Shut off water and power to both navien tankless units in basement. Hooked pump up to isolation valves and cycled descaling solution through heat exchanger for approximately 45 minutes. Removed pumps and ran hot water through isolation valve and into sump pit to purge out any remaining solution and turned both units back on. Ran faucet in basement to check and water is flowing hot. Everything is in working order at this time. We must schedule hvac tech to come perform inspection on air conditioning.