L
lame plumber
Another possibility is to give them the invoice on the second to last day of the job, reminding them that full payment will be due tomorrow. It might also be worth investing in those service interval programmers, and set it for +7 days before it shuts the heating off. When paid, plug an ordinary unit into the backplate.
Ray's new top seller for 2016, brilliant lateral thinking. Me I get all materials plus 20% paid before they are delivered or when delivered, then you are just looking at labour. If they quibble you can always walk away, I have twice now and been proved right on one of them as the lad who followed me in, never got paid!!