Hot Tub HX Sizing

Robert O'Connor

I am trying to understand HX sizing for hot tubs and would also like to know what additional load this will put on the boiler.

The Tub is approximately 6'x6' x 2.5'deep. It is insulated and is located outside. Our design temp is -10F.

It is presently heated with a 4kw electric heater. According to my heating pad this is 13,660 btu.

So if I get a HX that can deliver 13,660 btu's am I there?


Thanks as always for your input.

Robert O'Connor

Big Ed

Depends on how fast you want to heat it and temp rise

The fomula is...... Gallons of water x 8.33 x temp rise / hours to heat the pool = BTUH ........ok ......you get 7.48gal.per cubic foot of water ......You have a 6'x6'x2.5' pool= 90 cu.ft.of water ,90X7.48 =673 gals of water

Now lets say the owner wants it heated to85* in one hour....and the water is 55* ...... 673gal.x8.33x30*deltaT / 1=168,182 BTUH rating needed . Note....You also need a 168K net boiler to heat it that fast. A 13,660 BTUH heat exchanger would take .....673gal x8.33 x 30* delta T/ 13,660 BTUH =12hr and 20 min(if I did the math right).... By then the girls already left the party Me I would use the largest one that matches my boiler . While the girls are in the mood.. you know what I mean?....;)

Al Letellier

hot tub

Robert, that formula is OK assuming you reheat the pool each time you use it, but most hot tubs are filled and left heated. The BTU"s to maintain heat temp are far less that cold start up. We have a 3 section Peerless (that's right, even coming from a Weil guy) that heats a 35,000 gallon pool and a 300 gallon hot tub all season on 300 gallons of oil. Check with your manufacturer of choice for specs. We used Superstor pool heaters (20 Gallon) and it works great.

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Robert O'Connor

Thats how they use it

In this case the people do in fact keeep it hot all the time . They are not trying to save any money on setting back the temp. Thanks for your input Al, you have confirmed my suspicions.

Ed thanks for your input. You have shown me that we would have to add a section to the replacement boiler if they want to bring the temp up quickly. I will be clear with them that we are designing to maintain temp.

Thanks to both of you

Robert