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Thread: Vertex or 75 gal?

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    DIY Junior Member Novablue's Avatar
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    Default Vertex or 75 gal?

    Should I have installed a Vertex 50 gal HE unit or a 75 gallon conventional water heater? I have an 80 gallon tub and 4 persons in the house. The price points are similar after the tax rebate and there is no problems with space - the basement is fairly open. The existing unit is a powervent 50 gallon.

    Thanks.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Don't know, but consider that a typical WH can only dispense about 75-80% of it's contents before the temperature starts to drop significantly. Now, depending on the aquastat setting on the WH, you will be mixing at least some, and maybe a lot of cold water to make it comfortable. The first hour rating is more relevant when taking a shower, since the flow is lower. To fill a tub, depending on the supply lines, you'll be using water as fast as the pipes can flow it, and with a 3/4" supply, maybe as much as 18gpm. You might overrun either WH, as they wouldn't have much on-time to recover at a high flow rate. The way to maximize the hot water quantity, is to run the WH hot, and install a tempering valve to prevent that excessively hot water from hurting anyone. This way, you can get the equivalent of more hot water volume in the same tank since it is mixed with some cold on its output. This will increase your standby losses, though, and therefore decrease the overall efficiency. How much of the hot is in the percentage used will depend on how cold your cold water is...in the winter, you'll need more hot since the cold will be much colder; in the summer, the cold may be quite warm, and to cool the hot off, you'd need much more cold. So, it could be fine in the summer, and not enough in the winter.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    80 gallons is heluva tub to fill primarily with stored-heat. The Vertex squeezes more heat out of the ~75KBTU burner than the typical 75 gallon unit, but neither is likely to be able to fill it quickly in one shot. A Vertex 100 (with a 100K burner instead of the 76K) would do measurably better, and might just squeak in a full fill before running too cold. A condensing tankless with a 199KBTU burner would be better yet, and would also have tax rebates & other subsidies behind it.

    First hour ratings aren't as relevant in tub-fills as how much you can get out of the first 10 minutes (which isn't measured & rated.) At 8gpm it'd take 10 minutes to fill your tub, and burner on a tank would be firing for at least 7-8 minutes. With the Vertex 100 you'd heat another 20-25 gallons or so during that firing time. With the 76K burner only 15-18. Either would likely be a few gallons shy of a tub-fill unless you bumped the storage temp a bit. With a lower efficiency 75 gallon tank with a ~75K burner you'd be looking at only 10-12 gallons over usable stored heat on the warmest days of summer, which might make it. With a big-burner condensing tankless in VA you'd fill it in 10 minutes or less most of the year, but mid-winter when the incoming water is cooler it might take 12-13.

    If your heating system for the house is hydronic (pumped hot water with baseboards or radiators) you'll may get more first-10-minute gallons out of a 40 gallon indirect-tank running off the heating system's boiler. (Depends on both the boiler and the indirect.)

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    DIY Junior Member Novablue's Avatar
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    Thanks. I agree the flow rate is key here and with a 3/4 supply line to the tub I can quickly overrun either unit. If I turn down the flow rate a bit the conventional 75 gallon unit should meet my needs. This is also in line with the rule of thumb I've seen in other posts. I think I'll go with the Bradford White unit and be done with it.

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