HRV and air flow around the house

BarneyMc

What's the best way to allow air to flow around the house with HRV... cut the doors short at the bottom to leave a gap?

893bet

BarneyMc wrote: »

What's the best way to allow air to flow around the house with HRV... cut the doors short at the bottom to leave a gap?

I think that is standard. Should be 10mm clearance.

Calahonda52

There is a passive house design detail which passes the air through the top frame

BarneyMc

My friend suggested something the other day that I just half grasped. Something to do with taking a small slip out of the back of the top of the architrave that sits on top of the door. It would be invisible unless you stood up on a chair. I guess if you did the same on both sides of the door and left a small gap around the frame it might work? Sorry but I don't know enough to know if this would work.

BarneyMc

Calahonda52 wrote: »

There is a passive house design detail which passes the air through the top frame

Only saw this after I posted!!

rampantbunny

Calahonda52 wrote: »

There is a passive house design detail which passes the air through the top frame

I've also heard of this. Would you be relying on a gap between the top frame itself and the lintel, or do you cut a slot out of the top frame (if it sits tight to the lintel)?

BarneyMc

Only guessing here but leaving a gap between the frame and the lintel sounds like the easiest way.

rampantbunny

BarneyMc wrote: »

Only guessing here but leaving a gap between the frame and the lintel sounds like the easiest way.

...provided your brickies installed the lintels at the right height which wasn't the case in my build (bit lower than planned). Bit of a cock-up but I'm sure something can be done.

If you are buying door 'kits' which I believe come pre-assembled, then rebating/cutting the architrave might be more difficult. In the past I considered installing a small vent over/beside each door but don't think it would look too good.

I've said it before and I'll say it again; when installing MHRV there has to be a definite trade-off re. noise transmission. Even if you have the proper ducting and use dedicated runs from MHRV unit to rooms (as opposed to T-ing off a central duct) , the effect of closing the door shut to isolate you from noise in the other rooms is no longer possible. Something we'll have to get used to.

BarneyMc

How does the 10mm gap at the bottom of doors look? Might be ok upstairs but not so sure I'd like it downstairs.

gooseygander

I an totally missing the point about gaps in doors regarding mhrv, can someone explain what you mean and reason for same, I will have mhrv and doors currently being hung without gaps, have I made a balls of something.

hexosan

Calahonda52 wrote: »

There is a passive house design detail which passes the air through the top frame

Do you have a link

rampantbunny

gooseygander wrote: »

I an totally missing the point about gaps in doors regarding mhrv, can someone explain what you mean and reason for same, I will have mhrv and doors currently being hung without gaps, have I made a balls of something.

You need to allow air to flow between rooms. most rooms will have only a supply or extract.. If rooms are sealed, you'll end up with either positive or negative pressure in these rooms (depending on whether you have a supply or extract respectively).
If you have a room sealed stove, and have negative pressure, it can result in 'sucking' CO from the stove and could be fatal.

rampantbunny

BarneyMc wrote: »

How does the 10mm gap at the bottom of doors look? Might be ok upstairs but not so sure I'd like it downstairs.

I was advised to leave 10-15mm. Can't say how it looks personally as I've no doors installed yet. 15mm gap at bottom might look a bit like bad joinery.

I think I'll go around the house and for those rooms that have walls that back onto quiet spots I might put a vent provided the location is a sensible distance from the input/extract vent.
Otherwise, my preference is for a gap at architrave if going non-door kit.

gooseygander

Is there not sufficient air gaps around normal door installation, with key hole, regular clearance top and bottom for door to swing comfortably, also the door arcatrave has gaps behind it where it butts the wall whet I am sure air can move through also???

BarneyMc

rampantbunny wrote: »

I was advised to leave 10-15mm.
Otherwise, my preference is for a gap at architrave if going non-door kit.

I've just ordered a load of door kits!! :mad: Why will it be more difficult if using these kits?

rampantbunny

gooseygander wrote: »

Is there not sufficient air gaps around normal door installation, with key hole, regular clearance top and bottom for door to swing comfortably, also the door arcatrave has gaps behind it where it butts the wall whet I am sure air can move through also???

