NCM-300 heats for only for 3 mins

[n00brh]

The setup is an EWC NCM300 zone controller on a Rheem furnace with 2 zones (10 years old). A supply air sensor (SAS) is also used. When heat is called, it runs for for about 3 minutes and shuts off even though the zones are still calling. The supply air limit light comes on the board at this time and stays on until the blower turns off about 3 minutes later. The furnace then fires back up and this keeps repeating over & over until the thermostat is satisfied. The supply air temperature reads about 135-138F when the burners shut off (I checked with a thermometer about 5 inches below the SAS). The current setup using a supply air sensor and the settings on the board are as follows:

W2 timer set to 14
Low temp set to 40F
High Temp limit set to 125

The main limit switch spec is Rheem 47-25350-01 Limit Switch, SPST N/C 190' Open 170' Close. I suspect the high temp limit on NCM300 is set too low. I turned off the dipswitch for SAS on NCM300 and tried again. This time the furnace keeps running and the thermometer stabilized at 156F.

My question is what should be set on NCM300 for high temp limit - 170F as indicated on the main limit switch or something lower - is there a good way to determine that? I think 170F is the answer (max possible on the board as well) but want to check before I do so.

Thanks in advance!

[Steven W.]

I would set the high limit between 160-170

[n00brh]

Thank you!