How 'recently'?
If it is more than 30 days since you received your heater, then you are too late to open a return through Ebay.
If it is within 30 days you need to find the email you received from Ebay when you paid then use the 'bought as a guest' link in message 2.
If you have had the heater more than 30 days, you will have to try your payment provider and start a payment dispute on the grounds that you have bought an item which is not fit for purpose.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)