Simple air changes per hour, ventilation rate calculator

[MrMagic]

Howdy folks - I'll be upfront and admit I'm being lazy, but does anyone have a simple ventilation rate calculator?

 

i.e. if my volume is ...m3 and ACH is 6 to 8 then the required ventilation rate would be ... litres per second or hour

[SteamyTea]

1000 litre in a cubic metre.

3,600 seconds in 1 hour.

 

So 

 

Total Litres = m³ x ACH x 1000

 

Litres per second = Total Litre / 3600

 

So take a room that is 10m by 5m by 2.5m

 

125m³

 

ACH is 6

 

125m³ x 6 ACH 

 

750m³

 

Multiply by 1000

 

750,000 litres 

 

To change that in 1 hour, and express in litres per second, divide by seconds in an hour.

 

750,000 / 3,600

 

208 Lt/s 

 

Can call that Radio Luxembourg.

 

Or

 

lt/s = (Volume [m³] x ACH) / 0.2778

 

So

 

125 m³ x 6 ACH

 

750 / 0.2778 = 208 Lt/s

 

 

Edited May 10, 2022 by SteamyTea

[JohnMo]

Why would you need 6 to 8 air changes per hour.  With an airtight house you are looking at 0.3 or 0.5 to comply with building regs (depending on which part of the UK you live).

 

Normal housing stock my leak at the rate you quote at 50 Pa, but that is equivalent to the leakage rate in a howling gale in normal terms.

[SteamyTea]

8 minutes ago, JohnMo said:

Why would you need 6 to 8 air changes per hour.

Could be a catering establishment.

 

Have you ever wondered why it is so cold waiting for a takeaway.

 

https://www.lakeair.com/air-changes-per-hour/

 

Commercial kitchens are 15 to 30 ACH.

Edited May 10, 2022 by SteamyTea

[JohnMo]

Alway thought it was the doors never shut properly (or other customers couldn't be bothered closing doors behind them) and the owners refused to heat place.

[SteamyTea]

1 minute ago, JohnMo said:

Alway thought it was the doors never shut properly (or other customers couldn't be bothered closing doors behind them) and the owners refused to heat place.

 

No, it is the huge extractor fans we have to use, even worse if you cook on gas.

 

Not unusual to be 40°C by the hobs/ovens/fryers, and struggling to get to 16°C in the actual eating area.

 

There is often a double door between kitchen and restaurant for this reason.

Trouble is is gets propped open to help cool the kitchen.  Opening exterior windows and doors always seems like a good idea, but the orders then get blown off the rail.

Oh the life of a chef, it is a mugs game.

[JohnMo]

Every day's a learning day

[MrMagic]

Thanks @SteamyTea! Just what I needed.

Yes this is for an office so going by https://www.vent-axia.com/sites/default/files/Ventilation Design Guidelines 2.pdf we're looking at 6 - 10 ACPH. (basic) CO2 monitoring is also showing 1000ppm+ throughout the day.

 

I think we have to be careful to not conflate air leakage with air changes - not my area of expertise (as you can guess) - but an airtight building with low 'uncontrolled' leakage will still need a decent amount of 'controlled' ACPH.

 

Seal it up tight.. ventilate right etc.

[SteamyTea]

What are you doing for climate control and energy recovery?

 

There have been some studies that show the workforce mood is improved if you let them open and close windows. It gives the impression they have some control, though I would not want to work in an office, in winter, with 2 women going though the change. If the temperature swing don't get to me, the wrestling would.

Edited May 11, 2022 by SteamyTea

[MrMagic]

Home office rather than commercial. I can tell I'm not getting enough air through here so need to up the ante... I've got a dHRV already but wanted to do the calcs to see if I'm shifting enough air through here. If not then it'd be up to a larger MVHR... only a cheapy one tho.

 

Edit: Both your link and the ventaxia link settle on 5 ACPH for general domestic so need to make sure I'm at least achieving that

Edited May 11, 2022 by MrMagic

[JohnMo]

Still really confused why you need that many ACH for an office.  From the text above the table it is referring to intermittent rates, not continuous.

 

If you read the last sentence on the sheet.

 

"total extract airflow rate during normal operation of a continuous mechanical ventilator eg. MultiVent etc, should be 0.3 l/s/m2. This is based on the 
whole dwelling volume, with provision to increase the inlet volume, as required, in moisture generation areas.

 

A quick calc on a room with a ceiling height of 2.4m gives an ACH of around 0.3.

 

[JohnMo]

Our living room is airtight and has MVHR, the ACH is set at 0.5, even with 6 people in the lounge for several hours the ppm CO2 stays below 1000.

[SteamyTea]

2 minutes ago, JohnMo said:

really confused why you need that many ACH for an office

Maybe something is introducing something that should not be there i.e. leaky gas boiler flue.

[JohnMo]

But the document referenced is mentioning leaky anything.

[MrMagic]

Thanks for the feedback, reminded me to go read part F to see what I should be doing - note this is an existing small garden office/shed.

 

Part F actually makes it quite clear - 

 

Since this garden office/shed is a single room, you could argue it should follow the above and have a minimum fresh air delivery rate of 13l/s - currently I'm only putting in 6l/s with my dMVHR - which kind of explains why CO2 is rising so much. No combustion appliances or anything like that here and the graph tracks exactly when I start/finish work.... perhaps less gassing on Zoom will help too?

 

For reference its 17m3 so if we take the lower 0.3 ACPH - 

(17*0.3)/0.2778 = 18.35 l/sec of air.

 

So whichever way we cut it, I'm at least 7l/s of air 'short'. (assuming it's a perfectly sealed box)

 

Thanks @JohnMo and @SteamyTea !

 

Edit: I've got a https://www.vent-axia.com/range/lo-carbon-tempraselv fitted - looking at that it's 6 or 9l/s in trickle and 15l/sec in boost - so running in boost all day should be enough to clear the air.. will give it a whirl.

 

 

 

Edited May 12, 2022 by MrMagic

[JohnMo]

If too noisy just install a second one.  As a matter of interest how is air being allowed in the office replace the air removed.  Is natural leakage through the building fabric or a vent?

[MrMagic]

23 minutes ago, JohnMo said:

If too noisy just install a second one.  As a matter of interest how is air being allowed in the office replace the air removed.  Is natural leakage through the building fabric or a vent?

 

I'm assuming, perhaps incorrectly, that the dMVHR is balanced? It's got a heat exchanger and an inlet/outlet pipe in pipe (a bit like a combi boiler flue). Certainly it's 'extracting' from slots on the side and inputting via a grill

[JohnMo]

I missed looking at the previous link.  That explains why I was missing something.