Advantages
Quick heat up Reliable Medium cost installation Some can be installed outside without flue
Quiet Almost no smell All day energy at same price Much more powerful than electric
Minimal maintenance

Considerations
The boiler must be installed by registered CORGI installer and must not overload household gas supply.
The existing house boiler with a pool heat exchanger can be used if pipe runs are not too long. However, be very careful that if you put the pool heat exchanger is installed inside the house and there is a leak in the pool plumbing, it could flood the house.
If the heat exchanger is not in the pool room there must be a control cable between the pool plant and the heat exchanger valve with an interlock to prevent the heating valve from opening without a water flow.
The new designs of condensing boilers are more efficient than original types of pool boiler.
In my opinion a domestic boiler with a heat exchange is more efficient than a older style pool boiler with inbuilt direct pool water to flame heat exchanger because when the boiler is up to temperature the pool water still goes through the heat exchanger and air flows vertically through and cools down the pool water
There are now high efficiency pool boilers that over come this problem (be sure you get what you want!) prices vary

Disadvantages
Ever rising energy costs & on non condensing boilers a larger percentage of energy goes up the flue.

LPG Tank Stored Gas
This can be a dedicated flow through pool boiler – indoor, outdoor or a dedicated domestic boiler with heat exchanger.

Advantages
Quick heat up Reliable medium cost installation. Some can be installed outside without flue.
Quiet. Almost no smell. All day energy at same price Much more powerful than electrical. Minimal maintenance.

Considerations as above

Disadvantages
Ever rising energy costs & a percentage of energy goes up the flue. At present Most expensive fuel requires tanker deliveries.

Oil Boiler
This can be dedicated flow through pool boiler indoor or outdoor or dedicated domestic boiler with heat exchanger.
Advantages Quick heat up Reliable Higher cost installation Some can be installed outside without flue
All day energy at same price Much more powerful than electric

Considerations
Boiler must be installed by registered installer
The existing house boiler with a pool heat exchanger can be used if pipe runs are not to long be very careful if you put the pool heat exchanger in the house as if there is a leak in the pool plumbing it could flood the house
If the heat exchanger is not in the pool room there must be a control cable between the pool plant and the heat exchanger valve here must be an interlock to prevent the heating valve opening without a water flow
If using existing oil tank must be high enough to feed pool boiler

Disadvantages
Some noise, possible smell, annual service
Ever rising energy costs higher running costs than natural gas

Alternative Energies

Wood Burner
These are seldom used but can be with a heat exchanger

Advantages
Quick heat up on larger models
All day energy often using waste wood for fuel or wood pellets.

Considerations
The existing house wood burner boiler with a pool heat exchanger can be used if pipe runs are not to long be very careful if you put the pool heat exchanger in the house as if there is a leak in the pool plumbing it could flood the house
If the heat exchanger is not in the pool room there must be a control cable between the pool plant and the heat exchanger valve here must be an interlock to prevent the heating valve opening without a water flow
IN all cases I have seen it only gives a variable temperature output

Disadvantages
Hard work; usually requires manual stoking. Possible smell, erratic temperature control
Some energy goes up the flue.

Air to Water Heat Pumps
A well established method of heating a swimming pool. Two types are available, a low air temperature version
and a standard summer use unit, they work well their efficiency is dependant on air temperature

Advantages
All day energy at reduced costs due to the heater COP the electricity used is only about 30% of what heat that goes into water the rest is taken out of the air.

Considerations
These heat pumps have high start up amps and the supply cable must be large enough in some rural locations the electricity supply company may not want a heat pump without a soft start system.
They can find it slow to heat the pool in early summer but are very good once they get going, if the supply cable is not the correct size it may invalidate your guarantee.
Some people put them in sheds this can greatly reduce efficiency!
These units need a good & changing air supply be careful that the air does not go round and back into the input grill
When working well where will be condensation dripping from the drain outlet
Some customers want to use during off peak supply hours but at night when off peak is on the air temperature will be reduced so there may be a lower COP.
Often installed with a back up electric heater for early & late season.

Disadvantages
Can be noisy. Larger slow speed fans are now available. Neighbours have been known to complain about noise efficiency dependant on air temperature & location.

