Natural Ventilation Multiunit Residential Results

Lorey, I have had multi-unit residential projects approved utilizing natural ventilation, mechanical ventilation, and a combination of the two. It sounds like in your case you should only use natural ventilation. You would not want to use mechanical ventilation unless you are distributing the OA directly to the spaces for ventilation purposes. If it is only for pressurization purposes then for IEQp1 you can act as if it does not exist. The only time you should use a combination of the two is when you have spaces that do not have operable opening to the outside which receive ventilation air. We have used that configuration on a VRF system before. Note that in addition to the requirement of 4% operable area (to floor area), all occupied spaces must be within 25' (or 8m if you need the extra little bit).

As for increased ventilation, you will have to comply with IEQc2 requirements to document the effectiveness of natural ventilation through modeling or compliance with CIBSE. I have never personally done either of these or seen these documented.

We recently had a project approved where we used an engineered natural ventilation approach because a subset (<20%) of the residential units had spaces that were located more than 25' from the operable windows, and so did not comply with the prescriptive requirements for natural ventilation. We argued that the constant kitchen and bathroom exhaust, combined with trickle vents, operable windows, and some make up from the halls was sufficient to meet VPR OA rates for mechanically ventilating the space. We actually showed that this engineered approach allowed us to exceed VPR OA rates by more than 30% and were awarded IEQ c2 as well.

Michael, how did you document the engineered approach? Did you simply input the exhaust rates into a VRP calculation with the approach that exhaust must equal fresh air make-up?

Yes, we input the exhaust rates into a VRP calculation and assumed that exhaust would equal fresh air. Because of the location of the trickle vents and the exhaust fans we used a zone distribution effectiveness (Ez) of 0.8.

I'm working on a project in which some of the residential units have operable window area less than 4% of the occupiable floor area. I would like to show compliance through mixed mode ventilation, since there is constant exhaust through the bathrooms and corridor make-up air units which will be providing fresh air to the residential spaces. Any advice on how I can show compliance using mixed mode ventilation for the residential units?

Thanks,

Since corridors are defined by ASHRAE 62.1 to be unoccupied, I dont see how ventilation utilizing tranfered air would be a problem. So long as there is enough flow for both the corridors and the residential spaces. I sent a CIR for exactly the opposite configuration. (transfered air from residential spaces to corridors). I justified my configuration with ratios of outside air to supply air, bioeffluent contamination, and exhaust through corridor restrooms. I think your case would be less of a problem. It would have to be based solely on mechanical ventilation though. Windows dont factor in if they dont comply with the minimum opening sizes.

Thanks for the help, Rudolph. I was thinking there would be some way to combine the natural and mechanical ventilation.

Mike, if your spaces are separated then you can use natural ventilation for the individual spaces that qualify and mechanical ventilation for the others. Unfortunately, you cannot combine them for one space such as having 2% openable area and 50% of the ventilation flow.

Rudolph, when using the method that you suggested, how would I determine the design airflow rates to the residential units since air is being transferred through the corridors and not sent directly to the units?

Hi, Mike-
I think you would have to come up with a room ventilation effectiveness (Ez) number for each space in the residential unit. This is a little dicey, because you are not exhausting from each space individually, but only from the restroom. Hopefully your restrooms are located such that the route of makeup air from the corridor to the restroom goes through the other spaces in the unit. If not, it might be hard to justify anything other that a very dismal ventilation effectiveness, for example in a bedroom that is not located on the airflow route between the corridor and a restroom.