Koi Pond Heating Guide

Thermotec Inverter Horizontal Heat Pumps 9kw to 24kw


Heat pumps are an excellent way of heating koi ponds through the winter and summer months.

Using a heat pump as a pond heater will significantly reduce your pond heating costs compare with all other forms of pond heating.

Below is a guide to the costs to heat a koi pond using different heating methods. This is an average/guide as the cost of heating a pond will vary according to the time of year and the weather, however this still provides a useful illustration of the savings that using a heat pump can bring.

Maintaining a reasonable pond temperature throughout the winter and spring (eg +12c) can have significant health benefits for the fish, helping to prevent disease and parasite attack

For use with koi ponds, it is important to select a heat pump that can operate down to low temperatures in order to maintain the required temperature of the pond throughout the winter and spring.

Typically a koi pond will be kept at between 10c and 15c throughout the winter and around 20- 22c in the summer.

The range of heat pumps that we recommend for koi ponds are the excellent Thermotec Inverter Pond Heat Pumps or the Duratech Dura+ range. The Duratech Dura+ range come in different sizes of 7kw10kw14kw, 19kw, 22kw and 30kw of output heat. Beware of other heat pumps that will only work down to about +7c, as these will become inefficient in very cold weather.

Dura plus heat pump on koi pond

Dura plus heat pump on koi pond

Our Duratech Dura+ and Thermotec Inverter heat pumps provide the highest output at lower air temperatures compared with other heat pumps on the market.

thermotec inverter heat pump on koi pond

Thermotec Inverter 17kw on koi pond

Whilst there are cheaper heat pumps available on the market, the slight extra cost of the Duratech Dura+ and Thermotec Inverter ranges are worth it in terms of performance, reliability and suitability for use on koi ponds.

A heat pump can be used all year round to provide a stable pond water temperature. A stable water temperature provides siginiicant health benefits for the fish.

Our Thermotec Inverter and Duratech Dura+ ranges can operate down to –15c and have a “hot-gas” defrosting mechanism which will quickly clear away any ice that may form on the heat pump at low temperatures.


Thermotec Inverter 12kw heat pump on koi pond

Dura plus heat pump on koi pond

Dura+ heat pump on koi pond

For a koi pond that is heated all year round with a winter temperature of around 12c and a summer temperature of around 20-22c, we recommend the following sizing:-

Duratech Dura+ Pond Sizing 
Max Pond Volume
Dura+74800 gallons
6800 gallons
9800 gallons
13,800 gallons
Thermotec Inverter Pond Sizing Guide
Max Pond Volume
Thermotec Inverter Horizontal 9kw4500 gallons
Thermotec Inverter Horizontal 12kw6000 gallons
Thermotec Inverter Horizontal 17kw
7500 gallons
Thermotec Inverter Horizontal 19.5kw
9600 gallons
Thermotec Inverter Horizontal 24kw
11,600 gallons
Thermotec Inverter Vertical 29kw
13,800 gallons
Thermotec Inverter Vertical 34kw
19,900 gallons
Thermotec Inverter Vertical 34kw (3 Phase)19,900 gallons

(Note: This assumes that the pond is covered in winter and that a temperature of +12c is to be maintained over winter)

A cover should always be used on the pond in winter to help retain the heat and reduce heat loss (see below for our range of pond winter covers). In addition, any insulation that can be built into the pond walls and floor will be beneficial in reducing heat loss and hence reduce running costs.


Thermotec Inverter 12kw on koi pond

Heating a koi pond with a heat pump will always be significantly cheaper than using gas, oil or direct electric heating.

Our Duratech Dura+ heat pumps will return between 2.8 and 6.7 times their input energy as heat depending on the air temperature.

The Thermotec Inverter models can output up to 13 times their input energy as heat

This factor of efficiency is called the COP (Coefficient of Performance) and you can view a graph of how the unit performance varies with air temperature on the Dura+ product page Here

At an air temperature of 0c and water temp of 13c, the Duratech Dura+ heat pumps have a COP of about 4.

ie even at 0c air temperature, they will still output four times their power input as heat. The COP rises to over 6 as the air temperature rises.

This performance exceeds virtually all other heat pumps on the market. The output of other heat pumps will fall to virtually nothing as the air temperature falls. Many other makes of heat pumps that claim to be suitable for koi ponds are in reality only for summer use.

The Duratech Dura+7 has an input power of 1.2kw and therefore with an air temperature of 0c, it will still output 4 x 1.2 = 4.8kw of heat for only 1.2kw of input power.

The COP rises to over 6 meaning that the heat pump can produce 6 x 1.2 = 7.2kw of heat from only 1.2kw of input power.

