Warm Up an Intex Easy Set Pool Fast
Hi,
u/jack4607 recenty asked a question on how to heat his new above ground pool. I have noticed a few similar posts on the sub and thought that I could share how I enjoy swimming in 30'C water in my Intex 4.5m x 2.2m pool, heated cheaply and quickly. I live in the UK where we have less favourable weather than much of America so my thoughts reflect this.
My Intex Pool at 30°C seen by thermal camera. Red = Hot. Blue = Cold.
TLDR
Your domestic central heating boiler is the fastest and cheapest method of heating a small pool for temporary summer use if you can do some basic DIY plumbing. Skip to the section entitled "Making the pool heater" for the gory details, otherwise read the rest for my thoughts on other heating options for small above ground pools.
MAKE THE MOST OF WHAT YOU GOT
First of all, make the most of what nature gives you. If possible, place the pool in a sunny spot and preferably one that is sheltered from wind. This will maximise solar gains and reduce wind chill as far as possible.
HEAT LOSSES AND CONSERVING HEAT
The biggest source of heat loss from a pool is evaporation. When a material changes phase from a solid to a liquid, or from a liquid to a gas (as happens when water evaporates) it takes energy so do so. This is called latent heat. When water evaporates the energy supplied to do so comes from the body of water, so the water that is left behind is cooled (removing energy cools, adding energy heats). Every litre of water that evaporates takes 0.63kWh of energy. For the pool to maintain a constant temperature this lost energy must be replaced somehow. If the pool does not have an artificial heat source this energy must be sourced from the environment. Energy flows from hot to cold so for a pool to absorb energy from the environment, it must be cooler than the environment. Therefore, an uncovered pool with high evaporation rate can be significantly colder than the average environment temperature!
Covering the pool shall prevent evaporation and the associated heat losses. You can get solar covers which absorb energy from the sun and transfer it to the water to heat it, but anything that will cover the surface shall prevent evaporation. So, if you don't want to spend money on a solar cover using something like a tarpaulin or plastic sheet shall still be worthwhile. A dark coloured cover shall act as a poor man's solar cover, it will just provide less insulation as it does not have the bubbles like bubble wrap.
Other significant losses under your control come from things like the amount of surface agitation, bubbles and waterfalls. All of these increase the surface area of water that is in contact with the air and thus increase heat loss both through evaporation but also plain and simple radiated and convection heat losses. Do not run features unless you need them (I.e. aeration to increase pH of the water) or are actively using them for enjoyment.
Heat losses are proportional to the difference in temperature and the environment. A cold pool has no losses, so it is always cheaper to let the pool cool down when not required. The issue is one of convenience because there is a significant warm-up time from cold but depending on the size of your pool and the heating method the time can actually be quite short.
HEATING OPTIONS
The first and cheapest option is of course not to heat the pool at all but in such as case the pool will sit at the same temperature as the average daily temperature. According to the Met Office the average daily highs in July & August in East of England are 22.6'C and the average minimums are 11.4'C. As a result, an unheated pool will tend to sit around 18'C. Not a pleasant experience in my view!
So, you want to heat the pool...
In each heating method I shall use the following stats from my pool:
Volume = 7,200 litres.Cold Temperature = 18'CHot Temperature = 30'C
Daily energy requirements: 25 to 50kWh depending on weather if heated daily based on measurements from my own pool.
I shall use the following energy prices: Gas = 2.2p/kWh. Electricity = 14p/kWh. Approx. UK average prices for summer 2020.
Fist a bit of maths. It takes 1.16kWh to heat 1,000 litres of water by 1'C. You can easily calculate the energy required to heat your pool by multiplying the volume in thousands of litres by the temperature rise, multiplied by 1.16. This neglects losses.
Example. A 7,200 litre pool starting at 18'C and raising it to 30'C would take:7.2 * (30 - 18) * 1.16 = 100kWh.
