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heating koi ponds     (Heating koi ponds)


I wrote the following articles for Koi Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.

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Explaining the facts, the myths and simplifying the mathematics of heating koi ponds

The case for heating koi ponds in winter
Koi are not cold water fish that can thrive in any temperature down to near freezing.  Having been bred from carp, they have similar characteristics including the ability to adapt their behaviour to a wide range of temperatures.  We have all seen how much more active koi are in summer temperatures and how their appetites increase to match their activity.  We have also seen how, in unheated ponds as winter approaches and the water temperature decreases, koi become less active and their appetite also decreases until below about 10°C when they have little appetite at all.  As the temperature falls further, they spend less time swimming and more time near to the bottom of the pond until at around 4°C they are usually to be found huddled together, almost motionless resting on the pond floor, conserving their energy until warmer temperatures arrive.

Temperature stratification summer
Without currents due due filter pumps, in summer deep  ponds would stratify as above. With pumps running, circulating water currents prevent this from happening.

Temperature stratification winter
Without currents due to filter pumps, in winter, deep ponds would stratify and fish could retreat to the comparatively warmer water at the bottom but with pumps running, the water is continually being mixed and this “warm” zone cannot happen.

This is what I would call normal carp behaviour for the normal temperature range. Below this temperature, koi can do nothing more to protect themselves from the cold.  They have no biological mechanism to generate internal heat in order to keep warm, nor do koi posses the ability to shiver, which rapidly contracts and relaxes the muscles and generates some internal heat, as we do when we are cold.  In the lakes where carp evolved, no matter how severe the winter, carp didn’t need to do anything else other than wait for warmer temperatures because in deep natural lakes the water temperature near the bottom would not fall significantly below 4°C.

The temperature stratification that prevents deep water from falling below about 4°C cannot happen in a koi pond unless it is unusually deep.  Water circulation from the filter pump and from air stones will also be constantly stirring the water and mixing the colder water at surface with the slightly warmer water near the bottom. The result is that, whilst it will always be true that the coldest water will tend to be at the surface, the effect of mixing some of the surface water with water deeper down will be to cool the whole pond below 4°C and expose the fish to temperatures they have never evolved to cope with.

Temperatures below about 4°C are a threat to the lives of koi. Not all koi will die as a result of being exposed to these temperatures, the strongest ones will survive but a great many koi were reported to have died in unheated ponds during the severe cold weather of last winter [Dec 2010 - Feb 2011].  Although covering an unheated pond to prevent it falling below 4°C will often be effective depending on individual installations and location, it is a good idea to have a heater set at 4°C for ponds where covering alone is insufficient.  Even if we ignore the animal welfare aspect, i.e. whether or not it is cruel to expose koi to such temperatures, the financial cost alone of replacing koi can exceed the cost of the minimal amount of heat required to prevent a properly covered pond from falling below that temperature.

Heated ponds and the immune system
To give them their correct description, koi in common with other fish are poikilothermic which means that they can function over a wide range of temperatures and do not control their body to any particular temperature.  They can do this because they have a range of enzymes each of which operates at different temperatures and it is these enzymes that control the metabolic processes inside their bodies. As it becomes too hot or too cold for one enzyme to function, it “switches off” and another which is more suited to that particular temperature takes over. Some of these are more efficient than others as can be readily seen by a koi’s feeding response to temperature changes. It is obvious that the koi digestion and its appetite diminish as the temperature falls.

A koi’s immune system is also temperature dependent. In fact, what doesn’t seem to be particularly well understood is that koi have two immune systems; the specific and the non-specific immune systems. It is beyond the scope of an article on heating to go into great detail of how these work but the specific immune system is so called because, in simple terms, if it has previously encountered a particular pathogen, it “remembers” it.  Should it encounter the same pathogen again, this part of the immune system is able to mount a specific quick and effective defence. The non specific immune system is a less effective but general defence against any invading pathogen. It is this specific immune system that becomes far less effective as the temperature of the water falls. One study by Morvan, Troutand, & Deschaux found that the non-specific immune system may work harder in order to compensate at lower temperatures but, in general, whilst the immune system as a whole isn’t totally ineffective at temperatures below 12°C, it is very much impaired.

