Using F7 filters and impact on flow and noise

[Levo]

Please help!

I am trying to figure out the impact of using F7 filters rather that the "standard" G3/4 and I wonder if anybody had any experience with them and share their experience and comment to my thinking and calculations below.

Assumption-1: The manufacturers quote their flow rates using the "standard" G3 filters.

Assumption-2: I have found a Filter manufacturer spec sheet and as best I can interpolate using the sizes they specced, a G3/4 filter will cause roughly 35-50Pa drop. Assume being fluff, the pressure drop does not go up dramatically over time.

Assumption-3: From the same doc, an F7 filter will be in the 125Pa+ drop to begin with but will go up to as high as 450 Pa drop (@ rated flow) as it gets clogged up.

Now, there appears to be almost 100Pa of difference (125 - 35) between a G3 and F7 filter to begin with and over time, it may go as high as 400 Pa.

I have got the Ventaxia Advance SX performance chart below. I have plotted the flow/Pa for my installation, 130m3/h (0.036m3/s) and I assuming I could keep the system ducting  pressure drops to 60 Pa total, I would be running the system at 40% fan rate with stock G3 filters.

But if I decided to go to F7 filters (and the Ventaxia appears to have a piddly 365x135mm filter size), the initial drop will be ~150+ Pa which means the system will have to run nearly at 60% capacity (assume this to be the fan rate). And as the filter ages, even if I didn't wait to reach the +400Pa point and replaced at 350Pa, I am looking at 80% fan rate, and that is at nominal flow.

Has anybody done or seen this sort of analysis before? Any comments on the noise etc. from those who tried using filters other than the stock G3/4?

If I am correct in these calculations, I need to look for a different unit for sure.

 

Edited November 2, 2020 by Levo

[woodman]

Hi all, I've fitted an F7 filter to my mvhr system, which uses a nuaire xbox95wh1 (very basic unit, but bought very cheaply, £350 as new,  as an incorrectly specified installation)

 

The whole thing has cost less than £100.

Here are the key points of construction, results and next actions. 

 

I hope this is helpful. I'd also appreciate any further info you may have to assist with the next steps, as below.

 

SYSTEM DESCRIPTION

https://info.nuaire.co.uk/IandM/671686.pdf

The system has 5 extract and 5 intake ducts, semi-rigid ducting, external vents being on the gable end of the house.

 

THE PROBLEM

To try to eliminate wood and coal smoke pulled in from my neighbour's woodburners and open chimneys.

 I firstly researched the commercially available units such as this:

 

https://www.blauberg.co.uk/en/blauberg-cleanbox-nox-carbon-polution-pollen-indoor-air-quality-filter-box-for-heat-recovery-ventilation-systems

 

and this

 

https://www.airclean.co.uk/indoor-air-quality-filtration/indoor-air-quality-filter-system/

 

but they appeared to be vastly overpriced  for what they actually are, so I decided to make my own, initially using a g3 and f7 filter, as here:

 

ASSEMBLY

G3 prefilter and F7 particulate bag filters used:

https://www.addfiltration.co.uk/12x12x2-290x290x45mm-g3-grade-2-deep-dry-glass-panel-filter?search=g3

https://www.addfiltration.co.uk/12x12x12-287x287x300mm-4-pocket-12-deep-f7-grade-synthetic-bag-filter?search=f7 bag

Total cost inc postage, around £20

I  used an F7 bag filter rather than a panel filter to increase the interval between servicing.

 

The enclosure itself was fabricated using tilebacker board, as it's robust, light. cheap, easily cut, and impervious to moisture.  Much better than mild steel, plywood, MDF etc.

https://www.jtatkinson.co.uk/catalog/product/view/id/11068/s/multi-pro-moistsure-tilebacker-s-e-1220x800x12mm/

cost, about £15

I  used angle brackets to assemble the enclosure, which was then sealed with silicone sealant. Total cost about £10.

Removable filter access panel was fixed using these, as I had some lying around

https://www.powertoolsdirect.com/forgefix-cavity-wall-zinc-speed-plug-4-5-x-35mm-bag-10  

150mm duct flanges from here, about £10 delivered

https://easyhvac.co.uk/index.php?id_product=155&controller=product

Total cost around £60. Significantly cheaper than the above, although it does not have an activated carbon stage yet

I designed the enclosure to fit in the loft under the roof, so it's an interesting rhomboid shape.

