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

[Jason Quinn]

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

On Thu, Mar 29, 2012 at 5:33 AM, Brian Fountain <bfountain at greensim.com> wrote:
> 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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-- 
Jason Quinn
Lead Engineer
Sustainable Engineering Ltd.
82 Liverpool Street
Wanganui 4500
New Zealand

+64 21 1846 911