The situation
Illustrative example. Not a real job or customer.
A homeowner had a tankless unit installed in spring on the existing gas line. Through summer it worked well. On the first cold mornings, showers turned lukewarm and the unit sometimes showed an ignition code, but only when the furnace was running.
- The owner logged each event and noticed the link to the furnace.
- They confirmed there was no gas smell and that the unit's gas valve was fully open.
- They called a licensed plumber and shared the log.
The pattern pointed to gas supply, not the heater itself.
How the plumber checked it
- Totaled the load.Listed every gas appliance's input rating, including the tankless unit at full fire.
- Measured the run.Measured the pipe length from the meter to the heater along its actual route.
- Compared with sizing tables.Found the existing pipe too small for the combined load at that length.
- Confirmed under load.Measured pressure at the unit with the furnace and heater running together.
- Wrote up options.Proposed a new dedicated line from the meter and asked whether a meter review with SoCalGas was needed.
Symptoms and what they meant here
| Symptom | Meaning in this example |
|---|---|
| Fine in summer, lukewarm in winter | Furnace adds load only in cold months |
| Ignition codes with the furnace on | Gas pressure dropping when both fire |
| No problem with only the range on | Range load alone was small enough |
What the example teaches
In this invented case, the original install skipped a full load calculation. Because the problem only appeared when several appliances fired together, it went unnoticed for months.
The fix was a gas line sized for the whole house, installed under a permit with a pressure test. The owner also asked for the load calculation in writing to keep with the house records.
Common questions
Is this a real install?
No. It is an invented example written to explain how an undersized line can show up.
Why did it only happen in winter?
In this example the furnace only ran in cold weather, adding load that pushed the line past its capacity.
Does SoCalGas need to be involved?
Only if the meter or service side may need changes. The plumber can advise.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.