Arrived at customers home due to issues with water pressure on the hot side in the master bathroom and hot and cold water mixing. Performed vinegar flush on tankless water heater inspected cold water inlet filter, screen was clogged cleaned and reinstalled upon completion of flush Removed internal sediment with cleaning Upon completion of flush water pressure restored, water was still mixing, isolated faucet and hot water was working, replaced manifold tee. Tested water is heating properly, cold water sandwich when tub spout is turned on this is residual cooled water in hot water distribution being expelled then heated water is dispersed at fixtures. Remedy would be an external buffer tank (estimate to be sent from office) or tankless water heater with internal buffer tank Water pressure restored on hot side of water system Hot and cold water mixing resolved Strongly recommend full house filter between well and potable water system Tested water 5 grains per gallon see chart, softene