Electric Resistance Heating
At the bottom end of the market you have plug in electric heaters that go in-line with your pump and filter. Plug in heaters are limited power by the plug sockets themselves and therefore top out at about 3kW in the UK & EU. Intex make a 2kW heater. At 2kW power it would take 50 hours (just over two days) to bring the example pool up to temperature, neglecting losses. In the real world as the pool temperature rises so too do the losses. For this reason, the pool is unlikely to hit 30'C an in most likelihood would sit around 25'C in British summer. So to keep the pool at a moderate temperature ready for use the heater would potentially need to be left on 24/7.
Cost for 100kWh of heat = 100 * £0.14 = £14.00.
Cost of daily heating 25 - 50kWh = £3.50 to £7.00 per day / £106 to £212 per month.
Heat Pumps
Next up are plug in heat pumps. These are much more efficient than electrical resistance heating. They take heat from the air and move it to the water. The efficiency depends on the relative temperature difference between the air and the pool water with them being more efficient with the highest air temperatures and lowest water temperatures. The Coefficient or Performance (CoP) can be anything from 3 to about 7, so for every kWh of electricity put in you can get 3 to 7kWh of heat put into the pool. Clearly much cheaper to run than resistance heating. You can recover the cost of the unit fairly quickly if you use the pool often. Small plug in units are now available for a few hundred pounds and are pretty much plug and play. Hot Splash and Nano Action are two examples of small plug in heat pumps. Both are rated at 3kW power to the pool, but only draw 0.6kW of electricity giving a CoP of 5. The issue of long warm up times due to the low output power remains however, meaning it would need to be potentially left on 24/7, and may not attain 30'C but the running costs are slashed considerably. You can get higher power heat pumps, but the cost rises and they usually require permanent installation.
Cost for 100kWh of heat = (100kWh / 5 CoP) * £0.14 = £2.80
Cost of daily heating 25 - 50kWh = £0.70 to £1.40 per day / £22 to £44 per month.
Gas Boiler
Gas is about 15% of the cost of electricity, and an order of magnitude more powerful than plug in heaters. The high power means that gas is the fastest to heat the pool and as such losses are insignificant in the ability to reach your target temperature and the time taken to do so compared to theoretical calculation. Dedicate pool heaters are used on most full-sized pools, but they are expensive (£2,000+) and require permanent installation so are not suitable for a small above ground pool. What is the solution? See below...
Cost for 100kWh of heat = 100kWh * £0.022 / 95% = £2.35 in gas, plus £0.42 in electricity for pumps = £2.77.
Cost of daily heating 25 - 50kWh = £0.70 to £1.40 per day / £22 to £44 per month.
However, due to the very high power delivered by gas boilers there is no need to keep the pool at operating temperature all the time like you must with the plug in resistive electric heaters & heat pumps because you can heat it from cold in just a few hours. A cold pool uses no energy.
MAKING THE POOL HEATER
A typical domestic gas boiler in the UK is between 20 & 35kW power rating. This is 10x more than can be drawn from a plug socket. Heat is transferred from the gas into water running in the heating loop which is then pumped around radiators into rooms or to a coil in a hot water tank. We can use this to heat a small pool with the aid of a heat exchanger, a pump, and some plumbing. The project is suitable for anyone with basic DIY skills and work on the water side of the boiler (as opposed to gas) does not require formal qualifications in many countries. Check regulations before work and keep away from the gas supply!
The cost of this project shall be approximately £250, but you may be able to find things cheaper if you look around. Even so, £250 is cheaper than any heat pump and the low running cost savings compared to electrical resistance heating mean that it pays back very quickly despite the build costs being twice the price of resistance heater purchase.
A Typical British Boiler Cupboard. (Rarely sees sunlight let alone a vacuum cleaner! ;-) )
A boiler has two copper pipes that supply the radiator heating loop on combi boilers, or both the radiators and the hot water tank on traditional / system boilers. We need to tap into the output of the boiler to install a heat exchanger so we can transfer heat from the water in the boiler to the water in the pool whilst keeping the two water bodies separate. *You cannot under any circumstances run pool water through the boiler directly - you shall kill the boiler due to corrosion and it shall require replacement (£~2,000).* Not to mention that your pool would fill up with nasty rusty water that inhabits the average heating system.
