"Indirect heat exchanger" means combustion equipment in which the flame and/or products of combustion are separated from any contact with the principal material in the process by metallic or refractory walls which includes, but is not limited to, the following:
N.H. Admin. Code § Env-A 3203.31
#6811, EXEMPT, eff 7-17-98; ss by #7077, eff 8-25-99; ss by #8968, INTERIM, eff 8-25-07, EXPIRES: 2-21-08; ss by #9022, eff 11-2-07