Hello forum,
Our team received a note from our LEED reviewers that our equivalent full load hours (EFLH) in our baseline model were less than 8760, despite the ASHRAE 90.1 requirements that fans be run at constant speed 24 hours per day for multifamily occupancy type. Our baseline system type is PTHP. When we plot hourly fan energy usage for the building, usage varies hourly and loosely follows the profile of the outside air temperature. The EFLH is approximately 7517 hours (88%). When we switch the system to 'packaged single-zone heat pump' from 'packaged terminal heat pump' EFLH becomes 8760. This seems to indicate that the lower EFLH of our baseline model is somehow associated with the system type, however our team can't seem to figure out what characteristic of a PTHP causes fan energy to change hourly despite the 'constant speed' designation. Does anyone have any insight as to why PTHP EFLH would be below 8760 while packaged single-zone heat pump would not be? Thanks for any insight you might have.
Cheers,
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Harshul Singhal, LEED AP BD+C
Project Consultant
Thornton Tomasetti
386 Fore Street, Suite 401
Portland, ME 04101
T +1.207.245.6060 F +1.207.245.6061
D +1.207.245.6074
HSinghal at ThorntonTomasetti.com
www.ThorntonTomasetti.com
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Hello Harshul,
I think it is because the PTAC interior fan is running intermittently where the PSZ is running continuous. You can check this quickly by changing the "indoor fan mode" of the PSZ from "Continuous" to "Intermittent". Even though you have selected the PTAC fan as "Constant Volume" the fan is cycling on and off to meet the space load, so if the hourly space load is 50% of the design load the fan will run for 30 minutes of the hour. The indoor fan mode is not editable for PTACs, but you may try setting it manually in the .inp file, I am not sure if it will work or not but would be worth a shot, just double check your hourly results.
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Byron D. Burns, EIT, BEMP
Energy Engineer, Building Sciences Studio
H&A | Hankins & Anderson | www.ha-inc.com
d: (804) 420-1622 p: (804) 285-4171 f: (804) 217-8520
Hi Harshul,
If you specify some small of outside air at the zone, say
ZONE=OUTSIDE-AIR-FLOW = 0.001 cfm, the system (TYPE = PTAC, right?) fans
should run 24/7 if your SYSTEM:FAN-SCHEDULE is set to 1 for all hours.
As for the modeling the system as TYPE= PSZ, it sounds like you made
sure that SYSTEM:INDOOR-FAN-MODE = Continuous, which is correct if you
want the fan to run continuous regardless of heating/cooling demand.
- David
The PTAC system's default fan control is TWO-SPEED whereas the PSZ default fan control is CONSTANT-VOLUME. You may not have explicitly set the PTAC fan control to CONSTANT-VOLUME.
Christopher Jones, P.Eng.
Tel: 416.644.4226 * Toll Free: 1.888.425.7255 x 527
Thanks a lot, David. IT makes sense totally.
Harshul Singhal, LEED AP BD+C
Project Consultant
Thornton Tomasetti
386 Fore Street, Suite 401
Portland, ME 04101
T +1.207.245.6060 F +1.207.245.6061
D +1.207.245.6074
HSinghal at ThorntonTomasetti.com
www.ThorntonTomasetti.com