If a tankless water heater is installed in an older home with existing gas lines that serve the furnace and stove, how do you determine if the line is sufficient to supply or if it needs upgrading before the install? Potential problems if the gas line is not large enough: reduced equipment life, increased CO and flue gas condensation, delayed ignition, and reduced performance. Are there any safety concerns if all systems (hot water, stove and furnace) run at the same time?

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The answer is it depends - check the manufacturers specs, they will tell you the size of line needed, etc... if it isn't large enough or you ae not receiving enough through said line you won't get the BTU's needed to bring the temperature up to where you may need it an it can also downgrade the other components also.

I would say generally no on safety concerns though it will impact the burn though in most cases most by-products are caused by incomplete burn do to too little air. My biggest concern would be the stove as most people don't use the exhaust hood

This makes sense, thanks Sean!

Whe I personally installed my gas demand unit in 2010, they had to bring a 3/4 inch line to it, instead of the 1.2 inch line.    I had a 1 1./4 service entering the house, was not a serious issue.  Installed the same day the meter reader was there.   Usage went from 3 MCF per month in summer to 0.5 MCF per month.

Gas design should always take in consideration of all appliances running at the same time.

If all lines are proper size and delivery is normal then there should be no issues These items are normally overspeced. 

I would think net free air to gas fired appliances more of an issue as I have seen water heaters and furnaces slammed into tight spaces clearly not following specs countless times

First the problems - increased CO due to poor combustion, flue condensation delayed ignition, etc. Many potential safety issues.

The plumbing and mechanical code books show a method to size gas lines.The method assumes ALL combustion appliance are operating because that is the most critical condition. The method uses the longest length of gas line, pressure available and gas demand to size gas lines from the table. It is pretty easy to use this method compared to the "friction loss" method found in most fluids engineering text books. Since most plan reviewers and inspectors will compare to this method, it is usually easier to give them what they expect.

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