Gas pipe sizing and pressure need for adding tankless hot water heater

hot-waternatural-gas

I am trying to figure out if we can add a 199,000 BTU tankless water heater to our house/system.

We currently have a dryer (B in figure) and gas stove (C in figure). The water heater will be at A, where there is currently a 140,000 BTU gas boiler, but the boiler will be removed and heating is being switched to electric heat pump mini splits.

Schematic of gas system

I have been trying to use this guide to come up with the required pipe sizes, but not sure how having bigger than required pipes might affect things.

Using this, I calculated the demand for each point:
Table of distance and gas demand

Then, using the table in the linked to guide:

  • The pipe going all the way to C would need to be 1/2"; however, it is 3/4"
  • The pipe going to Y would have a total demand of 51 cfh, so requires 1/2"; however it is also 3/4"
  • The pipe going to X would have a total demand of 232 cfh, so requires 1"; it is 1"
  • The pipe going from X to A has a demand of 181 cfh for 26', so requires 3/4"; it is 3/4"

Key Questions

  • Does this design work? It seems like it does to me, but not sure about splitting the demand for A into two pieces: meter to X and then X to A.
  • Is there any need to change the pressure at the meter? Currently the meter pressure is 6 WC. The hot water heater states that it needs a natural gas pressure of "3.5 in. wc – 10.5 in. wc". If I understand the tables above correctly, there should only be a 0.5 in wc drop, so if it's 6 at the meter, it will be 5.5 at the hot water heater so everything is fine.
  • If there are problems with this design, would removing the dryer help at all?

Best Answer

Based on the 100 ft longest length from your system, the 1" pipe from meter to X is only good for a flow rate of 195 cubic feet per hour (from Table 1216.2(1), but you also fail under the other tables). This is inadequate for your 232 cubic feet per hour demand (assuming that's the correct demand).

It seems like you haven't figured out the "Longest Length Method." 100 ft is the longest length in your system. The flow rates vary as you've tabulated, but for navigating the tables you use 100 ft as the length for everything.

Bigger pipes are a good thing. They imply a smaller flow velocity (flow velocity is flow rate divided by cross sectional area). That translates to a smaller pressure drop.

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