Hot tub mod to make use of overnight cheap rate

FreeBear

bintyandthebeast said:

FreeBear said:

bintyandthebeast said:

- there is no timer on the internal heater, it’s set-and-forget. If I wanted to raise temp I would need to go out at midnight every day to do this and shortly before 5am to reduce temp again manually (taking my sorry self outside in the cold and wet in the middle of the night!)

A programmable (smart ?) thermostat with a remote temperature sensor would do the job. But.... They are not designed for use outside, so would need a suitably sealed box. Nor are they capable of switching large loads, so you'd need to use a contactor (heavy duty industrial relay) to switch the heating element on/off.

Point me in the right direction and I'll take one!

One at random - https://www.ebay.co.uk/itm/293794288892 - Comes with a remote sensor that you can dangle in the water (check that it is waterproof first). It claims to have a 16A relay fitted, but I would never trust a cheap Chinese relay to switch heavy loads repeatedly. You also need to check the range of temperatures that can be programmed in.

Or if you are feeling masochistic, whip up a control system using an Aduino, ESP32, or Raspberry Pi.

silvercar

Would it be too expensive to invest in a battery to store the energy generated by your solar panels?

ThisIsWeird

Section62 said:

There's no indication the device has any safety approvals, not even a 'CE' mark.

The only thing I'd do with one is to send it to BigClive, so he could do this with it...

https://www.youtube.com/watch?v=hIUJWIT9GrU&t=0s

I don't understand what Clive is on about there. Is he suggesting the actual element is in electrical contact with the water? Surely not. But, that heater is well scary, and clearly to be avoided.
If a suitably safe waterproof and earthed and accurately thermostated immersion element of around 1kW can be sourced, then I suspect that would do the job just fine. Connected via a plug-in timer and an RCD.
The element itself suspended under a float, so it sits deep in the tub to set up a decent convection, but a few inches from the bottom so as not to cause a hot spot. 
~1kW should be fine, imo.

FFHillbilly

does the solar diverter not have a timer function? that would be the simplest way of doing it. the Myenergi Eddi has a boost function, just set the boost times for the cheap rate period

just read it again and I see the solar diverter is currently installed on the hot water only. also I did have an idea of adding a contactor to control the power supply to the hot tub, but this coming on off in the middle of the night is unlikely to work as the hot tub may require buttons to be pressed to get it to turn on?

what about just adding a heat pump to it? https://www.thetubcompany.co.uk/heat-pumps/

Section62

ThisIsWeird said:

I don't understand what Clive is on about there. Is he suggesting the actual element is in electrical contact with the water? Surely not. But, that heater is well scary, and clearly to be avoided.

Yes, exactly that... its how it works.  The 'heater' has two electrodes and passes current through the water to heat it.  There are other videos online showing what happens when this type of device is used in water which is more conductive than typical tapwater - not pretty.

When working as designed there will be some voltage potential in the water outside the body of the device - which is dangerous enough.  But if the device partially fails you could have a situation where a 'live' electrode is energising the whole body of water at mains voltage.  If the water isn't 'earthed' then it won't necessarily trip any protective RCD and would effectively be a deathtrap waiting for its victim to dip a hand or toe in.

The problem I have with the item on that Amazon link is that whilst it appears like it may be a conventional ('immersion heater'-type) heating element, the "heavy-duty stainless steel guard" obscures the business end, so for all we know it may be a similar kind of 'electrode' heater to the one in Clive's video.  That might explain the "Do not touch water or heated containers when boiling water" instruction.

ThisIsWeird said:

The element itself suspended under a float, so it sits deep in the tub to set up a decent convection, but a few inches from the bottom so as not to cause a hot spot. 

Given the value of the hot tub I'd be unhappy relying on either i) the flotation device; or 2) the depth of water, to keep the heating element away from the probably meltable tub.

If the floation device fails, or the water level drops without being noticed, the 'few inches' could be replaced by direct contact between the element and the bottom of the tub.  Not pretty.  However unlikely that might be, if it does happen the resulting damage could easily wipe out all the potential savings from taking advantage of cheap-rate electricity.

ComicGeek

FFHillbilly said:

does the solar diverter not have a timer function? that would be the simplest way of doing it. the Myenergi Eddi has a boost function, just set the boost times for the cheap rate period

just read it again and I see the solar diverter is currently installed on the hot water only. also I did have an idea of adding a contactor to control the power supply to the hot tub, but this coming on off in the middle of the night is unlikely to work as the hot tub may require buttons to be pressed to get it to turn on?

what about just adding a heat pump to it? https://www.thetubcompany.co.uk/heat-pumps/

The heat pump was what I was thinking. Strangely we have just had a swimspa and heat pump installed by The Tub Company this summer. 

There's no timer function on the swimspa (like the OP's hot tub) - you set the required temperature and then the heater kicks in as and when to achieve this. But the heat pump I have installed (to replace the electric heater) has an integral timer within the app control, so I can either set it to run at cheap night time rates, or in the afternoon when I'm likely to have excess solar generation or spare capacity in the batteries. 

I'm also getting an efficiency of 800% from the heat pump on a warm day like today, so major running cost savings. 

ThisIsWeird

Section62 said:

ThisIsWeird said:

I don't understand what Clive is on about there. Is he suggesting the actual element is in electrical contact with the water? Surely not. But, that heater is well scary, and clearly to be avoided.

