Hot Tub Running Costs

gromituk

Erm, you do have the lid on all the time it's not in use, don't you?

As others have said, the rating in kW has rather little to do with the ENERGY consumption (in units) as opposed to the POWER consumption (how fast the meter spins) when the element is on, because the element is controlled by a thermostat. The bigger the element, the less time it will have to be on to maintain a certain temperature. What matters is the insulation - how fast the heat escapes. So adding an extra layer was a good thing to do.

The hotter the water is above ambient temperature, the more rapidly heat will escape - in direct proportion. So, if the outside temp. is 10 degrees and the water is at 20 degrees, the difference is 10 degrees, a certain amount of heat will escape, and it will cost a certain amount to run. If the water is at 30 degrees, the difference is 20 degrees, and it will cost twice as much to run. If the water is at 20 degrees and the night time temp drops to 0, it will again cost twice as much to run.

So, to save money, keep the temperature down - the same happens with central heating systems (it is a myth that it takes less energy to keep the house warm all the time because the heating doesn't have to work so hard to heat it up if you do). But of course, the disadvantage is that you need to anticipate using the hot tub to heat it up first.

Water has a calorific value of 4200J/kgK. What this means is that you need 4200 joules of energy to heat each kg (i.e. litre) of water by one degree celcius. 4200J is almost exactly what your 4.7kW element gives off every second, so, if you had 1l of water in the tub, it would heat up by a degree a second. If you have 1000l in the tub, it will take 1000 seconds (about 15 minutes) to heat up by 1 degree. The time it takes to warm up, in seconds, is approx. the temperature difference multiplied by the number of litres in the tub.

Notice I am making approximate calculations here, because there are other factors involved. The water will take longer to heat up in practice because of the heat constantly being lost through the insulation of the tub. You will lose a large amount more with the lid off, because the water will evaporate much more rapidly, and it takes a lot of energy to evaporate water. If you are blowing air through it, you are causing more evaporation by agitation, and cooling it down with the air, so your costs rocket again. Environmental nightmare.

For something as energy hungry as this, heating from a condensing gas boiler would be a much better idea. Not only would you be able to heat up more rapidly when required (20kW or more) thereby allowing you to save energy by keeping the water cooler the rest of the time, but gas is about 1/4 of the price per unit, and the conversion to hot water would be very efficient (as the boiler would always be in condensing mode).

It might be a good idea to see if your meter is calibrated properly. If the element is running for an hour, little else is on in the house, and the meter goes up by more than 5 units, suspect it. (By the way, units are in kWh [kilowatt-hours], not kilowatts per hour - it is a multiplication, not a division.)

gromituk

totalguitar wrote:

Spa Bath

This one on Ebay claims £6pm running costs if kept at a constant temp. How does yours compare in specification ?

Ugh - legalised lying. It goes on about how the water pump only consumes less than 100W, but fails to mention that the 1hp air pump is 700W and doesn't even mention the power of the heating element, which dwarfs both of these!

But we can calculate it, from "425 gallon-degrees per hour". Assuming this is UK gallons, and degrees Farenheit, that is approx. equivalent to 425x4.5/(2*3600) = 0.26 degrees C per litre (kg) per second. Multiplying this by the calorific value of water gives you about a 1.1kW element - pretty tiny. It couldn't be more than 2kW anyway, or it couldn't plug into a 13A socket (when pump and blower are taken into account) so could no longer be described as portable. That's going to take you more than four times longer to heat up the water than with the built-in spa, assuming the same amount of water.