A heat exchanger is a set of thin metal plates with 4 ports. The plates transfer the heat whilst keeping the water separate. Pool water chlorine is corrosive, so the heat exchanger needs to be stainless steel or titanium to resist the effects. Dedicated pool heat exchanges exist and are the best option, but they are often pricey. For a small pool we can take a view that as the system it not operated 24/7/365 we can get away with lower specified components and accept that they shall eventually need replacing due to corrosion. This gives options for cheaper heat not intended for pool use. The exchanger I have used is stainless steel but has copper brazing. The copper will corrode, but as very little copper is inside the exchanger and it is not wet 24/7 (only while actively heating and hoses connected) the corrosion rate shall be low.
On my heat exchanger two ports connect to the pool loop and two the boiler loop. I soldered my copper connections whenever possible and only used compression when component availability dictated it. This is simply because compression connections are large whereas solder takes up virtually no space. If you are not comfortable using solder you can use compression fittings, or you can use copper push-fit fittings providing you support the pipes so as not to stress the joints.
Locate the hot water flow and return on your boiler. We need to tap into the flow pipe before any control valves for radiators or hot water cylinders so we can add a Tee for the pool heat exchanger. If you do not have any control valves (as may be the case on combi boilers) you shall need to add one in order to isolate radiators when heating the pool as no-one wants the room heating on in summer! My system is a traditional / system boiler and thus has two motorised valves already in place (white boxes on pipes on the right and left sides of photo) - one for each heating and hot water, plus several manual valves. I have tapped in before those valves and they remain closed when heating the pool. The return pipe does not need valves and should be direct to the boiler return.
Install the heat exchanger taking care to note the recommended direction of flow. Heat exchanges work most efficiently when used in counter-current flow. That is where the two bodies of water pass in opposite directions.
There should be a control valve installed before the heat exchanger to isolate it from the system when the pool does not require heat. My system currently uses a manual valve with no automation, however this shall be replaced with a motorised valve, flow switch and temperature sensor for full boiler automation in the future, but that is beyond scope of this post.
Boiler Water Loop into Heat Exchanger.
Pool Water Loop into Heat Exchanger.
Close up of heat exchanger showing labelled plumbing components.
Thermal camera image of boiler cupboard whilst heating pool.
Note the temperature difference on each side of the heat exchanger. It can be seen that the hot water from the boiler is significantly cooled as it flows from right to left across the top ports, and the pool water is significantly heater as it flows across the heat exchanger bottom ports from left to right (counter current).
The boiler side is plumbed in 22mm copper, and the pool side is 1 inch PVC pressure pipe. You cannot use PVC waste pipe! Use at least 1" pipework to the pool to ensure sufficient water flow rate. Anything smaller shall be too restrictive and result in overly hot water to the pool which could damage the PVC pipe or cause a safety issue with scalding. The 1" PVC pipe shall need connecting to the heat exchanger with threaded fittings to suit and will also require pipe size reducers to adapt the 3/4" pipe to 1". My heat exchanger has 3/4" BSP threads on all 4 ports. I recommend 3/4" as the minimum on all ports. Some heat exchangers have mixed 1/2" & 3/4" ports. If you can only source mixed size port heat exchanger use the smaller port on the boiler side rather than the pool.
The PVC pipework it taken through the wall outside where it is terminated using GEKA Connectors. GEKA are a German brand of high quality quick-connect hose pipe connectors that can be used in both suction and pressure applications. I recommend using 1" Female GEKA connectors screwing into 1" BSP Male PVC threaded connector as the GEKA Female couplings have nice rubber seals on them which the male ones don't. Use GEKA connectors on all hoses.
PVC pipe should not get hotter than 60'C max, though I prefer to de-rate to 50'C. We also want to prevent scalding injury risk so we do not want water entering the pool hotter than 45'C. With this in mind we must ensure that the water flow rate from the pool through the heat exchanger is sufficient to keep the Delta-T (difference in temperature across the heat exchanger from pool inlet to pool outlet) to below 15'C max, and preferably below 10'C. This will allow safe usage for pools up to 35'C with 10'C Delta-T. For a Delta-T of less than 10 you shall need an extremely high flow rate and thus large or multiple heat exchangers.
