Tank vs. Tankless Water Heater in Maryland: The January Test, Hard Water and the Gas Line

The tank-versus-tankless argument is usually settled with national numbers: tankless saves energy, tanks are cheaper. Both are true, and neither decides the job in Montgomery County. What decides it here is how cold WSSC water gets in January, how hard the Potomac side of the county’s water is, and whether the gas line in your basement was sized for a 40,000 BTU tank.

This guide works through those three in order, then puts the running costs side by side. If you are already leaning electric, read our companion pieces on heat pump water heaters versus gas and gas versus electric tankless — the answers diverge more than most people expect.

Tank vs. tankless at a glance

The rest of this guide is the reasoning behind each row.

What each one actually does

A storage tank keeps 40 to 50 gallons hot around the clock. A gas tank’s burner typically fires at about 40,000 BTU per hour, and the tank’s capacity covers the gap between how fast you draw hot water and how fast the burner can replace it. The number that matters is the first-hour rating on the rating plate and the EnergyGuide label — how many gallons it can deliver in an hour starting full — not the tank size.

A tankless unit stores nothing. A flow sensor sees a tap open, and a modulating burner fires — up to about 199,000 BTU per hour on a whole-house gas unit — to heat water as it passes through. It never runs out, but it has a ceiling: a maximum flow rate at a given temperature rise. Exceed it and the water arrives lukewarm. It also has a floor: below its activation flow, often around half a gallon a minute, the burner does not light at all.

That ceiling is where Maryland comes in.

The January test: WSSC water is river water

The U.S. Department of Energy’s guidance on sizing a new water heater is to size a tankless unit by two numbers: the flow you need at once, and the temperature rise from incoming water to the temperature you want. DOE notes a typical gas unit manages a 70°F rise at about 5 gallons per minute.

The catch is that incoming water temperature is not fixed. WSSC Water draws from the Potomac and Patuxent rivers, not deep groundwater, so the water in your service line follows the seasons. We size tankless installations in the county for an inlet near 45°F in January, against roughly 70°F in late summer.

Now add up a real January morning: two showers at 2.0 GPM each (a current WaterSense head; older heads run 2.5) plus a kitchen faucet at 1.5. That is 5.5 GPM against a 5.0 GPM ceiling. The unit does not fail; the last fixture to open just runs cooler. Homeowners describe it as “the tankless can’t keep up in winter,” and the physics agree with them.

Maryland note: a private well holds roughly the mid-50s year round, so a tankless unit on well water in Olney or Glen Hills actually has more winter headroom than one on WSSC water. Well water costs you somewhere else — see the hardness section below.

None of this rules tankless out. It means sizing for January, not for the spec sheet, and being honest about whether your household’s peak is two simultaneous showers or three. Where the peak really is three, the answer is two units cascaded on one manifold, or a tank.

The gas line usually decides the budget

A 40,000 BTU tank was almost always fed by a 1/2-inch branch. Under standard low-pressure residential sizing tables, a 1/2-inch line cannot carry 199,000 BTU even at ten feet, so a whole-house tankless conversion nearly always means a new, larger line back toward the meter — sized against the furnace, range and dryer already on it. In some houses the meter itself becomes the question, and that is Washington Gas’s call, not ours.

Venting changes too. A condensing tankless unit vents through PVC out a sidewall and needs a condensate drain. That can be an advantage: if your furnace went high-efficiency years ago and left the old tank alone in an oversized chimney, a tankless conversion takes the water heater out of the chimney entirely. That orphaned-chimney problem, and the other ways to fix it, are covered in our guide to power vent vs. atmospheric water heaters; it is the same check we make on every furnace installation.

Where there is no gas main at all — much of Glen Hills and parts of Travilah in Potomac — gas tankless means propane, with a tank and regulator sized for full fire. In those houses the comparison usually shifts to a heat pump water heater instead.

Hard water: Potomac-side scale and Olney wells

Scale forms fastest where water is heated quickly in narrow passages, which describes a tankless heat exchanger exactly. WSSC’s tap water analysis reports put Potomac plant water in the hard range — yearly averages from about 120 to over 150 mg/L as calcium carbonate, roughly 7 to 9 grains per gallon — while Patuxent plant water runs at about half that. Most of Montgomery County is served primarily from the Potomac plant.

