Hi guys, currently extending our MVHR system to new basement incl moving unit. I see references on at least one site if needing a trap at bottom of vertical pipework carrying stale air to outside to stop any condensation running into unit. We are using 160mm round steel ducting which is inside heated envelope along with unit itself. The unit has a drain but I assume that deals with water from the heat exchanger.

I can see the theory that as the warmer air meets colder pipe as it exits the building but don't find any such drain product for steel ducting and also how would such a trap work as it would need to he airtight? Any thoughts/experiences appreciated?

I'd think a waterless trap would be a much better idea in an application like this as, presuming there isn't much condensation in practice, a wet one would dry out.

Repeat..

Condensation running into the outlet (to outside) of the MVHR unit should not be an issue as the MVHR unit should be deigned to cope with it, along with the much greater amount of condensation that is formed in the unit’s heat exchange.

Ian, I think it's not just condensation but also any windblown rain that has to be considered.

To add to the mix, has anyone experienced flies in their MVHR?

We've got a Kinetic plus and have had lots of fruit flies in the kitchen. Then I went to service the MVHR today and there were a few of the blighters in there. I'm wondering if they could be accessing via the waste pipe (although this is about a 10m+ drop, or more likely being sucked in by the fans I guess.

Yes there was one biggish fly stuck to the HE. :( Probably 10 or so very small fruit flies around too. They're so small they could get anywhere in the unit as the Kinetic Plus doesn't seem to be very well sealed.

That's the thing, I don't think there's any trap or HEPVO, unless it's been put somewhere out of site rather than near the unit, which I guess wouldn't be usual.

We haven't had any drain smells noticeably but the condensate pipe runs about 10m+ vertically and that may mitigate them.

The exception to the smell was when we had an air test done and in one room particularly there was a bad smell but there was a sink, washing machine and an old covered up toilet waste in the vicinity, so maybe air being pulled through one or all of those...

Note that condensation should happen in the outgoing air flow so the drain smells, if any, ought not to get into the house, only into the MVHR exchanger and out. Of course, if the MVHR is ever off rather than on trickle then smells could get back to the house. Hence DarlP's question was about drain smells in the MVHR unit.

after a couple of months our inlet filter is a mass of crane fly legs which I assume are sucked or enter through the inlet duct, and the extract filter is full of a very grey fine dust. I expect that with harvest time which brings any number of flying and jumping critters into our house that next time I wash it, it will also be thick with corpses. A few seem to get round the filters and into the exchanger but nothing that an annual wash doesn't take care of (note to self - not done that annual wash for a couple of years...)

I made the mistake once of applying linseed paint at harvest time - the internal door frame ended up a kind of dalmation effect with the bugs stuck to it.

-Steve

That's a good idea, thanks, although could that move the point of blockage from the filter to the duct that I can't easily access?

ah, yes, mine are not accessible without pain - the inlet was put equidistant between upstairs windows to minimise noise - but a side effect it maximises inaccessibility :)