Tankless water heater installation, done with the whole system in mind

How tankless actually works

There is no stored water. When you open a hot tap, a flow sensor detects the draw, the burner fires, and water passes through a heat exchanger and comes out hot. When you close the tap, it stops. Nothing is kept warm in between.

The genuine advantages follow from that. No standing loss, you are not paying to keep forty gallons hot around the clock. No capacity limit, so a long shower does not run out. A small wall-hung footprint that frees up floor space. And typically a longer service life than a storage tank, often twenty years or more, because there is no tank to corrode.

The real constraints follow from it too. Output is limited by flow rate rather than volume, so a tankless unit that cannot heat enough water per minute will deliver lukewarm water indefinitely rather than hot water for a while. There is a short delay before hot water arrives. Most units need electrical power, so they stop in an outage. And they need substantially more gas than the tank they replace.

The New England temperature problem

This is the part that gets glossed over, and in Massachusetts it is decisive.

A tankless unit is rated by temperature rise at a given flow rate, how many degrees it can raise the water while delivering so many gallons per minute. The colder the incoming water, the bigger the rise required, and the less flow the unit can deliver.

Massachusetts groundwater in July is mild. In February it is genuinely cold, and in much of the state it drops to the low forties. Heating water from the low forties up to a usable delivery temperature is a much larger rise than the same unit faces in a mild climate, and a model that comfortably runs two showers in September may only manage one in February.

This is the single most common reason people are disappointed by a tankless installation, and it is entirely avoidable at specification stage.

Sizing by flow, not gallons

Forget capacity in gallons; the question is how many gallons per minute you need at once, and what temperature rise that requires.

Add up your realistic worst case. A modern shower is roughly two gallons per minute, an older one more. A kitchen faucet is around one and a half. A dishwasher or washing machine adds its own draw. Two simultaneous showers plus a kitchen tap is a common family peak and is a substantially bigger ask than most people assume.

Then apply the winter rise. That combination, required flow at required rise, in February, is what determines the model, and it is why some households need a larger single unit and some need two units in parallel.

The other option worth knowing about is a small buffer tank on the outlet, which smooths out the "cold water sandwich" effect and helps with short intermittent draws that a tankless unit handles poorly. It costs a little space and solves a real annoyance.

Gas supply: the deal-breaker

A tankless gas unit fires at a far higher input rate than the storage tank it replaces, frequently three to five times higher, because it has to heat water instantly rather than slowly over an hour.

That means the existing gas line is very often inadequate. A half-inch line that served a tank perfectly well will starve a tankless unit, and the symptoms are exactly the ones that make people hate the technology: temperature fluctuation, fluctuating output when other gas appliances run, and intermittent faults that nobody can pin down.

Proper practice is to calculate the total connected gas load, the run length and the fittings, and upsize the piping if required, often to three-quarter inch or larger, and in some cases to confirm the meter itself can supply the total demand. Where the meter is undersized, the utility has to be involved.

This is the item most likely to turn an attractive quote into an expensive one, and it is also the item most likely to be quietly omitted. Ask directly: has the gas line been sized for this unit, and is upsizing included?

Venting and condensate

Tankless units cannot use a conventional chimney. They need either sealed direct venting through a sidewall using approved stainless or, on condensing models, approved plastic vent pipe.

Direct venting brings combustion air in and exhaust out through a sealed concentric or twin-pipe assembly, which is efficient and safe but imposes clearance requirements from windows, doors, air intakes and property lines. In Massachusetts there is an additional practical concern: the terminal must stay clear of drifting and accumulating snow, and a sidewall termination at the wrong height on the wrong elevation gets buried in a nor'easter and shuts the unit down.

Condensing models, which is most of what is worth buying, produce acidic condensate that must be drained safely, often through a neutraliser, to an appropriate drain. That drain line must not be run somewhere it can freeze, which is the same mistake that causes so many condensing boiler lockouts every winter.

Cost and payback, honestly

Installed cost for a tankless unit is meaningfully higher than a like-for-like tank replacement, and the gap is mostly not the appliance. It is the gas line upsizing, the new venting, the condensate arrangements and the additional labour.

Energy savings are real but modest in absolute terms, because standing loss on a modern well-insulated tank is not enormous to begin with. Payback on fuel savings alone typically runs long, and anyone quoting a short payback is being optimistic.

The honest case for tankless is not primarily financial. It is endless hot water for a household that genuinely runs out, reclaimed floor space where that matters, and a longer service life. Where those things are worth money to you, it is a good buy. Where you simply want cheaper hot water, a correctly sized modern tank or a heat pump water heater is usually the better value.

Massachusetts rebate programmes for high-efficiency equipment change from year to year and are worth asking about before committing, since eligibility often depends on the specific model.

Descaling and maintenance

Tankless units need annual maintenance, and unlike a tank, skipping it degrades performance noticeably rather than invisibly.

Scale builds up inside the heat exchanger, restricting flow and reducing heat transfer. The symptoms are gradual: reduced flow at temperature, longer waits, and eventually error codes. Annual descaling, circulating a descaling solution through the unit via the isolation valves, restores it, and it is a straightforward service visit.

Which is why isolation valves matter at installation. A tankless unit fitted without service valves cannot be descaled without cutting pipework, and installers who omit them are saving themselves an hour and costing you every year afterwards. Insist on them.

The inlet filter screen also needs cleaning periodically, and on hard water a treatment system upstream extends the interval between descales considerably. If your water is genuinely hard, factor that in at the outset.

Related: water heater installation for the tank comparison, indirect water heaters if you have a good boiler, and water heater repair if your current unit has just failed.