I have been reading this forum for two years and found it very valuable. For my own pool, water looks good and I am happy with my water clarity, but I suspect my old 1975 Hayward 22-inch sand filter is not working well. I want to avoid the trouble and inconveniences with a broken filter. I am asking for your guidance to make sure my filter does its job.
Here are some observations/symptoms:
1) When first open the pool (or clear a green pool due to negligence) and maintain shock level, I see no improvement on water clarity with the filter running continuously for a few days. Water is always milky blue color (full of dead algae, I think). To clear the pool, I do a one-time floc and vacuum to waste. After that, the water remains clear under routine pool care.
2) Filter pressure as shown by the gauge never increases (always 15 psi in filter mode), even after filtering the milky blue water for a few days. I thought the pressure should gradually increase as the filter catches more particles from the pool (and a backwash is recommended when the pressure increase by about 20%). The filter pressures with different multi-port-valve positions are
filter - 15 psi
recirculate - 15 psi
backwash - 10 psi
rinse - 10 psi
waste - 6 psi
3) Vacuuming through the filter in general makes the water cloudy.
4) I have a little sand on pool bottom. I am not very sure but perhaps a little sand could appear at the pool bottom after backwashing-rinse and turning back to filtering. I also have less sand (maybe only 60% of the sand) than the spec of the filter manual. I tried adding sand but it caused more sand at the pool bottom. So I reduced the sand level to 60%.
5) The backwash hose looks long (about 20 ft I think). Not sure if it is too long and prevents a good back wash.
6) I don't have a bottom drain but I think circulation is good. All circulation operations looks normal.
What concern me in practice are 1), 2), and 3). Overall, I suspect my sand filter is not working well. I disassembled the filter and this is what I found:
a) The spider gasket and the multi-port handle are new. I think they work well. They used to leak and I replaced them two years ago.
b) The plastic valve itself is old. I see no obvious leaks. But I am not sure where to look at exactly.
c) The stand pipe shows no obvious problems either.
d) The sand looks fine, loose and clean. I don't think "deep cleaning" is needed. But there is less sand than it should be because of the "sand in pool" issue. I guess having less sand is not detrimental. A shallow top layer of the sand does most of the work?
e) There are no laterals as commonly used in today's sand filters. My filter has a metal screen under the sand as the underdrain. I think it is much sturdier than plastic laterals. I don't think there is any way for the underdrain to go wrong.
How can I test a sand filter to make sure it works well? Should I take any steps to replace the filter valve, or buying a new filter is just simpler? but I feel not very comfortable doing any change without a good diagnosis.