Heating ponds in spring and autumn
Although the koi immune system is impaired below 12°C, the ability of bacteria such as aeromonas to infect koi isn’t so limited by temperature. The precise temperatures are the subject of debate, but in general, aeromonas bacteria can be considered to be inactive below 4.4C and, as temperatures increase above this, it become increasingly active until it reaches its peak activity at around 15.6°C. Since the koi immune system is impaired below about 12°C but aeromonas can still infect them until the temperature falls below 4.4°C, this temperature range is sometimes called aeromonas alley although aeromonas is not the only bacteria that is able to infect koi at these temperatures. If pond water stays in this temperature range for extended periods, these pathogenic bacteria have the advantage that they can attack koi but the koi defences are weak.

Even if a pond isn’t normally heated through the summer, the aeromonas alley effect provides a good case for some degree of heating during spring and autumn.  If a heater thermostat is set to, say, 13°C it will not come on during a normal summer, but as the water temperature begins to fall as winter approaches the heater will prevent it from falling below 13°C.  This temperature could be maintained all through the winter but there are two schools of thought on the matter.  One argues that the breeding of modern koi has removed them too far from their ancestors and that they should be heated throughout the entire winter. The second argues that koi need a “winter period” just as would have happened to their ancestors in the natural lakes where they evolved.

Those who prefer to heat a pond all through the year will naturally maintain their ponds at or above about 13°C and their koi will never experience aeromonas alley.  Those who would prefer their koi to have a winter period could simply set their heater thermostats to maintain 13°C for as long as they wish or can afford and then, reasonably quickly, turn down the thermostat in stages until it is set at 4°C.  This will prevent the temperature falling lower than that if covering a koi pond is insufficient in a severe winter.  In spring as the temperature starts to rise, the thermostat can be adjusted in stages back to 13°C to maintain that temperature until prevailing weather conditions take over and warm the water above that point. If the pond is covered during the coldest weather, this second approach will allow the koi keeper to give their koi a winter period and also enjoy them for longer without incurring a huge heating bill.

Heated ponds and dissolved oxygen levels
The growth in many species of animals is regarded as “determinate”, which means that, as a general rule, they grow to a particular size and then stop growing.  Fish growth is “indeterminate”, meaning that there is no limit to their ultimate size.  Although the growth rate of koi slows as their size increases, as long as they have a suitable diet and temperature range, they don’t lose the ability to grow throughout their entire lives.  The availability of oxygen is one limiting factor that determines rate of growth and this is where warmer water could have a negative effect on growth rate and ultimate size.

Figure 1 shows the saturation values of oxygen in fresh water. This is the maximum amount of dissolved oxygen that pond water can hold.  The table shows that, as the temperature of water rises, its capacity to hold oxygen decreases and, regardless of how many extra air-stones are added, the dissolved oxygen level cannot be increased above these values.  The effect on growth of a reduced oxygen level in the water due to increased temperature or any other reason is twofold.

Firstly, as a koi grows and doubles its length, its width and height will also double. Doubling its length and, at the same time, also doubling its width and height results in there being an eight-fold increase in body mass (2 x 2 x 2 = 8).  All that extra body mass requires oxygen for it to function but although the size of a koi’s gills will increase as it doubles its length, their size doesn’t also increase by eight times. This is a natural effect and it should not be taken to mean that fish suffer from oxygen deprivation as they grow.

However, it is this reduced ability for the gills to provide proportionately as much oxygen to the body of a large fish as they did when it was smaller that is one reason why small fish grow rapidly but the growth rate slows as they grow larger.  Bearing in mind that the warmer the water, the lower the dissolved oxygen level, when a koi pond is heated for better growth, in order to avoid oxygen becoming a limiting factor on that growth, the highest level of aeration possible is essential to counteract the reduced ability for water to hold dissolved oxygen.

