Hi James

Please accept my apologies, I have been browsing the forum recently but only on my tablet which isn't the most efficient device for typing lengthy entries. Borrowing the wife's laptop today so I'll make a stab at explaining the way the calculations were taught to me, based on the training at the Passive House Academy course.

There are 3 calculations to determine the total air flow requirements (which we work out in "boost" mode which is 30% greater than normal operation).

The simplest is the assumption that an air supply rate of 30m3/h per occupant is required for good air quality, so in a home for 5 people this would be 5 x 30 = 150m3/h.

You also calculate the extract requirements based on the number and type of extract rooms using some given values:

Kitchen - 60m3/h
Bathroom - 40m3/h
Storage room, WC, utility - 20m3/h
En suite - 20 or 40m3/h depending on intensity of use

So a house with kitchen, utility, bathroom and 1 en suite used daily by 2 people would have an extract requirement of 60 + 20 + 40 + 40 = 160m3/h.

Thirdly, you should perform a minimum check which ensures 0.3 air changes/hour will be achieved. This is calculated by multiplying the total Treated Floor Area (TFA) in m2 by a ceiling height of 2.5(m) to establish the ventilated volume and then multiplying this figure by 0.3ach and 1.3 (as boost mode is 30% higher than normal operation). Floor area with less than 1m ceiling height is ignored.

This means if the house is 100m2, the minimum check would be as follows:

(100 x 2.5) x 0.3 x 1.3 = 97.5m3/h

As 160m3/h is the highest value and safely exceeds the minimum requirement of 97.5m3/h this is the total flow rate needed at boost mode.

To allocate this to individual room requirements we deal with the extract and supply rooms separately.

Looking at extract first, the individual room extract requirement is divided by the total extract requirement and multiplied by 0.77 (to revert the flow rate to normal operation) in order to work out the % of the total flow rate.

This means for the kitchen in this example the calculation is (60/160) x 0.77 = 28.875% of 160m3/h, so the kitchen needs a flow rate of 46.2m3/h at normal mode.

Repeat for the other rooms and verify that the total in this case is equal to 123.2m3/h (160 x 0.77).

For supply rooms we divide the floor area of that room by the total supply TFA and again multiply the result by 0.77 to establish the % of the total flow rate in normal operation, so if the living room was 25m2 and the total supply TFA is 50m2, (25/50) x 0.77 = 38.5% of 160m3/h, so 61.6m3/h.

You can verify the supply room results by again checking the total is 123.2m3/h.

To ensure you maintain good air quality in a bedroom while sleeping just check you get 15.4m3/h per person, so a room shared by 2 people should ideally receive at least 30.8m3/h.

The following is sure to look complicated/messy, but I'll try my best to describe the process logically:) To calculate the minimum diameter for a round duct you need to divide the normal flow rate by (air speed x pi x 3600), then find the square root of this number and multiply by 2.

For effective quiet operation, the air speed should be 2m/s in individual ducts and 3m/s for trunk ducts.

So, for the living room above:
61.6/(2 x pi x 3600)
=> 61.6/22619
=> 0.002723
sqrt of 0.002723 = 0.0522 x 2 = 0.104m or 104mm.

Just choose the closest standard duct size up from this. For ducts containing air for more than one room you must first add the individual flow rates, then perform the calculation and remember that trunk ducts can have an air speed of 3m/s.

I hope that helps and if anyone else feels I have omitted something crucial or made an error please feel free to discuss or correct as appropriate.

Michael

Same here . Thanks for putting the time in to type that lot Michael, excellent info .
I'll save that somewhere useful and let you know how I get on.

I would like to fourth that!- very clear advice thank you.

One question - what happens if your extract requirement is lower than your minimum check?

Posted By: SteamyTeaTreat it as one volume I would have thought.

Any particular justification for that? It seems somewhat at odds with:

Posted By: slidersx200This is calculated by multiplying the total Treated Floor Area (TFA) in m2 by a ceiling height of 2.5(m) to establish the ventilated volume

which indicates that a double-height space should be treated as just 2.5 m high (ignoring the volume above 2.5 m).

Hi everyone, thank you for the kind comments and welcome to the forum, future former Mrs Jack

PHPP asks that you enter the average ceiling height for each room and finny is correct that 2.5m is just the convenient way of working with 8' ceilings (2.4m is probably closer, but a marginal overestimation of volume should be to the safe side). Apologies if lack of clarity caused confusion.

In your instance divorcingjack, for the ventilation system you are best to treat that space as one room with a ceiling height of 5m or whatever it actually is, unless you plan on supplying the mezzanine individually.

Incidentally, it would be more helpful for your overall heating requirements to count the mezzanine floor area separately as the transmission losses for the building envelope are divided by a larger floor area.

Jfb, the system would have to be sized to provide at least the minimum requirement, but in a domestic situation it would be unlikely for the extract requirement would be insufficient as even if you have only a kitchen and one bathroom, the house would need to be larger than 100m2 before you would under supply. A house of that size would probably have either an en suite, utility or at least a wc.

Offices and commercial premises are a different story as the supply areas could be much larger for the number of possible extract points and the number of occupants may require some major air flow rates meaning that noise is hard to eliminate.

Michael

Posted By: slidersx200For supply rooms we divide the floor area of that room by the total TFA and again multiply the result by 0.77 to establish the % of the total flow rate in normal operation, so if the living room was 25m2, (25/100) x 0.77 = 19.25% of 160m3/h, so 30.8m3/h.

You can verify the supply room results by again checking the total is 123.2m3/h.

I just tried to follow the instructions and got stuck on this bit. As I read it, this is only going to result in the total checking out if the air supply rooms are 100% of the TFA, isn't it? And that can't be right, since bathrooms and kitchens etc are part of the TFA. I must be misunderstanding something somewhere, but what?

Just updated the above post in question. My example was bad as it is only the TFA for supply rooms you divide the total flow rate by, not the total TFA of the house.

Hope it makes sense now:-)