Air Exchange Rate and HRV.

gears

I'm a little confused with specifications for HRV units.

Some manufacturers specify units for a floor area of say 300m2 but then the units can only supply 100-200m3 per hour depending on the fan speed. So if you take an average ceiling level of 2.5m that would mean you would need between 4 and 7 of these units to give one air exchange per hour.

Maybe I'm just totally confused with the whole thing but can any one shed some light on this.

sydthebeat

gears wrote: »

I'm a little confused with specifications for HRV units.

Some manufacturers specify units for a floor area of say 300m2 but then the units can only supply 100-200m3 per hour depending on the fan speed. So if you take an average ceiling level of 2.5m that would mean you would need between 4 and 7 of these units to give one air exchange per hour.

Maybe I'm just totally confused with the whole thing but can any one shed some light on this.

minimum standards 'will be' measured in litres per second, not in m3.

and there 'will be' different rates for different rooms.

Sizing
1.2.3.2 The minimum capacity of an MVHR system
should be based on the calculated general
ventilation rate, adjusted to allow for air infiltration
due to permeability of the building fabric. The
calculated general ventilation rate is determined as
the greater of
a) 5 l/s plus 4 l/s per person, e.g. 25 l/s for a five
person dwelling, or
b) 0.3 l/s per m2 internal floor area, e.g. 30 l/s for
a 100 m2 dwelling.

for example... to reach 25 litre / second

say your HRV is rated at 150m3....
thats 150 * 1000 = 150000
divide by 3600 (60 minutes x 60 seconds)

= 41.7 litres / second

gizmo555

gears wrote: »

So if you take an average ceiling level of 2.5m that would mean you would need between 4 and 7 of these units to give one air exchange per hour.

Isn't this quite a lot? I thought these were generally configured to give air exchange rate of about .35 per hour?

I'd be interested to learn more as I'm considering retrofitting a system to my house.

gears

gizmo555 wrote: »

Isn't this quite a lot? I thought these were generally configured to give air exchange rate of about .35 per hour?

I'd be interested to learn more as I'm considering retrofitting a system to my house.

An air exchange rate of approx 0.35 makes more sense with Syds figures. So an installation with 2 units or one large unit wouldn't be unheard of.

Borzoi

sydthebeat wrote: »

b) 0.3 l/s per m2 internal floor area, e.g. 30 l/s for
a 100 m2 dwelling.

So for the OP this works out to be 324m3/hr

I use the simpler: Vol * .5 = 300 x 2.5 x .5 =375m3/hr

So looks like those initial units are undersized for your house size - and in general a larger unit will be cheaper than 2 smaller units - shop around for a larger system

sydthebeat

Borzoi wrote: »

So for the OP this works out to be 324m3/hr

I use the simpler: Vol * .5 = 300 x 2.5 x .5 =375m3/hr

So looks like those initial units are undersized for your house size - and in general a larger unit will be cheaper than 2 smaller units - shop around for a larger system

where does that formula come from??

Borzoi

sydthebeat wrote: »

where does that formula come from??

Sorry, but it's just a quick Rule of Thumb

sydthebeat

Borzoi wrote: »

Sorry, but it's just a quick Rule of Thumb

don't you think its slightly inappropriate then, to advise someone to go off and source a larger product based on a 'quick rule of thumb'?

are you a supplier?

Borzoi

sydthebeat wrote: »

don't you think its slightly inappropriate then, to advise someone to go off and source a larger product based on a 'quick rule of thumb'?

are you a supplier?

Don't be so quick to disparage Rules of Thumb, they're endemic within engineering

Rules of Thumb in Engineering Practice - Wiley wrote:

An immense treasure trove containing hundreds of equipment symptoms, arranged so as to allow swift identification and elimination of the causes. These rules of thumb are the result of preserving and structuring the immense knowledge of experienced engineers collected and compiled by the author - an experienced engineer himself - into an invaluable book that helps younger engineers find their way from symptoms to causes.

Did you reference your calculation source? Both our sizing methods come out with similar results, so I'm not sure what your problem is with it.

I am not a supplier, though I am 2 decades into a career as a design engineer of HVAC&R systems. I thought I made a helpful contribution, which both you and the OP are free to ignore.

sydthebeat

forgive me borzoi.. i meant no offence, i suppose im a bit inquisitive by nature.

The reason i asked about the formula is because i have seen established engineer 'rules of thumb' go out the window over the last few years due to the more exacting science of domestic house building.

if someone is planning to build a low energy dwelling, then the more exact the sizing of energy consuming installations, the better. It sjust the way things are going.

again, i meant no offence, and a person with your experience would be a great addition to debate on this forum.

by the way my source material was the draft for the new updated building regulations part f 'ventialtion'.

Borzoi

sydthebeat wrote: »

The reason i asked about the formula is because i have seen established engineer 'rules of thumb' go out the window over the last few years due to the more exacting science of domestic house building.

by the way my source material was the draft for the new updated building regulations part f 'ventialtion'.

No offence taken.

Just to be clear, the RoT I used is not from the Wileys book - in fact these days I'm not sure of it's origin. You are right in that ROTs do change and adapt - but you still find them everywhere

With regard the actual case in point - really the best way to design is on a per room basis, I think the CIBSE guides have a recomendation, and then sum it up, apply a diversity load factor (ie another RoT :rolleyes:) and use that as your start.

However looking at the OPs situation it's a function of price, airflow and quality. Shop around, let the suppliers to the work for you if you don't have the technical expertise, then compare their offerings. The reality is that with a real world situation, it's unlikely that you will notice the difference between a system that delivers 0.5 AC/hr and 0.4 AC/hr.

sunnysoutheast

gears - we were just (app. 400m2) on the cusp of having one very big unit, but in the end we went for two smaller (app. 250m2) units. Our rationale being that one will be in "normal" mode for the heavily-used side of the house and the other for the lightly-used/guest rooms in "low" mode, but turned to "normal" when people are staying.

Can definitely recommend fitting HRV if you can get decent levels of air-tightness.

SSE