Ground Source Heat Pumps

Advantages
Energy available 24 hours a day all year round not dependant on weather
Energy at reduced costs due to the heater COP Electricity used is only about 25% of what goes into water.
This product can heat your pool in summer and your house in winter
Ideal for indoor pools can be used for outdoor pools & heat house during winter
Electricity used almost the same as what goes up the flue when compared with gas or oil
Your investment is used all year round 24 hours a day

Considerations
These heat pumps have high start up amps and the supply cable must be large enough in some rural locations the electricity supply company may not want a heat pump without a soft start system, if the supply cable is not the correct it may invalidate your guarantee
They are best located so that they can heat pool & domestic loads
There is more disruption during installation, but the long term advantage of all year use should show good rewards
The ultimate all year round Green Heater
The electrical supply must be installed by a part P registered contractor
Four types of collector can be used
Horizontal closed loop with circulator pump using antifreeze in the circuit with horizontal pipes 0.75 to 2 meters deep. The second more expensive system that can be used in a smaller garden vertical drilled deep holes
The third method is to pump water from one well position and release at a second bore hole less suitable in this area
The fourth is to use solar collector pipes with extra heat exchanger & combine high efficiency solar combined with ground loops and still use ground source heat pump as it is quiet and will give higher collector gain throughout the year apart from a few days where a back up fossil fuel may be required.
Care must be given to stop progressive icing of ground coils as this can reduce the collector fields efficiency for ever.

Disadvantages
Efficiency dependant on collector area available
More expensive to install
More ground disruption but it will all go back!

These heaters give most efficient output at 30-40’C If used for conventional house heating the temperature needs to be higher but this reduces the COP, to overcome this there are special fan radiators with more fins to make them more efficient
They are used for under floor heating
They can be used for duct heating with ducted air systems but the control has to be exact or they can take heat out, if the house boiler coil is on.
The pool heating heat exchanger has to be much larger than with a boiler as the temperature difference is much less.
In this part of the world our air temperatures are often much higher than the ground temperature so an above ground collector is very worthwhile with a diverter valve & differential thermostat and an extra central heating circulator pump this is set up so that if the system is up to temperature this small pump continues to circulate water round the above ground loop and the in ground loop to recharge the ground this is a very efficient way to keep the system very efficient.

With ground sourced systems the most important thing is the control
Electair have been building Pool control systems for 28 years and can give you what you need
If going Geo & this is a retrofit systems it is probably best to use them for 80% of the load so that you get the maximum from them & use your existing equipment to cover the top up during mid winter. This would probably give maximum return for your investment!!!

Rubber Mat Collectors
Usually piped directly into the pool circulation.

Advantages
Free energy Low cost can be rolled up for the winter Ideal for above ground pools

Considerations
If the pump is left on when the sun is not shining they can cool the pool
Slow heat up in early season best used with diverter valve and differential thermostat but these are quit expensive compared with an above ground pool.

Disadvantages
Some look a bit untidy Needs a fair bit of space, Summer use only

Framed Unglazed Collectors
Usually piped directly into the pool circulation.

Advantages
Free energy, low/medium priced, long life

Considerations
If the pump is left on when the sun is not shining they can cool the pool
Slow heat up in early season best used with diverter valve and differential thermostat
Dependant on sun . Wind will drop efficiency Panels can over heat pool in mid summer Water in the collector system can come to the boil if not in use in mid summer Must be well fixed as they will have a slope angle of around 30’ and the wind can get under them but installed properly this not a problem.

Disadvantages
Some look a bit untidy Needs a fair bit of space Usually installed on roofs Must be drained totally for winter.

Framed Glass Tube Collectors
Usually piped via heat exchanger with a circulator pump. www.sun-save.co.uk

Advantages
Free energy, medium priced, long life, can heat hot water circuit.
Not affected by wind far more efficient than mat or frame systems.

Considerations
Slow heat up in early season best used with diverter valve and differential thermostat
Dependant on sun . Panels can over heat pool in mid summer Water in collector system can come to the boil if not on in mid summer Must be well fixed as they will have a slope angle of around 30’ and the wind can get under them but installed properly this not a problem.
These systems are now far more cost effective as prices have dropped in the last 2 years if you are thinking of going solar for an in ground pool these are a far more efficient system
Running costs about Ł20 per year for circulator pump
These collectors will give a heat output on quite cold days, correct sizing is important

Disadvantages
Some look a bit untidy, needs a fair bit of space, usually installed on roofs

Other people may have different views - seek more than one person's views.