Using an Air Source Heat Pump (ASHP) to heat your koi pond

We see a lot in the news about heat pumps and how they are part of a lower carbon future for heating our homes in the UK.  They're available for pools and ponds too and we use a range of them here at Byer Koi Farm for koi pond heating.

They are not in any way an essential piece of kit for a koi pond, and its perfectly possible to enjoy the koi hobby immensely with an ambient pond in the UK.  And that is the case for the vast majority of our customers, and many of our ponds at Byer Koi Farm are not heated and remain ambient throughout the year; such as all the ponds in our sales room.

There are some benefits to being able to control pond temperature as a way to mitigate fish stress or to encourage faster growth, and if that's of interest then using an Air Source Heat Pump (ASHP) is an efficient way of doing that.

This blog post is aimed at those interested in how they would use an ASHP for their koi pond or have already taken the decision to buy one and need advice on how best to use it.

One of the benefits of using ASHPs for heating a koi pond is that they are extremely energy efficient. They can work in sub zero Celsius temperatures, and can provide up to five times more heat energy than the electrical energy they consume. This means that you can save money on your energy bills compared to a standard in-line electric heater and also other methods of pond heating too.  Another benefit of ASHPs is that they are relatively easy to install.   Advising on the right size of ASHP is another topic that we will not go into here and depends on what your aims are.  But in general, err on the side of caution and oversize it if you can to give you the greatest control in an ASHP's weakest situation which are times when the air temperatures are at their lowest.

There are 3 main reasons to artificially control the temperature in your pond.  Everyone will have their own reasons for using an ASHP and so the priority order may be different depending on those reasons.  However, I've ordered the reasons below in the priority of how we see their potential benefit to a customer who's made the decision to invest in an ASHP to heat their pond, and who want to use it  increase their enjoyment of the koi hobby.

If you are using an ASHP and have the budget to run an ASHP to achieve all 3 of the items below, that's fantastic.  But if you can only do 1 of them then we'd suggest no. 1.  If you can do 2 of them, then we'd suggest no's 1. and 2.

1. Improve koi growth

Koi are cold blooded and their metabolism is dependent on the water temperature.  For optimum growth, they benefit from being between 22C and 23C.  Up to the end of their second summer, a slightly warmer temperature might also work well but in general, between 22C and 23C is ideal for growth.

An ASHP works by taking the heat out of the air and transferring it into the water.  It's efficiency at doing that improves a lot when the air temperature is 10C or warmer.  That coincides nicely with our UK summer.

Our advice for the highest priority use of an ASHP is to use it in the summer to hold the pond at the optimum growing temperature, benefiting from not only it being the most efficient time of the year to run one but also the ambient conditions holding the pond temperature near to where the ideal growing temperature is too.

We would recommend using the spring and up to the end of June to ensure your koi are in good health and that the pond is working well and the koi clear of parasites and to build up the biological filter so it can take a period of heavy feeding; and then hold the pond at 22-23C from the end of June through to the end of September and feed heavily in that period.  Your koi will take note that the days are long, and the water warm and availability of food is good; and they will reward you with good growth.

That in turn helps them enter the autumn and winter in good condition, and after you've done another set of parasite checks and clear up if need be during the autumn and before the winter, that combined with a good summer of growing will help the koi go into the winter in a good condition.

2. Mitigate parasite risk in the spring

Koi can survive in a wide range of temperatures, including very low temperatures.   But below around 12C, their immune system is largely ineffective and at the cooler temperatures, parasites are also less active too.  But, when the pond temperature swings above and below 12C across the days and weeks at the start of spring, there are periods when the parasites are more active and the koi's immune system not fully turned on and able to defend.  So, this is one of the periods in the year when there is the greatest risk from parasites to your koi.

Using an ASHP during this period to hold a constant temperature at 12C or just above (we'd advise 14C to provide some head room), then the koi's immune system remains available to the koi and will help to defend against parasites.  This may not be sufficient to fully repel all potential parasite issues, but certainly mitigates it a great deal and also enables more flexibility with parasite treatments - many of which work better at 12C or above.

Like the principle in the summer, holding the pond at 14C in the spring is just asking the ASHP to heat the water to a little above what the ambient temperature is naturally doing for you.

Koi like consistency.  It helps to avoid stress.  And constant temperatures are more agreeable for them than fluctuating temperatures.  So, in terms of when to start this constant temperature, we suggest waiting until the pond it brought up to 12C naturally by the weather and then use the ASHP to hold the temperature to provide consistency for the koi.

3. Enjoy active koi all year round

If you have the budget, then why not consider heating your pond all year round to extend the enjoyment that your beautiful pond gives you for more of the year.

In this case, we suggest holding the pond at 14C throughout the winter.  In practical terms, use the ASHP to stop the pond temperature falling below 14C in the autumn.  And then in the spring, you are holding at a 14C minimum and allow the pond temperature to rise as the weather takes it up through the spring.  Then, see point 1. above for the timing and temperatures to set the ASHP for in the summer to optimise koi growth.