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Tankless meets Jacuzzi: who wins?

KB1186 Member Posts: 1
Hi, all. Hoping you can help me out here.

Our house is 20 years old and has a first floor "spa" with a two person Jacuzzi tub.

The original owners had a 50 gal tank dedicated for the tub, and a 40 gal tank for the rest of the house. The 50 gal is toast. The 40 gal is 5 years old and still cooking. We want to go tankless for the whole house.

<strong> <span style="text-decoration:underline;">Question: if we install tankless will it fill the Jacuzzi?</span> </strong>


- some folks told us we need to install 3 tankless units

- some folks told us we can use the 40 gal as a feeder for the tankless to increase flow rate

- some folks told us a tankless will never fill a Jacuzzi, or it will take forever

Other note:

- we estimate we will use the Jacuzzi 1-2 / month, and we realize we should not do dishes, wash clothes, or have everyone else take showers at the same time.

Any help is much appreciated.

Many thanks,



  • CMadatMe
    CMadatMe Member Posts: 3,086
    Simple Anwser

    Do the Math for the right size unit cause the math never lies...

    GPM x (Temp Rise x 500) = Btuh/hr needed

    A Roman Tub Filler for a Whirlpool/Air Tub could flow 15gpm at 40Psi water pressure. Suggest you figure that out first. Find some 5 gallon buckets, weigh them, get out the stop watch, run the water for one minute. Weigh the buckets of water. There is 8.33lbs in one gallon of water.

    Temp Rise is the difference between your incoming cold water temperature and what set point water temp you want. For instance in my area NY incoming is 50 degrees and we generally want to make 120 degree hot water. 120-50 = 70 Degree Rise

    15 x (70 x 500) = 525,000 Btu/hr. So if you install multiple tankless heaters that can produce 525,000 btu/hr for this example your good to go..

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  • Jack
    Jack Member Posts: 1,047
    Do the suggested flow test

    And determine the vol of water needed to fill your tub. A 199kbtu condensing tankless will make around 6gpm at a 70* rise (50-120). If you sit in a bathtub over 105* you are no longer bathing, you are parboiling. So at a satisfactory bathing temp you should be in the 6.5gpm to the tub. Will the tankless fill the tub? Yes! How long to fill the tub you have to test and report back.
  • heatpro02920
    heatpro02920 Member Posts: 991
    edited June 2013
    I agree with these guys..

    I have never installed a tankless that wouldn't make hot water... The only place they run into problems is with body jets... As far as filling tubs, check out the rinnai RU98 with the bath controller, this will allow you to turn it down for your tub fill and allow you to use the tub fill setting.... Also if you want it to fill faster, you can always turn it up and mix at the faucet...
  • HDE
    HDE Member Posts: 225
    How much time do you need?

    First off, the advice of turning up the tankless and mixing in more cold to fill faster.is not correct, they cant be used like a tank.

    Tankless fire based on GPM and temp rise. All quality tankless units have a water adjustment valve to control flow rate at the desired set point temp. Once they hit max BTU which it surely will it will control the flow rate. So turning up the temp likely will slow the flow rate leaving the tankless, and with the cold mixed in give you the same tub fill rate.  

    Just throwing out round numbers, if two water tanks can fill a tub at 15 GPM in 5 minutes, and a single tankless unit fills it at 7 GPM, is 10-11 minutes too long to wait for the tub to fill at the perfect temp?

    Unless you have a big GPM flow elsewhere, I wouldn't add extra tankless and all the added costs for the occasional spa tub filling, just wait longer or prepare your tub filling ahead sooner.

    And of course the tanks are full of cold water, with a tankless once the tub is done filling, you still have hot water for elsewhere.
  • heatpro02920
    heatpro02920 Member Posts: 991
    edited June 2013

    When filling our tub, if the rinnai is turned down to 98, the tub will fill much slower than if I turn it to 140 and mix in cold. Truth of the matter is the tankless itself can only flow so much water... Same goes for the shower, more water pressure when I turn it up and mix in cold....

    I have been over this before and had this conversation with the Rinnai rep, but when I showed him the difference he had nothing to say, I used to say the same thing, just set the temp you want {which is actually what I do since the flow rate and psi is fine for me}, but it does make a difference....
  • HDE
    HDE Member Posts: 225
    edited June 2013
    I will accept that argument

    I wasn't expecting 98 degrees as a set point.

    The pressure drop of the tankless waterways reduces flow by friction before max fire rate is reached.

    But I doubt 120 and 140 settings with blended cold you would see a difference in total fill rate at the same blended temp.
  • heatpro02920
    heatpro02920 Member Posts: 991
    bathroom temps are funny like that

    you fill a tub with 120 degree water, which the tankless will easily do, and you wont be able to fully sit in it without jumping out, fill it with 105 and you might last 5 minutes but will sweat the entire time, keep it around or under 100 and its comfortable and you can stay in there...

    Now a shower at 120 is comfortable and at 100 is freezing...

    I ask my customers what they usually set their rinnais at, and the average for showers are 110-115 {a few are 120 including my wife}, for tub fills most set them at 98 {the lowest setting} a few are higher but none over 105...

    And yes, you will not notice a difference between 120 straight and 140 mixed {obviously depending on the delta}, but with low temp tub fills you will, and also some spray nozzles you will also depending on the desired temp, water quantity and delta...

    To get back on the op's topic, the tankless is IMO the best option for tub filling... Ill take it over a tank anyday...
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