Koi pond heaters. How they work.

Despite the apparent efforts of global-warming nibbling away at either end of our winter, our ponds continue to experience several months of near-zero and freezing temperatures in winter. Our cold-blooded koi have no choice but to become inactive, well before our ponds freeze over, stopping feeding around 8 degrees C, sinking to the pond bottom to take refuge in the denser, warmer water.

Different koi keepers and koi professionals have very different opinions on the benefits and motivations behind heating a koi pond. We could argue that koi have been overwintering in the UK quite safely at ambient winter temperatures (albeit insulated by the obligatory 4 ft deep pond) and that the motivation behind heating our pond is that it gives us an excuse to buy and install yet more pond hardware.

    And there are those that maintain that we have ridden our luck so far with our koi (which are in fact warm water fish and would benefit from the comforts of a heated pond over winter). Yet if you are in the business of farming koi, then you will appreciate how important it is to expose your brood fish to a ‘winter period’ – advice I have heard first-hand from Sakai (of Matsunosuke fame) and something I have experienced for myself.

    But for the koi keeper, heating a pond over winter can provide many tangible benefits for ourselves and our koi, and finances permitting, should be something we might consider.

    The benefits.

    Total control over the water temperatures that your koi are exposed to, even in the harshest of winters.

    Unpredictable and wild swings in temperature at the onset and end of winter can be ironed out, again protecting these artificially selected carp from the trials of nature. Unlike the natural world, we do not want the winter period to subject our fish to a test of a survival of the fittest.

    If koi activity is maintained to some degree, then they will be able to resist disease better by having an active immune system.

    Your filter will not experience a period of inactivity, removing the need to jump-start it in the spring – something that could run the risk of a water quality problem.

    The Costs.

    Depending on how do you choose to manage your water temperature over winter, you run the risk of confusing certain koi bio-rhythms, which may go on to affect their spawning activity in the subsequent year. There are also risks in exposing your koi to a seasonless year which may result in you koi becoming spawn bound at autumn time.

    Your koi pond will not benefit from a fallow period. Instead, koi parasites and other pathogens will remain active throughout the year, putting your koi at a greater risk of disease.

    Cost. There is both a capital outlay for the heating hardware (and its installation if required) and also a significant running cost. You will have to compare this against the potential benefits before making your decision.

    What next?

    If, having weighed up all of the factors involved for your own pond and circumstances, that you consider heating is the option for you (like an increasing number of koi keepers) then you should reconsider the heating options. Knowing how they work, and how they are installed will be fundamental to you making the correct decision.

    What are the heating options?

    Based on how a pond heating system functions, there are two options to choose from: electric or gas/oil-fired boiler.

    1. Electric. Electric is the most straightforward to install and heats the pond water directly and efficiently. For many years, swimming pool heaters were used to heat koi ponds. They were (and still are) relatively straightforward to install and easily available in a range of sizes to suit different pond volumes. However, pond keepers started to notice a chronic deterioration in the health of some of their koi and after water testing identified an increase in metal contamination of the pond water. The source of this was soon traced back to heating elements of swimming pool heaters being manufactured and sheathed in copper and other non-ferrous materials that had started to leach and accumulate in the pond water. Consequently, all dedicated koi pond heaters are now made from complex stable materials containing titanium, making them super-resistant to chemical attack. All other associated casing and pipework is also made can high-grade ’316′ stainless-steel which is resistant to chemical attack. The criteria for choosing electric pond heaters are similar to those when choosing a UVc. They are also installed in a similar way.

    The wattage of the heater is determined by the volume of the pond that needs heating. The general rule of thumb is that every 1000 gallons of water requires one kilowatt of heating power. Up to three kilowatts, heaters can be plugged into the standard mains supply via an RCD or RCBO (circuit breaking device). Heaters above 3Kw will need to be wired back, directly into your house consumer unit (or fuse box).

    The most popular electrical heaters are plumbed in line between your pond pump and return pipework (as per a UVc) and it is advisable to install them after the filter (to avoid fouling up a of the inside). Every care must be taken to ensure they run at full bore.

    Safety Protection.

    Besides the circuit-breaking protection that is there for our benefit, the more advanced electric heaters, although they are essentially a very simple heating device, are equipped with several complex control and safety devices.

    a. Digital control thermostat: allows accurate observation and control of the ponds temperature, often to fractions of a degree C. b. High limiting thermostat. This is factory-set to ensure that the heater unit is not allowed to overheat. This could happen should it become blocked with blanket weed or other debris. c. Flow switch. If insufficient flow is passing through the heater then it will automatically be turned off, preventing the heater unit from overheating should your pump fail.

    Because electric heating is a very direct method of heating, it is regarded as an efficient method of heating your pond (i.e. a high proportion of the energy used by the heater actually heats the water of the pond). This does not however necessarily mean it has a low running cost as this is determined by the unit cost (kilowatt-hour) of electricity.

