HRV and air flow around the house

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.

BarneyMc

Yes, that's not bad I suppose. Is this common practice now for everyone going the HRV route?

delfagio

BarneyMc wrote:

Yes, that's not bad I suppose. Is this common practice now for everyone going the HRV route?

To be honest Barney I had never even thought about it until this I seen this thread about a year ago.

Mind you it did make sense that airflow should have a path to move around the house if doors are closed. So I suppose leaving the gap should be installed.

And I did get down on my knees around the different rooms and put my eyes/face as close to the gap at the bottom of the doors as I could and there is some airflow moving, you can feel the breeze on your face

BarneyMc

I think I've key holes so that might work also!

delfagio

Ha ha they should work too

murphaph

With balanced mvhr where each room has a duct for in and a duct for out I don't see a need for gaps under doors.

I always thought this was something only DCV systems depend on.

BarneyMc

murphaph wrote: »

With balanced mvhr where each room has a duct for in and a duct for out I don't see a need for gaps under doors.

I always thought this was something only DCV systems depend on.

Most of my rooms (all but one) just has one duct (in or out).

delfagio

Same here, our HRV has only fresh air supply ducts to all living spaces and bedrooms and extract ducts from all bathrooms Kitchen and utility. Not supply and extract to each room

ht9zni1gs28crp

murphaph wrote: »

With balanced mvhr where each room has a duct for in and a duct for out I don't see a need for gaps under doors.

I always thought this was something only DCV systems depend on.

You are thinking of a DCV not a MHVR. A Balanced MHVR is whole house comfort rather than singular rooms.

Its also prudent to allow airflow to move as it will maximise your heat recovery potential...

A year on in our house and I had completely forgotten about gaps at bottom of doors....that's how unobtrusive it is I guess.

BarneyMc

Frederick Nutty Orate wrote: »

You are thinking of a DCV not a MHVR. A Balanced MHVR is whole house comfort rather than singular rooms.

Its also prudent to allow airflow to move as it will maximise your heat recovery potential...

A year on in our house and I had completely forgotten about gaps at bottom of doors....that's how unobtrusive it is I guess.

Approx what size of a gap did you leave?

delfagio

Sorry, just going to throw another query out here in relation to HRV....

So HRV system basically gives fresh air to all living spaces, bedrooms etc. So let's say it's 23degree inside and 15degree outside.

So essentially the 15degree outside air is warmed up to say 90% of what the indoor temp is. So say the fresh air coming in warmed up to approx 20degree.

Then would I be right in saying that in the DEAP calculations for the living space size makes no difference as basically the entire house is been warmed by the HRV to approximately the same temperature throughout the whole house. Does that make sense.

So upstairs stats can be set to 18 or 19degree but realistically if it's 20degrees fresh air coming back into the house through the HRV supply ducts, then this warm fresh air is been supplied to all living spaces.