Secondly, in order to digest and metabolise food to maximum advantage, koi in common with other fish, need a good supply of oxygen.  If a koi pond is heated to promote good growth, it will be counterproductive if the dissolved oxygen level in the water is insufficient to allow the maximum amount of energy to be obtained from the food that the koi are being fed.  This is a second reason why, when a pond is heated to gain good growth, it is essential to ensure that the highest level of aeration should be achieved.

Temperature °C








Oxygen mg/L








Figure 1.  Saturation values of oxygen in fresh water

Heated ponds and growth
There is a principle in scientific circles known as Q10 Temperature Coefficient Theory which relates to many chemical processes and also to the biochemistry processes that control fish metabolism. Simply put, it says that these processes will happen twice as quickly if the temperature is raised by 10°C.  There are other factors that influence koi metabolism and so, in practise, increasing water temperature by 10°C doesn’t mean that its metabolism will be exactly twice as active.  One of those factors is the fall in available oxygen as mentioned above so, although koi metabolism can’t be guaranteed to follow this rule exactly, it does mean that its metabolism will be far more active if the water temperature is raised from 15°C to 25°C.

Opinions will vary as to the optimum water temperature for maximum growth but 25°C is a commonly chosen temperature if budget will allow.  The optimum temperature range for carp is often quoted as being between 20°C and 30°C and so 25°C is in the middle of this range. Increasing the water temperature slightly above this will increase a koi’s metabolic rate and its capacity to grow but there is a diminishing return regarding growth per gram of food as the temperature rises.

Although it is true that increasing the temperature by 10°C will approximately double a koi’s metabolic rate so that it has the potential to grow twice as quickly at 25°C as it can at 15°C, increasing the temperature by another 5°C to 30°C will not increase the potential growth rate by an additional 50%. Apart from this being near to the maximum temperature a koi can tolerate, the falling capacity for water to hold dissolved oxygen is an important factor in this relationship between food and growth.

Koi at rest require 6.0 mg/L dissolved oxygen but an actively swimming koi requires nearer to 7.0 mg/L to fully power its muscles. Higher amounts of dissolved oxygen above this can be used to metabolise food and build body tissue.  In figure 1, it can be seen that, at 30°C the maximum amount of oxygen that water can hold is only 7.5 mg/L, and this is under ideal conditions. Although highly aerated pond water can approach the values in the table, in practice, as long as there are fish and other life in the pond that are using oxygen as it is added, the actual values in the water will always be slightly lower.

Since digesting food and metabolising it into body mass requires a great deal of oxygen, it is the reducing amount of oxygen that is available above that required for actively swimming that limits the growth rate as the temperature rises much above 25°C.  At higher temperatures, appetite for food will reflect this limiting factor and will actually decrease.  Also some of the food that is eaten will not be fully digested and will be excreted in the faeces.

Koi rate of growth depends on how quickly it can eat and metabolise food, it isn’t sufficient just to double the feeding regime and assume that the growth rate will automatically double, the fish needs to be able to metabolise the extra food in order to build a bigger body. But if the water temperature is increased from 15°C to 25°C the metabolic rate will approximately double so there will be the capacity for the growth rate to double too.  A word of caution here, if the metabolic rate doubles but the feed rate and quality of the food doesn’t doesn’t keep up with the resultant increase in metabolism and appetite, growth will suffer.  This is because the koi will be swimming more actively so more of the energy that is in the food will be expended in providing the extra muscle power that will be needed for the greater activity in a similar manner to the way we might use extra exercise as part of a weight loss or weight control fitness program.

Keeping energy costs to a minimum

If a pond is only heated in spring and autumn to around 13°C and in winter to prevent the temperature from falling below 4°C the current cost can be less than £1 per day for a 3,000 gallon pond even with an electric heater but only for those days that the heater is used.  Those who heat their ponds all year can pay much more depending on the water temperature, the amount of water that is changed and the weather. With some koi keepers changing as much as 10% per day it isn’t surprising to hear of bills of £1,000 or more per year.

