Jenny
Transfer gaps work fine and the intumescent strips aren't a problem as they don't seal until high temperatures are experienced.
It is surprising just how much ventilation can pass through the gaps around a door. A standard door can pass 70 litres/sec at a pressure differential of 15 Pa.

Posted By: tonySounds like a bad idea to me, is it for ventilation or draughts?

I think he's talking about internal doors & maintaining MVHR driven circulation when they are closed.

David

Posted By: GaryBA standard door can pass 70 litres/sec at a pressure differential of 15 Pa.

Interesting and worth knowing but note that 15 Pa is a lot. 2 m × 0.75 m × 15 Pa = 22.5 N which means the force needed on the handle when opening or closing the door would be about the weight of a 1 kg thing¹. That'd be very noticeable. Also, those conditions would dissipate a fraction over 1 W, probably mostly as noise - energy which would have to come from the fans.

How does flow rate scale with pressure difference - I've a feeling it's either with the square or the square root but can't work out which?

¹ The door's a lever, the other half of the pressure being taken by the hinges.

Posted By: tonyI leave a gap at the bottom of the door for that!

But then it wouldn't be a fire door, which is the point of the intumescent seal?

When a blower-door test is done, are internal doors left open or closed? Open I'd guess. Would the efficiency of the MHRV be assessed with doors open or closed? Closed I'd guess.

If the door is fire rated then it should have intumescent strips rebated into the bottom of the door as well.

An MVHR slightly pressurises the habitable rooms which could help keep smoke out.

Joiner:

It was Tony suggested undercutting the doors, not me!

I was pointing out the level of leakage possible through internal doors even those which appear to be a tight fit.

It is also my understanding that fire rated doors are rebated and fitted with intumescent strips on the bottom regardless of whether undercut or not.

Gary, wasn't having a go at you mate, I was actually confirming what you said because it was a point I hadn't made.

HOWEVER

Just spoken to fire officer son (fresh in the pub after a morning's diving) and he says no formal checks are done now by fire officers unless there is a statutory requirement, in a residential home or hotel, or such like, so apologies to Tony. He did say that any gap under a fire door has to be "a sensible one of about 6mm" because it's assumed that smoke will probably not drop to that level within the room in which the fire is happening. He'll check with the enforcement officer tomorrow and get back to me if there is such a thing as a recommended maximum gap.

I wonder why no one makes anything like this to be 'triggered' by heat in the way of intumescent strip?

http://www.norsound.co.uk/Door-Bottom-Seals/

Do find the whole subject fascinating because leaving (or creating in this case) a gap under a fire door is counter-intuitive. It's just something I've never done and I've fitted scores of the things, taking pride in getting the door just skimming whatever floor covering is there.

Just texted my son and he says he's hoping to hear back from his enforcement officer colleague later today about whether there's a recommended max gap allowable as far as the fire service is concerned.

He did say in that earlier conversation that if he ever saw a gap under a door that he thought "a bit wide" he'd advise against it "for safety's sake", but that at the end of the day, as far as any prescriptive regulations went, it was down to the householder concerned to worry about the safety of his or her family.

I'd personally be concerned for the simple reason that if you're cutting a slice off the bottom of a door to allow an easier movement of air between rooms, then it surely goes without saying that if that air contains combustion smoke it's going to be shifting that around as well, courtesy of the MV air changes.

What am I missing here?

If you have intumescent strips there is there any risk of them jamming the door just when you need to open it most? Conversely, would strips so low down actually get any heat to make them work early enough to prevent the spread of smoke?

Out of interest, I've recently done a project which used computer room waste heat to heat some office space. The appropriate bit of the building regs (commercial fire bit), stated that if the fire alarm was triggered, then any ventilation system would need to either shut-down entirely, or switch over to extract-only mode.

I was thinking of tying my domestic MVHR system into the fire-alarm system to do something similar... In an MVHR system which I installed 8-or-so years ago, I stuck a heat-closing (70 degree C?) vent port in the kitchen - since the duct run then passed through a bedroom on it's way to the MVHR unit.