Tankless Water Heater Service in Frisco Square, Frisco, TX

Electric tankless units are frequently discussed as though they obey different rules from gas units. They do not. Heating water takes a fixed amount of energy per gallon per degree, and the only question is where the energy comes from and how fast it can be delivered. The arithmetic that governs a gas unit governs an electric one, with a unit conversion inserted at the front.

Kilowatt to BTU conversion

One kilowatt of electrical input equals 3,412 BTU per hour of heat. Because resistance heating converts essentially all of its input into heat in the water, an electric unit's rated kilowatts can be treated as delivered energy without the efficiency discount that applies to a combustion appliance. That is the one genuine advantage electric has in this calculation.

Put that through the same equation used everywhere else on this site. Flow in gallons per minute equals BTU multiplied by efficiency, divided by the temperature rise multiplied by 500. Against the 55F winter rise North Texas demands, the results line up as follows, with amperage calculated at 240 volts.

That final row is the whole story in one line. A gas unit delivering roughly six gallons per minute at a winter rise draws less current than a hair dryer, because the energy arrives as fuel. Matching it electrically means finding something in the region of 150 amps of dedicated capacity, which in most buildings is not a water heater decision at all. It is a service decision.

A residence with a 200 amp service does not have 200 amps available for a water heater. It has whatever is left after the air conditioning, the range, the dryer, the oven and the general lighting and receptacle load have been accounted for, and a whole-home electric tankless unit is a continuous load, which means the circuit and the conductors serving it are sized above the raw current rather than at it.

• The existing service size at the meter, which sets the ceiling on everything that follows
• The calculated load already on that service, not the breakers installed but the load actually served
• Available spaces in the panel, since a 27 kW unit typically wants several double-pole circuits rather than one
• Conductor size and run length from the panel to the appliance location
• Whether the building's service can be increased at all, which in attached construction is frequently the binding question

Every appliance in this category performs better in summer, because warmer inlet water means a smaller temperature rise and a smaller rise means more gallons per minute out of the same energy. The difference is that a gas unit carries enough surplus to absorb the seasonal swing without anybody noticing, and an electric unit, constrained by what a panel can supply rather than by what a burner can produce, generally does not.

Work the same equation twice on one appliance. A 27 kW unit delivers roughly 92,100 BTU per hour. Against a summer rise of about 30F that is close to 6.1 gallons per minute, which feels genuinely generous. Against the 55F winter rise the same unit delivers about 3.3 gallons per minute. Nothing failed and nothing changed. The appliance simply has to put nearly twice as much energy into every gallon, and its energy supply is fixed by the breaker.

There is a second way to use the same arithmetic. Rather than asking one appliance to heat every gallon the residence will ever draw, put a small unit immediately at the fixture that is causing the complaint. A single lavatory drawing about one gallon per minute at a 55F rise needs roughly 8 kW, which is an entirely ordinary circuit. The same fixture served from a central unit forty feet away needs no additional energy at all, but it needs the forty feet of standing water purged first.

The failure column is where the decision usually resolves. Point-of-use units are excellent at lavatories and small draws and are consistently disappointing when asked to carry a shower at a winter rise, because the arithmetic in the first table does not bend for them either. Used correctly they are a distribution tool, and used incorrectly they are an expensive way to reproduce the problem you started with.

In attached and stacked construction the exterior wall is shared property in every practical sense, and a termination there has to satisfy more than the appliance manufacturer. Two details govern more of these installations than anything else, and neither is obvious from inside the utility closet.

• Termination height relative to any walkway beneath it, because exhaust discharging at head height across a route people use is unacceptable regardless of what the clearance table permits
• Intake height above grade, because an intake set low on a street elevation collects debris and standing water far faster than one set higher
• The fact that both the intake and the exhaust are terminations, each with its own clearances, so a single-penetration concentric fitting resolves two problems at once
• Separation from any neighboring opening, which on a shared envelope is not always visible from your own side of the wall

None of that is difficult once it is surveyed. All of it is expensive to discover after equipment has been ordered and a wall has been opened, which is the argument for resolving the exterior before anybody commits to an appliance category.

A condensing gas unit produces liquid water as a by-product of running efficiently. Pulling the flue gas below its dew point is precisely how the second heat exchanger recovers the energy that lifts efficiency into the mid-nineties, and the water that drops out of the gas has to be carried away continuously while the appliance runs. On a suburban garage installation this is unremarkable, because there is usually a floor drain or an exterior wall within a few feet. In a closet stacked above occupied space it is frequently the hardest single detail on the job.

Why the drain cannot simply be teed into anything convenient

Tankless condensate is mildly acidic, typically in the pH 3 to 4 range, and it arrives at a low, steady trickle rather than in slugs. That combination is unkind to the materials it sits against. A neutralizer cartridge filled with limestone media raises the pH before discharge, and it is a consumable rather than a fixture: the media is consumed by doing its job and needs replacing on a schedule, not inspecting once and forgetting.

• A gravity path to an existing laundry standpipe or lavatory tailpiece, which is the simplest answer when the elevations happen to cooperate
• A condensate pump where they do not, which introduces a powered component and therefore a failure mode that did not previously exist
• A neutralizer ahead of either one, sized so it is genuinely serviceable rather than sealed behind the appliance
• An overflow or float switch that shuts the unit down rather than letting a blocked line find its own route into the ceiling below

That last item deserves its own sentence. In detached construction a condensate overflow makes a mess on a garage floor. Above a neighbor or a ground-floor commercial tenant it makes a claim. Where a pump is unavoidable, wiring its float switch to interrupt the appliance rather than merely sound an alarm turns the worst plausible outcome into no hot water for an evening, which is an entirely different category of problem.