Washing machine pressure switches directly control the hot and cold water inlet solenoids and so are rated for quite high currents - 15A at 230V AC is typical. So for low-voltage DC applications, the switches can certainly cope with (say) 5A. This means that, for most loads, a relay won't be needed.
A defective water heater element prevents the water from heating in a washing machine. Some hints are provided in this content to check whether you have an out of order heating element in your washer. There are ways to make sure if you have a water heater element in your washing machine, which is not working. If the fuse or circuit breaker blows within a few minutes of turning on your washing machine, there is a chance the fault is with the systems heating element.