Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I've read all relevant eQuest-user threads, and started my own here, and got extremely useful feedback.
It seems that the consensus is indeed to model bathroom and kitchen exhaust as a zonal exhaust, such as the following:
As you can see, I'm setting a flow (CFM) and a corresponding kW/CFM for the fan power. I'm setting the control as Constant Volume in my case, with a flow schedule that is always one.
The EXHAUST-SOURCE is set to "Infiltration", meaning that the make-up air comes from infiltration through the facade (which is the case for apartments without mechanical supply of air, such as apartments with trickle vents).
From eQuest help for the "EXHAUST-SOURCE" keyword:
INFILTRATION specifies that the EXHAUST-FLOW is made up via outdoor air infiltrating directly into the zone. The exhaust fan operates independently of the space conditioning system, and the exhaust requirement is not included in the calculations of outdoor air flows or VAV minimum terminal flows. The EXHAUST-FAN-SCH must be specified.
A common usage for this mode is to model residential exhaust requirements for bathrooms and kitchens.
When the exhaust fan operates, zonal air infiltration is modified by adding the infiltration and exhaust flows in quadrature:
Net infiltration = [ (Infiltration Flow)2 + (Exhaust Flow)2 ]1/2
As seen previously, it seems that the actual net infiltration through the wall is combined in quadrature (square root of the squares) between infiltration flow defined under "SPACE" and the Exhaust Flow.
Yet, I found out that I had low heating consumption and a bunch of unmet hours after adding the zonal exhaust.
To investigate, I played on the exhaust air:
On the following screenshot: On the left, "Exhaust Air" column, I set it as a zonal exhaust fan (like the picture above, except it's 400 CFM). On the right, I've set it as Outside air flow (on the central system).
I've got a good news and a bad news:
Since eQuest doesn't understand that it's supposed to size the system to handle the added infiltration, you need a workaround. One enlightened user on the previously mentioned mailing list suggested a smart one.
It's a little bit tedious if you have a lot of different zones with different zonal exhausts, but hopefully you won't have to repeat those steps more than 5-10 times.
The basic idea is to use a different infiltration, defined at SPACE level, for the design days.
In my case, this is what my space infiltration (Internal Load > Infiltration) is defined as :
As a reminder, an annual schedule refers to one of more weekly schedule. In this case, you'll have the same weekly schedule that applies to the entire year.
A weekly schedule has values for each day of the week, holidays, and Heating Design Day and Cooling Design Day.
What we're going to do is to play on the two Design Day schedules to trick eQuest into taking zonal exhaust in consideration: basically we're going to compute a Net Equivalent ACH and derives the appropriate multiplier.
Consider a Laundry room. The Laundry Room Space has a volume of 3000 ft^3. Under the space I have a value of 0.08 ACH for infiltration. Let's say the exhaust is 150 CFM continuous.
Infiltration:
Actual ACH: 0.08*0.5 = 0.04
==> Equivalent CFM: 3000*0.04/60 = 2 CFM
Exhaust: 150 CFM
Net Infiltration: (150^2 + 2^2)^0.5 = 150.01 (quadrature... see above)
Net ACH: 150.01 / 3000 = 5.16
Design Day Infiltration Multiplier: 5.16/0.08 = 65
So create a day schedule with all hourly values of 65.
Create a week schedule with all days using the Hourly 0.5 schedule, and for Heating and Cooling Design day you pick the 65 schedule. Put that into an annual schedule.
eQuest will now autosize your system correctly!
| | 2 | No.2 Revision |
I've read all relevant eQuest-user threads, and started my own here, and got extremely useful feedback.
It seems that the consensus is indeed to model bathroom and kitchen exhaust as a zonal exhaust, such as the following:
As you can see, I'm setting a flow (CFM) and a corresponding kW/CFM for the fan power. I'm setting the control as Constant Volume in my case, with a flow schedule that is always one.
The EXHAUST-SOURCE is set to "Infiltration", meaning that the make-up air comes from infiltration through the facade (which is the case for apartments without mechanical supply of air, such as apartments with trickle vents).
From eQuest help for the "EXHAUST-SOURCE" keyword:
INFILTRATION specifies that the EXHAUST-FLOW is made up via outdoor air infiltrating directly into the zone. The exhaust fan operates independently of the space conditioning system, and the exhaust requirement is not included in the calculations of outdoor air flows or VAV minimum terminal flows. The EXHAUST-FAN-SCH must be specified.
A common usage for this mode is to model residential exhaust requirements for bathrooms and kitchens.
When the exhaust fan operates, zonal air infiltration is modified by adding the infiltration and exhaust flows in quadrature:
Net infiltration = [ (Infiltration Flow)2 + (Exhaust Flow)2 ]1/2
As seen previously, it seems that the actual net infiltration through the wall is combined in quadrature (square root of the squares) between infiltration flow defined under "SPACE" and the Exhaust Flow.
Yet, I found out that I had low heating consumption and a bunch of unmet hours after adding the zonal exhaust.
To investigate, I played on the exhaust air:
On the following screenshot: On the left, "Exhaust Air" column, I set it as a zonal exhaust fan (like the picture above, except it's 400 CFM). On the right, I've set it as Outside air flow (on the central system).
Full size picture: Click here.