These heat pumps therefore have an efficiency of between 280% and 670%. A typical gas boiler will only be around 85% efficient and direct electric heating is about 95% efficient.

This high efficiency makes heat pumps the cheapest form of heating for koi ponds, even at low air temperatures.

The Thermotec Inverter and Duratech Dura+ heat pumps can be set to keep the water temperature between +7c and +40c and the temperature can be increased in 0.5c increments.

Note that some other heat pumps on the market have a minimum set temperature of +15c which is too warm for most koi ponds in winter. Most koi ponds are kept at around +12c in winter.

The ability to set the desired water temperature to within 0.5c accuracy is another important feature of the Duratech Dura+ units. The temperature of most other heat pumps can only be changed in 1 degree increments.

The controls on the Duratech Dura+ range therefore allow you to have precise control over the pond water temperature.

thermotec inverter 17kw on koi pond in winter

Thermotec Inverter 17kw on koi pond in winter

The Thermotec Inverter and Duratech Dura+ ranges of heat pumps can heat and cool the pond water.

In summer, if the pond temperature exceeds the desired temperature set on the heat pump (eg 20-22c), the heat pump will then automatically enter cooling mode to reduce the pond temperature.

The accuracy of the controls on the Duratech Dura+ range helps to prevent temperature fluctuations to the pond water temperature to provide a stable water temperature.


Maintaining your pond at a stable temperature will have significant health benefits for your fish.

Keeping the pond at a reasonable and stable temperature (eg 12c) during the winter and spring months will keep the immune system of the fish active.

This can reduce the risk of the fish contracting diseases and being attacked by parasites, particularly in the Spring.


As an example of running costs, the Dura+ 7kw heat pump has an input power of 1.2kw (5.2 amps).

If your electricity costs say 9p per kwh then the running costs of the heat pump would be 9 x 1.2 = 10.8p per hour while the heat pump is operating

Except in extremely cold spells, and if sized correctly, the heat pump should not need to run 24hrs a day.

The heat pump will heat the pond to the required temperature and will then switch off.

The running time of the unit will depend on the air temperature and how well the pond is insulated or covered.

Assuming the heat pump needs to run for say 7hrs per day, the running cost for a Dura+ 7 unit would be 7 x 10.8p = 75.6p per day


The use of the Thermotec Inverter and Duratech Dura+ ranges of heat pumps on koi ponds does not invalidate the warranty in any way.

The only provision in the warranty document states that for koi ponds, the PH must be kept between 7.0 and 8.5 which is fairly typical for most koi ponds.

The concentration of medicinal chemicals that may be used in koi ponds is not sufficient to cause any damage to the unit.

The Dura+ units have a titanium heat exchanger which is highly resistant to corrosion from all chemical types including chlorine in swimming pools.

A copy of the warranty document is listed at the bottom of the Dura+ product listing, or can be viewed Here


Please see below some customer videos from Wrighty of using a Thermotec Inverter heat pump to heat his koi pond. Many thanks Wrighty !


  • A backup heat source (eg electric heater) should be installed alongside the heat pump. This can provide additional heat in extremely cold weather conditions (eg below -5c) and also acts as a backup in the unlikely event that the heat pump fails.
  • In order to prevent damage to the heat pump, you must avoid water freezing inside the unit as this can crack the heat exchanger. The easiest way to avoid this is to keep the water circulation pump running when the air temperature falls below 0c.
  • A frost stat can be installed on the circulation pump to turn it on when the air temp falls below 0c.
  • The heat pump contains a pressure switch which detects when the circulation pump is running and water is flowing through the unit. In order for the pressure switch to work properly, it is advisable to install the heat pump on a pressurised water pipe as opposed to a gravity fed one. If the heat pump must be installed on a gravity fed water pipe, then we sell a separate water flow switch which can be used to detect water flow. Please Contact Us for details.
  • The Duratech Dura+7 require a flow rate of around 3 m3/hr (660 gallons per hour) to operate effectively.
  • A separate drip tray heating kit is now available which will melt any ice which builds up in the bottom of the heat pump. Please Click Here for details
  • Please also refer to the Installation Tips on our web site
  • Note that some heat pumps (eg Thermotec Inverter) require a minimum water temperature of +8c and above in order for the heat pump to start. If the initial pond water temperature is below this, then it may be necessary to use another heat source to raise the pond temperature above +8c so that the heat pump can then take over the pond heating.


In extremely cold weather, the condendsation that drips from the heat pump evaporator coil can freeze in the bottom of the heat pump causing an ice build up.

We sell an optional kit that contains a special  heating wire controlled by a low temperature thermostat.

The heating element only turns on when the air temperature falls below zero and turns off again when the temperature rises.

