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 Good water guide05   (Good water guide)


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

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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.

Part 8: Using gas or oil boilers or a heat pump to heat your koi pond

Last month I discussed what is arguably the simplest and the cheapest type of heater that can be installed on a koi pond; the in-line electric heater. Although they are relatively inexpensive to buy, they arenít the cheapest heaters in terms of running costs. If they are only used on Economy 7 in late autumn and early spring to prevent the water temperature hovering for too long in aeromonas alley as described last month, and then readjusted to prevent the temperature from falling below 4įC during the coldest part of winter, the electricity costs wonít be too high. But for sustained heating to temperatures greater than 20įC throughout the summer and especially if those temperatures are maintained through the winter, although initial installation costs would be much greater, a gas or oil boiler or a heat pump will reward that initial investment by way of lower heating costs.

Direct heating gas boilers
For larger koi ponds, swimming pool gas or oil boilers are sometimes used. They are called direct heating boilers because the water to be heated is pumped through an internal heat exchanger where it is heated directly and they donít require an indirect external heat exchanger. As shown in figure 1, gas is burned in an insulated combustion chamber and the hot combustion gasses rise through the heat exchanger which is just a network of heating pipes that are usually finned in order to collect the maximum amount of heat from the hot gas before it exits through the flue.  Using this method, efficiencies of around 95% can be achieved.  The burners are designed to burn natural gas but, where this is not available, alternative versions have a modified burner assembly that can burn propane.

Beginner gas boiler direct

Figure 1:  The essential elements of a gas boiler

Oil boilers are similar to gas boilers except that the burner assembly is replaced by a single jet of flame resembling a miniature flame thrower. Fuel oil is atomised into a fine spray of droplets and mixed with air which is pumped through a nozzle into the combustion chamber as in figure 2.  Otherwise the operation is the same and similar efficiencies can be obtained.

Beginner oil boiler direct

Figure 2:  The essential elements of an oil boiler

Direct heating boilers were designed to rapidly heat swimming pools in the region of 20,000 gallons or more and so they transfer heat to the water flowing through them very quickly. This can be too fast for small koi ponds.  Even a very small direct heating boiler has a rating equivalent to a 17 kW electric heater. Referring back to the electric heater calculations of last month; the normal electric heater size for koi ponds is 1kW per 1,000 gallons and this can heat the water by 1įC in around 5ľ hours which isnít too quickly for the koi.

If a boiler with a rating equivalent to 17 kW is used on a 17,000 gallon pond, it will give the same rate of temperature rise but if it were used on a smaller pond, say 3,000 gallons, it would raise the temperature by over 5 degrees per hour! Since the internal thermostat in a boiler is on its input in order to measure the temperature of the incoming water, it couldnít sense an increase in pond temperature until the heated water had been returned to the pond, had been through the filter system and then back to the boiler again. In this example, the boiler would have been heating the water at the rate of 1įC every 12 minutes and the delay before it realised it had reached, and exceed, the set temperature would lead to unacceptable variations in pond temperature.

Beginner heat exchanger

Figure 3: Inside the heat exchanger

The heat exchanger
Most koi pond installations that use gas or oil boilers use domestic boilers that heat water indirectly and less quickly by means of an external heat exchanger. All heat pumps also heat water via a heat exchanger although this isnít immediately obvious because the heat exchanger is hidden inside the equipment.

The way a heat exchanger works is shown in figure 3. Heated water from a domestic boiler is circulated through a finned or coiled stainless steel pipe inside a water jacket.  Pond water is passed through the jacket and picks up heat from the hot pipe.

This seems simple enough but there are complicated equations that govern transferring heat via heat exchanger.  For a pond installation these can be simplified to three rules. Following them will ensure the most efficient transfer of heat from the boiler to the pond via an external heat exchanger

  • The temperature of the hot water feed (primary flow) from the boiler to the heat exchanger should be around 80įC to 85įC:  Temperatures above this are unnecessary and only result in increased heat losses from the primary pipe work. If the temperature is significantly below 80įC, it wonít be hot enough to transfer heat efficiently.
  • The temperature of the hot water return (primary return) to the boiler should be around 10įC lower than the feed: This temperature difference represents the amount of heat that has been transferred to the pond.
  • For maximum efficiency, the directions of flow as shown on the diagram are essential:  The primary circulation flow is opposite to that of the pond water.  The primary flow should enter the heat exchanger at the top and leave at the bottom.  Although it is pumped by the boiler circulating pump, as it loses heat to the pond water, it will tend to sink.  It is more efficient for this to aid the boiler pump rather than to oppose it as would be the case if the direction of flow was reversed. Similarly with the pond water. If it is pumped in at the bottom and leaves at the top, the tendency for water to rise as it is heated will aid the pump rather than oppose it.  Also, the rate of heat transfer is greater when the two flows are in opposite directions because, at all times, the temperature difference between the two flows is at a maximum.

Heat pumps
There are different types of heat pumps according to the intended use but they all work on the same principle, they extract heat from a source where it is available and pump it to a destination where it is needed.  The easiest to understand and the most common type of heat pump is the air-to-water version as shown in figure 4.

