No Heating from PVVT Heat Pump

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I am trying to model a PVVT system, that uses an air-cooled heat pump for
heating and cooling. For some reason, the system refuses to supply any heating.
When I change the condenser type from air-cooled to water-cooled, the system
WILL provide heat. Has anyone experienced problems like this, or has
someone successful modeled a PVVT air-cooled heat pump? I have attached my IDF
and PD2 file for review.

Eddie Corwin's picture
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So I determined why I was not getting any heating, and the resolutions perplexes
me.

I am modeling an Air-to-Air heat pump, so in the air-side system --> Basics tab
I selected "NO" for the field "WL/GS Ht Pump". In doing so, I get no heating
out of the system. When I change the field to "YES" I now get heating.
Attached are two images showing the field i am referring to, and
showing that I have an air-cooled condenser. Does anyone know why this might be
happening. I am worried that eQuest is treating my HP like a ground or
water source HP instead of an air-to-air system. Any thoughts on the matter
would be appreciated. By the way, I am using eQuest 3.64

-Eddie

Eddie Corwin's picture
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Eddie,

The first thing I would do is open a new project (or a copy of your
project) in the wizard. Select DX cooling, DX heating and Packaged VVT
Heat Pump for the system type. This will show you the eQUEST defaults
for this system type, which differ in several ways from your overrides,
giving you clues to improving the model performance. The help item on
the PVVT system type gives good information on how these are controlled.
- Your ground level has floor surfaces that are exterior-type
instead of underground or interior/adiabatic, even for the spaces
located above the basement. Consider changing these to get more
reasonable envelope loads.
- You have some zones with zonal exhaust met by balanced
infiltration. These zones are likely to have high unmet hours using a
single PVVT system per floor and the same thermostat schedules
throughout. Baseboards or reheat should help.
- Your southern zones on each floor are listed first, making
them the control zones. Listing the northern-most zones first might
result in fewer unmet heating hours.

Regards,

William Bishop, PE, BEMP, LEED(r) AP

Bishop, Bill2's picture
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Water-cooled heat pumps, ground loop or not, are considered system type PVVT. Air-cooled heat pumps are considered type PSZ. Try changing your system type to PSZ and your heating source to ?heat pump?.

You can also create a temporary project and set up an air-source heat pump in the wizard, then go inspect the system type/settings in the detailed interface to see how eQUEST interprets your inputs.

Thanks,

DAKOTA KELLEY

Dakota Kelley's picture
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Bill and Kelly, thank you for your input.

Bill:
I have tried using the wizard to create the PVVT heat pump. I even used "system
per zone" so that each zone is controlled independently. Even coming strait out
of the wizard, there are plenty of heating hours not met. I understand that a
stand air-to-air heat pump will be unable to operate in very cold weather, but
that is where the supplemental heat should take over to get rid of any unmet
hours. I have attached this model.

The actual system I am trying to model is the Mitsubishi Hyper Heating Inverter
Y-Series Heat Pump. This system (in heating mode) uses electric resistance
heat to warm the outdoor air before it reaches the outdoor coils. This ensures
that heat transfer will occur between the OA and the outdoor coil, and allows
for operation at temperatures as low as -13*F. At first I thought that eQuest
can not exactly model a system like this, so maybe eQuest is not allowing any
heat transfer between the cold coil and the cold air. But then I tried again
using the RESVVT, and it seems to work well, even in the middle of winter. My
only objection to using the RESVVT is that I would like to have control over the
OA.

Kelly:

You say that I can not use PVVT with an air cooled compressor, but it says that
it is possible in the DOE2 help file, also, the wizard will set it up that way.
Normally I would just use the PSZ or even PTAC to model an air-to-air heat
pump, but I would like to use the variable speed compressor, so I need to use a
VVT system.

Has anyone modeled an air-to-air HP system that operates at such low
temperatures before?

Thanks,

-Eddie

Eddie Corwin's picture
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Eddie,
You can see with your attached model that at least you are getting
heating and the system is operating. To get rid of your unmet hours, you
will have to refine the model. First put the accurate loads and envelope
in (including getting rid of the exterior floor I mentioned previously)
and make sure your schedules (lighting, equipment, occupancy etc.) are
reasonable. Then change some interior walls to air walls as applicable
(interior walls that aren't actually there, such as corridors that open
into a space without a door) to more evenly distribute loads within each
floor. Finally, add system capacities and zonal air flows. Perimeter
baseboard heating may be needed in the design if the windows and walls
are as poor as your previous attached model suggests. You might also
create a custom capacity curve (HEAT-CAP-FT) for the Mitsubishi heat
pump.
Regards,

William Bishop, PE, BEMP, LEED(r) AP

Bishop, Bill2's picture
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I will continue on with you suggestions. I is just hard for me to get past the
idea that when all of the HVAC systems are auto-sized, I still have many unmet
hours. If it were just a few unmet hours, I would be ok with it, but this
system has more than a few unmet hours.

The problem seems to be in the "WL/GS Ht Pump" input. There is no DOE2 help on
the topic, so it is hard to know exactly what is going on, but when it is set to
"NO" The HP runs in heating mode for ~1600 hours (with many unmet hours). When
the option is set to "YES" the heat pump runs for 3218 hours in heating mode
(with very few unmet hours). It seems that this option determines weather or
not the heat pump is able to operate at low outdoor temperatures,
or something along those lines. With the option set to "YES" I am still able to
have an air-cooled condenser, and I still have curves that relate the EIR and
capacity to the OA temperature. If you want to try this out, I have attached
the latest model.

Again, thank you for your suggestions, I always find it very helpful to have a
3rd party's comments

-Eddie

Eddie Corwin's picture
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The best way to "understand what is going on" is
to examine the INP files to find how the system
is defined. That will help you to understand how
the simulation engine works. equest is a front
end to DOE2.2. It is a good idea to become
familiar with the DOE2.2 keywords and how they effect the model.

Chris Jones's picture
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