Hello,
I'm trying to use a PSZ system as a DOAS. The system I'm trying to model
uses and ERV tied to a heat pump to supply fresh air to common areas and
dwelling units. There are then separate VRF systems for space
conditioning. The DOAS is designed to supply a constant temperature
outside air (73F when heating, 75F when cooling). How can I make the PSZ
system in equest supply a constant temperature supply air?
Thank you
I also have a follow up question. Is there any system type that I can use
which will allow me to select heat pump as the heating source and also be
controlled by the supply air temperature? It appears that constant temp
control would be available for PMZS but I cannot use heat pump heat with
this type of system.
Hi Michael,
I have always used a Powered Induction Unit system type for DOAS systems.
(Or standard Induction Unit system if CHW instead of DX.) This should
give you the control and heat source capability that you need.
Good luck!
Elizabeth
*Elizabeth Gillmor PE, BEMP, LC, LEED AP*
*e n e r g e t i c s **consulting engineers, llc*
energetics-eng.com | 303.619.0091
Thank you Elizabeth!
I?m just getting back from a vacation, so sorry for the delayed response on this, but there is a way to force a PSZ system to have discharge air temp control. I?m ignoring the ERV portion of your system right now because that complicates matters.
We use this methodology frequently to model corridor/DOAS systems for multifamily projects. Note that this is only accounting for the energy use of the ventilation air. I?m assuming you will model separate systems for the dwelling units to meet the space conditioning demands (basically it looks like you are supplying neutral vent air, which won?t have much impact on how much load shows up on the in-unit VRF).
1) Create a new space (call it ?DOAS Dummy space? or something). Make the size minimal- 1?x1?x1? works well. Make sure there are no internal loads or walls/roofs/floors assigned to it.
2) Create a new zone (call it ?DOAS Dummy zone? or something). Assign the dummy space to the zone. Give the zone Heating/Cooling t-stat schedules that are equivalent to the discharge air temp range you want to control to. In your example you?d have a 24/7 heating schedule at 73 deg F and 24/7 cooling schedule at 75 deg.
3) Give this dummy zone control of your PSZ unit. Because there are no loads in the space, the PSZ unit should discharge air between 73-75 Deg to keep that 1 cubic foot space at the desired temp range. You can confirm this by looking at hourly reports.
4) If you have multiple conditioned zones assigned to the unit, they will ?come along for the ride? meaning they will get the same supply air temp, with no consideration for their space temps. You may have to assign a realistic ?relaxed? t-stat schedule to those space to avoid lots of unmet load hours.
5) Depending on how the vent air is being distributed in the dwelling units, you might have to play around with how the fan power is assigned. For example- do the in unit fans have to run 24/7 to distribute the vent air, or do they only cycle to meet the loads in the space?
Having an ERV can significantly complicate this modeling method depending on where the return air is coming from (and the expected temperature of that return air) for the heat-recovery.
Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com
Thanks for the input Nathan. Since I do have an ERV, I am trying to deal
with that more complicated issue. I actually ended up using a Powered
Induction Unit as suggested by Elizabeth and that seems to be working well.