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 Good water guide06   (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 7: Heating your koi pond

Some hobbyists heat their ponds to a higher temperature than 20°C throughout the year in order to achieve good growth. Other koi keepers subscribe to the school of thought that koi are a “four season fish” that must experience a “winter period” in order to reset their biological clocks, so they heat their ponds to greater than 20°C for most of the year but reduce the heat for one or two months during the winter so that their koi’s annual cycle can begin again as temperatures rise in spring. Both these strategies come with the expense of the resultant heating bill. Other koi keepers simply allow their ponds to follow the natural seasonal temperature changes throughout the year and rely on covering their pond and other forms of insulation to limit the fall and the variations in temperature.

Last month I advocated that all koi ponds should have some form of heating to limit the temperature fall during winter because fish in their natural environment live in much larger and deeper volumes than those of a typical koi pond and, as explained, they rarely experience temperatures below 4°C. Also large variations in ambient temperature don’t cause such great variations in water temperature because they are smoothed out by the large volume of water in a natural lake.  In order to give our fish as natural an environment as is possible, I suggested that even those who don’t heat their ponds during summer should have a heater to limit the fall and the variations in temperature during winter. Additionally, the heater could be used to maintain a minimum temperature of 10°C for as long as possible as winter approaches and then used to speed up the rise in temperature during spring in order to minimise the length of time that koi spend in the temperature range known as aeromonas alley.  In combination with covering the pond and good insulation this need not be as expensive as at first it may seem.

Electric heaters
Although they are not the cheapest way to heat water, for ponds up to around 3,000 gallons, in terms of ease and simplicity of installation, by far the easiest and least expensive to buy and install is the standard in-line electric heater. They come in a range of power ratings and a guide to choosing the correct size heater is that, under typical conditions, an electric heater should be rated at 1 kW per 1

Beginner Elecro

A typical electric heater. This one
is manufactured by Elecro

,000 gallons.  A 3 kW electric heater, which will be sufficient for a 3,000 gallon pond, draws a little less than 13 amps and so heaters up to this size may simply be plugged in to a standard 13 A socket.

A popular make of 3 kW heater costs under £300 if you shop around. Higher power heaters are available for larger ponds but these need a dedicated feed from the consumer unit with an appropriately rated fuse or miniature circuit breaker and electrical cable. Heaters rated at 1 kW per 1,000 gallons are suitable for heating ponds to temperatures higher than 20°C in summer but if the heater is just used for keeping the time spent in aeromonas alley as short as possible and preventing winter water temperatures from falling below 4°C the rating may be halved to 1 kW per 2,000 gallons as long as the pond is covered and well insulated to keep heat loss to a minimum,.  This means that a 3 kW heater, which doesn’t need a dedicated feed from the consumer unit, may be used for ponds up to 6,000 gallons or possibly even more depending on how well the pond is insulated.  

Calculating the cost
Whatever method of heating is used, the wide variations in pond installations mean that there are far too many variable factors to allow an accurate prediction or calculation of the running cost and it would be of no help whatsoever for the koi in a pond if a newly installed electric heater proved too expensive to run and was left switched off. But there are some simple rules of thumb with electric heaters that will help get an idea of running costs.

Firstly, a 1 kW electric heater consumes one unit of electricity for every hour that it is on. For the non electrically minded, that is to say that, apart from maybe a penny per year to power the display, a heater consumes no power at all when it is switched on at the supply and when the water temperature is higher than the temperature the heater is set for.  But when the water temperature is below the set point and the heater switches on, a 1 kW heater will consume one unit of electricity every hour until it raises the temperature above the set point and switches back off again. Similarly a 2 kW heater will consume two units per hour and a 3 kW heater will consume three units per hour.  A unit of electricity is sometimes, (more properly), also known as a kilowatt hour. Whether the power utility companies sell you electricity in terms of kilowatt hours or the less technical sounding expression, units of electricity, both expressions are the same and are interchangeable.

How electric heaters work
The principle is very simple.  Special types of wire, such as nickel-chrome wire, heat up when mains electricity is passed through them.  By adjusting its thickness and its length, the wire can be made to get hot and glow dull red. Modern heating elements consist of a heating wire inside a metal sheath and the wire is prevented from touching the sheath by an insulating powder such as magnesium oxide as shown in figure 1.  The sheath protects the wire from contact with its surroundings and is made of a material that suits its environment.  A copper sheath can be used in domestic water heaters but stainless steel is used, either for corrosive environments, or where copper can’t be used such as in pond heaters. The heating element is then coiled or bent to a suitable shape to fit into the design of the heater.

