House was re-piped with CPVC. Bathroom faucet has dual handles. When running full hot with no cold, if the toilet is flushed, the hot water will be interrupted with about 3 seconds of pure cold water, then revert to full hot. The bathroom is about 80 feet from the hot water heater. If the hot and cold lines are separated, how can this happen? Same thing happens with the shower which uses a continuously variable temperature, single handle faucet. Thanks in advance for any advice.