It will not hurt anything. Just very likely you will be tripping the breaker on occasion. But you can get by fine on a 20 amp outlet if you are very careful and do not use much in the way of high current devices. Meaning no A/C, electric space heater, toaster/oven, hair dryer, etc. that pull a lot of amps. And run the water heater on propane, not electric. You can operate the lights, furnace, and other relatively low current devices. And they could live in it under those restrictions. But I agree with above. It would be much better to get at least a standard 30 amp outlet installed.