My pressure gauge never seems to go up very much, even after I *know* the filter needs backwashing. For a long time I thought that the gauge was broken. Last week I finally got around to replacing it with a new one. The old one always read between 5 and 7, the new one mostly reads between 8 and 10. Then I got a PoolPilot SWG, which has a strainer in the manifold. Every couple of days the low flow warning goes on, I open up the manifold and the strainer is totally stopped up with the usual kind of crud I see in the skimmer sock. I didn't think that kind of stuff is supposed to get past the filter (the SWG manifold is last, between the heater and the return).
That got me thinking that perhaps the sand filter was channelized. So I opened up the filter, which by the by is so old that you can't even find a parts diagram on the web, let alone actual spare parts, and was rather surprised to see sand that looked just fine, a few stray pine needles and otherwise pristine (I had just backwashed). From the age of the pool and what I have read I was thinking that it would all be crusted over and clumped up. Apparently the sand was replaced fairly recently, well last several years anyway.
Then today, out of nowhere, the pressure drops to 6. I checked the pump impeller and it seemed fine. The PoolPilot always shows sufficient flow (if I have cleaned the strainer). Nothing about the valves or return has changed, the skimmer isn't blocked. Later in the day, playing with the PoolPilot, I ended up turning the pump on and off several times and each time the pressure gauge reads a different number between 6 and 10. Each number is steady, I can change it by turning off the pump, or playing with the valves, but leaving those things alone it stays steady until the next time I turn the pump off and then on again.
There was one other strange thing, when the pressure is 6 but not otherwise, there is an occasional irregularly repeating clank sound from the main valve. The main valve is ancient, one of those metal pull up for normal, push down for backwash, affairs. The pool inspector, when we bought the house, said that the filter and main valve were so old they stopped stocking the spare parts ten years ago, and they sure look the part. The pump is somewhat newer, but also ancient. Also, water slowly leaks out of the backwash pipe any time the pump is running.
I'm starting to suspect that this means the main valve is leaking internally, so some of the flow bypasses the filter entierly. I would rather not have to replace the main valve, as that looks like a fair amount of pumbing. More than that, I would *really* hate to go through all the work of replacing the valve only to still have problems with the manifold strainer constantly clogging. Any thoughts?