Very interesting!
Setting up stub-outs should be a no-brainer for the installer since they are working with PVC. They can be capped out with a glued-on cap that gets sawed off, or a screw end with either a screw in plug or a screw on cap, depending on whether it's a F or M fitting. PVC is SO easy to work it doesn't matter which.
They shouldn't charge you much for setting up for a future heater--just for the extra fittings and maybe 15 minutes labor--if they do it as they run the lines.
If you are having electrical setup done, have the electrician run the lines for the future heater as well. Even if all he does is pull the wire and leave it near the box in the house, it will help. I had my electrician run 220v 40amp cable. He ran it from the box to just inside the wall near where a heater would go. If I ever put in a heater, it will be a heatpump and the wiring is ready to go.
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