The kit comes with all necessary parts and full installation instructions

Click Here to Purchase this Kit

Below are some photographs of a heat pond being heated through the winter using a Duratech Dura+ heat pump.

Another example below of a Duratech Dura+ heat pump heating a koi pond taken March 2010


koi pond heat pump

The pond owner reported that the heat pump has worked extremely well and maintained the pond temperature at 15c despite the snow !

The running costs were reduced by approx 60%

Another recent installation was on a 14,000 gallon pond in Norfolk and used a Duratech Dura+14kw unit to heat the pool.

The pool owner reported:-

“I am writing to say I have been very impressed with the heat pump, I reckon over the past 3 weeks it has already saved me over £200 in heating oil”

The initial purchase cost of a heat pump may be higher than a traditional electric or gas pond heater, but you will soon recover the outlay through greatly reduced running costs.

Using a heat pump to heat your pond is also a more environmentally friendly method than using fossil fuels such as gas or oil and so you can also reduce CO2 emissions and do your bit for the environment !

Here are some quotes from our customers that are using Duratech Dura+ heat pumps on their koi ponds:-

Hi David,

AMAZING !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Took delivery yesterday morning, lucky for me my husband is on hols this week so it was plumbed in immediately, temp was at 15.5 degrees,went to work, returned at 2200hrs and it was up to 17.5 degrees, got up at 0600hrs – 19 degrees, by  early morning it reached the 20 degrees I had set it at, apparently it’s only kicked in once today to maintain the temp.

After seeing this unit working I now know that the old unit didn’t work as it should have from day one – all that time wasted – my poor fish – they’re now bombing around the pond at present (it’s going to cost me a fortune in food)

What an amazing piece of kit!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.

Over the next week I will attempt to take some photo’s for you, for you to use if you want, also I am chairman of South Devon Koi Club, although we are having problems with our website at present (nobody knows how to load anything on it) will quite happily get a link put on for your company when we can find a man that can.

Next month I have a pond visit – 2 clubs visiting, if you have any info you wish for me to pass around, I would be quite happy to do so.

Many thanks for your help in resolving my problems.

Thank you so much for your help, I’m so impressed with the pump, keeping a steady 24 degrees – fish are loving it. It was a big outlay for me, but one of the best I have made, would recommend this pump to anyone who is thinking of heating their pond


K Tovey

Hi David

Sorry for the delay in sending you some pictures of my Heat pump For my koi pond you supplied

May I say what a fantastic service you have supplied me.

Many Thanks

J Noel (June 2012)

Hi David

I thought you might like the attached photo. Now we are in the depths of ice and snow, the heat pump is really showing its worth by keeping my pond at a nice steady 10 degrees C even overnight when it is hitting –5 outside.

T Hogg (Jan 2013)

I have to say out of all the money I spent on my state of the art Koi pond, this is the best piece of equipment I bought. It surprised me how little I had to do with it. I just turned it on, set the thermostat to 23 degrees and left it to it. I never had to touch it again all the time it was on my pond.

Absolutely brilliant. I can’t ever imagine why anyone would use a gas system now these are available.

(Duratech Dura+19 on Koi pond 2013)

Dear Nigel,

Please find attached pictures showing heatpump maintaining 13 degrees centigrade on a very cold morning well below freezing.

It has been such a pleasure to have the unit since august as l could control the natural fall in temperature from September which has made health benefits to the koi.

I now wake up on freezing cold morning and the koi will feed as if it’s a warm day as pictures show.

So many people told me it’s to expensive to heat a pond. But the cost of insulation with a cover and to pipes and filter is not.

The additional electricity cost is only small.

So for the pleasure it has given me its all worth it.

Most importantly is the koi health, the benefit is huge.


Andy (Dura+7 on pond Dec 2016)

I purchased the InverterTec 9kw heat pump from heatpumps4pools for my 3000+ gallon Koi pond after very friendly discussions with their sales department.

It was very straight forward to set up, install and there is plenty of instructional help with the booklet supplied or YouTube videos.

I expect it will be very efficient and economical to run during the winter months with additional pond covering as advised helping to maintain my pond at a temperature that will allow the fish to continue feeding all winter.

The mobile App is very useful allowing monitoring the unit and any temperature adjustments can be made from the comfort of your armchair. Highly recommended.

Andrew Murton (Thermotec Inverter 9kw on koi pond Sept 2019)


On the example below the pipes from the pond to the heat pump were run underground in flexible pipe

Bypass kit and solid to flexible pipe connection shown



Duratech EU Series heat pumps



Note that we also sell an excellent range of winter covers specifically designed for koi ponds

Click Here for more details on the pond winter covers

If you have any questions regarding the use of heat pumps to heat koi ponds, please Contact Us and we will be delighted to answer them for you.