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Water Heater Thermostat
The water heater thermostat is set by the
installing contractor to provide the required
temperature at the hot water outlet of the water
heater. It is important that a warning label be
place near the water heater thermostat telling
the homeowner not to change the thermostat
setting. The label is included with the furnace.
Room Thermostat
The room thermostat controls both the water
circulation pump and the air circulation fan. It
should be on a centrally located, inside wall
away from any source of heat such as diffusers,
appliances and direct sunlight.
Energy Saving Room Thermostat
A set back thermostat or “smart stat” can be
used with a combo system, but care must be
taken in the timing of the temperature changes.
The timing of morning warm up should be early
enough that the desired air temperature has
been reached before the people begin to use
domestic hot water. The highest demand for
space heating is during the morning warm up
and the highest demand for domestic hot water
is during morning showers. Even if the water
heater is properly sized, it may not be able to
meet this combined load. Therefore, large set
backs should be avoided.
Air Handler Output Capacity
There are four factors that will significantly affect
the heating output of the air handler.
They are:
• Hot water supply temperature (EWT)
• Hot water flow rate (GPM)
• Air Handler return air temperature
• Air Handler air flow rate (CFM)
The hot water inlet temperature is typically 140˚F
(60˚C). If this temperature must be increased
to achieve higher outputs from the furnace
an anti-scald valve must be used to prevent
domestic hot water temperatures above
140˚F (60˚C). The manufacturer of the
Hot Water Tank should be consulted for
temperatures higher than 140˚F.
Hot Water Flow Rate
The hot water entering the water coil is the
source of heat to the air handler. The effect of
changing the amount of water entering the coil is
the same as changing the water temperature.
As water flow is reduced, the output of the
air handler and the air temperature rise will
both be lowered.
Air Handler Return Air Temperature
The return air temperature entering the air
handler is approx. 60˚F (33˚C) below the
hot water inlet temperature. If the return air
temperature entering the air handler is reduced,
more heat transfer will occur and the output of
the air handler will increase.
Air Handler Air Flow Rate
The air entering the air handler can only be
warmed by the temperature difference between
the hot water and the cool air. As the volume
(CFM (L/s)) of air is reduced, the amount of heat
which can be transferred is also reduced.
Air Handler Temperature Rise
In a fuel fired furnace, the combustion gases
can be 1000˚F (538˚C) above the return air
temperature. These units typically have a
temperature rise from 50˚F (10˚C) to 90˚F (32˚C)
and therefore delivers air at the diffuser at 120˚F
(49˚C) to 160˚F (71˚C).
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