An ideal way to keep the cost of heating a pond to a minimum during winter will be to cover it to prevent heat loss due to wind chill, but even when a pond is uncovered in the summer it is desirable that any heat in the pond is insulated from escaping.  The best opportunity the pond keeper will have to ensure that heat losses through the walls and floor are kept to a minimum is during pond construction.

It is natural that, once pond construction has begun, there will be a desire to get it completed as quickly as is possible so that it can be filled and stocked. But once the shell has been constructed, spending some time making sure that the future running costs are as low as is possible will pay great dividends in the future. Thermal insulation boards are a wise investment.

Extruded polyethylene insulation is made by mixing polyethylene pellets and an ingredient that causes a foaming reaction. Whilst the reaction is taking place it is extruded into flat sheets with a thickness dependent on its intended use and it sets into a closed cell plastic foam that is both moisture resistant and, to some extent, has a degree of flexibility. This type of insulation has good compressive strength which means that, when the pond is filled, it will be able to withstand the subsequent water pressure on it without crushing.

Polyurethane foam insulating sheets are produced by blowing a non CFC gas into urethane resin to produce foam which is then faced with aluminium foil to allow it to be passed through rollers to control the thickness as it sets into flat insulating sheets.

Where the pond walls and floor are flat and reasonably smooth, either of these types of board can be fixed in place by suitable mastics or adhesives. Care should be taken to ensure that the sheets don’t span any “voids” due to unevenness in the walls or floors.  There is a good degree of leeway if a liner is to be fitted as it will be sufficiently flexible to allow it to stretch into place if the sheets are pressed outward into these voids by water pressure when the pond is filled.  Although fibreglass finishes have great strength, they are not intended to flex when cured, and so whilst it is unlikely that a properly applied fibreglass finish would ever crack or split if the insulating board should be pressed outward where there is unevenness, these areas would be under unnecessary permanent stress.  It is good practice to ensure the sheets are properly supported so that this doesn’t happen.

Where the walls are curved or the floor is benched or shaped, large flat sheets cannot be fixed flat to the surfaces.  Since these boards can easily be cut with a sharp knife, a possible solution might be to cut a full size board into smaller pieces which can then be fixed individually.  Where the shape of the walls or floor is too curved for even this method to be practical, it is possible for polyurethane foam to be sprayed directly onto the pond surfaces by a specialist company. This isn’t cheap but the cost can be offset by a saving made on the cost of rendering the walls and screeding the floor.  The fact that this foam can be sprayed onto any surface, no matter how uneven it is, means that there will be no need to render a wall beforehand. Similarly it will not be necessary to screed a concrete floor in order to give the pond a smooth bottom; the foam can be sprayed directly onto the rough finished concrete surface and then easily sanded to remove any unevenness due to uneven application.

Heating ponds by electricity

The cost of heating a pond throughout the year is difficult to calculate since it will depend greatly on the size of the pond, whether it has been insulated to prevent heat loss through the walls and floor and the difference between its temperature and the ambient temperature. Water changes will have a great effect on heating costs too; the more water that is changed the greater will be the cost of heating new water.

Although not the cheapest to run, the in line electric heater is the cheapest and possibly the easiest form of heating to install. A good quality heater can be obtained for less than £300 by shopping around.  They are physically small, and are easy to retrofit into an existing installation since they can be fitted by cutting into a straight length of pipe work, but not just any piece of pipe work will do. Installation is simple but there are some basic rules that should be followed.

The elements in an electric heater are intended to be permanently immersed in water whilst they are heating and will quickly burn out if the water flow through the heater doesn’t fully cover them when they are switched on.  Good quality heaters usually include a flow switch which will switch them off if the water flow stops but these heaters and flow switches were originally designed for the swimming pool market.  This means that they were designed to operate in clean, sterile water immediately after a sand filter.

Where they are used to heat ponds, they are in a different environment entirely. Pond water cannot be sterilised by disinfectants such as chlorine nor will the water be as clean as that in an average swimming pool so it is possible that they can become jammed with pond debris, odd strands of blanket weed or the biofilm that can build up on them.  Any of these can render them unable to switch off if the flow through the heater stops.

