover heat but will still do the all important extraction in the wet rooms. Just gives us more flexibility in the system. The whole smoke thing may not even be an issue, but at least we have options either way, don’t want to spend a couple grand to realise we can’t use it at all for a big portion of the year.

 

the summer months make me think more about MVHR for it’s cooling ability, and I think that alone would be worth it’s weight in gold, but we must be able to extract as a minimum all year round

[le-cerveau]

I don't think that would be an option, to use MVHR you need a reasonably airtight house otherwise it just doesn't make sense!

 

If you have a reasonably airtight house you need to have the supply air (Building regs) as well as extract!

 

If you just have extract then you need a leaky house!

 

 

[Temp]

We find the main benefit of MVHR is the constant level of ventilation it provides. We still throw the windows open at night in summer to cool the place down.

[jack]

14 minutes ago, Temp said:

We find the main benefit of MVHR is the constant level of ventilation it provides. We still throw the windows open at night in summer to cool the place down.

 

I don't like leaving windows open overnight due to insects. If I were building again I'd spec insect screens on at least the bedroom windows.

 

We open everything up first thing in the morning and close up once the outside temp warms up. Peak temps are much lower than if we'd kept everything shut up.

[MJNewton]

2 hours ago, MikeGrahamT21 said:

as well as summer bypass mode, does anyone know of an MVHR unit which does summer bypass, full hr and extract only, which I appreciate won’t recover heat but will still do the all important extraction in the wet rooms. Just gives us more flexibility in the system. The whole smoke thing may not even be an issue, but at least we have options either way, don’t want to spend a couple grand to realise we can’t use it at all for a big portion of the year.

 

Is the extract-only mode desired because of the potential smoke issue? If so, are you sure it'll work as expected? For any air to be extracted there obviously has to be an equivalent amount of air sucked in hence won't the smoke outside still be a problem? I suppose there will be a difference between air coming in via gaps in the building fabric versus all from a single inlet but whether that will help I am not so sure!

[MikeGrahamT21]

We have negative pressure currently and don’t have any issues with the smoke coming in, at present there are huge gaps around the floors, soon to be gone tho.

 

maybe it just isn’t as much of an issue as I’m thinking.

 

didnt think about building regs for the supply as well as extract.

 

it may just have to be a suck it and see project, keep my eye out on eBay for leftover/second hand components and just give it a go and see what happens

[MikeGrahamT21]

I've seen a second hand MVHR unit on ebay, Vortice HR200, 8 years old, but reading the manual the heat exchanger should be changed every 6 years, even if the filters have been looked after. Is this normal practice? I would have thought the heat exchanger would last a lifetime, and perhaps just need the odd bit of cleaning?

 

What im wanting to do, and i think its the only way i'm going to come to an answer, is to mount an MVHR unit where its going to go, feed it a supply and extract, but don't do any ducting to the rooms, and see what i can smell come the winter time in the loft (which has never smelt of smoke), if its smells OK then its not likely to be an issue.

 

Before I saw this one, i'd considered a Brookvent Aircycle 1.2, with summer bypass, which can be had for around £700, which didn't seem too bad, but what do you think about the above unit for £250, and just needs new filters apparantly?

[PeterW]

It probably has a paper core heat exchanger that will be past it’s best. They won’t be  cheap either and they are basically 50% of the unit 

[MikeGrahamT21]

OK cheers for that, wasn't aware there were different types of core. Assuming they were all aluminium.

 

Not really seen much in the way in the specs either what type of heat exchanger they are, will have to keep a keen eye. Would aluminium core last forever bar a bit of cleaning?

[MikeGrahamT21]

17 minutes ago, PeterW said:

It probably has a paper core heat exchanger that will be past it’s best. They won’t be  cheap either and they are basically 50% of the unit 

 

Actually that particular one i looked at says: Heat Exchanger – Crossed Counter-Flow type, made of PS (Polystyrene). How can a heat exchanger be made of insulation material?

[PeterW]

Polystyrene is a plastic... EPS or Expanded Polystyrene is the insulation that everyone is aware of - the small white balls are polystyrene blown with a gas - nowadays they are CFC free but used to be blown with all sorts of weird stuff. 

[MikeGrahamT21]

Of course it is! My brain clearly isn't in gear today. Would a PS heat exchange need replacing every 6 years?

[A_L]

2 hours ago, MikeGrahamT21 said:

 

Actually that particular one i looked at says: Heat Exchanger – Crossed Counter-Flow type, made of PS (Polystyrene). How can a heat exchanger be made of insulation material?

 

Polystyrene is an insulator for two reasons. First it has a reasonably low thermal conductivity on its own. Second most of the insulating effect comes from reducing air movement, i.e. the small cells enclosed by the polystyrene do the work, this in fact is how most of our insulations work. This is enhanced by the fact that we use it in thicknesses up to several hundred millimetres.

 

Polystyrene can be produced like polythene as a clear plastic film, no air cells. It works in a heat exchanger because it may only be a few hundredths of a millimetre thick. It also obviously has a large surface area. Thus it has an adequate heat exchange.

In reality a mvhr heat exchanger is many heat exchangers thermally in series, each with about a 50% efficiency, the low thermal conductivity along the length of the heat exchange surface actually helps to isolate each series heat exchanger from the next/previous one, this is in fact an advantage over a metal heat exchanger.

