I have a 31/2 ton twin pac on a 1700sgft house. Two BRs are not used anymore so I closed the registers to concerve. Recently (last 1 1/2 yrs) the Heater has shut down (Hi Limit sw) twice. My question is, am I causing the limit switch to shut the system down because I closed off two registers (2 ea 6" ducts)?

It is possible and also note, you may be blocking the return air. The system is like a big closed tube, except for leakage, and blockage or pressure buildup will raise the plenum temperature. What temp is the hi limit set at? I'd look at that and also at least crack the doors where the heat is off to allow pressure to equalize.

Thanks ma0641 - I had to reset the HLimit, but this time I opened all vents and doors. I was surprised how much air volume I had closed. I agree with you that there was a significant BACK PRESSURE problem causing the HLimit to protect the system. Thanks again... so far everything is fine!!

Glad to help. You can "tip" me by checking Yes-is this helpful? Brian