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

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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

JasonQuinn's picture
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Without spending time on your input files, my first guess is to check our
heat exchanger fan static pressure. i had a similar situation on a model i
was working on in the past few months, and once i entered in 0 for my fan
energy (my heat exchanger worked as a DOAS for my system, and therefore the
supply fan was accounting for the HX fan) everything went back to normal.

Hope you find this helpful.

Rob

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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

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Brian,

Thanks for that. I've managed to obtain reasonable values by looking
at the ERV report - I essentially had two issues (1) schedules - the
ERV fan was on 24 hors a day 365 days/yr, and (2) there was no heat to
recover as you pointed out. I used your frost control re-heat
work-around to provide the energy to the ERV.

It makes sense to me that the outgoing air will be about 72-75F as
that is about what I condition to but I don't understand why you think
it should be some fraction of the " the heating on this meter is some
reasonable fraction of my lighting and equipment energy as that is
what it is mimicking." I would think the HRV is taking (depending upon
it's effectiveness) heat out of the outgoing air that was provided
into the model from all sources (boiler, people, equipment etc.). I
would think that I should look at the typical temperature difference
times the Cp air and the flow rate to see if the magnitude is about
right.

Does that sound good or am I missing something else?

Thank you very much,
Jason Quinn

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The issue might be that you haven't specified any controls on your ERV. Its on float with no capacity control.

On another note - one of the issues with the DOAS-Dummy zone approach is when you have a heat recovery systems attached to the DOAS. When you have a system borrowing air from another system, eQUEST cannot model heat exchange between the two. You need to figure out a way to trick the program into accounting for the actual heat exchange (possibly with a phantom internal load in the dummy zone).

Hope this helps.

Vikram Sami, LEED AP BD+C

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