Very irate gas heater client

I bought a Alva Gas Heater in March , I then started using the Heater 01.06.2024. Heater was switching itself off just after 10/20 minutes when it's on. For me that is a concern and very dangerous to sit with something that is not working. Took it back the 5.06.2024 Called Desiree at Woodmead Branch today only to be told the Heater is still in the store ,their technicians haven't came to fetch the Heater. I am now requesting my money back they telling about their terms. Very disappointed in how they are handling this situation, it's winter now and it's very cold. She then transfered me to a Manager,he didn't pick up my call.