That's something you'll need to calculate. But I've paid someone to design my system and he maintains a 10-15mm gap is required.

I read on another forum that standard doors can pass 70 litres/sec at a pressure differential of 15 Pa. Where does that leave you, I'm not sure.

Dardania

gooseygander wrote: »

Is there not sufficient air gaps around normal door installation, with key hole, regular clearance top and bottom for door to swing comfortably, also the door arcatrave has gaps behind it where it butts the wall whet I am sure air can move through also???

often times with modern doors, no, because they put those in-tumescent strips which expand upon the heat of a fire to seal...they need tight enough clearances I believe

rampantbunny

BarneyMc wrote: »

I've just ordered a load of door kits!! :mad: Why will it be more difficult if using these kits?

Because you'll have to cutting into the architrave. The alternative is that you leave your architrave sit out from the wall. How else are you going to leave a gap for air to flow behind the architrave - or am I missing something?

I'm thinking that you would need to router the top piece of architrave (but not the full depth as you don't want to look up from underneath and see a gap).

BarneyMc

rampantbunny wrote: »

I'm thinking that you would need to router the top piece of architrave (but not the full depth as you don't want to look up from underneath and see a gap).

That's exactly what I was thinking. I'll need to speak to my joiner to see what he says.

rampantbunny

BarneyMc wrote: »

That's exactly what I was thinking. I'll need to speak to my joiner to see what he says.

I suppose you could get fancy with the plaster work above the door (leave an indent) but using a router at the back of the architrave would be much easier.

sydthebeat

nobody thinking of sound transfer?

rampantbunny

sydthebeat wrote: »

nobody thinking of sound transfer?

..yes, but are there alternatives (hopefully no one will say trickle vents)?
If a gap is required, no matter where it is, then noise will get through. Unless there's a material that will stop the noise but allow the air...some fancy osmosis thing

rampantbunny wrote: »

I've said it before and I'll say it again; when installing MHRV there has to be a definite trade-off re. noise transmission. Even if you have the proper ducting and use dedicated runs from MHRV unit to rooms (as opposed to T-ing off a central duct) , the effect of closing the door shut to isolate you from noise in the other rooms is no longer possible. Something we'll have to get used to.

delfagio

Does anyone have a picture of what this gap at the architrave at the top of doors looks like, or a sketch of how it is done.....it sounds a bit confusing as to what it should look like???....

I don't like the sound of a 10-15mm gap at the bottom of the doors at floor level as personally I think that would look, in our great Irish tonque.....CAT. aka Sh#te. You could slide all your fingers under the door through a 15mm gap.

sydthebeat

i would be petrified what a 15 mm + gap under a door would do to toes caught in a door swing.

10-12mm is standard

if every habitable room has an air inlet from a central MHRV.... why is this gap in architrave needed at all? and why worry about air flow around a house?

ive only ever seen need for designed internal air flow where there is no central ducting system, and the extract and intake points are on opposite sides of a dwelling

rampantbunny

sydthebeat wrote: »

i would be petrified what a 15 mm + gap under a door would do to toes caught in a door swing.

10-12mm is standard

if every habitable room has an air inlet from a central MHRV.... why is this gap in architrave needed at all? and why worry about air flow around a house?

ive only ever seen need for designed internal air flow where there is no central ducting system, and the extract and intake points are on opposite sides of a dwelling

What about those areas that are not habitable rooms such as the hallway, the first floor landing, rear porch area etc. They also need ventilation and from what I've been told by various installers, there is not always a dedicated supply to these areas.
Plus how can you supply air to a room in an airtight house without giving it somewhere to escape to?
Surely your rooms are now positively pressurized, and those with extracts are negatively pressurized.

