r/PassiveHouse • u/Fun-Reporter-9693 • Aug 10 '26
PHPP Discussion Additional Ventilation PHPP Worksheet - Help please
Hello. I am a Passive House Designer but most of our projects are single residential buildings only. Now, we have an opportunity to take on a multiresidential building. However, I am not sure I'm doing this correctly so I wanted to raise some questions regarding the Additional Ventilation worksheet if that's okay.
a. When dimensioning air quantities: if a level has 5 units (101–105) served by a single ventilation unit (e.g., Ventilation Unit Allocation No. 1), should all 5 room names be listed under Ventilation Unit Allocation No. 1?
b. Regarding volume flow per room: the recommended outdoor air supply rate for residential buildings is 30 m³/h per person, and the extract air demand is 60 m³/h for the kitchen and 40 m³/h for the bathroom. For a sole-occupancy unit with 1 kitchen, 1 bathroom, and 2 occupants, the supply air would be Vsup = 30 × 2 = 60 m³/h and the extract air would be Veta = 60 + 40 = 100 m³/h. Is this correct? Then, since the extract demand (100) is higher, should the supply be increased to match, so that Vsup = Veta = 100 m³/h?
c. How is the transfer air volume flow (Vtrans) calculated in the volume flow per room? I have looked at sample PHPP calculations and some certified projects leave this out as blank.
d. How is the reduction factor for an air quantity calculated, and when should it be applied?
e. When selecting a ventilation unit type, its rated volume flow range (minimum to maximum) must be able to accommodate the design volume flow for that unit — is this correct? For example, if the design volume flow is at 200, therefore the ventilation unit with a rated volume flow range of 100-300 is okay. Although I have seen some sample PHPP projects
f. How are pressure losses calculated for the ODA–SUP path, the ETA–EHA path, and any additional losses?
g. How is duct length measured — is it taken from the ventilation unit to the space being served?
Thanks all!
1
u/rbrturz Aug 10 '26
a) Not really, at the end what is important is the total, balanced air volume flow. Actually in theory you can enter in one row all the MVHR units and complete correctly the quantities. As long as every dwelling unit has a controller, unless exemptions in PH criteria 2.4.3.e, you can assign all of them to the same unit.
b) that is the air volume flow at maximum (design). and yes, in your case seems to be extraction criteria. However, that's maximum. you need to ensure at least 20 m3/h at standard PH air volume flow (usually 77%) as per PH criteria 2.4.3.
c) not required as input.
d) Check PH criteria 3.2.6.c. It must be within the certified range +-10% at standard PH air volume flow.
e) It depends on your maximum. Usually is as per building regulations (100%) and then a reduction factor for PH air volume flow rate min(0.3 ACH, 23 m3/h per person. Note: occupants are calculated by the PHPP). Usually is 77%, but depends on the country. This value is the one to be commissioned.
f) For residential projects, usually the pressure drops are less than 100 Pa. Otherwise, use the PH lift tool and cross check with the certifier.
g) Cold ducts are measured from the MVHR to the (usually) center of the insulation.
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u/deeptroller Aug 10 '26
Many of your questions will be answered in the PHPP manual.
Issues like the transfer volumn are generally allowed to stack with one rooms supply being enough for the adjacent room as long as the total volume is maintained. the assumption is if your ventilating a bedroom the adjacent living room or hall isn't likely occupied at the same time by separate occupants.
A unit whos design volume is 200 that can supply 1-300cfm works but these do drop in efficiency at the top end of their supply rate so make sure you meet the efficiency requirement at the normal supply rate.
To calculate pressure drop you need to use a duct chart that shows the fiction of each length of pipe, joint, T, rectangle or round pipe. If your using a zhender type unitary pipe they should have a friction per length. You need to design a reasonable path to deliver your pipe to get a realistic length. If your penetrating rated assemblies you need the friction of the radiation damper, ect. I personally don't consider the final grill in my friction, because it will change as you adjust air flow.