 

[readiescards]

On 22/06/2018 at 13:35, MikeGrahamT21 said:

 

it may just have to be a suck it and see project, keep my eye out on eBay for leftover/second hand components and just give it a go and see what happens

 

I have a fair amount of red 100mm pipework and various other mvhr associated bits here in Lincolnshire ready to go on eBay. Pm me if your interested. 

[Onoff]

18 hours ago, MikeGrahamT21 said:

 

Actually that particular one i looked at says: Heat Exchanger – Crossed Counter-Flow type, made of PS (Polystyrene). How can a heat exchanger be made of insulation material?

 

There's a thread here on a DIY plastic sheet head exchanger I found a while back if you're intetested:

 

 

[jack]

Bear in mind that this is a hybrid counterflow/crossflow design (more the latter than the former). I'm not sure how you'd calculate efficiency, but I imagine you'd be lucky to get 60-65% in the real world.

 

Not much needs to be changed to turn this into a counterflow exchanger, which would likely be at least 10% more efficient.

[Onoff]

12 minutes ago, jack said:

Bear in mind that this is a hybrid counterflow/crossflow design (more the latter than the former). I'm not sure how you'd calculate efficiency, but I imagine you'd be lucky to get 60-65% in the real world.

 

Not much needs to be changed to turn this into a counterflow exchanger, which would likely be at least 10% more efficient.

 

Tell me more! What's the difference between the two types?

 

I was thinking to make o/of Buffalo Board and incorporate a st/st drip tray in the bottom.

[MikeGrahamT21]

19 hours ago, A_L said:

 

Polystyrene is an insulator for two reasons. First it has a reasonably low thermal conductivity on its own. Second most of the insulating effect comes from reducing air movement, i.e. the small cells enclosed by the polystyrene do the work, this in fact is how most of our insulations work. This is enhanced by the fact that we use it in thicknesses up to several hundred millimetres.

 

Polystyrene can be produced like polythene as a clear plastic film, no air cells. It works in a heat exchanger because it may only be a few hundredths of a millimetre thick. It also obviously has a large surface area. Thus it has an adequate heat exchange.

In reality a mvhr heat exchanger is many heat exchangers thermally in series, each with about a 50% efficiency, the low thermal conductivity along the length of the heat exchange surface actually helps to isolate each series heat exchanger from the next/previous one, this is in fact an advantage over a metal heat exchanger.

 

Thats good to know, thanks for the explanation

[PeterW]

4 minutes ago, Onoff said:

I was thinking to make o/of Buffalo Board and incorporate a st/st drip tray in the bottom.

 

If you mean the Correx type stuff it will never work as it’s just too thick and does not have a guaranteed sealed channel structure. 

 

Airflow between the layers is what causes the heat transfer - aluminium is used a lot where you have a large heat differential such as in condensing tumble dryers. In an MVHR heat exchanger the temperature differential is so low at 5-10c then the film thickness is reduced to 0.2mm or thinner to ensure heat passes between the air flows. 

 

In in its simplest form, a cross flow  heat exchanger has the two air flows at 90 degrees to each other, in a counter flow the two flows are parallel but flow from opposite ends. 

[MikeGrahamT21]

5 hours ago, readiescards said:

 

I have a fair amount of red 100mm pipework and various other mvhr associated bits here in Lincolnshire ready to go on eBay. Pm me if your interested. 

I think the end goal would be to use the 75mm Radial ducting for ease of installation, but thanks for letting me know up front. I think my first goal if I can get a reasonably cheap but good MVHR unit second hand, will be to mount in the loft, and give it external air, and then basically just see what comes out of the inlet, if it smells of smoke I can investigate further then, before I go cutting a load of holes in the ceilings and walls.

[MikeGrahamT21]

28 minutes ago, jack said:

Bear in mind that this is a hybrid counterflow/crossflow design (more the latter than the former). I'm not sure how you'd calculate efficiency, but I imagine you'd be lucky to get 60-65% in the real world.

 

Not much needs to be changed to turn this into a counterflow exchanger, which would likely be at least 10% more efficient.

So they quoted 89% efficiency i think for this unit, how can it only be 60-65? I'll keep on the lookout for counterflow, makes sense air at 90 degree angles to each other would pass more heat. All good information this!

[richi]

The "area" Vent-Axia I used in the garage reno uses what looks like a load of plastic straws bundled together (and, no, I didn't pay anything like £720 for it!)

[Onoff]

15 minutes ago, PeterW said:

 

If you mean the Correx type stuff it will never work as it’s just too thick and does not have a guaranteed sealed channel structure. 

 

No, I meant use Buffalo Board instead of normal ply for the enclosure. It's like a black / brown resin impregnated, phenolic faced, birch ply with one side smooth like Formica and the other non slip & embossed. 

 

http://www.advancedtechnicalpanels.co.uk/products/buffalo-board-birch-plywood

 

 

 

[PeterW]

15 minutes ago, MikeGrahamT21 said:

makes sense air at 90 degree angles to each other would pass more heat.

 

Thats crossflow - counter flow is where the air flows are parallel and  this is much more efficient. 

[PeterW]

3 minutes ago, Onoff said:

 

No, I meant use Buffalo Board instead of normal ply for the enclosure. It's like a black / brown resin impregnated, phenolic faced, birch ply with one side smooth like Formica and the other non slip & embossed. 

 

http://www.advancedtechnicalpanels.co.uk/products/buffalo-board-birch-plywood

 

 

 

 

For what ..? The enclosure for a heat exchanger ..??