Air Changes per Hour

CWatters

What's the recommended ACH for a house these days?

Part F appears to recommend different rates measured in L/S depending on how many bedrooms you have - which seems odd as bedrooms can vary in size.

Ed Davies

L/s goes with the amount of CO₂ and water vapour occupants emit so using number of bedrooms as a proxy for the number and size of occupants seems to make more sense than using the volume of the house (AC/h).

Actually, I wonder if there's any good use for AC/h. Can't think of one off the top of my head but maybe I'm being dozy.

Triassic

Posted By: Ed DaviesActually, I wonder if there's any good use for AC/h. Can't think of one off the top of my head but maybe I'm being dozy.

For over specifying kit, one supplier suggested I needed to two MVHR units to achieve the required AC/h, 2 units twice the kit, twice the profit etc.

Gotanewlife

Got to start somewhere. As a DIYer I decided to use this figure as the Max I might need (so sizing a Unit based on the Trickle being towards 75% of MVHR unit capability taking into account guessimated pressure loss) and then rely on the empirical evidence scattered across this Forum that less than regs is plenty (and best possible workmanship on the install) to allow me to set less. Without a start figure I wouldn't have had a clue!

snyggapa

we run our house about half the "building regs" compliant level , if that is any guide, and everything feels and behaves fine. We do have a auto boost on high humidity though so it ramps up when you take a shower

of course your milage may vary, but in our case the building regs minimums were in reality over specified..

Triassic

If I look at Part F, page 21, "Whole dwelling ventilation rates" based on the number of bedrooms, it states that I need 29 l/s. Yet one supplier has quoted that I need 116 l/s. I find this over specification a real pain, as everyone appears to do, the SE, the architect and suppliers.

tony

It is known as belt and braces approach, costing us millions, someone in an office decides to double something that is already safe.

CWatters

http://www.passivhaustagung.de/Passive_House_E/ventilation_and_humidity.htm

Seems to recommend 0.3 ACH

gravelld

Posted By: Ed DaviesActually, I wonder if there's any good use for AC/h. Can't think of one off the top of my head but maybe I'm being dozy.

If there's no good use for it why is it used in Ph recommendations?

Just trying to understand.

Greenfish

IMHO the Part F whole house ventilation requirements are a bit of a mess. Did it used to be expressed in ACH? Anyway now it is based on total house flow rate (l/s) given number of [edit: bathrooms ] bedrooms (regardless of size) and floor area of house. For a house over 100sqm it is generally the 0.3l/s per sqm requirement that dominates. I guess there are some assumptions about number of occupants and volume in that, and hopefully some science on the amount of stale air they generate. But in my experience the regs specify well over the flow required in practice.

Uncessary air flow, via MVHR, is a waste of energy. In my case the MVHR was under specified anyway, I am lucky that it seems to be enough and I can avoid running at flat chaff all the time. Triassic, I get your unhappiness with MVHR sales men, procrement was a right pain, but do take care not to end up with a fan that is too small for the volume of your house. It is nice to know you can ramp it up if you need too even if daily rate is gentle.

As a unit ACH is useful to give a clear view on how long will it take to totally replace the (smelly/humid) air with new. Of particular interest when evacuating a polutant, but less informative when continuously ventilating, and I have been unable to find an explanation why 0.3ACH is a good level. In a tiny room full of bodies this would be insufficient, in a vast empty room sucking cubic meters out every hour would be unncessary and drafty.

It seems to me that the amount of air movement you need depends on how much smell, CO2 or humidity you are generating. That would best be specified as a flow rate (l/s) but related to number of occupants or pollutant generation rate. A shower in a small bathroom creates the same rate of steam as one in a large bathroom.

Also just because I have MVHR (replacing random losses depending on wind and building air pearmeability), does not mean that I no longer want to clear the steamy bathroom in a reasonable time. I want a boost level necessary and sufficient for the volume of the bathroom, not just a set l/s regardless of the room size.

Finally I think I calcualted at some time that 0.3l/s per sqm floor area is around 0.3 ACH for the average hight of room. So I guess that I where the number comes from.

Ed Davies

Posted By: GreenfishAnyway now it is based on total house flow rate (l/s) given number of bathrooms…

Bedrooms? Though bathrooms would make some sense, too.

