Cheaper pond heater?

[Daintree]

So, just got a letter from the power company that our house uses more electricity than others of our size, and do we want an energy audit.
I know it is the greenhouse, and the biggest energy sucker is the pond heater, so I am hoping someone has an idea for me.
In the winter, the pond water in my greenhouse drops to 50-60 degrees.  Too cold for my tender plants. The pond is 175 gallons, partly underground and partly concrete blocks.
I have tried the following -
1. Running copper piping to my gas furnace, circulating the pond water through it and letting the furnace heat the water.  Made a HUGE mess with condensation and rusted the front of the furnace.
2. Solar heat. Can't heat ENOUGH water to the preferred 70-80 degrees.
3. Tankless water heater. Can't keep the water clean enough and it clogged the lines and burned up the heater.
4. Stock tank heater. Shuts off at too low a temperature.
5. Regular water heater.  Never could find room for it.
6. 300 watt aquarium heater.  Yippee! Works like a charm!  But not very efficient. Just the pond heater costs $20 per month to run.

Is there anything out there that may be more efficient?  Or just call the $20 per month good?

Carolyn

[brian]

Natgas is dirt cheap.  If you can run a new gas line to your greenhouse you can power heating devices there rather than trying to pipe hot water in from your house

[SeaWalnut]

20 per month isnt that much and you use it only in winter.I would keep it as it is.
I have a friend that pays 200 dollars a month just to keep the aquarium lights on.
You could use a heat pump( air conditioned used for heating- Daikin up to 400% efficient) to get up to 4 times more heat  for same wattage as the aquarium heater but the heat pump its too big and to costly for the size of your pond.

If its just 300 watts of heat needed,you coul simply buy a 2 meter long copper pipe and conect it to the heating boiler to make a soird of an small radiator to heat your pond.
Beware that copper harms aquatic invertebrates thogh so you will probably not be able to grow shrimps and crayfish.

[demingcr]

So, just got a letter from the power company that our house uses more electricity than others of our size, and do we want an energy audit.
I know it is the greenhouse, and the biggest energy sucker is the pond heater, so I am hoping someone has an idea for me.
In the winter, the pond water in my greenhouse drops to 50-60 degrees.  Too cold for my tender plants. The pond is 175 gallons, partly underground and partly concrete blocks.
I have tried the following -
1. Running copper piping to my gas furnace, circulating the pond water through it and letting the furnace heat the water.  Made a HUGE mess with condensation and rusted the front of the furnace.
2. Solar heat. Can't heat ENOUGH water to the preferred 70-80 degrees.
3. Tankless water heater. Can't keep the water clean enough and it clogged the lines and burned up the heater.
4. Stock tank heater. Shuts off at too low a temperature.
5. Regular water heater.  Never could find room for it.
6. 300 watt aquarium heater.  Yippee! Works like a charm!  But not very efficient. Just the pond heater costs $20 per month to run.

Is there anything out there that may be more efficient?  Or just call the $20 per month good?

Carolyn

can only really speak from pool experience, not pond, but...

1- easiest way to keep temps raised is to prevent evaporation. i'm assuming there's aquatic plants since you mentioned 'tender plants', but would a blanket or type of covering at night be feasible?

2 - a solution we see a lot in FL - unsure if it'd be appropriate in colder areas - is water is pumped up to the roof where there's solar concentrators, then back down into the pool... passive heating.