Heating an outdoor tropical pond

Introduction

 

All fishkeepers wish they had more tank space to keep more fish. For example, keeping some of the largest cichlids in conditions allowing them to show the range of behaviours they exhibit in the wild, or a collection of medium to large cichlids in a community, would require accomodation well beyond the largest home aquaria.

For some time I've been intrigued by the possibility of heating a garden pond to keep large tropical fish and have wanted to work out whether, given the UK climate, the cost of doing so is prohibitive.

Rate of heat loss from ponds

Estimating the amount of heat lost from an open body of water is more complex than I'd imagined. The total amount of heat loss is the sum of the heat lost by evaporation, convection and radiation from the water surface, heat lost by conduction from the walls and floor and solar heat gain.

Of these the heat loss at the water surface is by far the dominant factor, to the extent that the approaches I've found completely ignore heat loss from the walls and floor. In particular, this should be a safe assumption to make if we are assuming an entirely or largely in-ground pond where the walls and floor will be insulated by the ground. Making this assumption also has the benefit of making the resulting rate of heat loss linear with respect to surface area, so we don't have to know how big the pond will be for the main calculations.

Each of the sources of heat loss are governed by a bewildering number of factors and a series of complicated equations. Luckily, after a bit of digging, I managed to find some software (HeatSim, from the University of Dayton Industrial Assessment Centre) that helps with the maths. If you want to do it longhand, for some extraordinary reason, the same source has a case study that goes through it step-by-step.

The key variables for our purposes are the surface area of the pond, pond temperature, air temperature and humidity. If we assume the pond will be in the South of England we can use monthly averages for the second two variables. Average monthly temperature data is widely available on the internet but humidity data is somewhat hard to come by, I'm guessing because averages are not particularly meaningful. I've calculated the average of daily humidity data from Bracknell for 2005 downloaded from metlink (the observer who provides this data also has a Bracknell Weather Page).

The table below shows the rate of heat loss in Watts (1 W = 3.413 BTU/hr) per sq. ft. of surface area from an uninsulated pond in the South of England at four typical tropical aquarium temperatures.

Month		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Mean Air Temp (oF)		37	39	43	46	52	57	63	61	57	52	45	39
Mean Air Humidity (%)		88.2	87.1	79.8	76.5	67.6	68.7	71.9	73.1	80.6	91.4	91.2	93.1
Pond Temp(oF)	76	38.68	36.81	33.49	30.85	25.82	20.84	14.59	16.44	19.64	23.61	30.64	36.41
	78	41.78	39.88	36.49	33.80	28.67	23.57	17.15	19.05	22.33	26.40	33.58	39.47
	80	45.01	43.08	39.63	36.88	31.65	26.44	19.86	21.81	25.16	29.32	36.66	42.66
	82	48.39	46.42	42.90	40.11	34.77	29.45	22.71	24.71	28.14	32.40	39.89	46.00

Equipment options and costs

There seem to be four main options for heating large bodies of water: Oil heaters, gas heaters, electric heaters and electric heat pumps. I'm assuming that solar heaters won't be sufficient to heat a decent sized pond to tropical temperatures in the UK climate and that heat exchangers that run off the central heating boiler won't deliver consistent high temperatures.

The first thing we need to establish is the capacity of the heater that we need to maintain the temperature. This should be the output needed for the worst conditions encountered. Heater output is measured in kW or BTUs per hour (oftern referred to confusingly as just BTUs.

The coldest day encountered in England was -26.1 degrees C, or -15 degrees F. As a result 110W per SqFt or 375 BTU/H per SqFt of pond surface area seems like a reasonable, conservative target (The software I've used to calculate heat loss won't provide estimates with negative figures, but the value is 88W/SqFt for an 82 degrees F pond at 0 degrees F and 50% relative humidity and it is close enough to linear with 81W/SqFt heat loss at 10F and 71W/SqFt at 20F). Given this, for a 15*10 foot pond, we'd need a heater with a capacity of 16.5 kW or 57000 BTU.

This is well within the range of mid-sized swimming pool heaters. This makes sense given that swimming pools are often much larger than the size we are considering here and heated to a higher temperature. The best range of swimming pool heaters I've found on-line in the UK is poolstore.co.uk. The cheapest appropriately sized models for a 15*10 feet pool and prices as of April 2008 are:

Oil heater	Gas heater	Electric heater	Heat pump
Certikin 110,000 BTU Oil fired heater	Certikin MB105 93,000 BTU Gas fired heater	Thermalec 18kW Electric heater	Alto H60Y 18kW Heat pump
£2,400	£1,375	£1,150	£2,300

The ongoing cost of heating

Beyond the cost of the equipment, the cost of heating a pond will depend on the total amount of heat that needs to be put into the pond, the fuel cost (which can be measured per unit of energy the fuel provides, i.e. per KwH) and the efficiency of the heater in converting the fuel to heat.

The rate at which we need to put heat into the pond is proportional to the rate of heat loss. To get a total amount of heat we need to put into the pond, we can simply multiply the rate of heat loss by month (see above) by the number of hours in each month. However, I'm also going to assume that the pool is covered at night. There is considerable research that pool covers can save a significant amount of heating cost, with up to 70% savings reported. I'm going to assume that the pond is covered at night and that the cover reduces heat loss by 40%.

In KwH per SqFt of surface area, the monthly and annual total heat loss is:

Month		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Total
Days heated		31	28	31	30	31	30	31	31	30	31	30	31
Daylight hours		8	10	12	14	15.5	16.5	15.5	14.5	13	11	9	8
Total Energy Requirement by Pond Temp(oF), KwH/SqFt	76	21.1	19.0	19.9	18.5	16.5	13.1	9.3	10.3	11.6	13.8	16.6	19.9	189.5
	78	22.8	20.6	21.7	20.3	18.3	14.9	11.0	11.9	13.1	15.4	18.1	21.5	209.6
	80	24.6	22.2	23.6	22.1	20.2	16.7	12.7	13.7	14.8	17.1	19.8	23.3	230.6
	82	26.4	23.9	25.5	24.1	22.2	18.6	14.5	15.5	16.6	18.9	21.5	25.1	252.7

If we assume the following efficiency levels for each type of heater (from poolstore.co.uk and fuel costs (from poolstore.co.uk and British Gas), the cost to heat a 150 SqFt pool with a cover is shown below. (BTW, I would strongly discourage anyone from using British Gas as their electricity or gas supplier - their customer service is astonishingly poor).

Type of heater	Oil heater	Gas heater	Electric heater	Heat pump
Efficiency	75%	85%	92%	400%
Fuel cost p/KwH	4.2	2.7	10.35	10.35
Total annual fuel cost at 78(oF)	£1,760	£998	£3,536	£813

Conclusion

The annual cost of heating a medium sized outdoor tropical pond is likely to be around £1,000 assuming gas or heat pump heating. Over a 5-year period the average annual cost of these two methods is very similar (£1,270) and significantly below electric or oil heating (£3,770 and £2,240, respectively). I'd go for the gas heating option, as heat pumps are less efficient at low temperatures and may not be able to heat the pond at all during the British winter. Also, the lifetime of the pond should be more than 5 years.