Point-of-Use Tankless Water Heater Installation in Frisco, TX

The heater is a small box that costs a few hundred pounds and takes twenty minutes to hang. The circuit feeding it is where the money and the difficulty live, and it is governed by arithmetic that anyone can follow. Start from the fact that watts are volts multiplied by amps, so an 11 kW unit at 240 volts draws about 45.8 amps. A water heater is treated as a continuous load, so the branch circuit and its overcurrent device are sized at 125 percent of that — about 57.3 amps — which rounds up to a 60 amp double-pole breaker and the conductors to match.

Distance then modifies that. Voltage drop is current multiplied by resistance, and the resistance is the round-trip length of the conductor, not the one-way run. Copper 10 AWG runs about 1.24 ohms per thousand feet, so a 30 amp load 60 feet from the panel sees 120 feet of conductor and drops about 4.5 volts, which is under two percent of 240 and perfectly comfortable. The same arithmetic on a 120 volt circuit is far less forgiving, because you are dropping volts out of half the supply — 12 amps down 50 feet of 14 AWG loses close to 3.7 volts, which is over three percent of 120. That is the practical reason small 120 volt units disappoint at the end of a long run: they are not getting their rated voltage, and heat output falls with the square of voltage.

The whole case for a point-of-use unit rests on one quantity: the volume of cooled water sitting in the branch that has to be pushed out before hot water arrives. It is worth calculating rather than guessing, because the answer determines whether the unit is solving a real problem or an imagined one. Multiply the run length by the volume per foot for the pipe you actually have.

Two things fall out of that table. The first is that pipe size matters more than length. A house plumbed in three-quarter inch copper all the way to a guest lavatory holds nearly three times the water of the same run in half-inch PEX, which is why the complaint is far more common in the copper-trunk homes than in the newer manifold-plumbed ones. The second is that the purge time scales inversely with the fixture flow, so a modern low-flow aerator makes the wait longer, not shorter. Fitting a water-saving 0.5 GPM aerator on a lavatory at the end of a long three-quarter inch run turns a one minute wait into a three minute one.

There is a further effect that the volume calculation misses. The pipe itself has thermal mass, and on the first draw of the morning the copper is at room temperature and takes heat out of the water passing through it. That means the water arriving at the tap is cooler than the water that left the heater, and it stays that way until the pipe wall has warmed. On a long copper run in an unconditioned attic in January, that alone can account for another ten or fifteen seconds before the temperature stabilises — which is why people report that the wait is worse in winter even though the distance has not changed.

Nearly all the published advice about point-of-use heaters assumes one configuration, and there are two. Which one is right depends on whether the fixture currently has hot water at all.

Standalone cold-fed installation

The unit takes cold supply and feeds the fixture's hot tap directly. The main water heater is not involved at all. This is the right configuration for a fixture that has no hot supply — a garage sink, a detached workshop, an added wet bar — and it is the one that requires the full temperature rise from the unit, which is what drives the kilowatt requirement up.

Inline booster installation

The unit sits in the existing hot branch, downstream of the main system. It does nothing when the water reaching it is already at setpoint, and it fires only during the purge period when the branch is delivering cool water. Because it is only making up part of the rise once the system catches up, it is doing less total work — but for the first thirty seconds it is doing the full rise on its own, so it still has to be sized for that.

The booster arrangement is the one people rarely hear about and it is often the better answer for the classic far-bathroom complaint, because it leaves the existing plumbing intact and the fixture reverts to full main-system capacity once the branch is hot. The critical check is the unit's rated maximum inlet temperature, which on small electric heaters is commonly around 140 degrees F. Feeding preheated water into a unit not listed for it is a way to damage the element housing and void a warranty, and not every model is suitable. It also needs a control strategy that genuinely modulates rather than simply switching on at a fixed output, or you get an overshoot when the hot water finally arrives from the main system.

A fair number of the people who ring us about a point-of-use heater do not need one, and it is quicker for everybody if that gets established on the phone. The complaint is almost always framed as a wait, and there are three or four causes of a wait that cost nothing or nearly nothing to rule out.

