Thereare 2 differentbalancepoint temperaturesto
considerwhenprogrammingthethermostat:Economic
andLoad.
Economic Balance Point Temperature
The economic balance point temperature is the outdoor
temperature where the utility cost of running in heat pump
mode is the same as running in gas heat mode. If the
outdoor temperature is above the economic balance point
temperature, then the heat pump mode will be less costly. If
the outdoor temperature is below the economic balance
point temperature, then the gas heat mode will be less
costly. The economic balance point temperature is affected
by electrical utility cost, gas utility cost, and model size.
Knowing the utility cost of electricity and gas, the economic
balance point temperature can be determined using Figure
10.
Figure10- EconomicBalancePointTemperatureChart
EconomicBalancePointTemperature(°F)
Cost PDX324 PDX330 PDX336 PDX342 PDX348 PDX360
Ratio* 040 060 080 080 120 120
0.075 0 0 0 0 0 2
0.100 18 18 17 18 17 21
0.125 38 29 32 37 29 33
0.1375 45 35 39 45 36 38
0.150 52 40 45 56 43 49
* CostRatioisthe electricalcost,in$ perkilowatt-hour,dividedbythegas
cost,in$ per therm.
Example: A PDX342080 is installed in a residence where
the electrical utility cost is 9 cents per kilowatt-hour and the
gas cost is 90 cents per therm. Proceed as follows:
1. $.09/$.90 = .1
2. Using Figure 10, a PDX342080 with a .1 cost ratio =>
Economic Balance Point Temperature = 18°F
Some utilities have a sliding cost based on consumption. In
this case, take the total bill and divide by the total
consumption to determine the average utility cost.
Some natural gas suppliers sell gas by every 1O0cubic feet
(CCF) of gas. For an approximate gas cost per therm,
multiply CCF by 97. Example: A price of $.01 per CCF is
approximately equivalent to $.97 per therm.
Note: The 97 multiplier is based on a typical heating
value of 1030 Btu per cubic foot of natural gas. For a
more accurate cost, contact your gas supplier to obtain
the Btu content of natural gas in your area. Divide
100,000 by the actual Btu content per cubic foot to obtain
the correct multiplier.
If the economic balance point is chosen, keep in mind that
utility rates fluctuate substantially over time. Review
monthly utility bills and re-calculate economic balance
points as necessary.
Load Balance Point Temperature
The load balance point temperature is the outdoor
temperature at which the load may be met using either heat
pump mode or gas heat mode. Ifthe outdoor temperature is
above the load balance point temperature, the demand for
heat may be met using the heat pump mode. If the outdoor
temperature is below the load balance point temperature,
the gas heat mode is required to meet the building load.
To find the load balance point temperature, a load
calculation must be performed on the building. The load
calculation must be performed at 3 different outdoor
temperatures and graphed on Figure 11. Plot the three
load calculations at their appropriate outdoor temperatures
and draw a smooth line through the 3 points. NOTE: The
line connecting the 3 points may not be a straight line.
Locate where the building load line intersects the
appropriate model capacity line. This is the load balance
)oint temperature.
Figure Load Balance Point Temperature
-_-- PDX324040
PDX330060
PDX336080
-x- PDX342080
PDX348120
-o- PDX360120
70000 ............................................................................................................................................................................................................................................................
60000
o.
50000
_. 40000
Ix:
_ 30000
20000 :
I
10000 4
_3
0
PDX360120
PDX348120
PDX342080
PDX336080
PDX330060
_ PDN324040
10 20 30 40 50 60
Outdoor Air Temperature (°F)
No matter what the balance point temperature is set at, the
unit will automatically switch to gas heat if the heat pump is
not able to meet the demand of the house. Calculating the
load balance point temperature and programming it into the
thermostat will minimize temperature fluctuations in the
house.
Final Electrical Check
1. Make a final wiring check to be sure system is correctly
wired. Inspect field installed wiring and the routing to
ensure that rubbing or chafing due to vibration will not
occur.
NOTE: Wiring MUST be installed so it is protected from
possible mechanical damage.
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