BarneyMc

Some ideas here but they might be.... CAT! :rolleyes:

rampantbunny

BarneyMc wrote: »

Some ideas here but they might be.... CAT! :rolleyes:

..you can do better than that Barney!

http://www.kulgrilles.com/

I'm in no way affiliated with this company nor pushing their products. Result of a quick google search for 'modern wall vents'

sydthebeat

rampantbunny wrote: »

What about those areas that are not habitable rooms such as the hallway, the first floor landing, rear porch area etc. They also need ventilation and from what I've been told by various installers, there is not always a dedicated supply to these areas.
.

you mean the main circulation spaces?

rampantbunny wrote: »

Plus how can you supply air to a room in an airtight house without giving it somewhere to escape to?
Surely your rooms are now positively pressurized, and those with extracts are negatively pressurized.

the rooms are not made air tight from each other.

rooms with extracts are generally left with open doors to indicate availability.

much ado about nothing in my opnion

BarneyMc

rampantbunny wrote: »

..you can do better than that Barney!

http://www.kulgrilles.com/

I'm in no way affiliated with this company nor pushing their products. Result of a quick google search for 'modern wall vents'

Snob! :P

rampantbunny

BarneyMc wrote: »

Snob! :P

You could probably make your own vent with a bit of that expanded metal over a hole in the wall barney...just tell your plasterer not to cover it over...job done

rampantbunny

sydthebeat wrote: »

you mean the main circulation spaces?
the rooms are not made air tight from each other.

True, but air must be allowed flow between areas, agreed?
And this airflow should be accommodated, not restricted. The less restriction, the less draughts. The less restriction the less pressure on the system to pull and push air.

sydthebeat wrote: »

rooms with extracts are generally left with open doors to indicate availability.

Personally I wouldn't leave the doors to the bathrooms or ensuites open. Especially after a night on the beer I'd also close the door to the utility/plant room to hide the mess.

sydthebeat wrote: »

much ado about nothing in my opnion

Fair enough, but enough of us have been told by the experts we've employed about this gap, so we have to do something about the nothing.

delfagio

What size vent are we talking here does anyone know. If say a 10-15mm gap at the bottom of a door which is say 813mm or so long then this air gap equates to 0.813mm x 0.015mm = 0.0122m2. Is this adequate enough for allowing air to pass through from one room to another?? Would bigger or smaller be required😜

BarneyMc

sydthebeat wrote: »

you mean the main circulation spaces?

the rooms are not made air tight from each other.

rooms with extracts are generally left with open doors to indicate availability.

much ado about nothing in my opnion

Fair point I suppose. We're only talking about tiny amounts of air flowing in/out of rooms and it is air remember which is fairly free flowing. Surely any gap in and around a door, even when closed is not going to prevent air movement. Also, a lot of rooms have 2 or more doors so more scope for air movement.

Or, has this been proven to be the case, i.e. MHRV needs gaps at doorways?

sydthebeat

BarneyMc wrote: »

Fair point I suppose. We're only talking about tiny amounts of air flowing in/out of rooms and it is air remember which is fairly free flowing. Surely any gap in and around a door, even when closed is not going to prevent air movement. Also, a lot of rooms have 2 or more doors so more scope for air movement.

Or, has this been proven to be the case, i.e. MHRV needs gaps at doorways?

all doorways have gaps, which is my point.

im sure someone can do the maths on it, but its my understanding that when a MHRV system is balanced and commissioned, all doors are closed, therefore the whole house is non pressurised.

im quite sceptical then as to the significance of any slight room pressurisations within this envelope seeing as:
(a) the rooms are not air tight
(b) the all have vent grills (be they intake or exhaust)
(c) they all have gaps around normal standard doors , and electrical fittings, and skirts etc on internal walls
(d) the frequency of door openings in a normal day

and im more sceptical when there is a cost to sound proofing

i can fully understand air flow gaps being required where the ventilation is not balanced and requires cross flow.

rampantbunny

sydthebeat wrote: »

...its my understanding that when a MHRV system is balanced and commissioned, all doors are closed, therefore the whole house is non pressurised.

Do you know if balancing is done with doors that have gaps or not Syd?

IMO, you DO need cross-flow, for example if you are extracting from your ensuite and supplying your bedroom. Air must flow between the rooms to balance.

