Night setback thermostats are not recommended on
radiant floor systems due to the response time of the slab.
B. Baseboard Heating
Another application of hydronic heating is finned tube
baseboard heating. This is the same tubing used with
boilers with one major difference. The discharge
temperature of a boiler is much higher than geothermal
heat pumps. The heat pump system should be sized at
115oF hydronic leaving water temperature to maintain
efflrciency. At a l25oF leaving water temperature, the heat
pump is at a maximum operating temperature and may
start to trip offon high head pressures. Standard 3/4"
finned tube baseboard conductors have an average output
of 230 Btuhfft at 120T hydronic leaving water
temperature. In most cases there is not enough perimeter
area in the conditioned space to allow for the required
length of tubing to handle the entire heating load. There
have been successful installations using baseboard heating
as supplemental heating using the forced air side of the
GeoSource DualTEK unit to satisfy the heating load.
Cast iron radiators have also been used successfully. If
these radiators are rated for an output of70 Btuh/square
inch at a 130oF hydronic leaving water temperature, they
work well with geothermal systems. Although the
radiator may be rated at 130'F, the system should still
operate at the standard 115"F leaving water temperature
of the hydronic side of the heat pump.
C. Other Applications
Additional open loop hydronic applications such as
outdoor swimming pools, hot tubs, whirlpools, tank
heating, etc. are easily sized based on heat exchanger
operating temperature and flow. The worksheet in
Appendix I was taken from the ASHRAE 95
Applications Manual and can be used for outdoor
swimming pool sizing. In many instances, sizing a heat
pump to these applications comes down to recovery time.
The larger the heat pump (within reason to avoid short
cycling) the faster the system recovery time will be.
eNote: Installing a plate heat exchanger (see Figure 3
for an example) between the heat pump and an open
system is required when corrosive fluid is used in the
open loop, especially on swimming pools where pH
imbalance can damage the heat pump. eNote: Expect
the maximum operating temperature of an indirect
coupled application to be l0oF below the maximum
operating temperature of the heat pump.
Other forms of closed loop systems such as indoor
swimming pools, pretreated fresh air systems, snow melt
systems, and valance heating systems are also very
corrmon with GeoSource DualTEK heat pumps. The
sizing of the heat pump to these systems is more precise
and information from the system manufacturer is
required.
V.APPLICATIONS OF
HYDRONIC SIDE HEAT
EXCHANGERS
This section deals with some common practices used
when coupling the hydronic side of an ECONAR
GeoSource DualTEK heat pump to the space
conditioning heat exchanger. There are so many possible
applications for these systems that they cannot all be
covered in this text. Hopefully these ideas can help in
many of your system designs.
cNote: Actual systems must be constructed to all
appropriate codes and according to accepted plumbing
practices.
A. Storage Tanks
Coupling the hydronic side of the heat pump to the space
conditioning heat exchanger through a water storage tao_k
is very common. In fact, the only instance where these
storage tanks are not recommended is when the heat
pump is coupled to a large heat exchanger capable of
absorbing the entire heating capacity of the heat pump
(see Figure 2). In applications rhat use multiple smaller
zones, storage tanks absorb the relatively large amount of
energy supplied by the heat pump, in order to provide
longer run times and less compressor cycling for the heat
pump. Storage tanks also serve to dispense energy in
small amounts so that the conditioned zones have time to
absorb heat without requiring high discharge water
temperatures. Insulated hot water heaters are commonly
used for storage tanks.
cNote: While all hot water tanks are insulated on the
top and sides, many do not have insulation on the bottom.
An insulated pad beneath uninsulated tanks will reduce
energy loss to the floor.
When properly sized, a storage tank eliminates many
problems with multiple zone hydronic systems. These
problems include excessive leaving water temperature if a
single zone cannot dissipate heat quickly enough and
hydronic flow reduction through the heat pump when only
one zone is calling. This may occur because the hydronic
side circulating pump is normally sized to provide the
heat pump's required flow with all zones calling. When
sizing storage tanks to the heat pump, a good rule of
thumb is ten gallons of storage tank per ton of hydronic
capacity.
The tank temperature can be controlled with a simple
aquastat or a setpoint controller. The setpoint controller
senses tank water temperature and outside air temperature
to increase the tank temperatwe as the outside air
temperature goes down. This confrol scheme provides the
highest heating efficiencies by requiring the lowest
possible water temperature to heat the space. Setting the
optimal design temperatures in the controller is difficult,
5
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