Gas line 1" or 3/4" for Furnace & Water Heater?

Gas lines - I have two running from the meter:
a) 3/4" line feeds the 40Gal Hot Water Tank b) 1" line feeds the Furnace (Forced Hot Air)
Both appliances sit next to each other. Both are located about 20' from the meter. There are now two sections of gas pipe (3/4" and 1") running for about 20', right next to each other. It looks like the Water Heater line was added later. They ran that line from the meter because it was the easiest method (a spare connection after the meter) - they did not want to thread in a "T" into the 1" line, near the boiler to feed the HW Heater.
I have to temporarily remove both lines to get access to joists. I will install one new gas line (to replace the existing two) and will install the extra "T" to feed the HW heater.
My question is: should the one line be 1" or can it be a 3/4" line. I don't have the capacity of the Furnace handy right now, but it is a relatively small one - heats a 24' x 28' cape-style house. The HW Heater is a 40Gal unit.
Anyone know whether I should run a 1" or a 3/4" pipe to feed them both?
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If it is an old installation, it sounds like the water heater used to be a "Flat Rate" heater with the gas line bypassing the meter. You paid a flat rate per month no matter how much hot water you used, however the heater was quite small so that you actually had a limited supply of hot water. When it was removed they just tied the lines together at the metered side the gas meter.
In my house, I have similar distances with a 1" line feeding a furnace, hot water heater and another 3/4" line running about 25 feet to feed a gas range top with 5 burners. The installer said I had enough capacity left to install a gas BBQ when I chose to convert from propane tanks.

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