My stove has been plumbed up to a thermal store without a vent. The plumber originally had one and the system didn't work by gravity so when he put the pump on to circulate the water it must have pumped out the vent and he just cut it off and blocked it. I got another plumber to look at it who said it's not safe don't light the stove. The owner of the thermal stores said it's ok because the thermal store has a expansion vessel and it doesn't matter unless the electric goes out.