I thought readers might enjoy exploring the issues raised in a recent e-mail. At first glance, it seems like a fairly simple straight-forward exercise, until you realize the variables and pitfalls are numerous and that the math doesn’t support the running-out-of-hot-water issue. Here’s the e-mail:
Hi Dave:
I’m shopping for a water heater for a small condo building. We have six units, each has one bathroom with a shower/standard tub. I'm assuming a 2.5 GPM showerhead. Each unit has a newer dishwasher and a "thin twin" washer/dryer with 1.5 cubic feet capacity on washer (about the smallest out there). The building has 10 bedrooms total, we currently have eight adults and one toddler living in the building. We currently have a natural gas, 50-gal., 75,000 BTU Rheem tank style water heater with a FHR (first hour rating) of 109 for the whole building that is almost keeping up, but we are occasionally running out of hot water. Right now we have one person that showers at about 5:45 a.m. for about 10 minutes, and then three other showers happen in the morning between about 7:30 to 8:30 that are probably about 10 minutes or less. I have had contractors suggest everything from two 40-gal. tanks, two 50-gal. tanks, one 75-gal. tank, an 80-gal. tank, and a Triangle Tube Smart 60 indirect. Our boiler is a little over a year old, is a Slant Fin Mod# SE-210EDP, 210,000 BTU, about 84% efficiency. I am told the incoming water temperature is 40°F, and I would like to keep the water heater at 120°F as recommended by our utility company, but of course will follow manufacturers’ instructions for whatever new water heater we get. Naturally, we would like to keep the costs down, but get the job done. I am having trouble figuring out what FHR to go with. Recommendations have been all over the map. The info I have found online is also all over the map, so I thought I would try you! Let me know your thoughts, or if there's any other info you may need. Thanks!
A dilemma
Ah yes … The horns of a dilemma! As much as people try to stay of hot water, they sure don't like it when the hot water runs out. A single source for hot water is the first issue I'd want to have the condo association consider. You can pretty much be assured the single DHW source will fail at the most inconvenient time, resulting in added costs due to overtime holiday pay at midnight on Christmas Eve! The other downside is that they'll be stuck with whatever is available from your inventory or whichever supplier is willing to open their doors in the middle of the night. The fact that they are being proactive before a crisis develops presents you with an opportunity to appeal to their desires and people buy based on their emotions over presentations that only deliver facts. They’ve told you what they want: more hot water; costs within reason (you get to define what’s reasonable); and the best part is others have confused the folks who will make the decision, a sure-fire formula for getting the sale if you can provide some education that will allow them to sift through the choices. Fill those needs and desires and you can walk away with the sale.
The wide range of choices and options presented suggests that not one of the contractors took the time to provide a full-range of options while explaining ECV or ROI comparisons. No wonder they’re confused!
Two basic routes
Your two basic routes are tank or tankless style. Natural gas and costs are a consideration. No mention of operating efficiencies or EF ratings. Chimney-vented, indirect-vented, direct-vented with EF ratings ranging from .58 to .98, and 82% to 98% tankless direct-vent models, plus, you need to take a hard look at that indirect water heater’s ratings and adjust them to what they’ll be when coupled to the listed boiler. Sounds like a lot of homework, and it is, but be thankful they have limited the four corners of your exercise to just gas-fired models and only one brand of indirect water heater!
The math doesn’t look right for running out of hot water. Rheem lists a recovery chart illustrating continuous-duty recovery-rates, which shows a 65-gal. storage and recovery capacity of 120°F water (for the 75,000-Btu model) if the DHW demand is 10 minutes. That 5:45 a.m. shower (2.5 GPM for 10 minutes) will use 25 gallons of water and the mix ratio is .8125 if the bather adjusts to 105°F (providing the HW is 120°F and CW is 40°F), resulting in a DHW usage of 20.3 gallons. Next DHW demand doesn’t occur for another 1.35 hours. Sometime between 7:30 and 8:30 in the morning, three more bathers take showers. Let’s assume they live in separate units and all three start their 10 minute showers at the same time. Going back to the first single-use DHW demand, we can project a need for 60.9 gallons at 120°F. We’re getting close to the water heater manufacturer’s listed capacity, but still show a slight reserve. All it would take to run out of hot water would be a higher than 2.5 GPM flow rate and/or longer bathing times. It’s not uncommon for folks to remove flow restrictors from 2.5 GPM showerheads and higher than normal (in excess of 70 PSI) pressure will both result in higher flow rates. You can either ask the condo association to time the flows or offer to test a few where they can grant easy access. If you have a bucket with gallon markings and a watch with seconds shown, this is a simple and reliable means for solving the mystery.
Next month we’ll explore the ECV & ROI!
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