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 Post subject: Running costs
PostPosted: Tue Nov 19, 2013 8:25 am 
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I was wondering about the running costs for a hot tub,
1500 litres capacity with a 3Kw(?) heater.
Is it cheaper to keep it on constantly maintained at 38-39C, even when it is used only once in a while, or is it cheaper to switch it off between uses, and only heat it up the day before you know you will want to use it?

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 9:09 am 
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Difficult to calcuate unless you know the rate at which the tub dissipates heat. You 'could' find out by measuring the temperature drop over a period of time to see how much heat loss occurs - this would have to be adjusted for ambient temperature too. Since the amount of energy required to heat water is a known factor you can then calculate how much energy is needed to maintain the heat content.

Similarly you can calculate how long it takes to raise x litres to 39 degC (from ambient) and compare the two.

I don't know whether your hot tub is insulated or not but I filled the underside of my bath with loft insulation and it retains the heat far, far longer now :-P

4,184 watts will heat a liter up by 1C every second.

2,092 watts will heat a liter up by 1C every 2 seconds.

1,046 watts will heat a liter up by 1C every 4 seconds.

How many litres are in your tub?

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 9:29 am 
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Approx 1500 l. capacity.
Insulated of course. I don't know the rate of heat loss, and anyway as you say it depends on the ambient temperature. It would need to be measured umpteen times at different seasons of the year to get an average rate of heat loss per unit of time.
I guess my question boils down to, whether it is cheaper to heat it up from scratch (ambient) each time it is to be used, assuming a rate of usage of once in 7 to 14 days, or whether it is cheaper to let it run continuously?

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 10:12 am 
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My mate let his cool, which ended up making the thing a nuisance to use........they had to turn it on about 12 hours before they needed it!!!!!!

I'd just drop it to the lowest you can sit in it, make sure you keep the lid on and crank it up those degrees when you want it.

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 10:29 am 
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i think in the winter months you will be talking 2kw per hour day and night for 3 months or 25p per hour £6 a day
£42 a week
£180 a month
£540 for 90 days or 3 months

it may cost more in the winter if it struggled it could be £810 [full power all the time]
my guess is somewhere between £500-£750 for the 3 months but only time will tell
and thats at 12.5p a kw off electric

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:04 am 
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Its always going to cost more to keep it hot than to let it cool and re-heat it however depending on how often you use it and how quickly it cools down it may only be a little more or it may be a lot more. You have to balance this with the convenience of having it hot and ready to use whenever you feel like getting in. Every 7-14 days let it cool and re-heat will be a lot cheaper than trying to keep it hot though as I said it does depend on how quickly it cools down. If you leave it from hot to go cold and by the next time you come to use it it is completely cold then you have not wasted money keeping it warm. However if when you come to next use it it still has some heat left in it then would have only cost you a bit more to keep it hot. 3Kw will heat it by roughly a degree an hour. You might find 38-39 just a bit warm to stay in for very long 36-38 works better for me. I only use mine for 6 months of the year. If I'm not going to use it for 3 or more days I let it go cold. If I let it go cold I will usually drain 1/3 to 1/2 the water before switching the heating back on. To replace the drained water I turn the central heating tank stat in the house up to max and run a hose pipe from the hot tap into the tub. This way I can bring it up from cold to 36-38 in a couple of hours instead of a day and most of the reheating is done by (cheaper) gas central heating rather than (dearer) electricity.

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:12 am 
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To heat the tub from ambient to 39 degC will take:

11.6 hours from 0 degC (and cost £5.92 based on 17p per unit)
8.7 hours from 10 degC (and cost £4.44)
5.5 hours from 20 degC (and cost £2.80)

It takes 18 minutes to raise the water temperature in your tub by 1 degC at the full 3kW of the heater.

This means your heater can compensate for a total heat loss not exceeding some 3 deg C per hour AT FULL POWER - in other words, during USE you will be losing heat faster than you can put it in!

However, with the lid kept on I can only guess at the rate of heat loss which I reckon would be half :dunno: the rate??????

It wold cost you around 25p per hour (£6/day) to keep it at useable temperature.

Given it only costs around £6 to take it from bollock-shrinking to mammary-floating then I'd not bother with trying to keep it 'on tap' - just make sure you leave (up to) 12 hours to make the soup (on a freezing cold day).

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:23 am 
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I agree with what kellys_eye said except for the heat up time. You are assuming Zero heatloss during warm up. There will be some therefore it will take longer.

As I said with mine which is about the same volume and also a 3Kw heater it heats it by about one degree an hour, maybe 45 mins in practice. going from say 30deg to 37deg. Going from say 20deg to 27deg is a bit quicker because the losses (speed at which the hot tub is loosing heat) during that time are less.

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:27 am 
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.... ok, disclaimer. All calculations were based on 100% efficiency and zero losses :lol:

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:28 am 
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If you want to leave it on all the time it might be worth looking into economy 7 and heating it at night but either way hot tubs in the winter are yea expensive :sad:

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 11:29 am 
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I should have said I still ALWAYS turn mine off overnight and back on in the morning when I am 'keeping' it warm. In SUMMER it looses about 5 deg overnight (12 hours) and takes about 4 hours, maybe a bit less to come back up to temperature.

:thumbright:
:lol:

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 Post subject: Re: Running costs
PostPosted: Tue Nov 19, 2013 4:26 pm 
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Thanks everyone. It's all food for thought.

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