The warm air is not hot after being turned on for a long time, and the water temperature meter won't come up.

Check the thermostat first (preferably 88 degrees). You can also try to touch the upper and lower pipes of the main water tank with your hands, if the pipes above are very hot. The sewer pipe is cold, which means the thermostat is not on. This shows that the thermostat should work well when the water temperature is not very high. After eliminating the problem of bad thermostat, you can use two pieces of cardboard (each piece is about two-thirds of the surface area of the main water tank).