Swimming pool heater efficiency

[simonlangford]

Hello,

I am trying to decide whether I should heat a swimming pool using a heat pump, or using LPG. I've already ruled out oil (no room for the tank) and direct electric (too damn expensive!).

The heat pump is the Calorex 820 (or maybe the model up -- the 1220). These burn about 2.5kW of electric, but pump about 12kW of heat into the water. (They extract "free heat" from the air - see

I know these do work - my parents have one)

When the pool is cold, I estimate this will be burning for 8 hours. When the pool is up to temperature (at say 78F), it won't be burning at all. So I've assumed an average of 4 hours burning, for 150 days (May to Septemper) at 6p per kWhour. I make this =A390 onto the electricity bill. Not bad.

On the other hand I have no idea how to estimate the costs for LPG. We already have a 2000 litre tank, and we pay about 24p per litre of LPG. The central heating burns it quite fast, but domestic hot water hardly uses any.

The pool holds about 15,000 gallons of water. Pool dimensions are 10m x 5m (16' x 32'). Depth is from 1m to 2.1m (3'6" to 6'6"). We would probably keep the temperature at about 78 degrees F (25 degrees C).

How can I estimate the costs of heating with LPG?

Simon.

[Pete C]

Hi,

Is the pool in the sun? If so I'd try a solar pool blanket first.

Also have a cool shower before getting in to get acclimatised.

cheers, Pete.

[PeterK]

I am trying to decide whether I should heat a swimming pool using a heat pump, or using LPG. I've already ruled out oil (no room for the tank) and direct electric (too damn expensive!).

Simon.

More than happy to be corrected but here goes:- The pool dimensions etc are not relevant you just need the comparative costs of useable energy Density of LPG around .55 Kg/l Calorific value around 46000 KJ/Kg (I believe LPG is a blend and therefore no specific value - unless there is a BS spec.)

So 1 litre LPG = .55 Kg LPG Burn .55 Kg and you get .55 x 46000 = 25300 KJ of energy If the "burn" took 1 hour ( 3600 seconds) that would be the equivalent of

25300/36000 = 7 KWh and with 80% efficiency boiler 5.6 KWh of usable heat for LPG cost of 24p = 4.2p per unit Your "electric" costs show 12.4 KWh being released for 2.5 KWh at 6p per unit ie 1.2 p per unit

Pete K

[simonlangford]

Excellent. Heat pump it is then!

I hadn't thought of making the direct comparison in that way, but I see that

has some figures which are similar to yours.

Thanks for the logic!

Simon.

[<me9]

[snip]

The figure of merit used here seems too high, more typically 3:1 so nearer 2p per unit.

[Tony Bryer]

That was my initial reaction but an American government site on pool heat pumps suggests 5:1 as an average coefficient of performance

[<me9]

Am I too cynical to believe figures from US govt. sources?

[Tony Bryer]

They actually suggest a range of 3-7. The original post made me suspicious as I am used to the SAP Rating heat pump efficiencies of

300% or so, but on reflection I suspect that you do get a higher figure when heating a pool as you are using the heat pump to produce loads of lukewarm water as compared with a UK domestic heating installation where you'd be looking for 50C or more.

[Christian McArdle]

Do you have space for solar panels? I presume not, although they can often be placed above the roofing on buildings or sheds. This has the additional advantage of cooling the underlying building in summer. Panels for solar heating swimming pools run more effectively than those for domestic hot water. This is because the circulating water runs at lower temperatures (25C instead of 60C), making the heat transfer much more efficient.

In any case, ensure you use a solar blanket as well. It will massively reduce the extra push required by less environmentally friendly sources. Saving both your cash, and unnecessary damage to the planet.

Christian.

[The Wanderer]

Err, just one thing you might like to bear in mind. A heat pump will almost certainly have a fan to ensure an adequate flow of air across the heat exchangers. The fan will probably make quite a lot of noise. If you can site the thing reasonably out of the way then that won't be too much of a problem, unless it's near a neighbour's house!

I had a heat exchanger on the pool at my last house, which I used for an occasional day-time boost towards the end of season. Noise from the fan was something of a problem. Main heating there, and for my present pool is an E7 flowboiler (Thermalec).

I keep the pool, 10,000 gallons, at about 30°C and a five month season costs me in the region of £350. A solar blanket is essential.

The heat pump? It cost me a bottle of Scotch, and it's now sitting in my garage, one of these days I suppose I'll get round to plumbing it in....

[Sparks]

I guess this heat pump will throw out cold air - this could be directed into the house (Depending where it is!) and used as air-con too!

Sparks...