Gas Line sizing and bottleneck issue

I have a major bathroom remodel that I am doing mostly DIY, but when it comes to gas, my wife is adament that I bring in outside help. Anyway, one of the first steps in the remodel was swapping out an OLD tank for a new 200K BTU tankless water heater (thanks in part to the 30% stimulus kickback). So far I'm more than happy with the unit, but I am concerned with one aspect of the gas plumbing.

We did need to upgrade our gas meter from low pressure to 2 lb pressure (3 furnaces, 2 fireplaces. . . ). The plumber put in a little manifold at the first gas drop, and ran a dedicated line to the new water heater. So now there are basically two runs: one to the new tankless, and the other feeding right back into the existing gas plumbing. They used two regulators to drop the pressure back down to .5 lb at each line. This made since to me; it basically leaves the existing gas service as it was minus the old tank water heater.

So. . . here's my question. I was up there looking at it again when I noticed the bottleneck they may have created. We have 1 1/2" service into the house where they put the manifold. They built the manifold with 3/4" and then tied it back into the 1.5" existing service. The way they plumbed it, there is now about 4 feet of 3/4" black pipe from the manifold and regulator that then jumps back up to 1.5" to serve the entire house (except the new tankless on it's own run). Keep in mind that we had to upgrade the service because the house was sized pretty much appropriately for the original plumbing. Next winter, when I have 3 furnaces firing at 275K BTU plus the fireplaces and whatever else, is the the gas going to get pulled through that little section of 3/4" rapidly enough? I don't think the regulator will restrict it, but it sure does look like a bottleneck.

Before they did it, I really envisioned the manifold coming in with 1.5" and going right back out with the same. I'm a bit worried.

Much thanks for your thoughts.

Gas lines should go down in size as they continue to supply fixtures in the home. It does seem odd to me that it would go from 1 1/2" to 3/4" back to 1 1/2". Maybe poor workmanship at the very least. Then again I'm just a plunger.

pipe

The gas line should have increased to the proper size RIGHT at the regulator. In fact, there is a possibility that the regulator is too small for the system, depending on its capacity. Some are sized to feed a full system and others are designed for just a single appliance such as a stove or furnace. It depends on the orifice installed in the regulator.