Tankless Water Heater Service in Dominion at Panther Creek, Frisco, TX

Square footage is a terrible proxy for hot water demand and the industry keeps using it anyway. A long single-storey plan with two and a half baths and one occupant places far less simultaneous load than a compact two-storey with five baths and a family of six. What actually sets the requirement is how many hot outlets can be open at the same instant, and how much each of them draws on the hot side rather than at the tap.

That distinction matters more than it sounds. A showerhead rated 1.8 gallons per minute at the tap is not drawing 1.8 gallons per minute of hot water. It is blending, so at a 120 degree supply and a 105 degree shower with 52 degree cold, the hot fraction is roughly 78 per cent, or about 1.4 gallons per minute of genuine hot draw. Sizing on tap flow rather than hot-side flow systematically over-specifies the unit, which is one of the more expensive ways a quote can be wrong in your disfavour.

1. 1

   List the fixtures that can realistically run together

   Not every fixture in the house. The honest peak in most households is two showers, a sink and possibly an appliance, in a fifteen minute window on a weekday morning.

2. 2

   Take the hot-side draw, not the rated flow

   Roughly three-quarters of rated flow for a shower at a 120 degree setpoint. A tub filler is close to 100 per cent, because a tub is usually filled much hotter than a shower runs.

3. 3

   Add them up and compare against winter capacity

   A 199,000 BTU condensing unit at a 65 to 70 degree winter rise gives about 5.5 to 6.0 gallons per minute. That is the ceiling, and it is a January ceiling, not a July one.

4. 4

   Decide what happens when you exceed it

   A tankless unit does not run out of hot water when overloaded. It reduces outlet temperature, or on many models throttles flow to hold temperature. Neither is a fault. It is the unit telling you the demand exceeded the arithmetic.

Applied to a four or five bath plan, the sums usually come out as follows. Two simultaneous showers is about 2.8 gallons per minute of hot draw and is comfortably inside a single unit. Three simultaneous showers is around 4.2 and still fits. Two showers plus a primary tub filling at five gallons a minute is roughly 7.8 gallons per minute and does not fit, and that is the scenario that decides whether a second unit belongs on the quote. Everything else is noise.

It is worth separating the two components of the wait, because they have completely different fixes and only one of them is about the appliance. The first component is burner lag: the two to five seconds between flow being detected and the exchanger reaching setpoint. The second is purge: pushing the cooled water that has been sitting in the pipe since the last draw out of the way. On a long run the second component dwarfs the first, and no upgrade to the heater touches it.

Purge volume

The quantity of already-cooled water standing between the heater and the fixture. It has to come out of the tap before hot water can arrive, and it is a pure function of pipe diameter and pipe length. Nothing about the heater changes it.

Most large two-storey plans are piped as a three-quarter inch trunk running most of the distance with a half-inch branch dropping to the fixture, so the real figure lands between the rows above. A sixty foot path is therefore about a minute of purge at a lavatory flow rate, and rather less at full shower flow because the water moves faster. That is the number people are experiencing when they say the upstairs bathroom takes forever, and it explains why replacing a perfectly adequate heater never fixes it.

It also explains the water bill side of it. A minute of purge, twice a day, at a gallon and a half a minute, is roughly 1,100 gallons a year down the drain from one fixture. In a plan with two or three distant baths that is a genuinely meaningful volume, and it is the honest justification for recirculation quite apart from the convenience.

There are three workable answers and one common mistake, and which one applies depends on whether your house was piped with a return line. That is worth establishing before anyone quotes, because it is the difference between a straightforward job and an invasive one.

The mistake is worth quantifying because it is so widespread. A hot loop is a continuous heat leak. Bare three-quarter inch copper at 120 degrees passing through an attic loses heat continuously, and over a long loop running around the clock that adds up to a standing gas burn measured in hundreds of therms a year, for the benefit of nobody at three in the morning. Insulating the loop cuts it substantially. Running the pump on a schedule that matches when the household actually showers, or on an on-demand button at the fixture, cuts it far more. The efficiency you bought with a condensing unit is entirely capable of being given back through a badly controlled loop.

An interior closet on the upper floor is the hardest location in a house to vent, and it is worth understanding why before you accept or reject a price for it. A condensing tankless unit is a sealed-combustion appliance, so both intake and exhaust have to reach outside air, and the manufacturer imposes a maximum equivalent vent length that covers the pair.

Equivalent vent length

Straight pipe measured in feet, plus a penalty for every fitting. Manufacturers typically count a ninety degree elbow as three to five feet of straight pipe and a forty-five degree elbow as roughly half that. A run with six elbows can therefore consume twenty to thirty feet of the allowance before a single straight foot is counted.

Most condensing units permit somewhere in the region of fifty to one hundred equivalent feet in three inch pipe, with the exact figure on the model data plate. From an interior upper-floor closet the run has to leave the closet, cross to an exterior wall or up into the attic, and then either penetrate a sidewall or continue up through the roof. Each of those direction changes costs allowance. Going to four inch pipe where the model permits it buys back a substantial amount of length, which is often the cleanest solution and rarely the one a low bid has allowed for.

The other closet-specific constraint is service access. An inspector will look for the unit to be reachable and serviceable without dismantling anything, with the clearances the manufacturer specifies to the sides and front. A closet that fitted a tank does not automatically fit a tankless unit plus its vent, its isolation valves, its condensate trap and the room to work on all four. That is worth checking with a tape measure before the equipment is ordered rather than on installation morning.