My 3.5kW storage heater has a standard thermal fuse rated at 10A which keeps blowing after a few uses. Should I put 2 in parallel ?

Need more information. 120 or 240 volts? Do you have 1 thermal fuse on each heater and one is blowing? 3.5 kw @ 120 volts = 29amps, and 3.5 kw @ 240 volts = 15 amp. Either case a 10 amp fuse is overloaded. There is a reason the fuse is blowing, overheating or overloading,  and that needs to be isolated. Adding a extra fuse will create a safety hazzard and possible personal injury.

Thanks for that Stan. As we live in U.K. the mains supply is rated at 240V so as you say 3kW (not 3.5 as I mistakenly said!) would draw 12 Amperes. The heater is supplied via a switched spur fitted with a standard 13A fuse which has not blown at all. Although the thermal fuse is rated at 10A it-has a max current 'blow' rating of 20A so excessive current does not seem to be the problem. The heaters are not on any thermostat control and have quite a crude rotary input. Temperature does seem to be the problem and 2 thermal fuses in parallel does seem excessive so will try just the one and turn input down to a mid-point setting. Thanks again and will keep you posted! Chris Winters. Coventry England.

I failed to ask if this unit has a blower. If so, checked and see if it is stopped up or not running at the correct speed to move enough air.

No blower fitted - fan to you Stan. Also have put 2 new thermal fuses in parallel and heater seems to be working ok at present on mid input setting. Also checked temps with a probe which gives 60 degrees in area of fuses and 110 degrees over heat output flap/vents inside heater after being on for 7 hours at night. Will wait and see - hey ho. Thanks again - Chris.