AIR CONDITIONING | CONDENSATE Air conditioner room unit above ceiling Condensate line Overflow pipe Figure 3: Retrofit system being tested below a false-ceiling installation Water tank Drain pan CWS Water sensor Solenoid valve Pump with pressure switch Check valve Check valve Existing line Figure 2: Above false ceiling recommended retrofit installation air conditioners will produce between 19 and 76 litres of condensate per day.1,2 Considering an example in Dubai, a simple system (Figures 2 and 3) could collect up to 15 litres of water in a day from an air conditioning unit (12 hours running time) of around 5.3kW (1.5TR high ambient temperature) cooling capacity. This water would then be used to fill the toilet cistern, and any top-up, if required, will be supplemented by the cold water supply (CWS) from the mains. Figure 3 shows the installed retrofit solution being tested at a residential unit in Dubai. A 9V DC solenoid valve was used for this exercise, and an exposed arrangement was applied only for the testing purpose. This design solution does not require any major changes to the existing WC: a few check valves; solenoid valve; water sensor; associated electrical works; and a small water-storage tank (12 to 18 litre) added to a line from the condensate drain pipe and connected to the CWS pipe supplying the WC. This compact assembly, in may cases, could be installed in the space above the false ceiling, closer to the indoor unit, as illustrated in Figure 2. A small (booster) pump may be required if the toilet cistern fill (inlet) valve minimum working pressure is more than the available static head. The other, less favourable, option that may be considered is to replace the fill valve with one with low working pressure (for example, 0.01MPa). The working principle is straightforward; the water sensor keeps the solenoid valve The water sensor keeps the solenoid valve closed as long as there is water in the condensate tank closed as long as there is water in the condensate tank this stops the water flow from the CWS line. After a flush, the water stored in the water tank flows first via the existing supply line (Figure 2) into the toilet cistern. The solenoid valve opens only once the condensate tank is empty and lets the mains water flow into the cistern if required; a simple controller is used to carry out this operation. Check valves are installed so that mains water doesnt flow into the condensate line/ tank and vice versa. For this retrofit solution in Dubai, the payback is estimated to be two to three years, depending on the air conditioning running hours and WC use. For new-builds, an even shorter payback period can be expected because the cost of installation would be much lower. Water savings With a production rate of up to 0.32 litres per hour of condensate per kW of air conditioning3 depending on the geographical location, number of hours of air conditioning operation and WC use a possible water saving of 30-60m3 per year could be realised in a two-bedroom apartment with two bathrooms and indoor air conditioning units with a total cooling capacity of 21.1kW (6TR high ambient temperature). It may not seem huge, but considering there are thousands of existing residential units, hotel apartments and offices with new ones constructed every year the water savings could be significant. As well as villas, townhouses and hotels, this system can be used for buildings where an onsite central condensate recovery and distribution system is not economically feasible. Currently, the condensate from the air conditioning units for such buildings goes straight down the drain. CJ REHAN SHAHID is a director at P&T Group References: 1 A Leahy, How much water should drain from a central air conditioner? Hunker, bit.ly/CJOct20AC2 2 How much water should drain from your AC? A NJ tech explains, Air Experts, bit.ly/CJOct20AC 3 Condensate water recovery, ASHRAE Journal (2005) 62 November 2020 www.cibsejournal.com CIBSE Nov20 pp61-62 AC condensate.indd 62 23/10/2020 16:41