Refused to take back faulty applience

I bought a dynamic 2 plate stove in 07.2015, which is extremely slow and it doesn't work sometimes, I took it back on 19.09.2015 but the lady called Emily refused to take it back saying there was nothing wrong with it, I had to wait for about 3 hours while it was being tested with a pot filled with 500ml water to see how long it takes to boil which took about 12 - 13 mins on a preheated stove at the maximum temperature, all they cared about was being penalized and to HELL with my complaint.<br> I also had problems with the dynamic heater, I exchanged it on 12.2014 as it wasn't working, it's not even 12 months and 1 of the bars is not working and they tried to convince me that I can replace the bar while the heater is still under warranty, I suggested that I take a non-electrical appliance but they refused and forced me to take the same model and I was told that should the heater breaks before 12 months I should not return it as the new 1 covers the months of warranty of the previous ones, I mean I experienced problems with the same model of heater why should I trust that this 1 is different? It is like they are trying to get rid of the stock and to HELL with customers complaint