Which ACH is ACH?

[Sparrowhawk]

Rant posting after digging online for figures to use in a friend's thermal model.

 

When someone quotes their air changes as "1.5ACH" do they mean "at 50Pa" or "at normal atmospheric pressure"?

 

The web is awash with figures from people who don't specify. Some are helpful and use "ACH @ 50Pa" or "ACH (50)" and similar. Others use "ACH (nat).

 

I made the assumption because no pressure was quoted then it was normal atmospheric pressure... but from the range of figures I found it could be either.

 

Is there an accepted meaning for "ACH" on its own?

Edited September 2, 2023 by Sparrowhawk

[JohnMo]

Let's do some assumptions.

 

If inside is at atmospheric pressure and outside is the same. There is zero motive force to move air, so the ACH must zero. This will be true in all cases. As an example on a still day (no wind) you open your front door, the air doesn't move after the door is opened.

 

Now pressurise the house, so the air pressure is higher inside compared to outside. Now you have a motive force so air leaks through the building structure. Same example as before, you open your front door in a gale, you leave your door open, you gets lots of wind blowing in the house.

 

So the figure must be at 50Pa.

[JohnMo]

I am not voting, it's a waste of time.

Edited September 2, 2023 by JohnMo

[IanR]

1 hour ago, Sparrowhawk said:

Is there an accepted meaning for "ACH" on its own?

 

The standardised measurement is at 50Pa

 

Heat loss calculators will then use a ratio/percentage of that figure to calculate the average infiltration and therefore the averaged heat losses through infiltration. The "divide by 20" rule is often quoted, ie the average infiltration of a property is only 5% of the measured figure @ 50Pa

[Sparrowhawk]

1 hour ago, JohnMo said:

Let's do some assumptions.

 

...

 

So the figure must be at 50Pa.

 

Except. Sometimes the figure being quoted is the @50Pa already converted, i.e. divided by 20 as @IanR says, to ACH at atmospheric pressure.

 

When the figure is "0.35ACH" it's hard to tell if that's a good build, or a standard build "7ACH @ 50Pa". In this case it was the latter.

[IanR]

46 minutes ago, Sparrowhawk said:

Except. Sometimes the figure being quoted is the @50Pa already converted, i.e. divided by 20 as @IanR says, to ACH at atmospheric pressure.

 

That's not what I meant. 

 

I'd say everyone quoting a performance figure for their property will be quoting the standardised measurement figure that is measured at 50Pa, whether or not the 50Pa is stated.

 

The "divide by 20 rule", is a rule of thumb that some heat loss calculators will use, or use a range similar if it asks the question "is the property sheltered, exposed or normal?"

If someone is quoting 0.35ACH, they are quoting the @50Pa, measured value.

[JohnMo]

Does it matter really to anyone, if someone with a leaky house wants to make it sound better than it really is, the only one person their kidding - is themselves.  Let them live in a make believe world, and pay the heating bills.

 

An air test actually measures m3 leaked per m2 of all internal surfaces at 50 Pa.  It not even the same as ACH as building geometry also plays a part in that calculation.

 

Move on, look after your own house, and don't worry if someone you don't know, is telling porkies or not, to make themselves feel good. No one cares.

 

My house leaks x m3 of whatever units? You don't care and why should you.

[Sparrowhawk]

You missed the first sentence of my first post John "posting after digging online for figures to use in a friend's thermal model." but fine let's move on 🤷

[SteamyTea]

Is it not covered in BS 40101:2022

 

 

[JohnMo]

Use this calculation method 

 

As it says if reasonably airtight use the figure suggested, if old and drafty possibly double the figure. If has MVHR, you need to take heat recovery efficiency into account also.

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

[SteamyTea]

2 minutes ago, JohnMo said:

Use this calculation method 

 

As it says if reasonably airtight use the figure suggested, if old and drafty possibly double the figure. If has MVHR, you need to take heat recovery efficiency into account also.

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

I have always been a fan of the OU.

Really should see what else they are offering for free.

[Dave Jones]

Part L reg 50

 

The measured air permeability to be not worse than 5 m³/(h·m²) at 50 Pa.