pressurizing a space

[317069]

Hello All

how can we pressurize a space with typical HVAC system
(a unit, supply duct, return duct,and HRV unit).?
I mean if we have 1000 cfm of supply air that mean there will be 1000 cfm entering the unit fan in order to get a 1000 cfm from this fan.

 

[ChrisConley]

A space is pressurized by exhausting less air than is directly made up. The 'extra' air pressurizes the building as it escapes through the cracks and penetrations of the space.

In your scenario, if 500 cfm of your fan was supplied from outdoor air, and only 300 cfm was exhausted you would have a general pressurization of the space. How much pressure would depend on the size of the space (specifically the leak paths, building orientation, wind speed and direction and a half-dozen or so other variable).

 

[317069]

Chris
it is right if I have a make up air unit supply 100% of OA to a space.
but in cas I have asked about, the mentioned system above ( unit+supply and return duct+ HRV)in typical house for example
HRV will exhaust the same amount that it bring in.
so the air balance will be as following:
-1000 cfm supply air to the space- supply duct.
- 1000 cfm int return duct
no matter what is the OA cfm in this case. Imean the furnace inlet will recive 1000 cfm to be able to give 1000 cfm to the space.

the factors you mentioned are uncontroled factore, how can I provid certain value of pressurization to a space, forexample 0.1"

 

[mauricestoker]

It would be easiest doing it at TAB. Set either exhasut or supply as required, adjust the damper for the other until you get the 0.1 IWG.

 

[ChrisConley]

You can pressurize the space by reducing the amount of air you exhaust using the HRV.

quark has an excellent example of calculating pressure requirements at:

http://www.eng-tips.com/viewthread.cfm?qid=196510&page=1