Primarily we are only concerned with the afflicted energy based on flying stars. Key bad energies are 2, 3 and 5. 2, 5 are earth based Qi and 3 is wood based. What we dealing with are really Natural elements in world. Not some numbers that are symbolic in nature. (4 rods or 6 rods etc) Key we are only interested in the element that counters the bad Earth Qi. Metal is to counter Earth energy. At home, I bought those iron dumb bells. (For lifting weights) Those are already good enough. Key is get the correct element and place in the correct Bad Qi sector of the house. Feng Shui and Bazhi application is totally different. Thus for windchimes, they are effective if they are metallic in nature, copper, iron, regardless of 4,6 or how many rods. The real deal is to get the metallic element to dissipate the bad earth energy in the correct sectors. Joey yap's book is a good place to start with. As in any application, the external environment is actually key to affecting a house. so if you look at my perspective, Feng Shui is very scientific and not superstitious in nature. If you notice, one of the key areas in a house is really the kitchen. Why? because that's where there is a REAL fire there, when you cook. So the energy is real and if placed wrongly or in conflict with the sectors of the house, it may not be conducive. Don't be worried about if it is a hollow or solid, number of rods, etc. Key is the element. However, I would prefer a real hollow metal (copper, iron, steel, gold; yes gold if you are rich ) so the tinkling metallic sound can dissipate the bad Earth energy. (You need to find out how to determine the yearly bad energies for each house) Last but not least: 1. Bazhi - your destiny - can't change. understand your weakness/threats. make the best out of it. 2. House Feng Shui - a mirror image of your Bazhi. you can try to fine tune to ONLY cushion the bad and elevate some of the good. (you can't be a millionaire overnight) 3. In HDB, most of us stays there, there limit on how we can tune the Feng Shui to make it better. 4. If after understanding your HDB/house natal chart (Flying stars) and putting in place some of the cures, and if it still don't work. It may be the greater environment's influence that cannot be changed. 5. At this stage or even earlier stage, if you are sick, please see a doctor.