Hot tub

[cdbe]

Having got the bug for "hot tubs" after a few lodge breaks in the UK we have considered, and dismissed having one at home - initial purchase price, running costs, chemicals etc.

We are on holiday in Majorca and have this on our balcony:

It's basically a big bath with a plug hole and a filler "tap" plumbed into the hotel's hot water supply, and a pump for the jets.

Took about 20 minutes to fill and when it cooled we just pulled the plug and topped it up a bit. Seems like a good option for a few hours a week use it would get.

I'm just trying to get my head around the maths - I've googled them and typical volume seems to be about 900 litres - Google also tells me that the volume of a human is 66 litres so if we have 2 or 3 "first world" people in it I think around 200 litres. So I'd need to heat 700 litres of water from whatever it comes out of the ground at to about 38° for the initial filling and then presumably something per hour to top it up.

I have a 15kw heat only boiler with a SAP of 86% and a 145 litre megaflo with an 18.7 kW coil (so the limit here would be the 15kW boiler) temperature is set to 65°

What I'd like to know is

How long to produce 700l of hot water - starting from a full cylinder (I presume the recovery time is shorter because it's heating as it's being emptied).

Roughly how much it would cost to heat, and a guesstimate of cost per hour of topping up to maintain the temperature (obviously I would insulate the sides to death. Our gas is price cap - 6.2p/kWh.

And if it's workable, is there some kind of thermostatic filler that will only allow the correct temperature of water to flow? My thermostatic shower mixer reduces the flow when the cylinder empties in an attempt to maintain the set temperature but doesn't completely stop.

 

[ericmark]

I would say around 32 kWh to heat the water to start with, with no losses, but when I used an 18 kW main 7 gas water heater to fill a normal bath, it would take around 20 minutes, I would add no cold water, as by time enough water in the bath, it had already started to cool. Clearly ambient air temperature, and how much the air is moved will change how much it cools, so all a bit of guess work, and indoors you need to deal with water vapour, and outside higher losses, and earthing problems.

The length of pipe work also has a bearing, I was surprised that it cost less to heat my DHW in summer with electric to oil, due to losses with pipe work. I did consider a hot tub, the brother-in-law has one, and it seemed a good idea, then I saw him with the wheelbarrow taking the wood to the heater, and one hot tub session used my whole BBQ supply for the year and more.

I would say an hour in hot tub, likely costs £4 which seems OK, but my electric for the day is normally under £2, and I complain about that, as the bill comes each month, and £60 seems a lot.