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Thermal insulation and all that.

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Thermal insulation and all that.

Postby 9fingers » 22 Sep 2019, 15:52

I've been doing some back of envelope calcs to see if I can reduce the heat loss from a friends inflatable hot tub.
She loves using it but it is a real money pit to keep it nice a toasty. I'm looking into fitting a air source heatpump which should reduce heating cost to about 25% but there are still the heat losses to combat

When not in use it has an inflatable cover which (naturally!) is full of still air.
The thermal conductivity of air is 0.024 W/mK according to google.

I thought how about a replacement lid made from cellotex and looking that up the conductivity is 0.022 W/mK. Sort of makes sense as it is primarily air bubbles with a little bit of greater loss due to the bubble walls made of the foam stuff itself.

BUT why therefore do we fill our house walls full/part full of the stuff when it is no better than air, in fact slightly worse? For years building regs has required cavities to be closed at the top so the air is essentially captive in both a wall and the hot tub cover.

The only difference I can think of is that in the insulation the air is trapped in little pockets but in an unfilled cavity wall the air can move due to convection currents but it still can't escape due to the cavity closers?
Is it simply the difference between moving air and static air?
Is there a flaw in my thinking? May be one for you MikeG?

TIA

Bob
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Re: Thermal insulation and all that.

Postby Woodbloke » 22 Sep 2019, 16:14

9fingers wrote:Is there a flaw in my thinking?

Bob

Yep, she needs to move house to Iceland or Japan and have geothermal hot water on tap :lol: - Rob
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Re: Thermal insulation and all that.

Postby Robert » 22 Sep 2019, 16:25

Knowing nothing about hot tubs but... the base? Maybe conduction is taking a lot of heat away if it is just a sheet of vinyl on the floor?
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Re: Thermal insulation and all that.

Postby 9fingers » 22 Sep 2019, 16:54

Certainly the base is worthy of attention but I just can’t find the flaw in my insulation thinking.


The hot tub bit is just background to say what I was doing that led to the question.

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Re: Thermal insulation and all that.

Postby RogerS » 22 Sep 2019, 17:01

If opportunity doesn't knock, build a door.
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Re: Thermal insulation and all that.

Postby 9fingers » 22 Sep 2019, 17:46



Indeed it does Roger. thanks very much.

They make an interesting assertion that the closure of a cavity (required I believe for spread of fire reasons) actually increases the heat loss due to convection which I had not realised.

Bottom line for my hot tub project is that replacing the air filled lid with the same thickness of insulation is pointless but using celotex to thicken the lid will of course be beneficial. Doubling the thicness will halve the the lid losses.
The walls are quite thick already so possibly not a lot to gain there without making it difficult to get in and out of the thing.
I will look at an insulated floor but need to think carefully about that at there is about a tonne of water in there plus the occupant(s). I think on reflection, it will possibly be diplomatic not to mention the weight of my friend and just stick to the weight of the water. :lol:

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Re: Thermal insulation and all that.

Postby Mike G » 22 Sep 2019, 18:24

9fingers wrote:......... there is about a tonne of water in there plus the occupant(s)........


If there is an overflow (ie the water stays at a set level), then there is about a tonne in there whether occupied or not. The total weight will change only by the difference in specific gravity of the occupants and water they displace. If they were floating on the surface, then the weight wouldn't change at all.

A healthy adult male has a specific gravity of approx 1.071 according to this paper:

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.1550-8528.1995.tb00152.x

So, a human is only very slightly heavier than water, and thus the full weight of the pool thingy will only increase by about 7% of the weight of the water displaced by the occupants. I think.

Yes, I was bored....... :lol:

Indeed! I believe the norm is to fill to about 80% capacity and that copes with the displacement in most cases and rarely is there any loss of expensively heated water.

We had one a few years back on holiday - a fixed one and i dont think that had an overflow/level control either but I did not take a lot of notice.

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Re: Thermal insulation and all that.

Postby Andyp » 22 Sep 2019, 19:09

If you buried it a) you could insulate it better and b) you would not have to climb into it.
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Re: Thermal insulation and all that.

Postby 9fingers » 22 Sep 2019, 19:36

Andyp wrote:If you buried it a) you could insulate it better and b) you would not have to climb into it.


True but a number of issues including a solid concrete patio in a rental property and drainage come to mind.
The ASHP would also not be self priming as it has to be above ground to get the air flow through.

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Re: Thermal insulation and all that.

Postby Robert » 22 Sep 2019, 22:24

Is the electricity cost for what is essentially an air conditioner used for heating less than the running cost of a water pipe circuit from the gas central heating.. assuming there is GCH? Plus the kit needed would probably be cheaper.
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Re: Thermal insulation and all that.

Postby 9fingers » 22 Sep 2019, 22:40

Robert wrote:Is the electricity cost for what is essentially an air conditioner used for heating less than the running cost of a water pipe circuit from the gas central heating.. assuming there is GCH? Plus the kit needed would probably be cheaper.


A heat exchanger on GCH would be great but sadly the maisonette is all electric, There is E7 but unless the heat losses can be better managed then it will have cooled down between end of E7 and the most often used time for the tub in late afternoon/early evening.
A quick estimate suggests a loss of about 0.5 degree per hour.

It is a rental property so no chance of PV panels but again the heat loss in the hours between sunset and sunrise is too great.

A combination of reduced heat loss and ASHP with a COP of around 4 should be worthwhile with capital cost payback in the region of 1-2 years. More research and better sums needed to check this. I've got all winter :D

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Re: Thermal insulation and all that.

Postby Rod » 22 Sep 2019, 23:05

How often is the water changed?

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Re: Thermal insulation and all that.

Postby Andyp » 23 Sep 2019, 07:27

Salt water stays warmer longer than fresh or retains heat better. There are two salt water spa/wellness centres near here who offer all sorts of treatments based around salt water including jacuzzis and hot tubs. We went once.
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Re: Thermal insulation and all that.

Postby 9fingers » 23 Sep 2019, 07:31

I’m not sure Rod. Balance is checked daily chemicals being added as needed and there are various filters to keep the water in a healthy state.
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Re: Thermal insulation and all that.

Postby 9fingers » 23 Sep 2019, 07:56

Andyp wrote:Salt water stays warmer longer than fresh or retains heat better.



What is the science behind that statement Andy?

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Re: Thermal insulation and all that.

Postby Andyp » 23 Sep 2019, 08:46

Sea water freezes at a lower temperature then fresh. Which to me means it must stay warmer longer. Googling seems to confirm this.
I suspect that the volume of salt in seawater is probably far greater than the hot tub components could cope with. Just adding water softening salt in the quantities used in domestic softeners would have little affect.

Just trying to think outside the box.
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Re: Thermal insulation and all that.

Postby Mike G » 23 Sep 2019, 13:49

I'm not sure it stays any warmer. I think that salt water has a lower freezing temperature than fresh water, but that if you have a swimming pool of each side by side in the same conditions that you'll find they're the same temperature.
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Re: Thermal insulation and all that.

Postby RogerS » 23 Sep 2019, 15:21

Bob, could you use one of those indirect IR thermometers to see where the greatest heat loss is coming from ? I bought one for under a tenner and it seems pretty accurate.
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Re: Thermal insulation and all that.

Postby 9fingers » 23 Sep 2019, 16:44

RogerS wrote:Bob, could you use one of those indirect IR thermometers to see where the greatest heat loss is coming from ? I bought one for under a tenner and it seems pretty accurate.


The HT has yet to be purchased but I do have one of those gadgets so can sniff round once it is up and running.

Good Shout!

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