[Greenbuilding] PHPP Flow Rates

[John Straube]

I think the PHPP approach is a bit of a throwback to the old days when people measured required ventilation by ACH.
A more sensible approach, and the one used by the most common north American standard ASHRAE 62, is to assess the need for ventilation based on the number of people (since they produce pollutants) and the square footage served (since building pollutants, including cleaning supplies etc are roughly square foot based).
A good approach is 7.5 cfm per person plus 0.01 cfm/Square foot.  Real simple, time tested, logical.
So for a 1500 sq ft 3 BR home, that would be assumed to be 4 people, and hence 30 cfm.  Then there is the 15 cfm added for the 1500 sf of building to get 45 cfm.  This is continous operation.  A 90 ccm HRV would be set to run 30 minutes on the hour to achieve this.
We often install more capacity than this (hence your paraphrase of Joe's), but commission it at these rates in tight houses (and lower rates in leaky production houses), thereby allowing for a higher rate to set if the occupancy demands it.
Recently, the ventilation rates in 62.2 have increased, and these are not in my opinion justified IF you do a good job of mixing and filtering incoming air.  IF you do a bad job of mixing and don't exhaust large concentrations of pollutants (kitchen range hoods, bathrooms etc) then you would need to increase the baseline rate.  But that is bad design and should not IMO be recommended.

Many people want to increase the ventilation rate despite the initial cost and operating cost penalty to improve indoor air quality.  While there is some logic to this, it is usually better from a health, economics, and environmental point of view to manage the production of indoor pollution through material choices and sensible operation than try to solve pollution with lots of dilution.

my 2 cents worth
John

On 2013-03-05, at 1:13 PM, John Salmen <terrain at shaw.ca>
 wrote:

> Maximum flow rate is generally the maximum of your supply and exhaust
> requirements (based on occupancy, assumed volumes, leakage, etc.) As I
> understand it PHPP however calculates this just as volume (interior building
> volume) multiplied by .3 ACH (air changes/hour) multiplied by 1.3 (real
> world factor?). So oversizing by 1.5 keeps it in the real world and ensures
> adequate ventilation.
> 
> -----Original Message-----
> From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
> On Behalf Of Bob klahn
> Sent: March-04-13 7:21 PM
> To: Green Building
> Subject: Re: [Greenbuilding] PHPP Flow Rates
> 
> Eli,
> I can't answer your question directly.  I am dealing with a situation where
> I'm pretty sure the ERV flow is low and am equally frustrated.  
> However, I recall a recent posting by Joe Lstiburek saying that they
> oversized ERV's by 150%.
> 
> Given what my memory has done to me lately, you might want to find that
> posting  for the rest of the story.  Check the BSC site.
> 
> I hope the lead helps.  This thread should be interesting.
> Bob
> On 3/4/2013 4:38 PM, Eli Talking wrote:
>> I am trying to make sense of Ultimate Air spreadsheet describing how 
>> to size ERV flow rates.  It references the Ventilation maximum flow 
>> rate as determined from PHPP.
>> 
>> Does anybody know how to find Ventilation maximum flow rate  PHPP?
>> 
>> Eli
>> 
>> 
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Prof. John F Straube, P.Eng.
www.BuildingScience.com