Hot tub mod to make use of overnight cheap rate

Hi all,

We have a great hot tub that we use regularly but the cost of power is silly. 

Summer months are fine as we have solar panels and the tub itself is pretty efficient despite its significant size (1,725 litres). Now it’s getting colder and the sun is hiding it’s a totally different prospect. I reckon we will pay between ~£100-200 per month over the coming 6 months and I want to reduce this.

we have a solar diverted linked to our hot water by Apollo Gem which saves me a load of money through the year and I want to replicate this with our tub.

I don’t want anything too fancy, but my thinking is we get electricity for 5 hours overnight at 9.4p per kWh, which is around 2/3rds less than the remainder of the day. I’d like to use an extra heating element to heat my tub to, say 42 degrees at this cheap rate and then allow the tub to cool through the day and have the internal heater running at, say 37.5 degrees for the remaining “expensive hours”. 

What I need to do this is, I think, an element inside a protective sheath (to not damage the tub) linked to a thermostat and timer. Doesn’t sound like a big ask but I can’t find anything like this. Does anyone have anything ‘ know of anything like this that could work please/

if you are an electronics genius and want to build me one for testing before marketing it then I’m happy to be a guinea pig!

Many thanks for any help!


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  • Belenus
    Belenus Posts: 2,548
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    edited 5 October 2023 at 10:40PM
    Why do you want to use an extra heater? Can you not use the hot tub's existing internal heater?

    If by 'element inside a protective sheath' you mean something like an immersion heater element, that sounds extremely impractical and dangerous to me.

    How will you waterproof the electricity supply? Toaster in the bath comes to my mind.

    How will you ensure that nobody tries to use the tub while that element is live? You could find yourself on a manslaughter charge if someone dies, even if that person is a stranger using it without your knowledge or permission.

    Perhaps I have misunderstood what you are proposing.
    A man walked into a car showroom.
    He said to the salesman, “My wife would like to talk to you about the Volkswagen Golf in the showroom window.”
    Salesman said, “We haven't got a Volkswagen Golf in the showroom window.”
    The man replied, “You have now mate".
  • Jonboy_1984
    Jonboy_1984 Posts: 1,191
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    Can you not add a programmable timer to the existing heater?
  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    edited 6 October 2023 at 7:48AM
    BatB, could you tell us what controls the tub currently has? As suggested above, surely it has just an adjustable 'heat' setting which could be timed to come on overnight - how high can this thermostat be tweaked up to?
    I'm guessing your issue is that you cannot, for some reason, use the tub's own temp stat to get it up to the required ~40o, for it to remain warm enough for the following evening?
    Ok, assuming the temp you require won't damage the tub, either from the raised water temp or directly from the immersed element, then wouldn't it be advisable to go low voltage for safety?
    Alternatively, consider instead heating a separate insulated hot tank, and running the water into the tub before being required. You'd likely soon be able to work out what quant and temp of stored water you'd need, and set it for just this.
    It's an interesting one - at first glance seems 'easy', but actually has a number of tricky issues.
    I presume the tub already has a highly insulated lid?
  • Belenus said:
    Why do you want to use an extra heater? Can you not use the hot tub's existing internal heater?

    If by 'element inside a protective sheath' you mean something like an immersion heater element, that sounds extremely impractical and dangerous to me.

    How will you waterproof the electricity supply? Toaster in the bath comes to my mind.

    How will you ensure that nobody tries to use the tub while that element is live? You could find yourself on a manslaughter charge if someone dies, even if that person is a stranger using it without your knowledge or permission.

    Perhaps I have misunderstood what you are proposing.
    Thank you for your reply.

    in answer to your questions:
    - 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!)
    - an immersion heater idea is what I’m after. Fish tanks use them from what I can see
    - our tub in in a very large pergola with power inside. I would connect a device to side of tub or nearby and have the heater dropped in. There are some “floating heaters” on Amazon which can do this but they are rudimentary. They’re designed for heating baths or buckets of water. I would want one that has a build-in timer, although I could use a smart plug or mechanical timer as assume it would be 13amp. I have a 16 amp supply to the tub so could get my electrician to tap into that if needed
    - regarding the manslaughter charge, I’m not looking to do something unsafe. There are CE approved floating heaters and fish tanks have a scaled down version, so in my mind surely there’s a safe option out there. 

