Bosch Geothermal Heat Pumps Applications Manual page 20

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sensible and latent loads established, there is really
no need to add extreme safety factors. Nor should
the installer unnecessarily oversize the equipment by
introducing conservative design criteria to increase the
cooling requirements as this is detrimental to determining
the actual proper loads.
The required capacity is not necessarily the sum of the
peak individual room loads of a residential structure as
some homes may have large levels of glass, or rooms with
large loads that peak at different times than other rooms,
especially with newer larger homes. Upsizing equipment
in the belief that bigger is better is never a proper solution
and always a problem with customers who think getting
a bigger unit for nearly the same money is economically a
smart purchase. Contractors and installers must always
explain the bene ts of using properly sized equipment to
customers, especially with more expensive geothermal
heat pump systems.
As a guide, for geothermal heat pumps in warm climates,
generally good design is to select equipment that is within
15%-20% of the calculated cooling load. For geothermal
heat pumps in colder climates, generally good design
is to select equipment that is closer to 125% of the
calculated cooling load, increasing the systems heating
ability by compressor heat, not auxiliary heat. However, if
the equipment is oversized in colder climates more than
approximately 25% of the total structure cooling load, it
is possible to experience poor humidity control during the
summer months. On the other hand, sizing geothermal
heat pumps in colder climates nearer to the 115% of
calculated cooling loads mark can result in increased
electric supplemental heating requirements during winter
months.
Generally, the safest approach for geothermal heat pump
contractors and installers is to always observing the Bosch
sizing, selection and application guidelines, meaning
the equipment must be able to meet the sensible and
latent cooling requirements at summer design conditions
without being oversized. However, the selected equipment
should also have the capacity to handle the latent load at
full-load operation (peak dry bulb conditions) and at part-
load operation (peak dew point conditions).
When discussing moisture control, the actual application
sensible to total ratio occurring in the conditioned space
is really more important than the design sensible to total
ratio and is largely a function of the coil apparatus dew
point. This application sensible to total ratio is dependent
on how the equipment is actually operated and controlled,
and on the latent loads generated within the home,
including in ltration.
As the heat gain decreases, the home's sensible load
decreases while its latent load typically remains the same,
resulting in a decreasing space sensible to total ratio
requirement (i.e., 0.75 to 0.65, or lower). This means that
the latent removal requirement is a higher percentage of
the total load. Yet as the total load decreases, the latent
Data subject to change
capabilities of most single stage geothermal heat pumps
are generally less able to meet the changing sensible
to total ratio requirements of the structure. Thus, it is
extremely important to have appropriate and accurate
load calculations to ensure that the geothermal heat
pump meets both sensible and latent loads at full and
part-load conditions. This is critical when geothermal
heat pump systems operate with continuous fan while
the compressor cycles which is possible with many
installations. Bosch's two stage compressor systems solve
this issue in most situations.
In general, the following guidelines may be used when
sizing Bosch geothermal water-to-air heat pumps:
f
Heat pump sensible cooling capacity (shown as
"Sensible Capacity-MBtuH" in the equipment data)
should be within 100-125% of the design cooling
sensible load at the maximum loop EWT/EFT (for mild
and warm climates).
f
In most areas of the U.S., Bosch geothermal heat
pump total cooling capacity at design conditions
should not exceed 125% of the total sensible load.
In colder climates where heat loss may be more than
twice the heat gain, this may not always be possible,
and consideration should be given to installing Bosch
two-stage equipment which may utilize a variable
speed ECM blower.
f
Depending upon the locale climate, the Bosch
geothermal heat pump may need some amount
of auxiliary heat to satisfy the heating load of the
particular structure at design conditions. This auxiliary
heat is typically electric heaters sized for at least 90%
of the structure heat loss at winter design conditions.
f
In southern climates, the geothermal heat pump
may provide 100% of the heating, but for most
installations, auxiliary heat will allow the use of a
smaller geothermal heat pump and avoid over sizing
the equipment for cooling.
f
In some applications, fossil fuel (i.e., natural gas, # 2
fuel oil, LP) is utilized in lieu of electric heat as the
auxiliary heat source to satisfy the heating load of
the particular structure at design conditions, as well
as the emergency heating requirement if a system's
refrigeration cycle fails. This scenario is often utilized
with air-source heat pumps, and the configuration for
a self-contained (packaged) air-source heat pump is
referred to as "dual-fuel". If the application involves an
air-source heat pump split-system, the configuration
is referred to as an "add-on heat pump". Geothermal
heat pumps may be utilized for these applications as
well, but are less prevalent. Control of this form of
heating is critical (as it is with air-source heat pumps)
and should prohibit the simultaneous operation of
the fossil fuel furnace and the geothermal heat pump
when operating in the heating mode (compressor
heat).
Applications Manual
Bosch Thermotechnology Corp.