Thermal Dynamics of Water

When learning how to size a pool heat pump, it is important to understand some basic science. I do not mean to go back to school and know how it all works, but I think knowing a little about why will help you make informed choices and realise when the wrong product is being offered to you.

Water unlike air is actually very predictable.  Water properties do change under pressure and temperature changes, but there are some very standard properties that are almost taken for granted.

• 1 Litre of water weights 1 kg
• Water Freezes at Zero Celsius
• Water Boils at 100 Celsius
• Specific heat of water is 4.181 (4.2) J/g/K
• It takes 4,200 Joules per kg of water to raise the water by 1 degree.
• 1 Joules = 1 Watt/ second thus 3600,000 Joules = 1 kW

This is why a lot of pool companies will base pool heating on volume alone.

A 50,000 L pool would require 116.66 kW to raise the water by 2 Degrees in 1 hour, or about 9.7 kW per hour over 12-hour period.  Thus, the pool company offers a 10-kW heat pump. This sized unit will work, but may also fall short if things like sun, ambient temperature, shade, wind and many other things are not considered.  After all a pool in Hobart requires a lot more heating than the same pool in Broome.

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The Surface area is key.
Although volume is great to know and very important. Your pool’s surface area is where you will lose the most heat, especially in winter. Heat naturally rises, and the bigger your pool surface area the bigger the heat loss potential straight off the top of your pool.  The amount of heat loss is in direct proportion to the temperature difference between the pool and the ambient temperature. Higher losses occur over winter. Wind, especially high wind just makes matters worse as they increase this heat loss significantly. With this said volume also matters.

Pool Features & Situation
Most pools are straightforward. However, some pools come with features such as wet edges, waterfalls, water slides, and infinity edges. These must be considered when sizing up your pool.  Also, it is important to know if your pool is above-ground or inground and what material the pool is made from. The colour of the pool can also matter. Dark pools absorb heat, light pools reflect heat.

Location Location
Believe it or not, where your pool is located makes a huge different in the pool’s heat load. A pool in Darwin requires far less heating than a pool in Hobart. The same is true in reverse. A cold plunge pool in Darwin will take a lot me cooling than the cold pool in Hobart.

Pool Blankets Save You Money
Modern pool blankets are transparent and allow critical sun energy to pass into the pool, but at the same time prevent heat loss off the pool’s surface. Regular use of pool blankets, especially over winter, will reduce your heat loss by up to 40%. It also will reduce evaporation.  If you are going to heat your pool, a pool blanket will be your best friend. They typically pay for themselves in the first year alone.

Direct Sun & Shade Cover
Swimming pools that are in direct sun can gain 500 to 1000 Watts / m2 of direct sunlight energy and heating during summer. This is one of the main reasons why pool heating is either minimal or not even needed over the summer. Trees, Shade cloth, and buildings all provide full or partial shade cover. This may prevent you from getting sunburn whilst swimming, it does reduce the free heating the sun can provide. Knowing the percent of shade cover is very important, especially over winter

Indoor Pools Verse Outdoor Pools
Indoor pools do not suffer the same heat losses to the same degree as outdoor pools. Often needing smaller heaters to keep the pool warm. However, air-conditioning and ventilation, required by law to keep you healthy, do cause the heat to move off the pool as they act like the wind. Indoor pools also do not benefit from direct sunlight solar heat gains. Big windows can help during summer but also add to heat loss in winter.

Swimmer Load
People cause splashes, splashes increase surface area and thus increase heat loss. People also splash water out of the pool, and take some water with them as they get out of the pool, increasing the requirements for water top-up. Top-up water is often cooler than the pool and will require heating. People funny enough though do add about 100 Watts to the pool due to body heat, more noticeable in cold pools, but they can also absorb heat in hot pools such as spas.

Pool Temperature
If you are looking into how to size a pool heat pump, you are thinking about heating the pool. But what temperature is good?

• Lap Swimming-  25 to 27 is common.
• General use pool – 26 to 29 is common
• Kids pool or Swim School – 30 to 32 is common.
• Spa – 36 to 38. We do not recommend warmer.

Important to understand, that the warmer you make the pool the more energy will be required to heat the pool, the larger the equipment you may need, and the dearer the running cost. 26 to 28 is the temperature most people heat their pool too.

Power Supply
Most domestic pool heaters and only 1-phase or 240 V systems. But large pools may require more grunt and thus a larger heat pump. Having 3-phase power available can give you the freedom of choice when it comes to equipment selection.

