Na can't go under the house lol might just go under ground with 150mm twin wall drainage pipe into greenhouse!! And if I got a 40 amp sub panle that should be anuff for a 2 x 600w lumateks n a 2000w heater plus fans water pumps air pumps ect?can u vent under the house. its not too much of an issue have a duct blowing air out of a shed, as i said if you use the excuse its becoming a laundry room/larder you will have reason to be ducting hot air out (tumble dryer)