Yes, exactly that... its how it works.  The 'heater' has two electrodes and passes current through the water to heat it.  There are other videos online showing what happens when this type of device is used in water which is more conductive than typical tapwater - not pretty.

When working as designed there will be some voltage potential in the water outside the body of the device - which is dangerous enough.  But if the device partially fails you could have a situation where a 'live' electrode is energising the whole body of water at mains voltage.  If the water isn't 'earthed' then it won't necessarily trip any protective RCD and would effectively be a deathtrap waiting for its victim to dip a hand or toe in.

The problem I have with the item on that Amazon link is that whilst it appears like it may be a conventional ('immersion heater'-type) heating element, the "heavy-duty stainless steel guard" obscures the business end, so for all we know it may be a similar kind of 'electrode' heater to the one in Clive's video.  That might explain the "Do not touch water or heated containers when boiling water" instruction.

ThisIsWeird said:

The element itself suspended under a float, so it sits deep in the tub to set up a decent convection, but a few inches from the bottom so as not to cause a hot spot. 

Given the value of the hot tub I'd be unhappy relying on either i) the flotation device; or 2) the depth of water, to keep the heating element away from the probably meltable tub.

If the floation device fails, or the water level drops without being noticed, the 'few inches' could be replaced by direct contact between the element and the bottom of the tub.  Not pretty.  However unlikely that might be, if it does happen the resulting damage could easily wipe out all the potential savings from taking advantage of cheap-rate electricity.

Clucking Bells - that's a crazy design!

As for the essential principle of a suspended immersion, yes, there's always stuff that could go wrong, as you've suggested. But I am referring to a decent quality device with a 'stat set to the required ~40o. Yes it can go wrong, but so can the gubbins in the tub itself. 

bintyandthebeast

ComicGeek said:

FFHillbilly said:

does the solar diverter not have a timer function? that would be the simplest way of doing it. the Myenergi Eddi has a boost function, just set the boost times for the cheap rate period

just read it again and I see the solar diverter is currently installed on the hot water only. also I did have an idea of adding a contactor to control the power supply to the hot tub, but this coming on off in the middle of the night is unlikely to work as the hot tub may require buttons to be pressed to get it to turn on?

what about just adding a heat pump to it? https://www.thetubcompany.co.uk/heat-pumps/

The heat pump was what I was thinking. Strangely we have just had a swimspa and heat pump installed by The Tub Company this summer. 

There's no timer function on the swimspa (like the OP's hot tub) - you set the required temperature and then the heater kicks in as and when to achieve this. But the heat pump I have installed (to replace the electric heater) has an integral timer within the app control, so I can either set it to run at cheap night time rates, or in the afternoon when I'm likely to have excess solar generation or spare capacity in the batteries. 

I'm also getting an efficiency of 800% from the heat pump on a warm day like today, so major running cost savings. 

That would be ideal but ROI would be maybe 7/8 years based on what I can see. If I found one for say £1,000 that would be great but they're considerably more than that. Maybe one day when we can stretch to a swimming pool we'll get one 🤔

bintyandthebeast

ThisIsWeird said:

Section62 said:

ThisIsWeird said:

I don't understand what Clive is on about there. Is he suggesting the actual element is in electrical contact with the water? Surely not. But, that heater is well scary, and clearly to be avoided.

Yes, exactly that... its how it works.  The 'heater' has two electrodes and passes current through the water to heat it.  There are other videos online showing what happens when this type of device is used in water which is more conductive than typical tapwater - not pretty.

When working as designed there will be some voltage potential in the water outside the body of the device - which is dangerous enough.  But if the device partially fails you could have a situation where a 'live' electrode is energising the whole body of water at mains voltage.  If the water isn't 'earthed' then it won't necessarily trip any protective RCD and would effectively be a deathtrap waiting for its victim to dip a hand or toe in.

The problem I have with the item on that Amazon link is that whilst it appears like it may be a conventional ('immersion heater'-type) heating element, the "heavy-duty stainless steel guard" obscures the business end, so for all we know it may be a similar kind of 'electrode' heater to the one in Clive's video.  That might explain the "Do not touch water or heated containers when boiling water" instruction.

ThisIsWeird said:

The element itself suspended under a float, so it sits deep in the tub to set up a decent convection, but a few inches from the bottom so as not to cause a hot spot. 

Given the value of the hot tub I'd be unhappy relying on either i) the flotation device; or 2) the depth of water, to keep the heating element away from the probably meltable tub.

If the floation device fails, or the water level drops without being noticed, the 'few inches' could be replaced by direct contact between the element and the bottom of the tub.  Not pretty.  However unlikely that might be, if it does happen the resulting damage could easily wipe out all the potential savings from taking advantage of cheap-rate electricity.

Clucking Bells - that's a crazy design!

As for the essential principle of a suspended immersion, yes, there's always stuff that could go wrong, as you've suggested. But I am referring to a decent quality device with a 'stat set to the required ~40o. Yes it can go wrong, but so can the gubbins in the tub itself. 

I thought I'd done one which would be designed for the inflatable tubs  I could use but can't seem to find one.
They seem to have a water pass through system but that would mean not having the insulating cover fully in place so expect it would be far from efficient

bintyandthebeast

silvercar said:

Would it be too expensive to invest in a battery to store the energy generated by your solar panels?

It would, unfortunately.
£6-11k for something which would save around £3-4 per day doesn't compute for me.