If sales of high-efficiency tankless water heaters are an indicator, then the economy is definitely rebounding. The federal government’s mandated changes to the old-reliable tank-style water heaters have altered the up-front cost difference and while the playing field is certainly not leveled, the changes have tilted the field to give tankless players a more favorable position nearer the goal-line: close enough for field-goals!
Gone, for the most part, are the outrageous claims first put forth regarding maximum GPM (gallons per minute) many tankless manufacturers claimed. As you and all who are in the trenches know all too well, you are the ones held to account when any product comes up short in its performance. From Florida to Alaska and elsewhere, the incoming cold water will fall to a low of 40°F during winter’s coldest month. It’s all about how deeply buried the municipal service line is for any given region. If 120°F is to be the target delivery temperature, then we must calculate for an 80°F temperature rise.
Using the formula of 1-Btu raises 1-Lb. of water 1°F, each gallon of water weighing 8.34-Lbs. will require a net input in energy of 667.2-Btus. Is the .2 really necessary for our calculations?
Months ago I was called to a home because the owners were exasperated about running out of hot water. They had a tankless water heater. They referred to it as their “thankless” water heater! They also have a large walk-in shower with a huge overhead deluge rain-head fed by a 1-in. diameter line emanating from the 9-ft. high ceiling. 2.5-GPM? Fu-get-about-it!
The only way to calculate the actual requirement for this monster showerhead was to grab a garbage can (a clean one that is), turn on the shower and let it run for a timed period, and then measure the gallons captured after they adjusted the temperature to where it would be when they shower. So, 10-GPM and 110°F measured on that 1-in. supply nipple at the ceiling. The aspiration of air and lengthy distance from rain-head to people-head causes a temperature drop and it’s necessary to know the actual delivery temperature. The 10-GPM flow rate at 110°F will have 87.5% of its flow coming directly from the tankless. The percent formula: (110°F adjusted bathing temp – 40°F inlet temp) / (120°F target temp – 40°F inlet temp) = .875.
Their 199,000-Btuh tankless water heater was rated at 92% operating efficiency. Unable to keep up with demand from Day 1, an indirect water heater had been added and treated as the target for the tankless water heater.
The math is relatively straightforward: 8.75-DHW-GPM x 667.2 = 350,280-Btuh net energy required. No room in the tiny mechanical room to install a mega-sized water heater or several tanks, so tankless was the only best hope for a resolution. We did discuss the possibility of installing a modulating condensing boiler to work in conjunction with an indirect storage tank, but the existing tank was an irritant they wanted gone for floor space.
They do not want to have diminished flow, so installing a single tankless water heater that would cut down on the GPM rate to maintain its 120°F outgoing target temperature would result in a very expensive callback. A single 199,000-Btuh tankless will render a net output of 195,020-Btuh at 98% operating efficiency, which will deliver 4.87-GPM of 120°F water when the 80°F rise is required. We have a demand for 8.75-DHW-GPM at 120°F under design conditions. A second tankless is required. Working in tandem, they will be capable of delivering 9.74-GPM @ 120°F.
Tankless water heater manufacturers have made it easy and simple to set up multiple twinning with plug-and-play communication cables. Gas supply and venting were all that remained to complete the design-phase. Here too, installations have become faster and easier with the ability to utilize a common vent line and connect up to 10 tankless water heaters. Careful attention to the installation instructions is required to prevent flooding the last-tankless-in-line with too much flue-gas-condensation. If all 10 are firing at their maximum rating, you’ll be dealing with 20-gallons of condensate per hour!
Neutralizing the acidic condensate is strongly recommended and may be required by local codes. For our homeowner with the twinned tankless installation, the existing gas line was adequate and accessible within the mechanical room to support the addition of a second 199K-Btuh tankless water heater.
While this home was exceptional, we’re now selling more tankless water heaters than ever before and not just in high-end homes. Most are being installed in blue-collar homes and most sales are originating via customers wishes rather than being generated from my input. The proof is in the pudding: the tankless manufacturers’ advertising has had a real pull-through impact on sales.
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