I'm trying to work out the ventilation requirements for my house. I want to retrofit MVHR. I am using Part F to work this out. I am following the procedure for "New Dwellings", because the section on existing dwellings doesn't suggest anything other than when performing general building alterations... it doesn't appear to allow for ventilation for ventilation's sake.

The house is five bedroom, 285 m². Three occupants but I'd like to be a good citizen and size it for the house.

Extracts in three bathrooms, one kitchen, two toilets.

Inlets in all five bedrooms, the lounge, the office and a dining room.

Step 1 is the supply rate. This is the bit that's scary. Part F suggests 29 l/s is fine for a five bed house, then goes on to say "In addition, the ventilation rate should not be less than 0.3 l/s per m²[...]". This gives the rather different figure of 85.5 l/s!

Step 2, the extract rate, I get the minimum high rate as 49 l/s.

Step 3 means both the "maximum whole dwelling extract ventilation rate" and "minimum rate" are both 85.5 l/s.

The discrepancy between the 0.3 l/s/m² and all other figures (standard 5 bed estimate, extract high rate) is what worries me.

Is it even required to do this to Part F, given it's a retrofit?

My initial look for units that can deliver 85.5 l/s haven't gleaned a single product... Although finding what they will actually deliver is quite hard on the product pages.

p.s. How do I convert 85.5 l/s to m³/hour? Using Google gives 306 m³/hour but units at BPC suggest they do more but are not unsuitable for our internal floor area.

Posted By: gravelldThe discrepancy between the 0.3 l/s/m² and all other figures (standard 5 bed estimate, extract high rate) is what worries me.

It is scary isn't it. A quick search will reveal lots of threads where I moan about this aspect of Part F and how OTT it is.

The 0.3l/s per sqm looks like a last minute addition to the regs. It comes from work on VOCs although I have not seen the actual paper. Persoanlly I feel there are better ways of addressing VOCs, like avoiding material sources or evacuation venting when materials installed, than a permanently excessive background ventilation. I also question if the VOC problem really scales with floor area.

Is it even required to do this to Part F, given it's a retrofit?

I don't know, best to ask your BCO what he wants.

My initial look for units that can deliver 85.5 l/s haven't gleaned a single product... Although finding what they will actually deliver is quite hard on the product pages.

Yes it is, and a max flow at 0Pa back pressure will not help once you have ducting. Seems to me that many suppliers ignore the 0.3l/s per sqm requirement and will happily sell you something too small for the regs. And 85l/s is a lot of air gushing about!!

Feel free to join me in a rant about MVHR and Part F anytime!

I think the latter wookey!

Someone needs to write an ebook with tactical and practical advice. I'm not sure if the Green Building Bible is one such book but I'm not sure if it is (a) too old or (b) too high level to satisfy this type of thing.

As well as design, there's commissioning. I found it easier to get the supplier to design the system.

Wookey siad "Can anyone point me to a good design document or overview or do I just have to read 20 threads here, SAPQ, partF, enerPHIt docs, and 16 manufacturers' sites?"

I found the post and diagram from GreenPaddy on this thread particularly helpful
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6681

But why not do all the posts and docs anyway :)

Works fine for me.

Consequence of the forum break a few weeks ago, I think.

To follow up...

Having been in contact with the principal building regulations officer, it is clear they do not really know the answer to this. They said it *is* subject to building regulations, but would not explicitly say (they simply stopped answering my emails) whether it is the new build requirements that apply.

He, in parallel, referred me to a local energy assessor who has overseen a few MVHR design/installs/commissions in the area and has worked with building control. He said that typically the new build requirements are the ones that apply to get sign off.

Thanks, I did use Google but was confused by how although units claimed to shift that amount they weren't suitable for dwellings of this size. Might be something to do with back pressure? Haven't done these calcs yet, just room and house ventilation rates.

Looks like Vent Axia do some units that should cover us.

Oh, i don't know that it's worth taking any notice of the area quoted in the sales pitch.. You'll note that an airflow dv520se shifts 520 cube and is suitable for 280 sqm, and a vent Axia kinetic e shifts 440 and is suitable for 280sqm

Possibly the higher end manufacturers oversize their kit for a particular house size so it isn't working so hard.. Think of it like cars (they seem to cost the same!) - Ford Fiesta 1 litre and merc s320 will both reach 90mph but you'll probably feel it in the fiesta a lot more

Thanks both of you. Greenfish, any good references for calculating that? ISTR you posting a general guide e.g. n Pa for m metres, x Pa for a 90 degree etc. Just wondered if there was a comprehensive reference.

And some of our GB friends have been discussing this issue over on another forum
http://www.ebuild.co.uk/topic/15454-mvhr-estimating-static-preasure/

Thanks. Does an attenuator reduce the flow? Are they just used to reduce flow if the delivered air would otherwise be to much? I.e. to reduce noise and draughts. Of so are they kind-of an afterthought to complement the design?

Possibly worth pointing back to a discussion of duct losses here
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8513&page=2

What you need to do seems to depend hugely on the technology you choose and your attitude. Rigid branched ducts apparently involve calculations, then lots of measurement and adjusting valves. Semi-rigid radial systems involve plugging numbers into a spreadsheet, installing what it says and then if you care a lot about Part F and depending on what ducting system you chose, lots of measurement and replacing flow restrictors. If you buy the system from a sensible supplier they will design it and run the numbers. The installation manual for my system says, and I paraphrase, 'all this faff is silly, just design the system properly, install it and you're done'. So that's what I did.

My BCO showed no interest in the ventilation but I'm happy that it works since I did it myself.

The Part F calcs work out in the same region as the manufacturer's software. Minimising duct speed is also important for cutting down noise. The 0.3 l/s seems to work out approx the same as the old 0.44 ACH requirement and is reasonable for sizing (i.e unit shouldn't be at full speed at that rate, but it's the maximum comfortable cruising speed). I use the 30 l/s fresh air per person guide for setting the normal run rate of our system and it seems to work.