It is impossible to predict. You make a good review hub with Amazon links in it and hope it works. Putting Amazon links is free so there is no reason not to try. But you have to be patient and do more hubs as you wait. There is no immediate money making here. That's how hubpages works. You do something now. You start to get your sales when you nearly forget about the hub. I think your HVAC system hub has a good chance of selling some books. There are some HVAC system handbooks on Amazon. Good luck.