Ventilation in Multi-Family Building

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Hello,

I have 12 storey multi-family building in PA. Corridors are served by
Makeup Air units with high CFM (0.9 CFM/sq.ft). Apartment units are served
by split fan coil units (dX cooling and hot water boiler). Apartment
bathrooms have continuously running exhaust fans of 30 CFM each.

How will be apartment ventilation modeled for proposed case?

1) Will it be assumed that Outside Air is coming through infiltration?

2) Do I need to put Outdoor Air flow value for apartments in eQUEST
manually? Would this value be zero since apartments are naturally
ventilated?

3) Should I take supply fan on cycling for fan coil units?

4) What would be the source of exhaust for the bathroom? Would it be
infiltration or air handler?

How will be the apartment ventilation modeled in the baseline case?

1) Baseline system would be PTAC for apartments. Since Outside Air
will be provided through PTAC, should the supply fans be set for continuous
operation?

As I understand that this is pretty common situation in multi-family
buildings. I am interested in knowing that how other energy modelers work
on this scenario.

Thanks,

Sunayana

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

We model this type of building regularly for Seattle, WA projects. In our mild climates it is typical that there is a dedicated make-up air path for the continuously running whole house fan. We most often see trickle vents integrated into the window frames (again, mild climate, so introducing 30-50 CFM of ambient-temp air directly into the space is not considered poor-form).

In our model we run the whole-house-fan continuously, and model it as bringing in infiltration air, so the space conditioning system see?s the load from the vent air. Then we allow the space conditioning system to cycle to meet the load in the space (which is ventilation dominated, but also includes envelope, occupant, equipment, and lighting loads). You can modify the schedule if you want to represent some additional intermittently operating exhaust fans (such as kitchen ranges, laundry closets, or secondary bath fans) IF you believe the make-up-air source will be more outside air.

[cid:image001.png at 01D1B72D.5906BC40]

To my knowledge most codes do not allow for the corridor pressurization air to be considered the make-up air source for the whole-house-fan (unless the air is directly ducted to the apartment units). But if the source of the make up air is essentially from a DOAS unit, I probably wouldn?t model anything in the individual zones (presuming they are more or less receiving neutral air), and the ventilation load will be met by the DOAS system. The zonal system again would just cycle to meet the conditioning needs of the space.

Regarding the baseline system, it is a little contentious how much ?credit? we should receive for whole-house-fan systems (which for their many drawbacks, do tend to be low-fan energy). By my reading of Appendix G (and backed up by numerous LEED reviewed and approved submittals), the baseline HVAC systems are intended to provide BOTH conditioning AND ventilation. PTAC/PTHP are constant volume, with the fan sized to meet the space conditioning load. So we model the PTAC fan as running continuously, and zero out the exhaust fan kW/CFM (because all system fan energy is assigned to the HVAC unit). Typically we leave the other inputs the same for the OA exhaust scheme. This is based on noticing that if you switch to using the OA inputs for the baseline case (and not the exhaust air inputs) eQUEST tends to calculate a very different OA load in the space, even if the CFM is identical. We were never able to justify this, so we?ve stuck to modeling the OA flow same-same in the BL and PROP.

Hope this helps.

Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Nathan,

Thank you for putting this description below. This was very helpful.

In the proposed case, when I put the Outside Air (as in image below), SV-A
report shows zero outside air in the apartment zone. Instead of zero,
should not this value be 50 CFM? Any suggestions will be helpful.

Thanks,

Sunayana

*From:* Nathan Miller [mailto:nathanm at rushingco.com]
*Sent:* Thursday, May 26, 2016 9:17 AM
*To:* Sunayana Jain
*Cc:* equest-users at lists.onebuilding.org
*Subject:* RE: [Equest-users] Ventilation in Multi-Family Building

We model this type of building regularly for Seattle, WA projects. In our
mild climates it is typical that there is a dedicated make-up air path for
the continuously running whole house fan. We most often see trickle vents
integrated into the window frames (again, mild climate, so introducing
30-50 CFM of ambient-temp air directly into the space is not considered
poor-form).

In our model we run the whole-house-fan continuously, and model it as
bringing in infiltration air, so the space conditioning system see?s the
load from the vent air. Then we allow the space conditioning system to
cycle to meet the load in the space (which is ventilation dominated, but
also includes envelope, occupant, equipment, and lighting loads). You can
modify the schedule if you want to represent some additional intermittently
operating exhaust fans (such as kitchen ranges, laundry closets, or
secondary bath fans) IF you believe the make-up-air source will be more
outside air.

To my knowledge most codes do not allow for the corridor pressurization air
to be considered the make-up air source for the whole-house-fan (unless the
air is directly ducted to the apartment units). But if the source of the
make up air is essentially from a DOAS unit, I probably wouldn?t model
anything in the individual zones (presuming they are more or less receiving
neutral air), and the ventilation load will be met by the DOAS system. The
zonal system again would just cycle to meet the conditioning needs of the
space.

Regarding the baseline system, it is a little contentious how much ?credit?
we should receive for whole-house-fan systems (which for their many
drawbacks, do tend to be low-fan energy). By my reading of Appendix G (and
backed up by numerous LEED reviewed and approved submittals), the baseline
HVAC systems are intended to provide BOTH conditioning AND ventilation.
PTAC/PTHP are constant volume, with the fan sized to meet the space
conditioning load. So we model the PTAC fan as running continuously, and
zero out the exhaust fan kW/CFM (because all system fan energy is assigned
to the HVAC unit). Typically we leave the other inputs the same for the OA
exhaust scheme. This is based on noticing that if you switch to using the
OA inputs for the baseline case (and not the exhaust air inputs) eQUEST
tends to calculate a very different OA load in the space, even if the CFM
is identical. We were never able to justify this, so we?ve stuck to
modeling the OA flow same-same in the BL and PROP.

Hope this helps.

*Nathan Miller, PE, LEED AP BD+C** ? **Mechanical Engineer/Senior Energy
Analyst*

*RUSHING* | *D* 206-788-4577 | *O* 206-285-7100

*www.rushingco.com *

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
] *On Behalf Of *Sunayana Jain
via Equest-users
*Sent:* Wednesday, May 25, 2016 6:10 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] Ventilation in Multi-Family Building

Hello,

I have 12 storey multi-family building in PA. Corridors are served by
Makeup Air units with high CFM (0.9 CFM/sq.ft). Apartment units are served
by split fan coil units (dX cooling and hot water boiler). Apartment
bathrooms have continuously running exhaust fans of 30 CFM each.

How will be apartment ventilation modeled for proposed case?

1) Will it be assumed that Outside Air is coming through infiltration?

2) Do I need to put Outdoor Air flow value for apartments in eQUEST
manually? Would this value be zero since apartments are naturally
ventilated?

3) Should I take supply fan on cycling for fan coil units?

4) What would be the source of exhaust for the bathroom? Would it be
infiltration or air handler?

How will be the apartment ventilation modeled in the baseline case?

1) Baseline system would be PTAC for apartments. Since Outside Air
will be provided through PTAC, should the supply fans be set for continuous
operation?

As I understand that this is pretty common situation in multi-family
buildings. I am interested in knowing that how other energy modelers work
on this scenario.

Thanks,

Sunayana

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Correct, this is a downside to this methodology, the ventilation air will not show up in the standard reports as OA, because you aren?t using the OA inputs. However, if you go three columns to the left, you can verify that the exhaust air is 50 CFM, so as long as you assigned that the source is infiltration, you know that shows up as OA load on the system. If you want to check, compare heating/cooling loads and energy both with and without any exhaust air specified.

Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400