No, it should not be possible (if the process is defined to be of type CHP). Note that if you define CEH=1, the activity and capacity represent the total energy output (power+heat) in full CHP mode, as described in the documentation.
Can you maybe provide the model input files (*.DD and *.RUN) for investigating why you have the issue, or is there anything else I could do to help with the issue?
[EDIT:] I just tested it for a test process "MYCHP" of type CHP, using the parameters you have given:
NCAP_CHPR(REG,2010,MYCHP,FX)= 1.47;
NCAP_CEH(REG,2010,MYCHP) = 1;
In the results, the outputs were electricity and heat exactly in the fixed proportions defined, as expected. So, I cannot confirm your findings.
Can you maybe provide the model input files (*.DD and *.RUN) for investigating why you have the issue, or is there anything else I could do to help with the issue?
[EDIT:] I just tested it for a test process "MYCHP" of type CHP, using the parameters you have given:
NCAP_CHPR(REG,2010,MYCHP,FX)= 1.47;
NCAP_CEH(REG,2010,MYCHP) = 1;
In the results, the outputs were electricity and heat exactly in the fixed proportions defined, as expected. So, I cannot confirm your findings.