Do you know why there's no hot water

When you are using hot water, it fills the pipe and goes out the faucet. When you stop, there is hot water left in the pipe, but it will quickly give that heat away to the area around it (and it really isn't a lot of heat). The more likely scenario is heat coming in from a poorly insulated attic. During the day you build a huge amount of heat in the attic. Without good insulation all that heat will then radiate back down into the house. In the same vein, it could mean that you're not getting good ventilation in your attic. Normally your attic cools by pulling cool air in through vents in the eaves and then hot air exits the roof vent(s)

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