Why the swap is not what it looks like
From the outside it reads as a simple exchange. Remove a cylinder, hang a box on the wall, connect the same water pipes and the same gas line. Reclaim the floor space.
The water connections genuinely are straightforward. Almost nothing else is, and the reason is a single fact about how the two appliances work.
A storage heater heats forty or fifty gallons slowly and keeps it warm. Its burner is modest, because it has hours to do the job. A tankless unit heats water as it flows, on demand, which means it has to deliver the same temperature rise in seconds rather than hours.
That requires a far larger burner. A tankless unit typically draws several times the gas of the tank it replaces while it is running, even though it uses less over a day because it is not maintaining a standing reserve.
Gas systems are sized for peak simultaneous demand, not for daily consumption. So a gas line perfectly adequate for a storage heater is frequently undersized for the tankless unit replacing it, and that is the single most common reason a conversion runs into trouble after the old unit is already out.
The items that change the scope
| Item | Why it changes | What happens if it is missed |
|---|---|---|
| Gas line size | Peak draw is several times the old unit's | Unit starves under load; lukewarm water at demand |
| Gas meter capacity | Whole property demand, not just this appliance | Other gas appliances falter when the heater fires |
| Venting | Specific to the unit and to condensing versus non-condensing | The old flue is not reusable; a combustion safety issue |
| Condensate drainage | Condensing units produce acidic condensate | Needs routing and often neutralising |
| Electrical supply | Controls and ignition need power even on gas units | A circuit may have to be run |
| Isolation valves | Descaling requires them | Every future descale means breaking into pipework |
| Minimum flow rate | The unit needs a threshold flow to fire | Very low-flow fixtures may not trigger it |
The cheapest thing to get right
The isolation valve row costs almost nothing at installation and a great deal afterward, which makes it the easiest thing to insist on.
Without a pair of isolation valves, descaling means breaking into the pipework every time. With them, it is connect, circulate, disconnect. In water as hard as this, where descaling is a recurring requirement rather than an occasional one, that difference compounds over the life of the unit.
Ask specifically. Some quotes include them and some do not, and it is not always obvious from a line item.
STUCK ON THIS ONE?
Gas sizing is the item that most often stalls a conversion mid-project. Call (480) 806-2944 and you will get a range and an arrival window before anyone is dispatched.
Sizing it for a house rather than a brochure
Tankless capacity is expressed as a flow rate at a given temperature rise, and both halves matter. The rise is the difference between the incoming water temperature and the temperature you want at the tap.
Incoming temperature is where local conditions enter. Groundwater and stored water run colder in winter than people expect in a desert, and a larger rise at the same flow rate needs a larger unit. Sizing from a summer assumption produces a unit that disappoints in January.
Simultaneous demand is the other half. One shower is a modest ask. Two showers plus a dishwasher is a substantially different load, and the large custom homes common around here often have more fixtures than the household actually uses at once, which makes honest assessment of real peak use more useful than counting bathrooms.
Undersizing shows up as hot water going lukewarm when a second fixture opens. That is not a fault; it is a unit doing exactly what its capacity permits. It is also the most common post-installation complaint, and it is entirely preventable at the specification stage.
The maintenance commitment, stated plainly
There is no tank floor to fail, which removes the failure mode that floods a garage. That is the strongest argument for tankless in this area and it is a real one.
Scale does not disappear, though. It forms on the heat exchanger instead, in narrow passages where a thin deposit matters far more than the same deposit spread across a tank floor. The consequences arrive as performance rather than as a leak: falling output, unstable temperature, odd cycling, and eventually error codes. Left long enough, the heat exchanger itself is damaged, and that is the expensive component rather than a serviceable part.
The remedy is descaling, circulating a solution through the exchanger via those isolation valves. Industry guidance commonly puts tankless descaling and filter replacement at every twelve to eighteen months in normal conditions, and water widely reported in the mid to high teens in grains per gallon is not normal conditions. Gas units also have air filters that need changing on the same kind of schedule.
So the honest guidance is that this is a question about you rather than about the equipment. A household that keeps to a descaling schedule gets a genuinely good outcome here. A household that will not is better served by a tank, where neglect shortens life but the maintenance task is a flush rather than a procedure.
Pairing the unit with softened water is the single most effective thing you can do for its working life, and it stretches the interval considerably.
Permits, code and who inspects it
A conversion is permitted work, and the inspection is genuinely useful rather than a formality, because gas sizing and venting are combustion safety matters rather than preferences.
Arizona sets no statewide plumbing code, so the edition your work is judged against depends on your address. Inside the Town of Cave Creek, the Town runs its own building department and adopts International Codes by reference, having moved onto the 2024 family and the 2023 National Electrical Code at the start of 2026. Unincorporated parcels are inspected by Maricopa County against the 2018 International Plumbing Code and the 2018 International Fuel Gas Code, adopted in 2022 through the county's Local Additions and Addenda, plus the 2017 NEC.
Those editions differ on things that matter for this job, which is why confirming which applies before work is specified is a five minute question worth asking. Contractor licensing throughout the state runs through the Arizona Registrar of Contractors.
If the gas assessment shows the supply needs upgrading, that is a separate piece of work with its own requirements around sizing, materials and pressure testing, and it is worth understanding before committing to the conversion rather than discovering it midway.
Conversion questions worth asking first
Is tankless cheaper to run?
Generally yes, because there is no standby loss from keeping a tank hot around the clock. The saving is real but modest, and it takes time to offset the higher purchase and installation cost.
Where the economics improve here is longevity, provided the maintenance actually happens. A well maintained tankless unit outlives a tank in this water by a wide margin; a neglected one does not.
Will I really never run out of hot water?
Continuous, yes, within the unit's capacity. Unlimited simultaneous demand, no.
A correctly sized unit will run a shower indefinitely. Ask it to serve two showers, a dishwasher and a laundry cycle beyond its rated output and the temperature drops. That is sizing rather than a defect.
How much extra does the gas work usually add?
It varies too much to give a figure honestly, because it depends on the run length from the meter, what else is on the system, and whether the meter itself has headroom.
What is reasonable to expect is that it gets assessed and quoted before you commit to the unit, not discovered afterward. Any quote that does not address gas capacity has not been thought through.
Can a tankless unit work on a well or storage tank supply?
Yes, with attention to two things. Minimum flow rate matters more when pressure comes from a pump and pressure tank rather than a municipal main, because pressure fluctuates through the pump cycle.
Sediment matters too. Particulate from a stored supply reaching a heat exchanger is unhelpful, so filtration ahead of the unit is worth having rather than optional.
What if my unit starts throwing an error code?
Look it up in the manual first. Many codes are scale related and clear after descaling, particularly those relating to flow, temperature or ignition under load.
What is worth avoiding is repeatedly resetting a unit that keeps returning the same code. It is reporting a genuine condition, and running through it is how a descale becomes a heat exchanger replacement.
Is it worth switching if my tank still works?
Not urgently, and there is no need to scrap a functioning unit. It is worth planning for, because a conversion done at leisure is a very different project from one done the morning after a tank failed.
Getting the gas and venting assessment done now, while nothing is broken, means the decision is ready when the time comes rather than being made under pressure.