In practice:

• WSSC addresses: descale a tankless unit once a year. We install isolation valves on every unit so that service takes half an hour rather than a plumbing visit.
• Untreated well water in Olney, Glen Hills and outer Potomac: plan on twice a year, and test the water before buying. Iron-bearing well water needs different descaling chemistry than calcium scale.
• Tanks are not immune. Sediment settles on the bottom of a gas tank and insulates the burner from the water; on an electric tank it buries the lower element. An annual flush is the tank equivalent of descaling.

A tankless unit that has gone three years without descaling on Potomac water often shows up as a tankless repair call for “lost output” that a descale largely recovers.

Power outages and the cold-water sandwich

Two behaviours surprise new tankless owners more than anything on the spec sheet.

Outages. Every modern gas tankless unit needs 120-volt power for its controls, igniter and combustion fan. When a summer storm takes out Pepco or BGE service, a tankless house has no hot water until the power returns. A gas tank with a standing pilot and a millivolt gas valve has no electrical connection at all and keeps working. If your street loses power a few times a year, that is a real point for the tank — or for a generator circuit that includes the water heater.

The cold-water sandwich. Turn a shower off and back on within a minute or two and you can get hot water, then a few seconds of cool, then hot again. That is the slug of water that sat in the unit and the pipe while the burner relit. Units with a small internal buffer tank or a recirculation pump largely eliminate it. On a long run to a far bathroom, recirculation also cuts the wait for hot water — a recirculation loop on a tank does the same, at the cost of standby loss.

Running cost and payback, side by side

DOE’s tankless water heater guidance puts the efficiency gain over a storage tank at 24% to 34% for homes using 41 gallons a day or less, falling to 8% to 14% for homes using around 86 gallons a day. The gap closes with heavy use because a tank’s standby loss — the heat it leaks while waiting — matters less when the water is turning over constantly.

To put dollars on it, we use the medium-use pattern from the federal efficiency test (55 gallons a day, heated 67°F) and round illustration rates of $1.90 per therm and 20¢ per kWh. Substitute the all-in rates from your own Washington Gas and Pepco or BGE bills.

A gas tankless unit saves a typical household about $130 a year against a standard gas tank at these rates. In our market a tankless conversion, with the gas line and venting work it usually involves, commonly costs two to three times a like-for-like tank replacement. On energy alone that is a long payback.

The longevity side narrows it. DOE notes most tankless water heaters have a life expectancy of more than 20 years; a tank on Montgomery County water typically gives eight to twelve. Over twenty years you buy the tank twice. If you will be in the house that long, the math is closer than the first-year comparison suggests. If your current tank is failing and you are weighing a repair against either option, our repair or replace guide covers where the line usually falls.

Permits and who can do the work

In Montgomery County outside Rockville and Gaithersburg, replacing or converting a gas water heater is permitted and inspected by WSSC Water under its Plumbing and Fuel Gas Code — not by the county’s Department of Permitting Services. The work must be done by a WSSC-licensed master plumber or gasfitter; homeowners are not permitted to install gas appliances. Tank or tankless, the inspection looks at the same things: gas piping, venting, relief valve discharge and expansion control.

Where each one wins

Choose tankless when:

• the household regularly runs long back-to-back showers or fills a large soaking tub;
• space matters — the mechanical closet in a 1990s Germantown townhouse gains real floor area when the tank comes out;
• the gas line upgrade is straightforward, or already done;
• you will descale it every year and plan to stay well beyond ten years.

Choose a tank when:

• the existing tank is gas, vented properly, and you want the lowest installed cost;
• your peak is one shower at a time and nobody complains about running out;
• you want hot water through power outages;
• you are likely to sell within a few years.

Look hard at a heat pump water heater when you have no gas service, or an unfinished basement with room around the tank. It has the lowest running cost in the table above, and EmPOWER Maryland rebates through Pepco and BGE target it specifically. Our heat pump versus gas comparison covers where it works in a Maryland basement and where it does not.

How we quote it

On a water heater installation or a tankless installation we price the options side by side on the same visit: the first-hour rating you actually need, the January flow requirement, the gas line and venting scope, and the WSSC permit and inspection, which either option requires. If tankless only makes sense on paper, we say so.

Tell us what the house has now — tank size, fuel, age, and how many showers run at once on a weekday morning — and we can usually tell you which way it leans before we visit.