Using 40gal House Water Heater w/ point of use 12KW for TUB

palmbayMOctober 22, 2012

I am remodeling my bathroom and purchased a 90gal-97gal Jacuzzi air tub.

My 40 gal hot water heater for the house didn't do the job for our old tub (set at 130 degrees) so during remodeling we ran a 6/3 and 240V 50AMP service for a Bosch US12 - 12kW point of use heater to soley be used on the tub.

The idea...was to run the hot to the Bosch and when the hot water ran out or began to drop, the Bosch would take over.

Receiving the heater the instructions state cold water only (max 80 degrees). Speaking to Bosch they say it will work about 2-2.5 gpm if we mix the hot/cold to feed it with 80 degrees, but that will be about 36 minutes to fill.

What I really need is a valve that will divert my hot water from the 40 gal house to the Bosch when the temperature falls. The vendor suggested a manual valve.

This is a luxury spa like bath area and I cannot have some exposed valve like this and I don't want to increase my home unit because 40 gal in FL is fine for the house, the TUB is the problem.

Any suggestions on what to do? Still in rough-in stages, can do ANYTHING!

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My brother in a distant State had a similar problem and his contacts had no solution to offer. So he asked me. He had a point-of-use heater in the bathroom and that was the only hot supply for his 75 gallon tub. The tub was not used often. The point-of-use heater had such a low flow rate that the first water into the tub had cooled by the time the tub was filled and the entire thing was chilly. He also had only a low crawl space under the house for adding any equipment.
So I did all the calculations on the basis basis of the BTU required-- ergo, the tub required X BTU for a fill of Y gallons at temperature W. His plumber determined the largest electrical water heater he could find that would fit into the crawl space. I calculated the needed temperature for that size heater to store the required X BTU. And it turned out that it would have X BTUs at temperatures just under the maximum possible setting. So the heater was installed, becoming the only hot water supply to that tub. Power to the heater is controlled by a relay which is controlled by a wall switch in the bathroom for the occasional use. Keeping the heater hot all the time is possible if desired. He turns on the heater about 1 1/2 hours before using the tub. The heater supplies the required heat of X BTU and the cold water supply to the tub makes up the remainder of the required water volume.
That's the procedure. You can do your own calculations.

    Bookmark   October 22, 2012 at 5:48PM
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Install 2 water heaters in series, with total gallons equal to demand requirement.
First water heater receives incoming cold water from outside.
Hot water outlet on first water heater is connected to cold water inlet on second heater.
Hot water outlet on second heater supplies household hot water.
Use 240Volt timer on fist water heater to schedule hot water usage for large bathtub.
When first heater is turned off, it acts as tempering tank in winter to passively preheat incoming cold water.

Here is a link that might be useful: Two water heaters

    Bookmark   October 29, 2012 at 12:04AM
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