spa water heating

[bhm1712]

my mother has a large indoor fitted spa which has a 6kw heater inline. the water content is 600 gallons. The water no longer seems to heat up much like it used to do, it used to take about 72 hours to get to 40 degrees. now even after 7 days it only reaches 25 degrees. i have tested it all electrically and everything is fine. there is 230 volts at the heater terminals and each element has 31 ohms resistance. if i frig the control circuit so the water stops flowing but the heater stays on, the heater tube gets red hot and the water boils. so therefore electrically it is fine.

any ideas as to what could be preventing the heating? i thought maybe the tube was airlocked but it appears not to be as i have bled any air out. there is also adequate flow through the heater.

i have not posted in electrics as i feel its not an electrical problem anymore.

 

[seco services]

is it scaled up ?

 

[bhm1712]

no it is not scaled up, 2 elements are copper coloured with no scale and 1 of the elements is slightly blackened as if in the past it has been heating dry. however the black element gets hot too so it is not burnt out, you may ask how i know that, well i did a finger test and ended up with a burnt finger lol

 

[seco services]

can you alter the speed the water flows through it ?

 

[bhm1712]

no its preset by a fixed speed circulation pump which if there is not enough flow cuts out a flow switch and turns off the heater, this test fine

 

[seco services]

what make/model etc is it ?

 

[bhm1712]

right this is where i have a problem, the system is a hotsprings system but its 15 years old.

the pumps are hayward, the heater sticker has peeled off mostly and all i can make out is "BEH..." against make, and the model V2R

I have searched BEH and heaters but nothing comes up.

 

[bhm1712]

initially i thought maybe although the control circuit was fine, that the elements may have been rewired in series instead of parallel last time it was removed but the problem started a while after this. i have checked the wiring and the elements are in parallel as they should be, so yet again it does not point to electrical!

its also drawing around 26 amps which is correct for 6kw

 

[seco services]

the only thing that appears strange is you say each element is giving 31 ohms.
a 3kw element should only give about 19 ohms. same as an immersion element.

 

[simond]

The resistance of the element suggests the bank of immersions would have an output of 1858 watts at 240v, slightly less at 230v.

That therefore equates to 5574w total load.

These are clearly 2Kw elements, so the impedance of a 3Kw element is not particularly relevant.

If you were designing a hot tub you would build in a thermostat so that the insulated water container did not get unpleasantly hot or boil.

I have a 3000litre Hotspring hot tub (your Spa is 4800 litres) and due to the good insulation it uses just a 2Kw element and heats up from scratch in around 24-36hrs.

In other words, the answer is pretty obvious, it is the thermostat.

 

[bhm1712]

there is no thermostat, i have linked it out as that was faulty years ago. now the heater just heats the water to 40 degrees and cuts out on hi limit, or should do.

the heater is actually on.....even if there was a thermostat it should still heat the water to the setting on the stat, it just does not reach a temp which is suitable for bathing anymore.

 

[bhm1712]

even if there were a stat, the way i frig the heater in order to test its heating up would bypass the stat as i am frigging the flow switch and forcing on the contactor.

 

[simond]

You describe the fault but keep certain information back, like you've previously butchered it from the makers spec.

What would happen to your mother if the high limit stat failed? Is she aware that the unit could boil?

Getting back to the problem, you cannot destroy energy.

So if you believe it is on all the time, where is your 5.7Kw going, exactly?

If there is a high/low limit stat it is most unlikely to be 6Kw rated, is there a contactor? Have you checked it?

 

[bengasman]

A 6 kw heater will increase the temperature of 2.5 tons of water approximately 0.5 C per hour which roughly matches the original duration of the time needed to heat up.
It only takes a very light flow of air over warm water to evaporate enough to cool it down significantly.
I can not recall the exactly what the energy is, required to get water from liquid to gaseous form at the same temperature, but I think it is something like 40 times what is needed to warm it 1 degree.
If you want to experience first hand how much that means, get the room to a nice 24 degrees, step out of the shower without drying and walk from to the other side of the room. Those few seconds will be enough to make you unpleasantly cold, despite the high temperature in the room.
Chances are that there is draft above the spa, or below it.

 

[simond]

Yes, that's probably another modification he's made and conveniently forgot to mention. Thrown the insulated lid away.