Beemer,
The types of heat pumps that are suitable for ponds were developed for swimming pools for summer use and are very efficient under those conditions so a heat pump on a pond will be equally efficient. You won't be disappointed in summer.
Winter is a different matter and you have to understand how a heat pump works so as not to have unrealistic expectations which lead to disappointment.
Heat pumps work by extracting heat from somewhere and using that heat to heat something else. There are different types but the type of heat pump you are thinking of using is an air-to-water heat pump which extracts heat from the air and uses that to heat water.
The ratio between the energy used and the effective output is called the COP (coefficient of performance). Extracting heat from the air is easy in summer and the COPs are typically 5 to 6 which means that for every 1 KW of electricity it uses it gives a heating output equivalent to a standard 5 to 6 KW heater.
COPs aren't fixed, they depend on the available heat, they can't extract heat that isn't there in the first place and so the COP of an air-to-water heat pump falls as air temperatures fall in winter.
As a very rough guide you could expect a COP of 5 to 6 in summer, 3 to 4 in spring and autumn, falling maybe as low as 2 to 3 in winter provided that the weather isn't bitterly cold.
This means that, for every 1 KW used, you might only get 2 to 3 KW of heat out which will reduce the electricity bill to a half or one third of what it would have been if you had used an electric heater instead of a heat pump which is still a very good saving but nowhere near the saving you would get in summer when the bill would be only one fifth to a sixth.
Heat pumps usually have a protection feature built into them that prevents them running if the air is so cold that they aren't producing any amount of useful heat. You wouldn't want a heat pump that was using, say, 1KW but only producing 500 W output!
Under very cold conditions this would mean that when your pond needed heat most, the heat pump might just sit there waiting for better weather and the pond would go cold for a few days or more.
This probably wouldn't happen very often but it's possible that, with a longish period of very cold weather, the heat pump might switch off for a few days, then on for a few days, then off again and so on. This could result in the pond temperature yo yoing during cold spells which obviously is something best avoided. For this reason, I have always advised that there should be a separate back up heat source.
This yo yoing would be more likely if you tried to heat the pool all year rather than heat it during summer and just maintain 8C in winter as you are intending but bear in mind what can happen in winter if you don't have a back up heater to take over and maintain the temperature if the heat pump switches off.
So, by all means buy a heat pump, they are a very good way to heat a pond and most pond owners are happy with them, but don't fall for over-enthusiastic sales talk and become unrealistic in your expectations. They can reduce your heating bill to one fifth in summer, one third in autumn, but may only halve it during a cold winter and may even just switch off during very cold spells.
As for enclosing it in a pit - don't. Air-to-water heat pumps draw massive amounts of air through them in order to extract the heat from it. I've seen people fit them in sheds and greenhouses thinking that:- "it gets very hot in there so the heat pump can use that heat...."
All that happens is that the heat pump switches on and within a few minutes has taken out all the heat. The temperature inside drops below freezing (literally) and the heat pump switches off. If you want to enclose a heat pump you have to arrange it so that the input and / or the output air is suitably ducted and with massive ventilation so that there is no restriction to air flow.
If you particularly wanted to do it in order to deaden the noise or to avoid having to see a heat pump plonked by the pond, it could be done and, if you provided pictures, I could tell you how to go about it in your situation. But it would be a lot of buggering about and even a small amount of air-flow restriction reduces the efficiency.
As part of a series of articles I'm writing, I've covered different methods of heating ponds and heat pumps are in the article on this link:
http://www.mankysanke.co.uk/html/good_w ... _pt_8.html
Ask if there is anything else you want to know.
