with it in the heating mode(inside coil is hot) the water side needs a load because of it being the cooling side now.with no water running thru the condenser the unit will trip on low pressure(if it has one) or you crack the condenser piping in that "that CAN looking thing,or race track piping setup" within the unit.for temporary heating hook up a water hose to the condenser.......IN>>>>>>>>>> and a hose to OUT>>>>>>>>>to drain it.you don't need to completly open the water to it,just enough to get the LIQUID LINE warm

with the unit in the heating mode.the LL is the thinnest copper one that runs from the condenser to the evaporator coil.if you can mix hot/cold off a faucet that would be better.then you can get the water to about 75F-80F going into the condenser....that thing will bang the heat with that water entering the condenser at that temp