For anybody having a swimming pool...

[Peter]

The standard lines from pool builders is that 90% of the heat loss
from a pool is via the surface, only 10% into the ground.

It turns out this originates from some website in Florida, and is
probably true in Florida if you have a pool at +30C and totally open,
no cover.

The rest of the time it is bollocks and is propagated by pool builders
who are too thick and lazy to bother with insulation. The whole UK
pool business works on the principle of the rolls royce buyer: if you
need to know the mpg you obviously cannot afford one.

So they stick a 40kW boiler in there and waste your money.

I have been operating two outdoor pools over 10 years, and done some
very careful measurements. They are quite an eye opener.

Case 1:

12x5m concrete pool, depth from 1m to 2.5m
NO insulation anywhere.
Cover is 6mm air-bubble PVC (standard £400 sort of thing).

Heat loss is around 10kW for a 20C water-air difference, and about 2/3
of this is into the ground, through the pool sides and bottom and the
pipes which are traditionally buried directly in the ground. The
air/ground loss split was determined by laying heavy insulation on top
of the surface (2 layers of industrial 1/2" thick bubblepack) and
seeing how much difference this makes - not a lot.

This pool is expensive to heat... you would spend £5000/year at
current gas prices heating it all year round, and that is assuming you
have one of those inflatable domes on it for much of the year.

Case 2:

10x4m concrete pool, depth from 1m to 1.7m
Insulated with 50mm Sellotex around sides, no insulation underneath
(there doesn't appear to be a suitable material which would carry the
weight). One idiot pool builder walked off the job because he didn't
want to be bothered. Most others wouldn't even quote. Eventually I
found one. It's really easy of course.
Cover is a 'solar' slatted cover (£13000).

Heat loss is around 2kW for a 20C water-air difference, and about 3/4
of this is into the ground.

The division of this loss is interesting.

There are 4 pipes coming out of the pool, and one going back in. All
were insulated with fibreglass wool but only crudely. For half their
run they are bare but laying in an air-filled buried duct. So not
directly in soil anywhere. But the pipes are exposed in the plant
room, where the ambient is just that (due to big vents for a heat
pump).

About 1/2 of the loss is via the pipes! IOW, if I stop the filtration
pump at night, the total heat lost from the pool is halved. One
achieves significant savings by closing all but one of the outlet
pipes.

Of course the expensive solar cover makes a hugh contribution; around
15-20kW from the 40m2 area in any sunlight. But one pays for that in
the purchase price.

The thing is that insulation costs only a few hundred quid on the
purchase price... for that you could insulate ALL the pipework with
100mm Armaflex too.

 

[Peter]

While I accept most of what you say, you are not comparing like with like.
One pool has more than twice the amount of water in it than the other, so
heating costs for the same change of temperature will reflect that.

Yes, but not twice the surface area. I was measuring the cost of
holding a *constant* temperature, which is not affected by the water
volume but merely by heat losses.

It'll be interesting to see if your pipe theory is correct by experiment -
I guess you'll be
checking losses with the pump off at night etc over a period of time.

The pipe losses are measured, not theoretical. I have been measuring
the temperature drop, for a given water-ambient difference, for
various combinations of open pipes, with no heating going on.

In fact the pipe losses are greater than it appears because when the
pump is running, one is putting about 500W (3/4HP pump, which draws ~
600W according to a true power wattmeter) into the water from the pump
only.

Unfortunately there is nothing I can do to add insulation - except
inside the plant room.

I am doing this with a PT100 thermometer, accurate to 0.1C.

I suppose the key point is that insulation is a piece of cake. It
takes only minutes extra to stuff a load of Cellotex down the side of
the pool before filling the cavity up. And insulating pipes with some
pipe insulation is equally quick.

I think the reason the pipe losses are so significant is this:

Say the soil temp is +10C. The pool is +25C. The heat pump produces an
increment of about 1.5C (a Calorex 910X, 6kW on a good day...) at the
particular flow rate. If the loss along that pipe is just 0.3C (which
doesn't sound much, given +25C water) you have wasted 1/5 of the heat
pump output, just like that When dealing with such small
temperature changes, heat losses become critical.

