at my company, and maybe this one too, we charge on a flat rate. which means if we replace a gas valve you pay XX if it takes us 15min we wont only charge you X (however we may go ahead and make sure your whole furnace is clean and in working order) and if it takes us 2 hours we wont go and charge you XXX still just XX. we do this because this is how we get payed. my boss has it all worked out to say changing a pressure switch you get payed for "this long" so no matter if you are slow or do it really fast we still get payed the same amount and charge the same amount. and we include diagnostics into part replacements.