Also, it is good practise to turn off a heater before turning off the filter pump or doing anything that may interrupt the water flow through it, but haste or inattention can sometimes cause us to forget the obvious. Another possibility is that the pump may deprime or the water flow stop due to a blockage.

It is situations such as these that the flow switch is intended to protect the heater elements against.  Whilst a clean flow switch is very reliable, one that is unable to switch off, because it is jammed, cannot protect the heater and a simple error such as backwashing a filter or flushing it to waste without first turning off the electric heater could prove to be an expensive mistake.

As a precaution, in line electric heaters shouldn’t be installed in any section of pipe work that can drain down if the filter pump is switched off, or the water flow is interrupted or if a valve is inadvertently left in the wrong position.

It is better that electric heaters are only installed in pipe work that is permanently flooded so that, even if the flow of water through them should stop, the elements will remain under water.  The benefit of plumbing an electric heater into pipe work in this way is that, should the water flow be interrupted whilst it is on, the water that remains inside will begin to heat up. This will be sensed by the built in temperature sensor which will switch off the heater even if the flow switch is stuck in the on position.  All installations are different but, as a guide, any pipe work that is higher than the water level in the pond has the potential to drain or to collect air when the pump is stopped and is best avoided when considering where to fit in line electric heaters.

They are rated in terms of kilowatts, (kW), and the rule of thumb is that 1kW per thousand gallons will suit all but the most extreme conditions.  Bigger isn’t better!  Assuming there are no significant heat losses, a 1kW heater can raise the temperature of 1,000 gallons of water by 1°C in a little over 5 hours, which is a reasonable rate of change of temperature for koi but if left on for a whole day on a cold pond, this equates to a rise of 4.5°C in 24 hours which is at the limit of what koi should experience. A 3kW heater would raise the temperature of 3,000 gallons at the same rate but, if used on a smaller pond, say 1,500 gallons, it would heat the water at twice that rate which is too fast.

Pool boiler
A typical swimming pool boiler

Direct heating gas boilers

Direct heating swimming pool boilers cost from around £2,000.  Water is pumped into the boiler and is heated directly by the gas burner.  They are designed to heat swimming pools up to around 30,000 gallons to temperatures around 30°C.  They can obviously be controlled to lower temperatures by a thermostat but they are designed to heat large volumes of water very quickly so the rate of temperature change will be very fast if they are used on small koi ponds.

Thermostats are not fitted on the output of these boilers because that would cause them to keep switching off as soon as they began heating the water. The control thermostats have to be fitted where they can sense the water flowing into them which means that they cannot sense the new temperature of the water until the heated output water has circulated right through the pond.  With a two hour pond turn-over rate, this means that when these boilers switch on, they won’t sense a temperature rise and switch off until the heated water is pumped back into them again two hours later. Since the rate of temperature change will be very fast and since the boiler will have been heating for at least two hours, the resultant temperature fluctuations on a small pond can be extreme.  For this reason, I wouldn’t advise using direct heating swimming pool boilers on a small koi pond. 

Domestic gas boilers
Domestic gas boilers cannot heat pond water directly.  They have to heat via a heat exchanger where they heat a coil of narrow bore pipe inside a water jacket to, typically, 70°C to 90°C. The pond water is pumped through the water jacket. It doesn’t mix with the heated water from the boiler which is sealed inside the internal coiled pipe but it flows over the pipe and picks up heat from it.  Heat exchangers come in a range of sizes according to the volume of water they are intended to heat and the temperature they are expected to achieve.

Standard sized heat exchangers range from 60,000 BTU for less than £200 up to in excess of 400,000 BTU costing over £600.  These sizes refer to the rate of heat that they can transfer from the boiler to the water that is to be heated.  As with electric heaters there is no advantage in choosing a larger BTU rating than is required since doubling the rating will double the rate of temperature change when they are heating the pond. A 60,000 BTU heat exchanger when fed from a boiler that has sufficient spare capacity to supply the correct temperature primary flow, (the water circulating between the boiler and the heat exchanger), is more than sufficient for the average sized pond.

