Why won't it turn on?

I own a hz 7021w heater, I purchased it online the first one I received I plugged it in, it worked for a second and a spark flew inside, and it instantly shut off , never to work again, I sent it back to receive a replacement, and the other was working for about 4 weeks and now it won't even turn on, the only way to get it to turn on is to unplug and wait a while and turn it back on. And that only works once the next time you want to use it again you must unplug, wait awhile and turn on AGAIN. so disturbing, when your trying to keep a babys room warm

please help