Removing the original Weber regulator on these BBQs is pretty straightforward and I can't imagine a BBQ/propane shop doing something wrong, but you never know.
Below is what our Weber Q looks like which has the regulator removed for connection to the low pressure piping under the TT. I used a short "whip". I also used 3/8" dia. hose and not 1/4". Our hose is the flexible rubber jacketed type which is easy to coil up and is longer than 10' (20' IIRC)..
Our Weber heats up pretty quickly and maintains good heat, as long as the lid is kept closed. Is your Weber an older one? If so, maybe it needs the orifices cleaned? Are the furnace, water heater and stove working fine? I find that I usually have to open the valve for a bit to purge out any air in the hose.
