I'm looking for clarification on required ventilation levels in residential settings.  For instance if I have a home that indicates an ach rate of considerably less than .35 that would suggest the addition of mechanical ventilation. I'm unclear, however on how much is enough, how much is too much?  It would seem that the additional air exchange rate should be based around the actual natural rate difference between the measured amount and .35.

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Always check with your local AHJ, but the most common one is 7.5 CFM per person (based on bedrooms - 1 bedroom = 2 people, for each additional add 1 person) + 1 CFM per 100 SF

Truly you want the natural rate to be 0, and the exchange to happen via an ERV/HRV

Now the above is just a standard, but if they use a room for a nail salon, that is not enough - if they have dogs, cats, smoke, etc... it also may not be enough which is definitely something to consider in a retrofit or if one is building a custom home

If everybody in the house smokes, they obviously don't give a rip about air quality.. Just sayin'

Seriously? Have you ever heard the saying about not judging a book by it's cover - it really is good advice

It is what it is, no excuses. If they give a rip about indoor air quality they smoke outside. Eliminating the source of indoor pollutants is on the top of the list for IAQ issues.

*rant mode mode on* It really gets under my skin when people smoke indoors with children present. Cars, homes, and even smoking areas of restaurants. The political idiots in our state legislature saw fit to force restaurants to set up separate smoking areas, but yet let parents to bring their minor children into the smoking areas. If you are going to force restaurants to go through this expense, at least prohibit minors in the smoking area.

I think the number for ASHRAE for residential is 15 cfm per person so a 3 Bedroom house would be 60 cfm or bedrooms plus 1

Using the calculations for a 2000 sq ft house with 8 foot ceilings the calculation would be Volume or 16000 cf * .35 divided by 60 or 93.33 round up to 94

We would use the larger number of the two so 94 cfm

Keep in mind cfm loss on duct runs when correctly sizing a system

Thats the old ASHRAE one & except for a few rare jurisdictions & BPI that still hasn't updated there standards it really isn't used

In reviewing my BPI notes I think I have what I'm looking for.  If I convert my test out level of cfm by my local conversion factor it will give me an natural cfm rate.  That rate will compare to my BAS level as a percentage of ventilation rq'd.

I'm assuming that since the BAS level is achieved with .35 ach as a component of the equation, then the calculated cfm rate would be on an hourly basis.  (24 hrs)  Thoughts?

Eric, correct, you figure the BAS for the house, use measured CFM50 and n-factor to determine how much of the required airflow is already occurring naturally, and if inadequate add mechanical ventilation. In doing all of this you are multiplying CFM by 60 to get ACH, or the reverse if necessary.

The new ASHRAE standard is suggesting 7.5cfm per person(with given occupants or # of BR +1), plus a factor for the house size(square footage x.01) and then a credit for infiltration based on the blower door reading and a new N factor based on location(not the same as the N factor used in the ASHRAE 62-1989 calc).   You are correct though, on the old BAS calculation which was based on (volume x.35)/60 or # BR + 1, whichever was higher.  

As Sean said, ideally the CFMn is 0, and your required BAS is provided by ERV/HRV, and that your system was sized with this fresh air in mind.

http://www.heyokasolutions.com/kb_results.asp?ID=28 I sat in this man's seminar last summer.I believe he even helped co-author the 62.2

As discussed in other replies, ASHRAE 62.2-2010 requires 7.5 cfm per person, usually based on number of bedrooms plus 1, plus 1 cfm per 100 square feet of living space.  So a 1,000 square foot two bedroom unit needs 7.5 X (2+1) cfm for the occupants plus .01 X 1,000 cfm for the building, so 22.5 + 10, rounded to 33 cfm.  That is the continuous mechanical rate.  62.2 assumes a default leakage rate of 2 cfm/100 sf, which is a moderately tight house.  Appendix A to 62.2 addresses existing buildings and tells you how to apply the standard to existing buildings, especially in relation to air sealing.  It also provides guidance on how to increase the continuous whole house rate to make up for the lack of say, a bath fan or a vented range hood.  You can view 62.2, including the Appendix, at http://www.ashrae.org/technology/page/548 for free.  A good rule of thumb is if the house tightened down to about 5 ACH50, just do the ASHRAE whole house number from above.  If it is in the 5 to about 9 ACH50 range, it is probably worth doing the calculation to reduce the required mechanical rate, although reducing a required rate of 33 cfm seems hardly worth the effort. If the house tests out at 10 ACH50 or more, you probably do not need to add mechanical ventilation but you are leaving a lot of energy savings behind.  As a point of reference, I am the Chair of ASHRAE SSPC 62.2.

Mr Stevens,Thank you for chiming in. I see that Paul Rayner's calc uses 15cfm and ASHRAE is using 7cfm.Can you explain?..is it possible the codes will be changing?

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