Tankless water heater for whirlpool tub

I am having a 60 gallon bathtub installed and have only a 30 gallon tank water heater. I am considering installing a 12 kw on-demand tankless water heater to serve just that bath. My husband, and our plumber (who seems to have no experience with these units) do not think it will do the job, but I have done the math and, though close, I think it will work.
We live in Florida and the tap water temp is around 70 degrees in the winter, 80 plus in the summer. The flow rate for the faucet is 2.5 GPM. When I apply the formula, (Kw x 6.83 divided GPM) it appears I would get a 32.8 degree rise in temp, which would be just barely enough in the winter and ample in the summer.
Does anyone have experience with these units for this sort of application? What happens if the flow rate exceeds the unit's ability to heat to the desired temperature? Does the water cool off, or does the flow decrease?
Any suggestions?
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you might be better off to use the tankless as a pre heater for the main hot water tank for everything
have you checked your main service entrance capacity to make certain the load isnt too big along with everything else in your home?
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I think we've determined that the electric can handle it.
The other issue is that this bath is the some distance from the hot water tank, having the on-demand would eliminate the wait for the water to heat up in that bath. It is a small unit that will fit in a hidden spot in that bath. My concern is whether it can handle the flow rate.
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You are probably looking at a cheap sink heater, a one shower Bosch takes 120 amps. If flow is more you wont heat, if voltage is poor, you wont heat. If electric supply is small you wont heat. You are just calculating, so figure in temp drop from heater to tub and cool down, maybe 20f, maybe more. 70f incomming, check that at different areas with water running 10 minutes and still take off 5f for saftey. Are you sure voltage is at rated needs, no you cant be. Improper-to small a wiring will reduce heater output also. Do you have Ng or propane, these fuels are still cheaper than electric. Unless you have a 200a service and 120 free forget tankless. People that hate tankless did not size, plan, test or install them right. So far your numbers are incomplete and have no margin for error. You also need a plumber that does this everyday and is not "learning his trade" on your pay and possible mistake. I would have a written temp output guarntee from the installer.
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I can't say anything about your calculations, but if you think it is going to be close you will be disappointed in the results. I have hooked up a few of these over the years and based on my experiences the customers were not satisfied with the units that were marginal. With instant electric water heaters you should oversize it to get the results you want. Did your calculations take into account any losses while the tub is filling up? If this is a hydromassage bath tub I recommend the optional built-in heater.
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Thanks, that's what I needed to know, but not what I wanted to hear. Unfortunately I can't get the built-in heater, because I have already purchased the tub.
I'll have to look for a different heater.
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Get a regular waterheater with a tank. I was involved with a tankless WH install at work. That thing is a worthless piece of $hit. I'd never have one in my house.
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On Sun, 26 Mar 2006 21:17:06 -0600, "J.A. Michel"

Back to the electrical discussion, its not so much the volts as it is the Amps. To get any decent kind of flow, you are probably looking at a two 12 kW unit in series which is approximately 24 kW or 81888 BTU.
Remember you are talking about filling a tub (not just shower flow).
At 240V, your heating array is going to draw 100A which is a significant peak load to all but the most heavy duty residential electrical installations.
In my opinion, you are better off with a tank. It will heat up more slowly, but you will have 80 - 100 gallons of hot water readily at your disposal without all the problems of peaking current in an effort to get instant hot water. Gas or propane would probably be a better choice for you still.
Beachcomber
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read about natural gas units 15,000 BTU/H 180,000 BTU - (Fully Modulating) first at: http://alsheating.com/Continuum.htm
your electric 12kwh is only 40,945.7 btuh. you will be filling the tub from a tub spout at a much faster rate than 2.5 gpm i hope. if there are no kids in the house just crank up the thermostat on the water heater you've got. this way you will be diluting the hot with some cold and filling the tub quicker. for those others with kids visiting the house: "TO TEST YOUR WATER HEATER'S TEMPERATURE Check it in the early morning before anyone has used the hot water. Go into the kitchen and turn on the hot water tap and leave it running for two minutes. Hold an outdoor thermometer or candy thermometer in the stream of running water until the temperature stops rising. If the temperature is between 120 and 125, good. If higher, find the thermostat on the water heater and turn it down. Gas water heaters have an external thermostat, near the bottom. Electric water heaters have two panels screwed to the top and bottom of the tank or one panel on the side. Set it to "low" or "Energy Efficient." Wait 24 hours and then test the water temperature again to see it is in the safe range. Consult a professional if the temperature did not go down." this and more at: http://www.cdc.gov/nasd/docs/d000701-d000800/d000702/d000702.html
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