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.
The above mentioned signs may or may not indicate a faulty heating element. The reasons for the above noted symptoms might be some thing else. The sure way to confirm a faulty heater is using the ohmmeter. It is a device for checking electrical values like resistance between 2 conductive appliances. To test if an element is working or not a professional connects neutral wires to the related connectors on the element and verify it for a positive reading.