Your current maximum gas demand is only about 215,000BTH/h based on tables from the International Fuel Gas Code.

Your proposed maximum gas demand will rise to about 315 BTU/h based on the same tables.

Your actual maximum gas demand in BTU/h needs to be determined from the manufacturer's rating on the appliances.

1 CFH of natural gas is approximately 1000-1070 BTUs/h.

Using 1000 BTU/h, you have a maximum gas demand of 315 CFH or 295 CFH using 1070 BTU/h.

For sake of simplicity, let's assume your maximum demand is calucated using 1000 BTU/h and that your new maximum demand will be 315 CFH.

The problem with trying to answer your question is that not enough information has been provided to to offer any answer.

We would also need to know what the street inlet pressure is as well as what pressure drop occurs through any regulators along with the type of pipe that connects from the manifold and the lenght of pipe from the manifold to the new appliance(s).

For example, if you have a "low" pressure installation, your 3/4" main pipe is currently adequate since it can provide 247 CFH but would be inadequate to supply the new demand.

If you have a "high" pressure installation, the 3/4" main pipe already can provide 2150 CFH...well above the amount currently and potentially needed.

My guess is that you have a "low" pressure installation and that the current 3/4" line will be insufficient to supply the maximum demand.

That said, while gas distribution systems need to be sized for maximum demand, you are correct in believing that demand will be seasonally adjusted.

You will not likely be using your gas grill during heating season, so you can reduce your winter estimated actual maximum demand from 315 CFH to about 275 CFH.

The problem is, that even using this reduced demand, your current 3/4" pipe can only provide 247 CFH if you have a low pressure system...well short of the 275 CFH required with your newly added appliances.

All that said, it appears that you will need to increase in size your main supply pipe to a minimum 1" diameter to be able to provide the gas flow required to properly run all your appliances.

The size of the new individual gas supply pipes would then need to be determined based on each unit's actual gas demand and developed length of pipe from the manifold.

would one be able to put in a "section" of say 2" instead? I think I understand the thing here is "volume" and short of doing it all over in 1", would an option be to have a reserve close?

if you mean can you feed some 2" with say the original 3/4", no it won't provide the volume. having some reserve close won't help, as it would soon be exhausted and then be no different than having the 3/4".

trying to find answers on mine.. see.. I have a 3/4 coming IN to the meter. then OUT of the meter..it is also 3/4 inch, then about three feet of 1 inch inside the house... then it goes down to 3/4 again then goes down at an appliance take off as normal.
I am going to be adding a signifaicant load to the downstream side. so.. I can come off the 3/4 available, or what. have the gas co. give me a bigger supply coming IN and then also change all my pipe size up? thank you in advance for your input! dan

the gas company side may not hav eto upgraded, as it is higher pressure than your side. but then again it may. best thing i can tell you is to get quotes from several local plumbers and let them know exactly what you have and what you want to add.