The short answer
- A gas tankless heater can draw several times the gas of a tank, so the gas line often has to be upsized.
- Tankless units need their own venting, usually a new wall or roof penetration, and most need an electrical outlet.
- Winter is the test: colder water coming in means less hot water out per minute. Size for January, not the yearly average.
- Spartanburg Water's lake-fed supply is soft, which is easy on tankless heat exchangers. Well water varies and should be tested.
How each one works, in one paragraph
A tank heater keeps 40 or 50 gallons hot all the time with a burner of roughly 40,000 BTU. When you run hot water, cold water enters the bottom and the burner catches up over the next hour or so. A tankless heater stores nothing. When a faucet opens, a burner that can run near 200,000 BTU fires and heats water as it passes through. You never run out, but you are limited in how many gallons per minute it can heat at once.
Where tankless wins, and where a tank still makes sense
Tankless is the better fit for a house that runs out of hot water, with back-to-back showers or a big soaking tub; for a house short on space, since a wall-hung unit frees up a closet; and for owners planning to stay a long time. The Department of Energy puts tankless life at around 20 years, against 10 to 15 for a tank.
A tank still makes sense when the gas line or venting would need major work, when the household uses modest amounts of hot water, when you would rather avoid a bigger installation, or when hot water during a power outage matters. Most tankless units need electricity to run. Many standard tanks with a standing pilot keep heating without power.
There are a few tankless habits to know. There is a short wait while the burner fires, very low flows may not trigger the burner at all, and quick on-off use can send a brief slug of cold water between hot. None of these are defects, but some people notice them.
The gas line is usually the first hurdle
The line that fed a 40,000 BTU tank is often a half-inch pipe that cannot carry a 199,000 BTU tankless unit, especially with a furnace and range on the same system. A proper install checks the whole house load against the fuel gas code sizing tables and, if needed, runs a larger line back toward the meter.
Sometimes the meter is the limit. If the total house load with the tankless unit is more than the meter can carry, the gas utility has to decide on an upgrade. That is on their schedule, so it should be checked before the unit is bought.
Venting and condensate
Most gas tanks vent up a metal flue, often sharing a chimney or B-vent with the furnace. Tankless units cannot use that flue. Condensing tankless units vent through plastic pipe out a side wall or the roof and make acidic condensate that needs a drain and usually a neutralizer. Non-condensing units use special stainless vent pipe.
Watch the furnace when the old tank comes out. If the two shared a flue, the furnace may be left alone on a vent now sized for two appliances, and the vent has to be checked and sometimes relined so the furnace still drafts properly.
Upstate water: cold winters and soft water
A tankless heater's output is a trade between flow and temperature rise. The colder the water coming in, the fewer gallons per minute it can bring up to shower temperature. Spartanburg Water draws from Lake Bowen and Lake Blalock, and lake water runs much colder in January than in August. A unit sized on a summer or yearly average can feel undersized on the coldest mornings, so size it for winter and for the number of showers you actually run at once.
The good news is the water itself. Spartanburg Water's supply is soft, around 15 milligrams per liter of hardness, so scale builds up in heat exchangers much more slowly than in hard-water regions. Tankless units still want periodic flushing per the manufacturer, and a tank still collects some sediment. Homes on private wells can have very different water, so test before you choose.
When to call a pro
Talk to a licensed contractor before you buy either kind. The gas line size, the meter, the vent route and the furnace flue decide what will work in your house, and a tankless conversion needs a permit and inspection for the gas and venting work.