Vent Axia declined to give me a design, referring me to a couple of resellers (I was told they only give direct design help to developments with multiple dwellings). From what I've read on other threads, several here have designed their own. Can anyone outline the approach to sizing the unit? My house is 285m2, the unit will be located in a central position and I'm erring towards a radial system with ducts run between the intermediate floor level joists and along the eaves (loft conversion). I have 5 extracts serving 3 bathrooms, a kitchen and utility. I have 7 supplies serving 4 bedrooms and three regularly occupied ground floor rooms.

I got a reply fairly quickly, within a week or so I think.I did send a nudge after a week and got a reply and got a design on my drawings the following day.

fresh forward are the in wall fans which carry air from room to room.

Posted By: MarkyPCan anyone outline the approach to sizing the unit?

Calculate the required flow rates for Part F - they will typically be greater than needed most of the time. Then pick a unit or multiple units that is/are big enough to produce that rate and still have some spare capacity. Then do the duct design, thinking about flow speeds (for noise) as well as the volume rates required (hint: you'll probably want two ducts into the kitchen). Work out the pressures involved and check that the MVHR unit is still comfortable.

Personally, I'd get the supplier to design the system and check it carefully myself.

You've had good advice from various people, but here's my (fairly recent) experience.

I fitted MVHR DIY last year. In a 1st-floor cupboard using rigid ducting. Mostly spiral steel, but one section in rectangular plastic (because it fits well between joists). I did my own design sums, and the spreadsheet is here: http://wookware.org/openbuilding/MVHR/MVHR.xlsx

It's quite comprehensive, calculating quite thoroughly from 1st principles, and caters for a whole range of square and round piping in various materials (but only rigid ducting). You'll have to change it to deal with your structure (ie. which pipe segments branch into other pipe segments, then fill in pipe materials and sizes and lengths, and list bends etc). It also confusingly has several ways of calculating flow-rates, because there is partF, and 'sensible' and per-room-divvying-up. I tried various to see how they reconciled. You can use it to ensure that a given unit will supply enough air down runs and that the velocity in the pipe is not too high. And see how well-balanced things are. It makes no allowance for fabric loss, assuming than in+out match, but that's easy to change. It may well still have bugs, but my setup-works and is quiet, so it did its job.

PartF is _way_ more ventilation than any sane person needs IMHO. But if you design it so that that doesn't make excessive noise, then the thing will be almost silent in real use. Like RobL I made the pipes large, and the net result is that you can't hear the unit anywhere on normal (low) setting except the bathroom (because that pipe is ~1m long). You can just about hear it on boost (medium) setting because my house is a quiet place.

Rigid ducting is a pain to fit. I spent about £330 on ducting in total (so nearly as much as the unit - I got an ITHO HRU ECO-4 cheap for £400). I spent about 8 days cutting and fitting (and glueing and taping and insulating) pipe in a cupboard and loft - which was quite dull. If I was doing it again I'd seriously consider djh's posh semi-rigid pipe + plenum setup, just as it'd be easier to fit and purchase (not so many bits of pipe of different sizes). I bought most of my stuff from UK supplier ducting online, who make their own pipe and have a good range at very reasonable prices. http://www.ducting-online.co.uk/PBSCCatalog.asp?CatID=2285504
They do plastic too, although you can get bits (slightly) cheaper elsewhere.

You could be a tight-arse on ducting sizes, but as it's lots of fitting and not that much cost I don't think there is much point. I used a bit of 180 (for main in/out) as that's the machine spigot size, mostly 150 for major runs and quite a bit of 100 for individual bedrooms. Fittings matter for overall pipe resistance so try not to have more than you need. I missed out the downstairs bog as getting a pipe to it was too much of a pain. It could be done later if I really cared enough. No pipe is visible in the house - it's all in the cupboard, the loft, or the floor/ceiling. Loft piping is a compromise on building airtightness and especially pipe insulation. Insulate it carefully, but it'll still be colder than is ideal.

I spent £30 on an anemometer for commissioning (which to be honest isn't finished - I measured everything, but haven't gone back to the sheet to see how the sums compare with reality, and try to do balancing, because it obviously 'basically works'). I've also bought a CO2/temp/humidy vthings sensor to see how rooms change when it's on/off, but haven't got round to plumbing that in yet either...

I have a small (4.5kW) woodburner which is not room-sealed and used daily. The MVHR does not seem to have caused any change in the operation of this. Maybe there is marginally less draft when it's cold - hard to tell.

Posted By: wookeyI have a small (4.5kW) woodburner which is not room-sealed and used daily. The MVHR does not seem to have caused any change in the operation of this. Maybe there is marginally less draft when it's cold - hard to tell.

Unless the MVHR is greatly out of balances and/or your home is very airtight it will not effect a woodburner. It is when a home is refitted to a good standard of airtight along with quality new builds that issues are seen with woodburner not getting a good draft.

wookey - thanks for the detailed post and sharing your calcs sheet.

Welcome to the forum, coruh82.

That's not quite right. The building regs are based on a 3 m³/m²/hr as-built limit and suggest that you should base expectations on that if you design to less than 5 m³/m²/hr in terms of requirements for mechanical ventilation. The suggestion that it is not worth fitting MVHR if the building has above 3 m³/m²/hr leakage is based on whether or not the MVHR will cost more to run than it saves in heat recovered. But many people on this forum have first-hand experience of the improvement to indoor air quality that an MVHR brings regardless of airtightness. Humidity is an imprtant part of this, of course.

BTW, have you read the guidance about commercial posting on this forum?