Minimum Flow Rate (L/min) = (Boiler Power in kW * 60 seconds) / (4.2 * Delta-T)
Where Boiler Power in kW, Delta-T in °C.
Example: My boiler is rated at 24kW, so for a Delta-T of 10'C I must achieve a flow rate of:
(24 * 60) / (4.2 * 10) = 34.28 litres per minute minimum.
To keep the system efficient by lowering temperature differentials between water in the boiler loop and pool loop I recommend over sizing the heat exchanger by a factor of 2. Ie for a 25kW boiler target a 50kW heat exchanger. The cost addition is minimal. My 24kW boiler uses a 20 plate 44kW rated exchanger.
To pump water from the pool through the exchanger I recommend a 1-inch self-priming surface water pump. Driving water through a heat exchanger requires high pressure. Pool filter pumps are high-volume low-pressure systems and may not be suitable. If you have a powerful pool pump you may be able to do it, but my Intex pump is too weak so I have a separate pump driving the heating loop and I leave the filter pump alone. I went with a Clarke SPE1200SS Stainless Steel Self-Priming pump as it is capable of 4.6 bar pressure (46 metres head) and can allegedly shift 60 litres per minute at zero head. This pump is also IPX4 rated so is waterproof against splashing for all directions so can be left out in the rain. Check your pump curves in the manuals to see what flow rates vs head they can pump before buying. I say allegedly shift 60 litres per minute because I have never seen this type of self-priming pump move more than 45 litres per minute with any useful length of pipe attached.
Clarke SPE1200SS 1" Self-priming water pump.
My pump thrown in the bushes near the pool, using GEKA connectors on pipework.
My system drives about 50 metres of pipework (round trip length) which is a lot. I achieve 38 litres per minute which is just above my desired minimum flow for my boiler power & desired Delta-T.
The pump should always be placed before the heat exchanger so that you are pushing water through it rather than sucking water through it. The suction pipe from the pool to the pump should be as short as possible for best performance but far enough to keep electricity away from the pool. Use common sense! If you must have a long suction pipe from pool to pump, step up the pipe diameter to compensate.
For now, the pipes are just clipped to the pool wall and dangled over the side. In future I shall plumb them in properly and install local isolation valves so I can disconnect the pipes.
Hot water return to pool clipped to frame using plastic spring clamps.
Toasty Warm! Pool temp = 30°C. Hot Water Inlet = 37.3°C.
To get the boiler to fire up I simply fool it into switching on by turning my room thermostat to max, closing the valve to the radiator heating loop and opening the valve to the pool heat exchanger. As the house never reaches max temperature on my room thermostat the boiler runs continuously. I manually watch the pool temperature and turn it off when I'm happy. Pools change temperature slowly so manual control is possible. I tend to use the 1-hour heating advance feature to limit the burn time and protect against accidentally forgetting to turn it off and having too hot pool. In future I shall add a thermostat to the pool which will automatically turn on the pump, which in turn shall activate the boiler via a flow switch sensor installed in the PVC pipe in-line with the heat exchanger. A thermostat on the PVC pipework inlet to the heat exchanger shall remotely sense the pool temperature from within the boiler cupboard. The flow sensor prevents the boiler from running in case of pump or pipe failure. The thermostat makes the system fully autonomous so I don't have to nip out every 30 minutes to take a pool temperature reading.
That's it! I can bring the pool up from stone cold to tropical in a morning, and daily top ups only require an hour or two. Now you too know how to.
Here is an Excel spreadsheet of all the parts I used (or similar as I had a lot of bits already).
Parts List.
Parts List can be found at: http://www.timwarren.co.uk/Pool_Heating_Reddit_Post_Shopping_List.xlsx
PS: Are you still reading? Do I win the prize longest Reddit post for today?
Source: https://www.reddit.com/r/pools/comments/gr698b/how_to_heat_intex_bestway_above_ground_pool/