2024-05-30

Hot tubs are expensive (again)

Yes, my hot tub is expensive.

Our whole house total power consumption was, typically, 55 to 60 kWh per day. Which is a lot. I have some excuses, servers in the loft, air-con for heating and cooling in various ways, and, of course, the hot tub.

The average hot tub usage 20kWh per day.

Simple change

The simple change anyone can do is insulations. The hot tub bucket has some foam coating stuff to insulate, but there are a lot of pipes connecting and holding (hot) water. These are inside some simple panels, and are not insulated.

The first surprise was how much difference the panels make. They are just thin fibre board of some sort, not obviously designed to insulate.

This is previous normal hot tub power profile :-

As you see, it is high when heating in the morning after being off all night, and when in use, but when idle is around 25% duty cycle maybe.

We removed the panels (to help turn it around, and ready for installing a heat pump). This was a surprising difference :-

The duty cycle, when not in use, when idle, was more like 75% or more. I emptied and refilled the tub from cold and it took 24 hours of full power to get to temperature. Yes, the lid was on.

This shows the side panels make a massive difference!

You can see why!

So what is the simple fix?

Lagging, multi layered loft insulation in fact, and a lot of silver tape, and quite a few hours.

The problem is I don't know how much this has helped, but it was done on the same day as the heat pump conversion - two changes at once. But it is a cheap change and I bet it helped a lot.

I should have done this years ago!

More expensive fix

The more expensive fix is a heat pump conversion. I spent £2299 in total on heat pump and installation.

It took a few hours...



It works by sitting in line with the circulation pump and with the internal resistive heater disabled (it actually has a relay to allow it to be used if really too cold for heat pump to work). The heat pump then operates whenever the circulation pump is on, leaving the hot tub to control temperature as normal, thinking it is working the resistive heater.

So, what's the difference

Firstly the power usage is way lower, the total for heat pump and the circulation pump, is around 1kW. Before it was 3.5kW. The other change is the duty cycle, which was lower. But I cannot be sure how much is down to heat pump and how much is down to insulation.

One big statistic is heating from cold, after a change of water.

  • With no side panels, resistive load 24 hours at 3.5kW. So around 84kWh.
  • With side panels, resistive load, back in January, 12 hours at 3.5kW. So around 42kWh.
  • With side panels, insulation, heat pump, 6 hours at 1kW. So around 6kWh.

So what doing the bigger stats say?

Average usage for May, 43kWh/day. I am seeing examples as low as 30kWh/day though. It seems the whole exercise has saved maybe 15kWh/day. But May is disproportionate with over 102kWh of tumble drier not a normal 42kWh due to someone having a broken bathroom :-)

It also means I am now regularly making enough solar, with battery storage, to run the house on overnight charge only, and have next profit on export, even in May, even on some gloomy days!

Last week's total electricity bill was 41p.

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