Modeling Multstack Units in eQUEST

4 posts / 0 new
Last post


What solution did you end up using to model the Multistack heat pump chiller
coupled to a vertical bore field in eQuest?

Any pointers would be greatly appreciated.


Steven Savich, LEED AP

Has anyone had any luck modeling a Multistack
( unit in eQUEST?

We have a project that uses a Multistack unit to provide hot and chilled
water to a project and is coupled to a vertical bore field. eQUEST does
not have the capability to model system directly, so I have been trying
to look into other methods, all which are falling short. So far I have

* Using the "Loop-to-Loop Heat Pump" chiller; however, this
system does not work like the Multistack, as the Multistack does not
have internal heat exchange, has multiple compressor stages which are
all piped in parallel with the condenser water, and can only be
connected to a Lake/Well loop.

* Using a separate chilled water loop and hot water loop, each
of which are connected to a WLHP loop with a vertical bore HX. This
would be perfect, as I can put as many chillers and boilers on the loops
as there are stages in the Multistack; however, eQUEST does not have a
heat-pump boiler, so the boiler cannot be connected to the vertical bore

* Using a dummy space/zone/system connected to a dummy heat
recovery chiller. The chiller would then reject heat to the hot water
loop that supplies the rest of the building. This would work, however
the chiller demand is driven by the cooling load of the dummy system and
it is impossible to create a dummy cooling load that would exactly match
the heating load required by the hot water loop.

* And finally, using a domestic hot water loop because the
boiler type can be set to "heat pump", however, the heat cannot be
rejected to a condenser water loop.

So I guess the question I am asking is has anyone either successfully
modeled the Multistack unit OR modeled a heat pump boiler?

Thanks in advance.

Dana Troy

Steven Savich's picture
Joined: 2011-09-30
Reputation: 0


We successfully developed a methodology for simulating ground-coupled
Multistack heat pumps in eQUEST. I have included below portions of our
SEED Report (Oregon's stated required program for publicly funded
buildings). A lot of the text is specific to the project we were working
on, but the general idea is there.

The project ended up pursuing a standard chilled/heating water
arrangement for the final design, so the Multistack heat pump was not

I hope this information helps.


Heating and cooling will be provided by hot and chilled water,
respectively, from a ground-coupled heat pump Multistack unit and will
be connected to a condenser water vertical bore field. A Multistack unit
is essentially a unit that contains a number of heat pump modules linked
together in parallel. Depending on the load, each module can provide
either heating water or chilled water. The condenser water loop is then
connected to a vertical bore field onsite, where 200 ft deep bores
located 20 feet on center is anticipated. The Multistack on this project
utilizes seven modules.

As eQUEST cannot directly model a Multistack unit, an alternate approach
had to be taken in order to approximate the operation of the equipment.
In summary, the Multistack was modeled in eQUEST by simulating each
module of the Multistack as an individual chiller for heating and
cooling. A general diagram on how the system is modeled in eQUEST can be
seen in Figure 8 on the following page while more detailed explanations
can be seen on the following pages.

Figure: How the Multistack was simulated in eQUEST

Chilled water

The chillers have a total flow of approximately 640 gpm and provide
45-degree chilled water with a 57-degree return water temperature.
Connected to the chilled water loop are two pumps, in parallel that
provide the 640 gpm flow.

Each chiller module is modeled as an electric hermetic centrifugal
chiller with a capacity of 70 tons. Each stage is scheduled remain off
until the stage before has been loaded to 100% and has an EER of 23 (EIR
of 0.1483).

Default capacity curves were used for the preliminary analysis, however,
customizes curves will be used once the information becomes available. A
screenshot of the eQUEST layout in the Water-Side HVAC tab can be seen
below in Figure 9.

Heating water

Since there is no heat pump boiler, an alternative method had to be
developed to simulate the production of hot water. The heat pump chiller
(LOOP-TO-LOOP-HP) in eQUEST is a heat pump that provides heating and
chilled water simultaneously, with the larger load rejecting heat to the
smaller load. This system could not be used for both the heating and
cooling of the Multistack unit because it internal heat recovery when
the dominant load rejects or adds heat to the small load. The Multistack
has multiple smaller modules that operate in either cooling or heating
only, and therefore do not provide integral heat recovery.

Since the LOOP-TO-LOOP-HP needs to be connected to a chilled water loop
to work properly in eQUEST, the chillers had to be connected to a
"dummy" chilled water loop. In order to avoid a "loop has zero flow"
error, the loop had a minimal process chilled water flow that reset down
to zero after the first hour of the year. The chillers were then
connected to the hot water loop that serves the building. Since the hot
water demand will always be higher than the dummy process chilled water
flow the chillers will be driven by the hot water loop.

The chillers have a flow of 715 gpm and provide 110-degree hot water
with a 90-degree return water temperature. Connected to the hot water
loop are two pumps, in parallel, that provide the 715 gpm flow.

In eQUEST, it is required that secondary hot water pumps and chilled
water pumps be connected to each heat pump chiller. Since the design is
a variable primary flow and the chilled water is a dummy load, the
secondary hot water pump power and the dummy chilled water pump power
was zeroed out.

eQUEST also does not allow the heat pump chiller to be connected to any
condenser water loop except the type "LAKE/WELL". In order to keep the
ground coupled performance level, both the chilled water and heating
water chillers connected to lake/well loops. Details of the condenser
water loop can be seen in the next section. A screenshot of the eQUEST
layout in the Water-Side HVAC tab can be seen below in Figure 9.

Also included in the design is a hot water condensing boiler to provide
additional heating capacity during extreme peaking cooling. Since eQUEST
uses TMY2 weather data that generally uses hourly averages, extreme cold
days are never modeled, and therefore the boiler never turns on.

Condenser water

Due to modeling limitations of the heat pump chiller, each condenser
water loop was modeled as a LAKE/WELL loop. Each heating and cooling
module was given its own loop to track and verify performance in the
hourly reports. Each loop was scheduled to supply 55-degree condenser
water back to the chillers, as scheduled in drawings.

The design consists of two condenser water loops connected in parallel.
In the model, the two pumps were divided up amongst the chillers, with
each pump set to single speed. Since each pump would only turn on when
the chiller it was attached to was operating, it would loosely represent
a variable speed drive pump with multiple stages.

Dana Troy

Dana Troy's picture
Joined: 2011-09-30
Reputation: 0

Very busy week here, but I have to take a minute to say: Dana, that is
a really nice write up! Thanks so much for sharing with the community



Nick-Caton's picture
Joined: 2011-09-30
Reputation: 605

Steven, Dana,

You could also consider using the Heat Pump chiller type (instead of
Loop-to-Loop), attached to a two-pipe loop. From the description, it
sounds like each stage/compressor of the Multistack unit supplies either
HW or CHW to a two-pipe loop. You still have to use the Lake/Well loop
type to represent the vertical bore field, but you avoid creating
separate HW and CHW loops and duplicating the pumps, and you deal more
realistically with the limitations of a two-pipe loop.


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

Bishop, Bill2's picture
Joined: 2011-09-30
Reputation: 0