I've got a good news and a bad news:
Since eQuest doesn't understand that it's supposed to size the system to handle the added infiltration, you need a workaround. One enlightened user on the previously mentioned mailing list suggested a smart one.
It's a little bit tedious if you have a lot of different zones with different zonal exhausts, but hopefully you won't have to repeat those steps more than 5-10 times.
The basic idea is to use a different infiltration, defined at SPACE level, for the design days.
In my case, this is what my space infiltration (Internal Load > Infiltration) is defined as :
As a reminder, an annual schedule refers to one of more weekly schedule. In this case, you'll have the same weekly schedule that applies to the entire year.
A weekly schedule has values for each day of the week, holidays, and Heating Design Day and Cooling Design Day.
What we're going to do is to play on the two Design Day schedules to trick eQuest into taking zonal exhaust in consideration: basically we're going to compute a Net Equivalent ACH and derives the appropriate multiplier.
Consider a Laundry room. The Laundry Room Space has a volume of 3000 ft^3. Under the space I have a value of 0.08 ACH for infiltration. Let's say the exhaust is 150 CFM continuous.
Infiltration:
Actual ACH: 0.08*0.5 = 0.04
==> Equivalent CFM: 3000*0.04/60 = 2 CFM
Exhaust: 150 CFM
Net Infiltration: (150^2 + 2^2)^0.5 = 150.01 (quadrature... see above)
Net ACH: 150.01 / 3000 = 5.16
Design Day Infiltration Multiplier: 5.16/0.08 = 65
So create a day schedule with all hourly values of 65.
Create a week schedule with all days using the Hourly 0.5 schedule, and for Heating and Cooling Design day you pick the 65 schedule. Put that into an annual schedule.
eQuest will now autosize your system correctly!
| | 3 | No.3 Revision |
I've read all relevant eQuest-user threads, and started my own here, and got extremely useful feedback.
It seems that the consensus is indeed to model bathroom and kitchen exhaust as a zonal exhaust, such as the following:
As you can see, I'm setting a flow (CFM) and a corresponding kW/CFM for the fan power. I'm setting the control as Constant Volume in my case, with a flow schedule that is always one.
The EXHAUST-SOURCE is set to "Infiltration", meaning that the make-up air comes from infiltration through the facade (which is the case for apartments without mechanical supply of air, such as apartments with trickle vents).
From eQuest help for the "EXHAUST-SOURCE" keyword:
INFILTRATION specifies that the EXHAUST-FLOW is made up via outdoor air infiltrating directly into the zone. The exhaust fan operates independently of the space conditioning system, and the exhaust requirement is not included in the calculations of outdoor air flows or VAV minimum terminal flows. The EXHAUST-FAN-SCH must be specified.
A common usage for this mode is to model residential exhaust requirements for bathrooms and kitchens.
When the exhaust fan operates, zonal air infiltration is modified by adding the infiltration and exhaust flows in quadrature:
Net infiltration = [ (Infiltration Flow)2 + (Exhaust Flow)2 ]1/2
As seen previously, it seems that the actual net infiltration through the wall is combined in quadrature (square root of the squares) between infiltration flow defined under "SPACE" and the Exhaust Flow.
Yet, I found out that I had low heating consumption and a bunch of unmet hours after adding the zonal exhaust.
To investigate, I played on the exhaust air:
On the following screenshot: On the left, "Exhaust Air" column, I set it as a zonal exhaust fan (like the picture above, except it's 400 CFM). On the right, I've set it as Outside air flow (on the central system).
Full size picture: Click here.
I've got a good news and a bad news:
Since eQuest doesn't understand that it's supposed to size the system to handle the added infiltration, you need a workaround. One enlightened user on the previously mentioned mailing list suggested a smart one.
It's a little bit tedious if you have a lot of different zones with different zonal exhausts, but hopefully you won't have to repeat those steps more than 5-10 times.
The basic idea is to use a different infiltration, defined at SPACE level, for the design days.
In my case, this is what my space infiltration (Internal Load > Infiltration) is defined as :
As a reminder, an annual schedule refers to one of more weekly schedule. In this case, you'll have the same weekly schedule that applies to the entire year.
A weekly schedule has values for each day of the week, holidays, and Heating Design Day and Cooling Design Day.
What we're going to do is to play on the two Design Day schedules to trick eQuest into taking zonal exhaust in consideration: basically we're going to compute a Net Equivalent ACH and derives the appropriate multiplier.
Consider a Laundry room. The Laundry Room Space has a volume of 3000 ft^3. Under the space I have a value of 0.08 ACH for infiltration. Let's say the exhaust is 150 CFM continuous.
Infiltration:
Actual ACH: 0.08*0.5 = 0.04
==> Equivalent CFM: 3000*0.04/60 = 2 CFM
Exhaust: 150 CFM
Net Infiltration: (150^2 + 2^2)^0.5 = 150.01 (quadrature... see above)
Net ACH: 150.01 / 3000 = 5.16
Design Day Infiltration Multiplier: 5.16/0.08 = 65
So create a day schedule with all hourly values of 65.
Create a week schedule with all days using the Hourly 0.5 schedule, and for Heating and Cooling Design day you pick the 65 schedule. Put that into an annual schedule.
eQuest will now autosize your system correctly!