In the trade, the builder chucks in a big boiler which costs a fortune
to run, and when this is running it is producing a big temperature
increment so that is OK, but one has to run the filtration pump all
the time even if the boiler is not on, and that is where the heat loss
goes.

 

[Peter]

Rate of heat loss will depend, amongst other things, upon the temperature
differential
between the water and the air. The higher that differential is, the greater
the rate of loss will be.
Therefore a large volume of water, which will maintain it's higher
temperature for longer, will have a disproportionately
higher rate of loss than a smaller volume over the same period of time.

Of course. Newton found this. Conductive heat flow is proportional to
temp difference. Convection and radiation are more complicated.

What I reported was a 5x energy saving due to the most basic
insulation. Dead easy to achieve.

The difference in the surface area (applicable to conductive losses)
between the two pools is only about 1.5x

The funniest thing (for some) was when one pool builder resigned from
the job on the day before he was due to start, and after the contract
was signed, because he thought the insulation requirement was too
difficult for him.

All we need now is a lot of Polish pool builders

 

[Peter]

Therefore a large volume of water, which will maintain it's higher
temperature for longer, will have a disproportionately
higher rate of loss than a smaller volume over the same period of time.

actually the above is incorrect!

 

[Peter]

Why?
TonyB

Taking the steady state condition, all that matters is heat loss from
the boundary of the object. The object volume, specific heat capacity,
etc, don't come into it at all.

The heat loss (assuming the simple case of conduction only) is
proportional to the temperature gradient across the boundary.

 

[Palindrome]

Why?

Let's see -

volume of a cube = a^3
surface area of a cube = 6a^2

As volume increases, surface area increases at a lower rate.
As heat loss can only occur from a surface, the greater the volume, the
proportionally less is the increase in surface area and thus the
proportionately lower will be the heat loss, compared to smaller volume
of water over the seame period of time.

Mice suffer from heat loss - elephants suffer from over-heating..

 

[gazz]

Mice suffer from heat loss - elephants suffer from over-heating..

dunno about mice, but i keep pet rats, and they suffer over heating easily,
because thye have a large body but only a small part of it can transfer
excess heat... their tail, they dont pant like dogs, and dont sweat like
humans, so their tail is their heat regulator,

How does that work into the equasions

 

[Palindrome]

TonyB wrote:

Now take two different volumes of water.....they both have the same
surface area

That's a neat trick.

 

[Palindrome]

I see what you are saying - but since he is using a pump to *keep* it
warm, it doesn't matter to him.

Palindrome,

a cubical pool 2x2x2 has a total area (underground and to air) of 12,
and a volume of 8.

A cuboidal pool 1x1x5.5 has the same area, but a volume of only 5.5.

Of course - different geometrical shapes have different equations for
volume and surface areas. However, for any particular simple geometric
shape, as volume increases, surface area increases at a lower rate.

Swimming Pools all tend to be near enough the same very approximate
depth, independent of volume. Few are spherical. Most have width and
length far greater than depth. There are, of course, always exceptions.
The Royal Navy has a pool with a depth far greater than width or length,
IIUC. Used for submariner escape training.

Hence discussions about how heat loss from cubical swimming pools
increases with volume tends to be somewhat theoretical/academic.

 

[Palindrome]

TonyB wrote:

I think the Navy's training pool for submariners is an indoor one, so
heat losses would be
less anyway.

Nope. It's a bloody great tower, visible for miles.
http://www.kentscuba.com/sett.htm

However, unlike it may appear in the picture - it doesn't actually
levitate..

70 year's old.

Rather a bargain "experience of a lifetime" at <40GBP.

And yes, as a taxpayer, I pay to heat the water....

 

[pwatercare]

While I accept most of what you say, you are not comparing like with like..
One pool has more than twice the amount of water in it than the other, so
heating costs for the same change of temperature will reflect that.
It'll be interesting to see if your pipe theory is correct by experiment -
I guess you'll be
checking losses with the pump off at night etc over a period of time.

TonyB

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