Without taking each individual installation on its own merits it isn’t possible to make general rules but it will most likely be the case that, even with the minimum sized 60,000 BTU heat exchanger, the rate of temperature change will be too great. There is a simple solution when plumbing in heat exchangers. Using two tees, fit a bypass loop directly across its pond water input and output and put a gate vale in the loop so that, with the valve closed, all water has to flow through the heat exchanger, but with the valve fully open, much of the water will bypass it.

When the valve is closed, the rate of heat supplied to the pond will be at its greatest and this will cause the maximum rate of temperature rise.  With the valve fully open, the rate of temperature rise will be far less. When the boiler is initially heating the pond, and with the valve set half open, the rate of temperature rise should be carefully monitored and the valve adjusted to obtain an acceptable rate. This can be tweaked slightly over the course of a few weeks, after which it may be a good idea to remove the handle/wheel to prevent it accidentally being altered from this optimum setting.

A domestic gas boiler and heat exchanger can be installed specifically for the pond or, if the pond is near enough to the house and the central heating boiler is conveniently located, an extra feed can be run to the pond heat exchanger. If this involves running the primary feed from the boiler to a heat exchanger near to the pond, these pipes must be well insulated to prevent heat loss.  This loss would not only unnecessarily increase the heating bill for the house but, if the primary feed to the heat exchanger isn’t in the correct temperature range, it will be ineffective as a heat source for the pond.

A slightly more efficient way to run a heat exchanger from an existing house boiler is to fit the heat exchanger in the house near to the boiler and run the pond water to the house.  This will generally not be convenient in practise because of the size of the pipes that will have to be run and the extra trouble involved in insulating them but, if it is possible, there will be an energy saving. This is because, assuming that the insulation thickness is the same, pipes carrying water at normal pond temperatures lose far less heat to their surroundings than pipes carrying water with a temperature in excess of 70°C. There will also be the advantage that the heating controls for the heat exchanger can be easily linked into the central heating controls with the central heating and the heat exchanger controlled by separate zone valves so that the central heating and the pond heating can operate independently of each other.

Heat pumps

There are a few installation constraints on fitting a heat pump.

A heat pump needs to continuously draw in large volumes of air so it should be sited where there are no restrictions to the airflow.

The exhaust air will be very cold so it shouldn’t blow into an area where you might like to sit.

The continuous cold air from a heat pump will feel like winter to shrubs or garden hedges, don’t let the exhaust blow directly onto them or they might die back.

Don’t fit a heat pump in a shed or greenhouse thinking that it is warmer inside them so the extra heat will be transferred to the pond; the heat pump will quickly cool the air then be unable to draw in warmer air from outside.

Originally designed to heat swimming pools, these are a very efficient way to heat ponds in summer.  They work by pumping huge volumes of air through two internal heat exchangers which extract heat from the air flow. This heat is passed to a third heat exchanger which heats water in a similar way to those described above. They don’t need sunshine and will operate on cloudy or even rainy days as long as the air temperature is warm.

A small heat pump can cost less than £500 and will use typically in the region of 1kW of electricity, whilst it is running. The output is rated in terms of COP (coefficient of performance) which is a measure of how many times more efficient it is than an electric heater of the same rating.  For example, a heat pump with a COP of 6 will produce an output equivalent to a 6kW electric heater for each 1kW of electricity it uses.  Small ones typically have a COP rating of between 4 and 6 when the air is warm.

The COP is dependent on air temperature and can easily fall to 3 or less in cold weather. This would mean that they only give the equivalent of 3kW out for every 1kW of electricity they use which is still a good saving but this may not be sufficient to heat a pond during winter when heat losses are at their greatest and air temperature is at its lowest. Anyone intending to purchase a heat pump for winter use is best advised to question the supplier about its performance in winter to ensure that it will maintain the desired pond temperature without temperature fluctuations caused by there being insufficient heat output on the coldest days.

