water heater upgrade for new large whirlpool bathtub

Old 10-13-11, 02:30 AM
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water heater upgrade for new large whirlpool bathtub

I'm a mechanical engineer and I specialize in fluid and thermodynamics. I thought I would share my story of how I increased hot water capacity to accomodate a new whirlpool tub.
We recently purchased and installed a 100 gallon whirlpool bathtub in a house we just purchased. The ten year old 40 gallon water heater was woefully inadequate. So I had to decide how to increase the available hot water capacity. Here are the choices I came up with:
* tankless gas fired water heater
* 75 gallon water heater
* add another water heater in series or parallel with existing 40 gal heater

I could purchase a Rheem 8.4 gpm water heater for about $900 and install it myself for another $200-$300. Or I could install a 75 gallon water heater for around $1100.
Incidentally, I thought I would see what water heater contractors recommended. I had a contractor look at my situation and he offered to install the Rheem 8.4 gpm tankless. He would install it for $4000, so he was charging $3000 to install a tankless water heater. This is a job that would take a contractor perhaps four hours. Another contractor insisted that I must have a high efficiency boiler combined with a storage tank, and he wanted $13000. He kept insisting that my solution wouldn't work, until I told him I am a mechanical engineer. He suddenly changed his mind. Two contractors offered to install a 75 gal water heater for $1350 or $2800. At those prices, these plumbers make more than I do, and I spent six years in engineering school to get a master's in mech engr, and I have been practicing for 25 years. All I can say is buyer beware! Get many quotes and spend some time studying, or you might end up paying a contractor $500-$1000 an hour to do a rather simple job.
To make a long story short, I decided to install a 50 gallon water heater and connect it in series with my existing 40 gallon. I did the work myself. Took me about six hours, including a couple trips to the box store.
The total cost was $600. I now have 90 gallons of hot water and 80,000 BTU/hr input to heat water quickly. That is more than enough for our whirlpool and the rest of the house.
The series design works well because the new 50 gallon heater does most of the work. I turned the temp down on the old 40 gallon to make it last longer. It won't fire much anyway because it is being fed hot water from the 50 gallon. The only time the 40 gallon will fire is when the 50 gallon is depleted and starts sending cold water to the 40 gallon.
The tankless option was a close second, but I found that it is not as economical as many tankless manufacturers claim. Consumer Reports had a lab do a study comparing tankless water heaters with regular tank heaters. They found it would take 22 years to recover the additional cost of a tankless, and the tankless is warrantied to last only 12 years, so essentially you never recover your investment.
I thought I would share my story with others. I was able to get 90 gallons of hot water supply for $600. I spent a great deal of time studying the options, and since I'm a mechanical engineer I understand all of the concepts. It's a simple problem compared to the work I do everyday.
Good luck!
Old 10-18-11, 12:22 AM
Join Date: Mar 2000
Location: Arlington, WA
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Welcome to the forums.

While you're at it, install a recirc system and save a bunch of water!! How long do you have to wait for hot water at the farthest valve from the water heater??

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