We had a hot water tank that had a mixing valve built in. The claim was it made a 6 gal. act as a 10. The valve failed within 6 months and all we had was lukewarm. The purpose of the valve is to let in a little cold to mix with the hot and if it gets any calcium in it you get too much cold. Ask me how long it took to find that! The valve is at the top of tank at rear and there is a plastic hose that runs down to the cold in at bottom. I crimped the hose with vice grips to try, problem solved! New valve was expensive and we sold trailer 3 years ago, pretty sure vice grips still there Dave