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Primary causes of nuisance tripping automatic limits are:

- Insufficient air flow (see label on heater’s door for airflow minimums). Make sure airflow meets minimum and pressure differential is met.
 - Improper installation, i.e. too close to a fan, damper, elbow/turn, transition
  (4ft minimum). Move the heater further downstream from the device causing turbulence. If moving the heater is not possible, air straighteners or pressure plates (perforated sheet steel) can be installed upstream of the heater to reduce the turbulence at the heater.

- Incoming air temperature too high – IAT of 110°F or higher can cause issues. 

There are two possible causes for this problem:

1. The first is that the transformer is weak, is being used for functions in addition to the heater control, or the wire length from the heater to the thermostat control is excessively long causing more than usual voltage drop. To remedy this, remove the additional load from the transformer or replace the transformer with one having more VA available.

2. The second cause is that there is turbulence in the airstream which is causing the airflow switch to “flutter” open and closed rapidly. This is usually caused by the heater being installed too close to a fan. It is necessary to ensure that the air is calmed by the time it reaches the heater either by moving the heater further downstream or by means of pressure plates (perforated metal placed just upstream from the heater face) to alleviate this problem. An additional measure which may be necessary is to add a second pressure sampling hose to the LOW port on the pressure switch and run it to a point upstream of the fan to sample the air on the return side.

There is usually a serious system problem that causes the manual limit to trip. The heater components and entire HVAC system should be checked for problems including fan functions and filter status.

At least annually the wire terminals should be checked for tightness. Additionally, a visual inspection of the heater controls and heating elements should be performed. System filters should be changed/cleaned at regular intervals so that proper airflow is always maintained at the heater.

Pressure drop is determined by how much resistance wire is in the airstream. Consult Greenheck for pressure drop

Once a heater has been built it cannot be converted.

Yes, as long as the control signal used previously is the same as on the new heater. A staged heater without an internal transformer will need to be provided with low voltage signals to bring on the stages. For a staged heater with an internal transformer the only thing needed to turn on the heater will be dry contacts for each of the stages – usually via a thermostat but can be as simple as an on/off switch or control relay. For a heater that has SCR or Electronic Step Control an analog signal such as 0-10VDC or 4-20mA must be provided to control the heater.