A tankless water heater heats water only when you open a tap, so it never runs out the way a tank does and takes up much less wall space. The trade-offs are a higher installed price, a gas line and vent that often need upgrading, and yearly flushing to keep scale from clogging the heat exchanger. For a household that runs out of hot water or wants floor space back, it can be a good fit.
For a small household with a healthy tank, it often is not. Here is the balanced version, with the parts that sales brochures tend to skip.
Pros and cons at a glance
Neither type wins in every house. This table weighs the points that matter most to homeowners.
| Factor | Tankless | Storage tank |
|---|---|---|
| Hot water supply | Continuous, but limited by flow rate when many fixtures run at once | Fixed amount, then a wait for recovery |
| Space | Small wall unit frees floor area | Takes a footprint of a few square feet |
| Installed price | Usually higher because of gas line, venting and sometimes electrical work | Usually lower, especially when replacing like for like |
| Energy use | No standby loss from keeping a tank hot | Loses some heat through the tank wall all day |
| Maintenance | Yearly flush to remove scale, filter cleaning | Occasional drain to remove sediment |
| Lifespan | Often longer when maintained | Shorter, and a leak means replacing the whole tank |
| Power outage | Needs electricity even on gas models, so no hot water without power | Many gas tanks keep working without power |
The gas line and vent: where the real work is
A tankless heater burns a lot of gas in a short burst, often several times what a tank burner uses. The old gas pipe may be too small to feed it without starving other appliances. The plumber sizes the pipe by the unit's input rating and the length of the run, so a bigger line is a common line item.
Venting changes too. Many tankless units are condensing and need plastic vent pipe that runs out a side wall, with a drain for the acid condensate. An old chimney cannot be reused without a liner. If you smell gas at any time, leave the building and call 911 or your gas utility from outside before calling a plumber.
The cold water sandwich and other quirks
Run the hot tap briefly, shut it off, then open it again a few seconds later and you may get a burst of cold. That is the cold water sandwich. The slug of cooled water left in the pipe passes through after the unit has shut down and has to restart.
Longer pipe runs make it worse. Fixes include a small recirculating pump, a buffer tank or relocating the unit closer to the bathroom. Very low flow, such as a trickle at a sink, can also fail to trigger the burner, so test the fixtures you use most before you decide.
How to decide if tankless is right for your house
Work through these steps before you call for quotes. They keep the conversation about your house and not a sales pitch.
Count your peak demand
List how many showers, dishwashers and washing machines run at once on your busiest morning. Tankless units have a maximum flow rate, and the number must cover that peak.Check your gas and vent path
Look at the gas pipe size at the old heater and note where a vent could exit the wall. A plumber can tell you in a visit whether the line can feed a larger burner.Think about your water
Ask the plumber about scale protection and a flush valve set. Plan yearly flushing as a normal chore.Compare total installed cost
Ask for a clear price before work starts that includes gas line, vent, condensate drain, permit and haul-away of the old tank.Weigh the payback honestly
Savings come from lower standby loss and a longer life. They may not cover the higher installation cost within the years you plan to stay.
Scale, flushing and what neglect costs
Minerals in water bake onto the heat exchanger over time, narrowing the passages and making the unit work harder. Errors, rumbling and cooler water are the common signs. A yearly flush with a pump and a descaling solution clears most of it.
Skipping the flush is the most common way tankless units die early. Ask the plumber to install isolation valves so the flush is a short job and not a major one. Also ask who will do it, how long it takes and what a missed year can do to the heat exchanger.
Who installs it and what the paperwork looks like
Gas work and piping changes must be done by a plumber or gas fitter licensed by the state. The local plumbing inspector issues the permit before the work starts, and the permit goes only to a licensed plumber. Ask to see it, and check the license on the state eLIPSE portal.
Our tankless water heater and water heater installation pages describe how the work goes if you decide to move ahead. If you only need your current tank fixed, there is no shame in keeping it. According to the Board's policy statement, installs must follow the manufacturer's instructions and sizing guides.
Milton, MA specifics: state rules, house age and heating systems
Source type: Mass.gov Board of State Examiners policy statement, Town of Milton Housing Production Plan, EIA residential energy survey, MWRA treatment page
The Massachusetts plumbing board treats on-demand water heaters as an alternative method for domestic hot water, so they do not need to meet the minimum flow pressure and flow rates in 248 CMR 10.14(4) Table 4. Installs must follow the manufacturer's instructions and sizing guides. For you, that means the manual is the rulebook, so ask the plumber to show you the sizing sheet they used.
About 46.8 percent of Milton homes were built in 1939 or earlier, and the median home is 71 years old. Older basements often have a small gas line sized for a tank, a masonry chimney and tight clearances. Those are the three parts that decide whether a tankless unit goes in smoothly or needs a larger gas pipe and new venting.
Statewide, 55 percent of occupied homes heat with a furnace and 26 percent with a steam or hot-water boiler. If your house has a boiler or furnace, its flue may already be shared with the water heater. Ask whether the new unit can share that vent, because it usually cannot.
MWRA water is treated with sodium carbonate and carbon dioxide to adjust pH and alkalinity. The sources we trust do not give a hardness number for Milton, so we do not state one. Ask Milton Water and Sewer at 617-696-5731 for the current water quality report, then ask the plumber how to flush the unit if scale shows up.
Questions readers ask about this topic
They provide continuous hot water, but not unlimited flow. Each unit has a maximum number of gallons per minute it can heat to your set temperature. Run too many fixtures at once and the water cools or the flow drops. Sizing to your peak demand avoids that.
They often do, when flushed every year and installed correctly. A neglected unit with scale on the heat exchanger can fail sooner. Parts can also be replaced separately, whereas a leaking tank usually means replacing the whole heater.
Not legally if gas or piping is involved. Massachusetts requires a licensed plumber or gas fitter, and the plumber must pull a permit through the local inspector. A mistake with gas or venting can cause carbon monoxide or a fire.
Usually not. Even gas models need electricity for the controls, fan and ignition. If outages worry you, ask about a small battery backup or a gas tank model that keeps working on its own pilot or standing flame.
Nothing directly, but it often cannot share the old flue. If a boiler already uses the chimney, the tankless vent may need a new path through the wall. Ask the plumber to check that the boiler still drafts properly after the change. The state code is the same in Quincy and Braintree, so the vent question comes up everywhere.
It can be, but the chimney will not serve the new unit as it is. Condensing units vent through plastic pipe out a wall, so the chimney is left to the boiler or lined for it. Have the plumber look at wall access and gas pipe size first, since the older houses usually fail on those.
Rather talk it through? Call and ask a plumber.
(617) 752-0573