Beginner electric heater

Figure 1.  Inside a typical electric heater

The mysteries of Economy Seven solved
The cost of a unit of electricity varies according to a multitude of different tariffs but, as a guide, at the time of writing, the cost per unit including vat for my Economy Seven tariff is around 11.6 pence per unit during the day and around 5.4 pence per unit during the seven hours from midnight until 7 am [typical UK prices in 2011].  Both times are G.M.T. and since electricity supply companies don’t send someone round to all their customers twice a year to alter their Economy Seven time switches when British Summer Time begins and ends, this means the times are one hour later during summer when the clocks have been put forward by one hour.  To save my swimming pool customers who use Economy Seven tariffs from having to alter their pool heating time switches twice a year and then work out the new time for the cheap rate, I tell them to set their heater time switches to G.M.T. and set the heating time from midnight to 7 am and then never alter them. The “on time” will always match the cheap rate period. The same will apply to pond owners who don’t want to keep altering their pond heater time switches

There is only one difficulty with Economy Seven times but it’s a very small one. In order that the entire country doesn’t all switch on their heating devices at exactly the same time with disastrous consequences for the National Grid, it is common to give individual customers slightly staggered times.  There doesn’t seem to be any hard and fast rules about this but whilst we always refer to the cheap rate period as “midnight to 7 am”, some customers start the period anywhere from a quarter to midnight to a quarter past midnight and cease exactly seven hours later.  Your electricity supplier will tell you exactly what times your cheap rate period starts and ends so you can set the heating period of any time switches accurately.

Heating times
A principle that applies to heating water by means of an electric heater is that a 1 kW heater will raise the temperature of 1,000 litres of water by 1°C in 70 minutes or it will raise the temperature of 1,000 gallons by 1°C in 317 minutes.  Strictly, this refers to heating water in tanks where there is good insulation so that there are no heat losses but, if we consider typical heat losses in the average koi pond, we can get a good guide as to how long a heater will need to be on in order to maintain any particular temperature.

The next couple of sentences may seem to be a bit technical and daunting but if you can follow them everything else becomes easy.  Taking the example of a 1 kW heater raising 1,000 gallons by 1°C in 317 minutes, it is possible to say that if the temperature of a 1,000 gallon pond drops by 1°C, it will take a 1 kW heater 317 minutes to bring it back up to that temperature. A 3 kW heater will need exactly the same time to reheat 3,000 gallons by 1°C and so on.

This leads us to a second rule of thumb for pond heating times which doesn’t involve complicated thermodynamic equations.  We can look at it more simply in terms of the quality of the insulation and say that if we could limit the heat losses to 1°C per day, it would take the appropriate size heater just 317 minutes, (approximately 5¼ hours), to reheat the pond. If the heat losses were 2°C it would obviously take twice as long and this would use twice as much electricity, but to limit heat losses to 1°C per day during winter is easily achievable by covering the pond and protecting exposed pipe work and equipment from the chilling effects of cold winds.

Electric heater running costs
If an electric heater is used only during the cheap period of an Economy Seven tariff and if heat losses are reduced so that it only needs to be on for 5¼ hours per night, the number of units used is surprisingly low. A 1 kW heater would only use 5¼ units costing around 28 pence per night, a 2 kW heater would cost almost 57 pence, and a 3 kW would cost 85 pence per night. It must be emphasised that this isn’t the cost of electrically heating the pond to greater than 20°C whilst it is uncovered during most of the year.  This is only the cost to keep the temperature from falling below 10°C as winter approaches, then to prevent the temperature from dropping below 4°C during the coldest months and then, as the temperature begins to rise in spring, to raise it to 10°C until the ambient temperature can take over and the heater is no longer necessary.

Another point to emphasise is that this is all based on temperature losses of 1°C per day. During the winter, koi forums are full of posts passing on tips about ways to insulate koi ponds.  Many koi keepers manage to prevent their pond temperature from falling purely by covering their ponds and making some inexpensive but very ingenious poly tunnels or bubble wrap enclosures.  If methods such as these are used and the heater doesn’t need to switch on at all during the night, the running cost will plummet, but the heater will always be there as a back up to protect your koi from harm if there is a sustained cold spell as was the case last winter.

Gas boilers and heat pumps are more expensive to install than electric heaters but their running costs are lower especially where ponds are heated throughout the summer, so next month I will describe how they should be used to heat ponds to gain maximum efficiency.  Also solar heating is almost free heat but can you use it?


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