[Equest-users] Heat Exchanger added to simple SD model with DOAS added - 3000% increase in energy for fans

[Brian Fountain]

Jason,

Couple of things (not having looked at the file yet)...

Firstly, on the 2nd tab of the heat recovery are static pressure values 
for added static (supply and exhaust) across your heat recovery heat 
exchanger -- these are added to the total fan static pressure.  Further, 
they are NOT reported in the SV-A report, but are on the ERV report and 
are included under ventilation fan energy in the BEPS etc.

Secondly, OA-FROM-SYSTEM does not do what the name suggests.  There is 
no actual air transfer.  What DOE2 really does right now is set the OA 
temperature for the borrowing system equal to the return air temperature 
of the lending system.  That much is fine.  The issue comes in when you 
do heat recovery -- it is recovering heat based on the dummy zone which 
has none of your internal heat gains.  So, the return air temperature 
you are recovering from is not correct.  The exhaust air from the 
borrowing system does NOT go back to the lending system (in fact there 
is no air transfer as already mentioned).  So, to model heat recovery 
correctly, you need to watch the dummy system exhaust air stream 
properties.  Being from Canada, I am much more interested in heat 
recovery than cooling.  As such, I am pretty happy to ignore the cooling 
impact but that may not be the case for you.  To model heating heat 
recovery correctly you have 2 options.  One is to put a dummy internal 
heat gain in the dummy zone and tweak it until the exhaust air 
properties meet your requirements.  I am lazier than that.  What I do is 
use the frost control of the heat recovery unit tab, using the EA 
preheat to fix the exhaust air stream at a set value.  It should likely 
be 75F but I tend to use 72F to be conservative on the heat recovery.  
So, I am preheating the exhaust air to 72F.  I use an electric coil to 
do this (again, still in the 2nd tab of the heat recovery coil) -- and I 
put this electric heat on a separate meter with no cost.  I also verify 
that the heating on this meter is some reasonable fraction of my 
lighting and equipment energy as that is what it is mimicking.

I cannot wait until eQUEST does DOAS for real but for now it is 
important to understand what it is doing and workaround accordingly.

Regards,

Brian

On 3/28/2012 6:31 AM, Jason Quinn wrote:
> So this is pretty odd I'm running a simple SD wizard generated office
> building and adding the DOAS to it in DD edit mode. The DOAS works as
> expected (Thanks to Brian Fountain) slightly lowering the space
> heating&  increasing vent. Fans. over a case without the DOAS. Now
> when I add a heat exchanger to the DOAS (PVVT) it causes the fan usage
> to go nuts increasing Vent. Fans from 0.2E6 kWh to 6.5E6 kWh while the
> space heating drops from a resonable (I think) 3.7E9BTU to 0.4E9BTU.
>
> If someone could point me in the right direction to sort out why the
> heat exchanger is causing such a huge increase I'd really appreciate
> it.
>
> I have played with the DOAS fan schedules and can effect the increase
> downwards but not enough to make this reasonable. I've attached the
> DOAS with Heat exchanger .pd2&  .inp file. To see the results without
> the heat exchanger just turn it off. I'll attach my weather file as
> well so that it's easier to look at my file.
>
> Thank you for the help.
> Jason
>
>
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