    Possibly there is a need to get someone to build a separate timer module to run my existing heater but I don’t know of anyone with this expertise and would be fearful for my tub’s warranty. 
  • Can you not add a programmable timer to the existing heater?
    I’d love to but there’s nothing from the manufacturer to do this.

    we have a 2021 Hotsprings Envoy if that makes a difference.

    I expect I would need to have something bespoke built to do this
  • BatB, could you tell us what controls the tub currently has? As suggested above, surely it has just an adjustable 'heat' setting which could be timed to come on overnight - how high can this thermostat be tweaked up to?

    I'm guessing your issue is that you cannot, for some reason, use the tub's own temp stat to get it up to the required ~40o, for it to remain warm enough for the following evening?

    Ok, assuming the temp you require won't damage the tub, either from the raised water temp or directly from the immersed element, then wouldn't it be advisable to go low voltage for safety? 

    Alternatively, consider instead heating a separate insulated hot tank, and running the water into the tub before being required. You'd likely soon be able to work out what quant and temp of stored water you'd need, and set it for just this.

    It's an interesting one - at first glance seems 'easy', but actually has a number of tricky issues.

    I presume the tub already has a highly insulated lid?
    - low voltage supply I don’t think would be sufficient as wouldn’t get 3kw + to heat the volume of water
    - the chemical balance in my tub would be disrupted significantly if i were to drop a couple of hundred litres of tap water into it. Unfortunately not an option
    - insulated lid installed already

    like you say, sounds straightforward but it’s driving me nuts.

    our solar diverted linked to our immersion tank has all the required programmable features. It allows a minimum temperature all day; 2x 3kw outputs which can heat elements when we have excess solar power: a boost timer function (which I could use to heat the tub at night); and a plug in thermostat to controls shut off to stop overheating. Downside is it’s expensive and the business has closed down. Plus I would still need an immersion element which is sheathed and insulated to avoid damage to the tub or anyone touching it.
  • Section62
    Section62 Posts: 7,482
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    - regarding the manslaughter charge, I’m not looking to do something unsafe. There are CE approved floating heaters and fish tanks have a scaled down version, so in my mind surely there’s a safe option out there. 

    The problem is there is a fair amount of stuff which is CE marked (note, not "approved") which is dangerous. A CE marking is no guarantee of safety.

    Water+Electricity+People is not a good mix.  Add in heat and there are so many dangerous situations you need to avoid that I wouldn't be confident you'll be able to find anyone professional willing to help you, other than in installing the hot tub manufacturer's supplied heater unit (if they did one).

    Aside from the risk of electric shock, you would need to ensure the heater fails safe, and that there is some form of anti-scald protection.  It isn't enough to rely on people dipping a toe in the water and going 'ouch!'... you (as the owner) would be expected to ensure the water temperature never exceeds a dangerous scald temperature.

    On the point about there being a 16A supply - the electrician would have to take into account the load from the tub itself before adding a 13A socket. The tub plus 13A is likely to be more than 16A, therefore you will probably need to either have the supply upgraded, or else run a separate supply to the socket.
  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    2.5kW - if it does have that power - should be fine.
    Your tub will already be at - what? - 35+ degrees at the time the cheap leccy kicks in, and your timer switches on. A few hours to bring the water up to your required temp should be very doable.
    Well done - you have found your own solution :-)

    (It would need to be reliable, tho', with little risk of overheating.)
  • How is your hot tub connected? What about something like smart plugs and you can set it to turn on at specific times (or just set via your phone).

    Apologies if this is a silly or crazy suggestion - I have never had a hot tub before so no idea how it is powered!
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