▁hence ▁the ▁name ▁' direct ▁mel ter '. ▁Since ▁these ▁mel ters ▁apply ▁heat ▁directly , ▁they ▁generally ▁cannot ▁be ▁larger ▁than ▁ 2 0 - 4 0 ▁lit res . ▁ ▁Water ▁jack et ▁Water ▁Jack et ▁Mel ters ▁function ▁more ▁like ▁a ▁large , ▁commercial ▁double ▁bo iler ▁in ▁that ▁they ▁keep ▁subst ances ▁even ly ▁he ated ▁but ▁generally ▁heat ▁up ▁to ▁only ▁, ▁since ▁that ▁is ▁the ▁bo iling ▁point ▁of ▁water . ▁They ▁can ▁be ▁made ▁of ▁al umin ium , ▁but ▁are ▁generally ▁made ▁of ▁st ain less ▁steel . ▁Un like ▁direct ▁heat ▁mel ters , ▁water ▁jack et ▁mel ters ▁can ▁be ▁almost ▁un limited ▁in ▁size , ▁with ▁ 5 8 0 ▁lit res ▁being ▁common ▁and ▁up ▁to ▁the ▁larger ▁~ 3 8 0 0 ▁lit re ▁mel ters ▁and ▁more . ▁ ▁Con ve ction ▁There ▁is ▁another ▁type ▁of ▁mel ter ▁which ▁uses ▁con ve ction ▁he ating ▁similar ▁to ▁an ▁o ven ▁but ▁these ▁are ▁generally ▁more ▁expensive ▁than ▁normal ▁he aters , ▁even ▁though ▁they ▁are ▁very ▁in efficient ▁and ▁take ▁a ▁long ▁time ▁to ▁heat ▁simply ▁because ▁they ▁rely ▁on ▁air ▁to ▁heat . ▁Since ▁w ax ▁is ▁fl amm able , ▁another ▁concern ▁with ▁using ▁con ve ction ▁mel ters ▁is ▁that ▁the ▁flash ▁point ▁of ▁w ax ▁will ▁be ▁triggered ▁and ▁there ▁will ▁be ▁an ▁explos ion . ▁Con ve ction ▁mel ters ▁are ▁dangerous ▁when ▁he ating ▁fl amm able ▁subst ances ▁and ▁should ▁not