Ive also provided for inclusion of an activated carbon filter stage within the enclosure, should it be needed. 

I used f7 rather than F8 to keep the pressure loss within reasonable limits, my thinking being that I can replace the F7 bag with an F8 at minimal cost after seeing how the NuAire unit handles the pressure drop.

I have removed the existing filter media from the  MVHR intake filter, ensuring that the edges of the filter frame still have the original G3 media present to prevent internal leakage within the MVHR unit.

 

RESULTS

1) The filter still allows some smell of smoke to penetrate, although to a significantly reduced  degree than previously

 

2 )  OK Intake flow rate been achieved despite the increased F7 filter resistance, although as expected the intake fan speed had be increased to maintain airflow as before.  The MVHR clogged filter warning has not been activated, therefor the system must be working within its design parameters, ie the fan is not seeing excessive load. 

 

3) Increased intake fan speed has resulted in increased noise level. This noise level is still very low, but any further increase, say to accomodate f8 and/or carbon filter may become unnacceptable for sleeping

 

 

NEXT ACTIONS

 

1) SMOKE/SMOKE ODOUR  I may add an activated carbon filter, using media such as this:

https://www.calgoncarbon.com/products/ap-4-60/

I'll make a tray with seive material in a an open frame. i've already included supports for such a  frame in the MVHR enclosure

I may also experiment with an F8 filter to see which solution gives the better result at the lowest pressure drop.

AND/OR

I may substitute the F7 for an F8 bag filter. Does anybody have any experience of the effectiveness or otherwise of F7/F8 for smoke particles? 

 

2) NOISE The filter is fitted before the MVHR, so I may seperate the G3 and F7, and move the F7 filter stage to  post MVHR, this will probably eliminate this noise issue, and allow for further fanspeed increase if necessary. I also wonder whether the MVHR fans are better at 'pushing' or 'pulling' the air. I'd appreciate any further info on this point  if there are any experts out there

3) Further down the road, if the carbon and/or F8 filters prove to have to much resistance for the stndard Nuaire fan, I'm thinking that maybe an additional intake fan, seperately housed, immediately before the MVHR, would overcome this problem. I've not researched this as yet, so I have no idea how to specify such a thing to produce the appropriate pressure and flow at minimal energy consumptiion.

 

I hope this is of use.

Any  info or comments you may have, especially on the fan'pushing vs pulling' aspect, or the implementation of an additional fan would be greatly appreciated

[Savage87]

great post @woodman please add some photos

 

I have the same problem with one of my neighbours and their log burner. It's not as bad anymore but at times, depending what they were burning it was coming in thick.

 

I done a lot of research, like you, but I didn't want it to be a permanent fixture to the system. I only wanted to use the extra filtration in the winter months. In the end I bought a filter box and a car carbon filter to fit in the box.

 

This website helps you find car filters based on the required dimensions

https://www.wixfilters.com/Lookup/FilterBySize.aspx

 

If you are worried about the MVHR being to loud at night you might be able to turn it down within certain hours.

 

Do you know at what percentage were your fans set before and after you changed the filters?

 

[woodman]

Hi Savage 87, good to share this stuff

I didn't take any photos as I built the unit, howver I'll tke a shot of the finished item next time I'm in the loft. Not much to see though, as the whole thing is covred in rockwool insulation!

The intake fan was originally set at aroud 25% trickle, it's nowup to about 35% trickle. Boost has always been 100%

Unfortunately, I cannot easily adjust the fan settings for nightime, as the controls are mounted on the MVHR unit, in the loft. My electronics expertise does not extend to making an additional control unit, although i suppose I could re-mount the existing controls remotely....

The filter size link is a great resource, I had not consideed using a car filter.

I've just looked up a couple, are they all US spec applications? I'm in the UK, but I guess most are global spec anyway.

Did you use the tool to find a filter of the size you want, then do a seperate search to find a carbon filter, rather than particulate?

I would have thought that car filters have a pretty high resistance- What filter size did you use, and what effect did it have on fan speed in your application?

[Savage87]

The way I done it (in the UK): I searched for the filter size that I needed, then I got the code for the filter and googled it and it came up with the car model it's used for, them I looked for a carbon cabin filter for that car model, 2 days later I had suzuki swift filter delivered.