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Pre filter between MVHR vents and MVHR unit


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My MVHR vents will be going through the gable end and the current terminal of choice has a very coarse mesh that may stop a horse, but not a horsefly, which means the horsefly will be stopped by the main filter within the MVHR unit.

 

I was thinking of making a couple of large boxes to go inside the thermal envelope, positioned just after the ducts enter the thermal envelope and just before the MVHR unit. Within the boxes I would have a fine filter (net curtain, stockings?) that is easily accessible and cleaned and will keep the horseflies out of my MVHR unit.

 

Has anyone done such a thing?

 

If the home made filter box is airtight enough is this a viable plan?

 

 

 

 

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Yep plan to do the same with some MDF, sealant and gloss paint. Then use a car pollen filter from eBay to do the prefiltering as they can be had for £3-4 each so swapping every 3 months  is no hardship. 

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My neighbour that installs MVHR systems insists on a coarse filter before the 2.5 filter. Massively increases the life of the filter, but more importantly delays the pressure drop as the filter clogs.

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Interesting, I wonder if one could use the same stuff as a prefilter in the MVHR and in a rainwater harvesting system. Having a good stock of spare material for both uses would encourage keeping both clean and changing before it all gets too icky.

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8 hours ago, SteamyTea said:

 

Get some reticulated foam

 

 

That looks like a great idea, I especially like the washable aspect.

 

My fledgling plan has the duct from the external vent entering the box horizontally and a separate duct then leaving the box vertically (en-route to the MVHR) with a pre filter horizontally mounted between the two.

 

When I open the door to (Pandora’s) box I will be able to inspect/clean the horizontal duct to the external vent, and slide out the pre filter to clean it.

 

It seems great in my head...

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Air filtration is pretty simple really, it is just a matter of slowing the flow rate down until particles stick to something.

So the key thing is that you just need a larger surface area.

I was on the production side, but the foam manufacturers would tell our design engineers the filtration properties of their materials.  I seem to remember that they refereed to it as the Q value, but may be wrong, it was over 20 years ago.

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non-woven mat (as used in spray booths as paint catcher, and prefilters in fumecupboards etc,  costs next-to-nothing and can even be washed if you're gentle. Basically the same stuff used in the MVHR units anyway...

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4 hours ago, Russdl said:

 

Thanks. Seems like I'll have quite a choice of pre filter material to choose from. I'd better get on and make the boxes.

Are you putting those inside in the attic? There are some good ready made filter boxes with circular ( 160 / 180mm ) flnges on ready to go. sealed lids with clips etc.

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Might save some work ;)

 

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On 27/08/2019 at 20:23, Russdl said:

Sounds like its a goer then, I'll get cracking. ?

 

@Conor Do you know exactly where/how that coarse filter is fitted?

Unfortunately I've been working in Dublin this week and not had a chance to speak to him... Will be speaking to him this weekend tho as I need his input on my proposed vent locations.

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8 hours ago, Nickfromwales said:

Are you putting those inside in the attic? There are some good ready made filter boxes with circular ( 160 / 180mm ) flnges on ready to go. sealed lids with clips etc.

 

Thats the plan Nick.

 

The idea of the box was to have a much greater surface area for the filter so as not to restrict the airflow too much, the boxes in the link you provided would seem to filter the air as the duct enters the box. I have no idea but wouldn’t that potentially restrict the airflow too much?

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  • 1 year later...

So, a year and a bit on from my initial thoughts on this I've been at it on SketchUp again.

 

The plan is to make these boxes out of ply or MDF, any suggestions as to which would be best, and why? Sealed with airtight tape and closed cell foam for the removable door.

 

My filter material will be, in the first place, unladdered stockings (if I can get them off without snagging them). The design will allow me to rip the box out and replace it with a standard Ubbink 180mm 90° bend if it all goes a bit Pete Tong. I welcome your comments and advice.

 

 

Screenshot 2020-11-15 at 20.31.46.png

Screenshot 2020-11-15 at 20.38.16.png

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?  Thanks @PeterW 

 

I always like to find complicated solutions to simple problems so that is way too straight forward for me, aside from that, it doesn't seem to do the 90° turn that I'll need.

 

And no little shelve underneath for spare filters - or something.

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