A good rule for amount of cash to have in reserves is two months of expenses. In the case of an HVAC business I'd say enough for parts and operations but not systems given how expensive those are. Im assuming you buy your systems on credit, if not then you may need to consider cash reserves for those as well.

As to where to keep it if you keep your books in QB Online they have 5% interest on cash held in "envelopes" in their checking account count. It's completely liquid and you can move it in and out of your operating account via ACH so within a couple of days to get it back if needed. To this point they haven't lowered rates even after the Fed lowered the base rate. If you aren't in QB then make sure to consider liquidity when making a decision. CD are great til you need the cash.