Posted By: GreenfishIt seems to me that the amount of air movement you need depends on how much smell, CO2 or humidity you are generating. That would best be specified as a flow rate (l/s) but related to number of occupants or pollutant generation rate. A shower in a small bathroom creates the same rate of steam as one in a large bathroom.

Yes, that's how I look at it, too.

Posted By: GreenfishFinally I think I calcualted at some time that 0.3l/s per sqm floor area is around 0.3 ACH for the average hight of room. So I guess that I where the number comes from.

Yes, roughly. 1 l/s is 3600 l/h which is 3.6 m³/h so with a 2.4 m room height that's 1.5 AC/h. So the 0.3 l/s probably comes from rounding the conversion from 0.4 AC/h.

snyggapa

commission it to meet building regs, then once the paperwork is filled in , adjust to suit your lifestyle...

borpin

In Scotland you do not need it officially commissioned :)

I am running mine well below the calculated rates - do not even boost for showers and they still clear quite quickly. I have a CO2 sensor from our old friend JSH but I have just not had the time to set it up and see what the values are.

In reality, despite the expense, I'd drive my system by CO2 levels if I did it again.

Triassic

Posted By: Ed DaviesYes, roughly. 1 l/s is 3600 l/h which is 3.6 m³/h so with a 2.4 m room height that's 1.5 AC/h. So the 0.3 l/s probably comes from rounding the conversion from 0.4 AC/h.

So what's the situation regarding vaulted ceilings? I have two in my design and they appear to be driving the need for two MVHR units.

Greenfish

Posted By: Triassic

So what's the situation regarding vaulted ceilings? I have two in my design and they appear to be driving the need for two MVHR units.

Part F for whole house ventilation does not consider volume at all. The regs require a total flow of at least 0.3 l/s per sqm floor area or a value based on number of bedrooms (which ever is greater), and minimum continuous extract rates in kitchens and bathrooms etc.

A MVHR unit can only move so much air for a given static pressure provided by the ducting connected to it and the room vents. I expect that the suppliers are looking at the volume of air in your house, including the vaulted rooms, and the length of ducting needed to reach each room and concluding that one unit is not sufficient to move that much air at the flow rates required. Be glad they are doing this, otherwise you would end up with an under powered noisey system.

Ed Davies

Scottish regs also mostly use l/s but for the few cases where they do give AC/h they say (3.14.1.5.9.26 [¹])

http://www.gov.scot/resource/buildingstandards/2013Domestic/chunks/ch04s15.html

Calculation of volume - for ventilation purposes, a storey should be taken as the total floor area of all floors within that storey, including the floor area of any gallery or openwork floor. Where an air change rate is recommended, the volume of the space to be ventilated may be required. The volume of any space is the internal cubic capacity of the space. Any volume more than 3m above any floor level in that space may be disregarded.

[¹] OK, so I extended the section number a bit. :tongue:

SteamyTea

Should have written that on Pi Day

joe90

I do like the breathing window from Viking simply because it is run by stats and does not run 24/7, only when required. I wish I could find an ordinary MVHR that did this.

tony

Add a programmer!

GaryB

Sorry folks, some of the calculations of air change rates in this thread are a bit off.

The Part F whole house 0.3 l/s requirement equates to 0.45 air changes per hour for a dwelling with a standard 2.4 m high ceiling.

For a 100 m2 house, the 0.3 l/s rate gives 100 x 0.3 = 30 l/s or 108 m3/hr

House Volume = 100 x 2.4 = 240 m3

AC rate = 108/240 = 0.45 AC / hr.

Paul in Montreal

Posted By: GaryBAC rate = 108/240 = 0.45 AC / hr.

So that would be roughly 9ACH @50Pa .... pretty leaky!

Paul in Montreal.

Ed Davies

PiM, you're joking, right?

tony

Plenty of homes in the UK have air tightnesses that bad, sadly even new ones.

borpin

Posted By: Paul in Montreal

Posted By: GaryBAC rate = 108/240 = 0.45 AC / hr.

So that would be roughly 9ACH @50Pa .... pretty leaky!