Yes, during the day you'd open and close your doors which is fine, but in bed at night my doors are closed and I want ventilation to be as effective then as during the day.

What is the tightness of a standard door? A good carpenter would take pride on installing a tight fitting door that almost brushes the floor.
Even manufacturers of MHRV units indicate that a gap should be maintained under the doors. If gaps are not required, why are more fellas in that industry indicating that they are.

It all boils down to the amount of air that needs to move between spaces. If you know what you're supplying to a room, you know what should be flowing out of the room to maintain balance. If it's a large room, with one door the air movement might be larger than a tight fitting door would allow. So maybe the 10mm is a 'just to be sure' measure. Coming from a house with mould and damp walls, I'd personally put up with some noise for the sake of being certain.

sydthebeat

rampantbunny wrote: »

Do you know if balancing is done with doors that have gaps or not Syd?

IMO, you DO need cross-flow, for example if you are extracting from your ensuite and supplying your bedroom. Air must flow between the rooms to balance.

Yes, during the day you'd open and close your doors which is fine, but in bed at night my doors are closed and I want ventilation to be as effective then as during the day.

What is the tightness of a standard door? A good carpenter would take pride on installing a tight fitting door that almost brushes the floor.
Even manufacturers of MHRV units indicate that a gap should be maintained under the doors. If gaps are not required, why are more fellas in that industry indicating that they are.

It all boils down to the amount of air that needs to move between spaces. If you know what you're supplying to a room, you know what should be flowing out of the room to maintain balance. If it's a large room, with one door the air movement might be larger than a tight fitting door would allow. So maybe the 10mm is a 'just to be sure' measure. Coming from a house with mould and damp walls, I'd personally put up with some noise for the sake of being certain.

i think we're coming from close to the same area.... what im saying is that there ARE gaps in standard door installations (im not talking fire doors here)... and that gap is usually at least 10mm under door. Im also saying that there are more areas for air flow than just gaps under doors.

what i would question though is the requirement for a continuous 10 air gap at the top of architraves, at the cost to sound proofing... especially in rooms such as en suites etc;)

as an aside to your above comment.... this would make absolutely no difference to mould or damp, i dont know why you think it would?
continuous positive pressure throughout the 'whole house' certainly would push vapour into the envelope.. but thats not at all what we are talking about here.

edit: just measured the two doors in my office, one with carpet under has a gap of 20mm, one with a timber floor under has a gap of 15mm

Calahonda52

hexosan wrote: »

Do you have a link

No, I saw it in a passive house in Mount Merrion the was open for viewing last year.

I will ask the architect.

rampantbunny

sydthebeat wrote: »

i think we're coming from close to the same area.... what im saying is that there ARE gaps in standard door installations (im not talking fire doors here)... and that gap is usually at least 10mm under door. Im also saying that there are more areas for air flow than just gaps under doors.

what i would question though is the requirement for a continuous 10 air gap at the top of architraves, at the cost to sound proofing... especially in rooms such as en suites etc;)

as an aside to your above comment.... this would make absolutely no difference to mould or damp, i dont know why you think it would?
continuous positive pressure throughout the 'whole house' certainly would push vapour into the envelope.. but thats not at all what we are talking about here.

The 10mm under door gap was to be substituted in favor of (not in addition to) the gap at architrave. It can be hidden at the architrave but cannot be hidden under the door.

I thought poor ventilation meant mould?
And it's not that I'm saying this exact topic would make the difference, but I am saying for the sake of a 10mm gap to guarantee air flow is not restricted, then why not.

10mm under the door isn't a lot to be fair, but I want mine closer to the ground when installed. I mightn't have a choice given that my lintels were installed lower than they should've been (and i def do not want someone cutting the doors).

sydthebeat

rampantbunny wrote: »

The 10mm under door gap was to be substituted in favor of (not in addition to) the gap at architrave. It can be hidden at the architrave but cannot be hidden under the door.

thats fair enough, your talking about exceptional workmanship of the carpenter to hang the door and the floor finisher to maintain that perfect gap

rampantbunny wrote: »

I thought poor ventilation meant mould?
And it's not that I'm saying this exact topic would make the difference, but I am saying for the sake of a 10mm gap to guarantee air flow is not restricted, then why not.
.

mould is caused where vapour hits cold surfaces and condenses.. and gathers over time. Your internal walls should not be causing condensation.

