Last month’s column addressed the proper sizing issues associated with "side-arm" style domestic water heaters (pg. 18). This month, we’ll explore combination systems and tankless water heaters.
You can also run into and/or create hybrid systems for the production of DHW that utilize all or some of the components that I’ve previously described. For example, if you have a large dump-load demand, and other back-to-back low-flow loads, you could use a semi-instantaneous, low-volume heat exchanger, a large un-powered electric storage tank, some well-placed pumps and aquastats and, voila, you’ve got the best of both systems.
A word of caution is due here. Make sure that you set up this hybrid system just as it’s depicted in my drawing, or you may end up with an extremely dissatisfied customer from the L.L.W.W. (Lotsa Luke Warm Water) syndrome.
Although not considered actual "boilers" per se, a plethora of wall-mounted instantaneous DHW boilers are also out there on the market. Among the most popular ones are Paloma, Aqua Star, Takagi and Rinnai. These water heaters come in various Btuh inputs varying from 43,800 Btuh all the way up to 235,000 Btuh.
At present, no set standards exist for comparing efficiencies and capacities for the different models and manufacturers. One thing for sure, a Btu is still a Btu, regardless of whether it’s being generated from a boiler or a wall-hung instantaneous water heater and, for the most part, numbers don’t lie. Output Btu divided by input Btu equals net appliance instantaneous efficiency. Gallons times pounds times temperature differential still equals Btu delivered.
Another way of looking at it is Btu divided by pounds divided by degree Fahrenheit temperature rise equals gallons per hour. Gallons per hour divided by 60 equals gallons per minute delivery of "X" degree Fahrenheit water.
Let’s do a little math here. For the sake of simplicity, let’s use 235,000 Btuh as our input and 84% as our instantaneous efficiency. So 235,000 (Btuh input) times 0.84 (efficiency) equals an hourly delivery of 197,400 Btuh; 197,400 Btuh divided by 8.33 (pounds per gallon of water) equals 23,697; 23,697 divided by 100 (anticipated degree Fahrenheit temperature rise) equals 237 gal. per hour of 100°F water. And 237 divided by 60 (minutes per hour) equals a flow rate of 3.95 GPM flow at 100°F temperature rise.
What does this mean to end users? It means that it would only take about 10 minutes to fill their large soaking tub with very hot water. It also means that they could probably run three or four low-flow (1.5 GPM) shower heads at the same time from an appliance whose size is just less than that of a medium-sized piece of luggage! Now that’s what I call a box full of power!
A word of high-altitude caution here. These appliances do have to be de-rated for use at higher altitudes. Make sure you’ve taken this into consideration in the net hourly delivery capacity.
The basic water heater efficiencies of these beauties vary from a low of 76%, all the way up to about 86%, with 82% to 84% being pretty common. Tankless water heaters are not rated in the same way as space heating boilers. Tankless water heaters have something similar to AFUE, but it’s not the same.
Inasmuch as standby losses are efficiency killers, these gems are extremely efficient due to the fact that they’re either on or off. And when they are off, there is little to no standby loss up the flue pipe. Compare this to the continuous standby loss of a standard gas-fired stand-alone storage tank, and you can see why they are becoming so popular.
Jack Bellanti of Low Energy Systems in Englewood, Colo., says that most of the customers he is seeing come through the company’s showroom are making the switch to instantaneous primarily due to increasing fuel cost, backed up by the "virtually endless supply" of DHW.
Jack should know. All he does is sell tankless water heaters. He said the Takagi TM1 has the capacity to handle three people showering at once. That equates to an output of 9.6 GPM of 50°F rise water. All this is in a package that comes with a built-in inducer fan, and with physical dimensions smaller than most laundry baskets.
Speaking of laundry baskets, the TM1 is making big waves in the commercial and coin-op laundry scenes. It also has a major potential in small- to medium-sized restaurants for handling the continuous demands of automatic dishwashers. If set up correctly, these units will provide many years of trouble-free, highly efficient service.
Unlike many of their predecessors, these units have a much higher degree of control logic intelligence, and they are easier work on.
Another word of caution is due here. These tankless heaters are essentially low-mass copper tube boilers, and as such they are very much subject to the scaling tendencies of hard water. Jack urges installers to find out what the water conditions are in the area they intend to install them in, and he recommends they follow the manufacturer’s instructions on pre-treating the water before it gets to the tankless units. Feel free to call Jack or his brother Dennis at 800/873-3507 for more information.
Until next month, Happy Instantaneous Hot Water Hydronicing!!
Mark Eatherton can be reached via e-mail at [email protected] or by phone at 303/778-7772.