Product data

Operation

The Stuck Gas Valve sequence is used to allow the system

to properly react to a failure mode of the HVAC unit when

the gas valve becomes stuck in the open position. When

this condition exists, the unit is unable to shut off the heat.

To minimizethis problem, theinternal limitswitch tripscaus-

ing the indoor fan to turn on and remain on until the gas is

manually shut off and the heat exchanger cools to normal

temperatures.

Without this feature, a VVTt system operating normally

would close each of its zone dampers as space conditions

warmed to set point. This causes the heated air to be re-

circulatedback throughtheunit, further aggravatingthe over-

heating problem. With the Stuck Gas Valve sequence, the

VVT dampers open to their conﬁgured maximum damper

position. This allows space temperatures in the building to

rise to higher than normal temperature levels. However,

this sequence reduces the temperature build-up in the unit

and keeps the heat exchanger operating within tempera-

tures to prevent failure.

During no system mode — If the duct temperature rises to

greater than 175 F, the indoor fan is started. The fan will

remain on if that temperature does not drop below 150 F

after 3 minutes. The bypass damper will be closed, and all

zone dampers will opento theirmaximum position.A Stuck

Gas Valve alarm (SE-13) will be initiated.

During a cooling mode — If the duct temperature does not

drop below 100 F within 10 minutes, the cooling mode will

be dropped for 10 minutes. The ‘‘No System Mode’’ logic

will test for this condition.

During a heating mode — No action is required in this

mode.

Typical VVT system installation

19397

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