Its my understanding that this organised air flow is to prevent the pressurization of rooms which causes a stress on the mechanical system and can cause them not to run efficiently. But again, how air tight can an internal room actually be.

hexosan

I'll be installing fire doors throughout my house as all except two doors open out onto a protected stairwell.
Will this have an effect on the MVHR system as fire doors won't have that 15mm gap.

sydthebeat

hexosan wrote: »

I'll be installing fire doors throughout my house as all except two doors open out onto a protected stairwell.
Will this have an effect on the MVHR system as fire doors won't have that 15mm gap.

In this case I would certainly think so.

Calahonda52

hexosan wrote: »

I'll be installing fire doors throughout my house as all except two doors open out onto a protected stairwell.
Will this have an effect on the MVHR system as fire doors won't have that 15mm gap.

Yes it will so you are looking at extract and supply in each room.
You may decide to put supply in floor under windows and extract on opposite side of room in ceiling.

rampantbunny

hexosan wrote: »

I'll be installing fire doors throughout my house as all except two doors open out onto a protected stairwell.
Will this have an effect on the MVHR system as fire doors won't have that 15mm gap.

I read that fire doors can have intumescent strips at the bottom which expand when hot to seal the gap.
But I've also read that there's no guarantee it would be hot enough quick enough in this area, to trigger this seal. In this case smoke would escape long before flame, and it is the smoke that kills in most cases.

Not sure I can post this but here goes..
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9298

rampantbunny

sydthebeat wrote: »

thats fair enough, your talking about exceptional workmanship of the carpenter to hang the door and the floor finisher to maintain that perfect gap

German door kits preassembled, with large tiles at the doorway giving a very flat uniform surface. Can't be that hard to get the gap consistent..or can it :rolleyes:

thatslife

Is this an issue with DCV? My architect is recommending it but I'm still undecided.

ht9zni1gs28crp

I really think there is too much thought going into the pressurization internally. Like Sidthebeat says there are plenty of gaps between architrave, door itself to allow general air movement.

Age old thinking has to be forgotten now, ie a tight door to prevent uncomfortable draughts! its a modern build with modern processes...if an open door causes you too much stress then the trade off of an over the door vent is your other option.

Dardania

I have DCV in my house. Strongly in favour of it generally, be that in new build (for simplicity) and retrofit in old properties (as in my case)

With the hot weather we had lately, and the fact that we closed our bedroom door at night (to keep a pet from our bed) the air tightness of our door frame led to issues with air flow, not overcome by the DCV.

Tempted to buy this, which is apparently an acoustic grille: http://www.waterloo.co.uk/product-models/dsr-acoustic-air-transfer-grilles/

BarneyMc

My chippy is starting 2nd fix this week and I just remembered about this issue. Any new thinking since this post was last updated? To gap or not to gap, that is the question.

delfagio

BarneyMc wrote:

My chippy is starting 2nd fix this week and I just remembered about this issue. Any new thinking since this post was last updated? To gap or not to gap, that is the question.

Hi Barney,

I left about 15mm at the bottom of each door to allow airflow. Just make sure you also allow for floor finishes .i.e carpet, laminate, tiles or wooden floors.

BarneyMc

delfagio wrote: »

Hi Barney,

I left about 15mm at the bottom of each door to allow airflow. Just make sure you also allow for floor finishes .i.e carpet, laminate, tiles or wooden floors.

All floors are in so whatever I leave now - that's that. So you should will have a 15mm gap at the bottom after finished floors are in?

delfagio

BarneyMc wrote: »

All floors are in so whatever I leave now - that's that. So you should will have a 15mm gap at the bottom after finished floors are in?

Hi Barney,

Yes all the gaps under my doors are 15mm. +/- 2mm depending on levels etc.

You may think 15mm is big but you wouldn't even notice it at all.