    2. Using a boiler (gas or oil-fired) Compared to electric heating, heating your pond using a boiler is indirect and less efficient (but because of the relative cost of gas, and may still prove to be a cheaper alternative). The capital outlay will be significantly greater than using an electric heater as there is considerably more hardware to purchase and install. Furthermore, the installation is less likely to be a DIY project, requiring a CORGI registered gas installer to connect your boiler to a gas supply. And compared to the single pumped circuit of an electric heating system, the boiler heating system typically has two. One for the hot water circuit running between the boiler and the heat exchanger and the other between the heat exchanger and the pond. These two pumped circuits are completely independent and will not (and must not) come into contact with each other.

    Equipment list.

    1. Heat exchanger. We come into contact with heat exchanger every day in other fields. A heat exchanger can take on many forms, being designed to transfer heat from the heat source to the target area. A central heating radiator or car radiator are both examples of a heat exchanger. The hot water comes through one pipe, into the radiator (which is engineered to have a large surface area for heat exchange), dissipates it’s heat into the cold air, cools down and returns to the heat source ready to be heated up again.

    Heat exchangers in aquaculture will generally take one of two forms.

    a. Shell and Tube. The shell is the outer casing in which the pond water will flow and the tube is the pipe work containing the heated water which conducts it’s heat to the pond water. There can be different shapes of tube inside the shell to increase the efficiency of the heat exchanging process. The vast majority of koi pond (and smaller swimming pool) heat exchangers use this type of heat exchanger.

    b. Plate-type heat exchangers. These are found in large applications such as fish farms (and Sea Life Centres). These are far more efficient at transferring heat from warm to cold, the most common design being the gasketed plate type (which looks like the press used for making dried flowers). They consist of a series of channelled plates that are mounted on a frame and clamped together. Each plate is made from a pressable material (eg stainless-steel or titanium) and the gasket arrangement of each plate distributes the hot and cold water into alternating flow channels throughout the pack of plates. To aid heat exchange, the hot and cold water flows counter to each other.

    The power output capacity of a heat exchanger will be described in BTUs (British Thermal Units) which in turn will be related to a recommended flow rate of pond water through the unit. The smallest heat exchangers generally start at about 60,000 BTUs with a recommended flow rate of 900 gallons per hour, suiting a pond up to approximately 2000 gallons. At the top end of koi pond heat exchangers, 460,000 BTU will take a flow rate of 4600 gallons, suiting a pond up to 10,000 gallons. Pond water will be pumped through one channel of the heat exchanger continuously (just as it would do through a UVC).

    What are BTUs?

    BTUs are the unit of measure for heating equipment that exchange heat. The higher the number, the higher the heat output. For comparison, a barrel of crude oil contains a 5.8 million BTUs whereas a kilowatt hour of electricity equates to 3413 BTUs. The world’s energy consumption in 1995 was estimated to be a total of 316 quadrillion BTUs!!

    2. The boiler. A boiler it is required to produce the hot water circuit that will be pumped through the other channel of the heat exchanger. To determine the size and output of boiler required (again measured in BTUs) you will need to work back from the volume of your pond. For example, a 5000 gallon pond will have a 2500 gallons per hour turnover of water, suiting a heat exchanger of 170,000 BTU. Consequently, you will need to install a boiler with a gross output of at least 170,000 BTU. However, sometimes boilers are rated by their power input rather than their gross output. If this is the case then as a rule of thumb, allow approx. 80 per cent efficiency so that if an output of 170,000 BTU is required then choose a boiler of approx 210,000 BTU input for that system.

    Heating efficiency of boiler/heat exchanger systems are far less than electric systems as heat and energy are lost in the production and transference of hot-water to the heat exchanger (in warming pipes, hot gases up the chimney, heat escaping from pipes etc) as well as at the heat exchanger itself. With these inefficiencies in mind, keep pipework lagged and pipe runs as short as possible.

    The boiler will be thermostatically controlled by a probe in the pond, feeding back to the boiler control/timer clock (just as it does in a central heating system).

    Now that you’ve got your heating installed, how do you use it? 3 strategies.

    1. Provide summer temperatures all through the winter. – Expensive – Unnecessary – Higher koi food costs, without significant increase in size – Seasonless year may lead to spawning problems and even spawn bound koi in autumn

    2. Provide 10 degrees C throughout the winter – cheaper running costs – Koi appetite and food costs reduced – Still avoid harsh winter conditions – Give koi and pond environment some sense of season

    3. Provide a controlled cold period (4-6 degrees C for two to three weeks) in winter and then provide 10 degrees C either side. – further savings on your heating bill – A further reduction in food costs – Allow koi and pond to experience a winter period, with a proven subsequent growth and spawning response – Still prevent koi from experiencing a really extreme winter

    If you are one of the many koi keepers who have decided to heat your pond, then with a full understanding of how the two heating methods work, you’ll be in a better position to choose the system that best suits you and your pond. Each method has its own benefits, be they financial (as upfront or running costs) or in the way they are installed or operate. Once installed, the decision-making process is not over as you have then to choose how you are going to operate your heating system through the winter.

    Kill blanketweed and string algae.