Beginner heat pump

Figure 4: Simplified diagram of the essential elements of a heat pump


The system contains a refrigerant gas which is continuously recirculated by a compressor whilst water is pumped through a heat exchanger similar to the one described above but with the input and output connections on the same side to make plumbing easier.

The compressor compresses the gas causing it to become hot and to flow through the heat exchanger where it gives up its heat to the pond water.  As it leaves the heat exchanger it is a very much cooler gas. It is still under pressure so it is forced through the expansion valve causing it to become a cold liquid.  As it continues, the air passing over the evaporator brings it quickly back to a gas at air temperature again before it is recompressed by the compressor to continue the heating cycle.  In this way, heat is continuously taken from the air passing over the evaporator and pumped into the heat exchanger to heat, in our case, pond water. Heat pumps are driven by electricity but are rated in term of COP (coefficient of performance).  This is the relationship between the power used by the unit and its effective output. E.g. a heat pump that draws 1 kW of electricity but produces an output equivalent to a 5kW electric heater will have a COP rating of 5 and the running cost to heat any particular pond installation will therefore be one fifth of the cost of heating that pond by an ordinary electric heater.

Beginner heat pump COP curves

Typical COP curves:  The effective output greatly depends on air temperature and to lesser extent on pond water temperature

Heat pumps are a very efficient way to use electricity to heat a pond, but since they extract heat from the air and transfer it to the water, the output very much depends on the air temperature at any particular time.  Just as with the advertising hype of a new washing powder or household cleaning product, beware of exaggerated claims.  Some time ago, a salesman tried to sell my company a range of heat pumps with COPs of 8 and 9.

Iím not easily fooled and I know that salesmen mustnít lie but they do present facts in the most favourable light for their product so I asked how these figures were obtained. In short, these were made in Spain and when the prototypes were tested in the factory in the middle of a heat wave they did indeed produce that kind of output.  Import them into the UK with our much cooler summer air temperatures and the COP would be nearer to 6.

Lest anyone misunderstand, let me restate the case.  Heat pumps are an efficient way to heat ponds. If a heat pump has a COP of between, say, 4 and 6, it will cut the summer heating bill to between one quarter and one sixth of what it would cost to heat the pond with an electric heater. But donít assume you will get this kind of saving if the heat pump is used in winter when the air is cold. You might only get a COP of 3. That will reduce the heating bill to one third, which is still a good saving.

Also figure 4 only shows the principle of operation of a heat pump.  The latest heat pumps incorporate extra features that allow them to produce useful heat when the air temperature is below freezing but there will come a temperature when it will switch off and produce no heat at all until the air temperature rises.  Other heat pumps are only intended for summer use and donít produce any heat at all when the air temperature is below 8įC. Before buying that on-line bargain, especially from a non koi dealer, question the supplier to make sure how it will perform in winter. Better still, buy through a good koi dealer, they will understand what a koi keeper wants from a heat pump in winter. Even so, a backup form of heating, such as an electric heater, may be advisable in areas that get particularly cold in winter.  This may hardly ever switch on but it will be available to stop the pond going cold if the air temperature should ever fall below the heat pumpís normal range.

Solar heating - good or bad?
There are many different ways to obtain heat from the sun. One way to gain solar heat is by spraying water directly onto black collector plates or trays angled towards the sun and draining the water into the pond. Another way is by pumping water through hose pipes or similar pipes that are exposed to the sun and returning the warm water to the pond.  These and other ideas will provide a great deal of heat when the sun shines strongly but none when the sun doesnít shine. If a pond is already heated by another form of conventional heating, any heat obtained from the sun will save on the heating bill to some extent and therefore would be an advantage, but if the pond isnít normally heated, solar heating may not be such a good idea. A few sunny days will warm the pond considerably but if they are followed by cloudy days, as is typical of our climate and if there is no conventional heater to maintain that temperature, the pond will go cold again.  With our uncertain climate, the pond temperature will vary up and down according to whether the sun shines strongly or not which isnít conducive to good growth or good health.

Every part in this series so far has referred to pumps.  How do pumps work and why can some low power pumps manage to pump the same amount of water as other higher power pumps?  Or can they?  Find out next month.

Some additional green information about using biodiesel as a fuel for oil boilers
Oil boilers used to be less popular than gas boilers due to the higher cost of heating oil and were only used where natural gas supplies were not available, but advances in biodiesel fuels and the rising cost of gas production will undoubtedly change that in the near future.  Petro-chemical companies are currently producing biodiesel from algae and estimate that, as early as 2017, it will be cheaper for them to farm diesel fuel than to refine it from fossil oil. Whether or not that timetable is met, when biodiesel actually does become available, it will have a zero carbon footprint.  Carbon dioxide is used by algae to grow and when it has been processed into fuel oil and burned exactly the same amount of CO2 is released.  This means that the CO2 released when a gallon of bio-fuel oil is burned will be exactly balanced by the CO2 needed by the algae to make the next gallon of bio-fuel.


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