Measure from edge to edge

Knowing how to size a pool heat pump begins with measuring your pool. This process is relatively simple by measuring the maximum water level length and width, of the main body of water. When measuring the pool it is important to measure the water surface area from the edge of the water wall, not just the tilled edge above.

Regular Pools

If you have any step areas, then measure the step area separately as set out below.

Shaped Pools

If you have a shaped pool, it is often easy to treat it as a rectangle. This may oversize the pool a little, but it is better than under sizing.  Round pools you can take the diameter measurement.

Pool Depth

Many pools have varying depths. In a basic pool just take the shallow ends depth and the deep ends depth and average the two. This gives a good estimate for your pool.  Some pool builders will provide these details in paperwork. When it comes to pools with beaches, steps, and other shallow areas, where possible add them as separate measurements.

Infinity Edges, Waterfalls and Wet Edges

Also, measure all features. All features cause water to be exposed to the air, and Waterfalls are the worst for direct heat loss. They look fantastic, but we recommend their use in Sumer and avoid their use in winter where possible.

Contact Toyesi For your Heat Load 

It may seem like a lot of information, however, the more you have the better the outcome you will have. Once you know all this information you will be well-armed to perform a heat load on your pool.

You can email or call us or enter your information online: Click Here 

However, if you are still having trouble understanding how to size a pool heat pump, feel free to give us a call or drop us an email and we would be more than happy to help: Contact Toyesi

High Initial Costs:
Heat pumps often have higher upfront costs compared to conventional heating systems due to equipment,  installation, and setup expenses. These costs can be up to double to four times dearer than many people anticipate.  However, it’s important to consider the long-term cost savings and energy efficiency benefits. With typical payback periods from as low as 2 up to 3 years, depending on setup and use. Although adding more to the initial expense, adding solar panels to offset the grid electricity use, nearly all the ongoing power running costs can be reduced to nearly zero.

You will find that you either invest today or continue to keep paying tomorrow.

You Get What You Pay For.

The old saying that you get what you pay for is very much true in the heat pump industry. Many brands of heat pumps are imported from huge factories that mass produce units by the 1000s. They are designed and built with a budget in mind. They may promise long warranties and throw catchphrases and huge COPs. But in truth, low-cost systems may cost you in the long run.

• Budget systems – 3 to 7 years are their typical lifespan.
• Mid-Range Units – 5 to 8 years is expected.
• High-End Units – You can expect 10 to 20 years.

Servicing your heat pumps regularly can also extend their life.

Maintenance and Repairs:
Heat pumps require regular maintenance to ensure optimal efficiency and effectiveness. Filters, coils, and fans must be inspected, cleaned, or replaced periodically. In case of malfunctions or breakdowns, repairs could be more costly and time-consuming compared to conventional heating systems. Preventative maintenance and regular service up to 4 times a year is advisable to keep the equipment as efficient as possible.

Noise Levels:
Heat pumps emit a certain level of operational noise, which may be a concern for farmers or greenhouse operators located in residential areas. Noise reduction measures, such as locating the heat pump away from living spaces or implementing sound barriers, and sound attenuation may be necessary to address this issue.

Installation Location
Most heat pumps are installed outdoors.  They need a stable floor, often a concrete plinth is great. They also need good airflow around them. Heat pumps in heating mode produce cold air. If this cold air cannot dissipate it gets sucked into the heat pump and can over cool the unit reducing performance or even causing freeze-ups. This is why it is best to avoid top-discharge units where possible.

You can also install heat pumps in a plant room. However, you will need to duct fresh air in and the cold air out of the plant room. This will add extra cost for the ducting. However, your equipment will tend to last longer.  Toyesi is the indoor heat pump installation specialist.

Heat pumps are the most efficient way to heat your pool, allowing you to all year long enjoyment.  However, it is important to size your pool correctly. Many pool companies undersize pool heaters, which may save on equipment costs but lead to unhappy pool heating.

Pool heating can add up ongoing costs, however, investing in a pool blanket will save you up to 40% of these costs, and if you couple a heat pump with solar power your running costs may become nearly nothing.

Toyesi has over 30 years of tacit knowledge in sizing heat pumps and chillers. We have a range of Toyesi Australian-made equipment, as well as a range of imported more domestic-oriented systems.

We can take your project, and your working conditions and choose the correct equipment for you. We know that a 50kW heat pump in Tasmania is very different from a 50kW heat pump in Darwin. Not all off-the-shelf heat pump solutions fit all projects.  So why not drop us a line (Click Here)