Unarmed verbal combat
On this magazine’s forum, a couple of years ago, I entered into a discussion with a manufacturer/supplier of a popular heat pump.  He was very convincing and I felt better about the performance of his product in cold weather after our discussion but there was one guarantee I couldn’t get him to make.  That guarantee was that his heat pump would always maintain a pond temperature without fluctuations in severe winters.  He very wisely said that, to ensure a constant temperature in the severest of winters, it would be best if there was a back up form of heat such as an electric heater that could take over if the weather didn’t allow his heat pump to produce enough heat.  I agree.  Heat pumps are designed to still produce heat even when the air temperature is well below freezing and so they will probably be able to maintain a stable temperature throughout most winters. But since the health and welfare of your koi is at stake, it would be a wise investment to also install an electric heater. This would probably not be needed in most winters but would be there as a back up, just in case.

Solar heating

One way to monitor and record water temperature is with a pen recorder that will draw a line representing the temperature on a continuous roll of paper.  Variations will be shown by the “waviness” of the line but this equipment is expensive. An inexpensive way to monitor pond temperature variations is by purchasing the type of digital thermometer that, not only displays the current temperature, but also stores maximum and minimum temperatures that were reached in any particular period since it was last reset. These can be obtained on-line for under £20.

If a pond is to be heated it shouldn’t be done in a half hearted manner. Solar heating is a good example of where heating pond water can be detrimental rather than a benefit. Solar heating can be a valuable addition to a pond that is already heated by some other means since whatever heat can be gained from the sun will reduce the running costs of the main source of heat.  But the problem with solar heat in the United Kingdom is that the amount that is available varies greatly from week to week and even from day to day.

It is tempting to feel the warmth of the water in a hose that has been left in the sun and imagine ways in which this heat could be put into the pond.  The simplest way to do this is to have a coil or zig-zag of hose in the sun, then simply pump pond water through it and allow the heated water to flow back into the pond.

In summer, on a sunny day, a great deal of heat can be gained this way and, if this is the sole form of heat, the pond will be much warmer on those days. But the sun doesn’t always shine. It is quite common for there to be good sunshine one day and very little the next so the pond will warm up on sunny days and cool back down again on cloudy days. Unless there is another, more reliable, heat source to take over when there is no sun, the constant varying of pond temperature will alter the koi’s metabolism very quickly and this will cause it unnecessary stress.

Using solar heat in the winter without a reliable alternative could be even worse. When the pond is cold, a couple of days of reasonable sunshine in the winter could raise the temperature of the pond, but the several days in between sunny periods will allow it to go cold again.  The koi will be in their “winter mode” one day, then the water temperature will warm and they will think spring has arrived early only to find that they are back in winter temperatures straight after. Repeated wild fluctuations in pond temperature during winter could be extremely detrimental to them.

Fitting a heater; some hints and tips

Adding an electric heater to an existing filter system requires little more than picking an appropriate pipe, cutting into it and fitting the heater.

Heaters up to 3 kW can be plugged into a 13A socket.

Larger heaters need a separate supply, usually run directly from the house consumer unit.

Gas regulations recognise that, if a gas boiler is fitted by someone lacking proper experience and training, the installation could be dangerous and it is consequently illegal for non-Corgi registered gas engineers to do any work that involves altering a gas supply or boiler flue .

It isn’t illegal for non-Corgi engineers to fit heat exchangers to existing systems as long as this doesn’t involve altering the gas supply or boiler flue. Anyone competent at plumbing could do this work.

A tip for the less experienced before commencing to fit a heat exchanger is to lay it out on the floor along with all plumbing and other equipment in the required positions so as to make sure that it will fit neatly into the available space on the wall.

Don’t cut the boiler or pond pipe work until the rest of the heat exchanger plumbing is finished and you are sure that you have the correct fittings to complete the job.

A house without heating in winter until a missing fitting can be obtained could be an unhappy house and a pond without filtration for a similar reason could be an unhappy pond!


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