Not a like for like comparison. the @50Pa is under test, the 0.45 SA/hr is steady state with the ventilator running.

Greenfish

Posted By: GaryBThe Part F whole house 0.3 l/s requirement equates to 0.45 air changes per hour for a dwelling with a standard 2.4 m high ceiling.

Thanks Gary, sorry for mis-remembering this rather than going the sums. I have higher than average ceilings.

Posted By: borpin

Posted By: Paul in MontrealSo that would be roughly 9ACH @50Pa .... pretty leaky!

Not a like for like comparison. the @50Pa is under test, the 0.45 SA/hr is steady state with the ventilator running.

It does seem daft to me that regs want so much ventilation as 0.3l/s per sqm in houses over 100 sqm. Maybe we can't compare it directly to air permeability, but I did wonder if it was derived from the "good" air quality of old drafty houses.

More seriously the minimum ventialtion of habitable rooms of around 0.5ACH seems to be an international standard. See http://www.aivc.org/sites/default/files/members_area/medias/pdf/Conf/2004/2004028_Yoshino.pdf. There was a graph in another thread, but can't see it at the moment. But I have not been able to find any research that establishes *why* such a rate is needed. Has it ever been justified?

Greenfish

A survey of the minimum ventilation rates in 15 countries is here http://www.aivc.org/sites/default/files/members_area/medias/pdf/Conf2004/2004028_Yoshino.pdf.

The rules were expressed in different ways but for a model house came out as around 0.5ACH in most places. What research justifies this rate, or is it just tradition?

DarylP

Greenfish, you cannot really compare controlled ventilation, 'Part F Building Regs', with uncontrolled ventilation, measured at m^3/m^2h q50.
They are opposite ends of the ventilation 'equation'...:smile:

Greenfish

Posted By: DarylPGreenfish, you cannot really compare controlled ventilation, 'Part F Building Regs', with uncontrolled ventilation, measured at m^3/m^2h q50.
They are opposite ends of the ventilation 'equation'...

OK, but I don't think I was.
Let's stick to ventilation.

From that survey of ventilation requirements it would seem that most countries demand we ventilate our houses more than I, and several others, currently are. Many nations base the requirement on floor area or ACH, which I believe model the amount of smells/pollution/humidity generated very poorly - think one person in a large house with high ceilings. This means if we comply then we are either wasting energy through vents, or wasting (less) energy running MVHR fans. Even if we turn the fan down we stil have to buy powerful enough units (sometimes multiple) in the first place with the embodied energy that entails. My guess is that the various bodies have just copied standards from each other, I can't find any supporting research.

It seems to me that the world needs some research on how much ventiation we really need, based on occupants and activities, measurements of CO2, formaldehyde etc. , not arbitrary floor area etc. and over cautious rules of thumb.

DarylP

Yes, I think you are broadly correct. Any metric that uses m^2 IFA can only be a rough guide as it cannot distinguish between a 'tall' house and a 'short' house....:bigsmile:

Paul in Montreal

Posted By: Ed DaviesPiM, you're joking, right?

I thought your building regs only required 10ACH@50Pa?

Paul in Montreal.

Ed Davies

Nope, AFAIK, none of the UK building regs put an upper limit on the AC/h rate. There are, I think, maximums on the airflow through the envelope at 50 Pa which might well be 10 m/h - I can't be bothered to look as it's not of any interest from the point of view of green building. Scottish regs say that if the house is tighter than 5 m/h at 50 Pa then you need some sort of artificial ventilation (passive stack or mechanical).

However, your comment which I thought was a joke was mixing up very different things. Firstly, I think it mixes up a discussion of the minimum ventilation required for health under any conditions with the maximum allowed under windy conditions (represented by +/- 50 pascals internal pressure) for energy conservation reasons.

Secondly, it seems to mix up the way in which incidental leakage tends to scale with pressure (which itself depends on the geometry of the leaks) with the way an MHRV system will react to pressure and wind. Basically, if you pressurize the house it doesn't make the fans blow harder so you wouldn't expect the flow through an MHRV to react in anything like the same way.

So, if your comment wasn't facetious/a joke then perhaps you could explain it a bit more fully because I don't get what you're saying.