After 24 months after an intermittent ignition device has been certified by the commission, no person shall sell or offer for sale in this state any new gas appliances, as defined in Section 25950, without obtaining the proper seal of certification from the commission, unless the commission otherwise permits such action. Beginning 24 months after an intermittent ignition device has been certified by the commission, no city or county, city and county, or state agency shall issue a permit for any building to be equipped with any new gas appliance, as defined in Section 25950, unless such building permit shows that the gas appliance complies with this chapter. However, any new gas appliance which does not comply with this chapter may be installed if the appliance was purchased pursuant to a contract executed prior to June 17, 1978, and if the building permit was approved prior to July 8, 1978.