Disappointing after sales service

I have had a 16l Constant Temp geyser for almost 3 years now and am very happy with the performance. Based on the experience with the 16l, I recently bought a 12l Constant Temp geyser from the Durban factory. This geyser responded very quickly, reached the set temp and stabilised quicker than the 16l. However, only for 3 minutes before it shut off. I tried 2 x new regulators as well as new batteries to no avail. After just 3 days I returned it to the factory - and now the struggle. I was even told that I would have to pay for the repair to an ‘un-tampered with’ unit. I demanded a refund instead of a replacement, for I really do not wish to deal with people whose warranty does not seem to be worth much. And now, Dewhot have gone silent..