1. 1

   Time it with a watch and measure the flow

   Fill a jug for fifteen seconds and multiply by four to get the fixture's real GPM. A wait of twenty seconds at a lavatory nobody uses twice a day is not a problem worth spending money on. A wait of ninety seconds at the fixture a family of five uses every morning is.

2. 2

   Check the main heater setpoint

   A system set to 115 degrees F delivers water that feels tepid at the tap after a long run has cooled it, and people read that as a delivery problem. Raising the setpoint on a properly tempered system is free and sometimes it is the entire fix.

3. 3

   Look at the aerator

   If somebody has fitted a 0.35 GPM aerator to a fixture at the end of a long run, the purge takes four times as long as it did with the original. Swapping it back is a few pounds and two minutes, and on a fixture used for handwashing the water saved by the low-flow aerator is being thrown away down the drain during the purge anyway.

4. 4

   Ask whether there is already a recirculation system

   Plenty of larger Frisco homes have a pump that was switched off during a power cut or after a service call and never switched back on, or a timer with a dead battery still programmed for a schedule from three years ago. That is a free fix and we would rather tell you where to look than sell you a heater.

5. 5

   Consider whether one fixture is really the problem

   If three fixtures in the same wing all wait, five separate point-of-use units is a poor answer. A return line serving that wing is the right one, and it will cost less than the five heaters and their five circuits.

Insulating the run is the suggestion people expect us to make, and it is worth being straight about what it does. Insulation does not reduce the volume of water in the pipe, so it does not shorten the purge on the first draw of the day at all. What it does is keep the water in the pipe hot for longer between draws, so the second person into the bathroom twenty minutes later gets hot water almost immediately. That is a genuine benefit and it is cheap where the pipe is accessible in an attic. It is not a substitute for a heater on a fixture that is used once a day.

A point-of-use unit sits closer to the outlet than anything else in a plumbing system, which changes the safety picture in a way that is easy to overlook. In a conventional arrangement the water leaving the heater is tempered by distance, by mixing at the valve and often by a thermostatic mixing valve at the source. A small heater a foot from the spout has none of that between it and a pair of hands.

Children and older adults burn faster than those figures, which are for healthy adult skin. The practical consequence is that a lavatory heater should be set for handwashing — 105 to 110 degrees F is plenty — rather than to the 120 or 140 degrees F that a whole-house system runs at to serve dishwashers and laundry. Setting it low also has a pleasant side effect: because the required rise is smaller, a modest unit delivers a usable flow rate that it could never manage at a higher setpoint.

Where a small unit serves a fixture that a child or a vulnerable adult uses, a point-of-use tempering valve on the outlet is the belt-and-braces answer. It caps the delivered temperature regardless of what the heater's thermostat is doing or what somebody has set on the dial, and it is a small in-line device that fits inside the same vanity cabinet. On a unit with a simple non-modulating control it also smooths out the temperature swing that occurs when the flow rate changes mid-wash.

The failure mode of this product category is not mechanical, it is expectational. The units do what they claim, and people buy them for jobs they were never sized to do. These are the situations where we say no.

• A kitchen sink used for washing up at 2.2 GPM, which at a 55 degree winter rise needs roughly 18 kW and a 90 amp circuit
• Supplying a dishwasher, which needs water near 120 degrees F at a flow rate no small unit can deliver at winter inlet temperatures
• Supplying a washing machine, for the same reason plus the fill rate
• A full-size shower head, where a 2.5 GPM head at a 55 degree rise works out around 20 kW
• Any fixture where the only available power is a shared 120 volt receptacle on a bathroom circuit
• Adding capacity to a house where the main system is struggling, since the two are unrelated problems
• A whole casita or guest suite with a shower and a kitchenette, which wants its own properly sized system rather than a collection of small ones

The casita case is the one worth thinking about carefully, because it is common in Frisco and the intuitive answer is wrong. A detached structure with a shower, a basin and a small kitchen has a genuine simultaneous demand, and stringing three point-of-use units through it means three dedicated circuits, three sets of scale-prone elements and a total connected load that will need a subpanel anyway. Running a properly sized gas line or a single adequate electric unit for that building costs less and works better. Ring us on (972) 430-9024 and describe the fixtures rather than the building, and the answer usually becomes obvious in a couple of minutes.