MHRV - worth oversizing?

philnixon

Hi,

About to make a decision on MHRV units. The house is approx 175-180m2 with high ceilings, so on the edge of the top capacity of smaller units (such as the Brookvent Aircycle 1.2). I could get a bigger unit, but the house is a victorian refurb so not guaranteed to be airtight. It's a 4 bed family house but there will only be 2 occupants at the moment so I think there would be plenty of air circulating with a smaller unit.

Is it worth oversizing the unit at this point, or going with the cheaper, smaller model? Are there any pros / cons with oversizing?

Thanks!
Phil

SteamyTea

Con is it costs a bit more, pro is that it will be quieter and more efficient.
Do the sums, or as they said in wood work classes, measure twice, cut once.

philnixon

The sums say a smaller unit will work if operating at maximum capacity - I am slightly concerned about the noise though.

tony

I would oversize, even double!

snyggapa

we had to run our vext axia sentinel kinetic at about 65% fan power to get it signed off for building regs - and it was intrusively noisy - fortunately we have down-rated it to run at about 35% after it has been signed off, which is fine.

However with hindsight I would have bought the "plus" model which has a higher rating, and if I read the specs right a lower noise level even at roughly double the air capacity (although of course if you try and suck or blow a gale through a restricted outlet you will generate significant noise at the terminal itself)

-Steve

borpin

Oversize and run slower.

tony

Good MHRV will usually be be designed to run at 60% of full blast which will be the purge rate.

Greenfish

philnixon said:

 The house is approx 175-180m2 with high ceilings, so on the edge of the top capacity of smaller units (such as the Brookvent Aircycle 1.2).

If just looking at the quoted max unit capacity  then I would definitely oversize significantly. In real life the ducting and terminals reduce the flow the fan can deliver, and manufacturers seem to forget the 0.3l/s per sqm floor area needed for building regs. The question you need answered is will the fan deliver 0.3*180 l/s with the static pressure that the route of ducting and vents you are installing will present.  Also look at how this flow will divide between terminals (proportioned by room floor area, but with minimums for kitchen and bathrooms), and unit needed to meet the flow of the limiting terminal.

I mistakenly thought a Silavent HRX2, with a max flow of 406 m3/h was more than adequate for my 540m3 house (that is 0.75ACH), but I was looking at the wrong parameters. Actually Polypipe didn't provide the more useful details, but I assumed the designers (salesmen) would know what they were doing.  In practice it had to be at full blast to deliver the 65l/s required for regs and was very noisey. I have now turned it right down, experimenting  if that is adequate for providing IAQ. So far so good, regs seem OTT, but I have no way to test if it is removing any VOCs adequately, and that is what drives the 0.3l/s per sqm regulation. If it is inadequate then I will have to open windows, which will make MVHR a wasted investment. So...

Unless cost is preclusive invest in a unit that can deliver your needs well within its limits, and be careful that the unit you choose will do that in practice. Also ensure that the system design is not trying to force a gale through a single vent, that is noisey too, install multiple terminals instead.

Oh and when it is all done come back and tell us how it worked out.