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 aren’t 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 4°C during the coldest part of winter, the electricity costs won’t be too high. But for sustained heating to temperatures greater than 20°C 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 don’t 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.
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Figure 1: The essential elements of a gas boiler
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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.
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Figure 2: The essential elements of an oil boiler
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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 1°C in around 5¼ hours which isn’t 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 couldn’t 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 1°C 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.
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Figure 3: Inside the heat exchanger
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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 isn’t 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 80°C to 85°C: Temperatures above this are unnecessary and only result in increased heat losses from the primary pipe work. If the temperature is significantly below 80°C, it won’t be hot enough to transfer heat efficiently.
- The temperature of the hot water return (primary return) to the boiler should be around 10°C 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.
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Figure 4: Simplified diagram of the essential elements of a heat pump
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