Bizarre dirt problem in pool

[MattW]

Hi all,

I have a fairly typical pool setup - hot tub attached to the main pool. I can adjust where my filters suck from, and where the water is pumped to. I can have the water sucked from the hot tub and pumped back into the hot tub, so the pool isn't affected at all, or I can suck the water from the pool, pump it into the hot tub, which causes a nice waterfall overflow back into the pool, and so on.

A few weeks ago, I woke up to a spotless pool and returned home from work to a pool that had a considerable amount of fine dirt-type particles all over the bottom of it. This puzzled me, although it had been quite windy, so I put it down to that. I vacuumed it. Next day, same deal. This then continued to happen every single day and so, I surmised, there must be a hole in one of the filter bags in the main tank filter (I'm sure there's a proper name for it). I was planning on taking it apart this past weekend when an odd thing happened.

On Saturday morning, the kids wanted to use the hot tub first thing, so I switched the pumps to have water only circulate in the hot tub, allowing it to warm up quicker. Once the pump had been on for half an hour, I went out expecting to have to vacuum the hot tub, but to my surprise it was clean. The kids were in the hot tub for an hour or so after this, and it remained clean the whole time. When they were done, I switched the pumps so water was taken from the pool and pumped into the hot tub. By the end of the day, there was a tiny amount of dirt in the pool, but very, very small.

The following day, the pump ran as normal - water from the pool and into the hot tub - and within an hour of it running, the pool floor was covered in fine dirt.

Today, I tested the hot tub circulating for an hour or so again before switching to normal, and I got the same result - clean hot tub, and virtually no dirt in the pool. Oh, and just to clarify, the hot tub NEVER gets dirt on the floor, regardless of what the pump setup is.

So, I'm a bit baffled. The hole-in-the-filter-bag hypothesis made some kind of sense, but this new information is odd. My second (horrifying) thought was that the pipe taking the water from the pool filter had a crack in it, but that doesn't fit either now that running the cycle through the hot tub for an hour first seems to stop dirt from getting into the pool later on.

If anyone has any vague idea what might be causing this, I'd be very grateful to hear it! One other data point is that I had just cleaned out the main filter tank prior to this happening, hence my thought that I'd punctured a filter bag.

Cheers,
Matt.

[Watermom]

Could it be that the slight bit of dirt is just in suspension while pool/spa is in use and then when all quiets down again, it resettles back to the floor?

[waterbear]

Can you describe the dirt (color, etc.) and where in the pool it is located? Also could you post a set of test results so we can possibly rule out some causes. Remember that there is a lot more turblance in a hot tub so any dirt would remain suspended in the water while in the pool it would tend to settle to the bottom, as Watermom pointed out.
Could you also let us know how you are chlorinating (SWCG, trichlor feeder or floater, liquid chlorine or bleach, etc.) and the type of filter (sand, cart, DE--not sure what you mean by 'filter bags; but suspect you either have a DE filter (the term is 'grids' or 'fingers' depending on design) or a cartridge filter. Also, does your pool have an automation system to switch from pool to spa automatically (and if so what kind) or do you have to manually switch valves?

[JimK]

... Remember that there is a lot more turblance in a hot tub so any dirt would remain suspended in the water while in the pool it would tend to settle to the bottom, as Watermom pointed out.
........

I was thinking the same thing.

[MattW]

Can you describe the dirt (color, etc.) and where in the pool it is located? Also could you post a set of test results so we can possibly rule out some causes. Remember that there is a lot more turblance in a hot tub so any dirt would remain suspended in the water while in the pool it would tend to settle to the bottom, as Watermom pointed out.
Could you also let us know how you are chlorinating (SWCG, trichlor feeder or floater, liquid chlorine or bleach, etc.) and the type of filter (sand, cart, DE--not sure what you mean by 'filter bags; but suspect you either have a DE filter (the term is 'grids' or 'fingers' depending on design) or a cartridge filter. Also, does your pool have an automation system to switch from pool to spa automatically (and if so what kind) or do you have to manually switch valves?

That's a very interesting point about the water turbulence in the hot tub; I hadn't thought of that.

In answer to your questions:

1) The color is light-ish brown, a typical dirt color (sorry I can't be more specific - I'll try and take a picture next time) and it seems that it's quite fine matter (it's not clumped up as lumps of mud or anything like that). The location of the dirt seems consistent - it appears first slightly to the left and right of where the water from the hot tub enters the pool, and then over the course of the next hour, spreads pretty much across the entire bottom of the pool. At the weekend, I will do 2 test cases on Saturday and Sunday and will photograph the results. Saturday will be hot tub only for 1 hour, followed by pool. Sunday will be all pool. I'll take a photo before, after 1 hour, and at the end of the cycle.

2) Chlorination is done via a floating feeder, and I'm using 4-in-1 tablets (chlorine, algaecide, etc.).

3) The main filter is a series of 8 'bags' (sorry, that was the first word to describe them that came to mind - perhaps cartridges is the proper word?) that are set in a circular propeller-type design (when viewed from above). Each one can be removed and is a very fine mesh 'bag' with a tube that connects to the bottom of the filter. They seem to catch a whole lot of silt and fine matter and eventually clog up and need cleaning - I hose each one down to clear all of the muck out of it and put them back in. I wondered if one of these had a tiny hole in it that allowed some finer matter to pass through?

4) Switching from pool to spa is done via a switch in the house. This activates a motorized lever on the valve.

Oh, and one more thing I've noticed - the amount of dirt put into the pool is not consistent. In the first half to full hour of the pump being on is when 95% of the dirt ends up in the pool. If I vacuum the pool (not using the hose vacuum, so none of it is going back into the filter) about an hour after the pump has been running, the pool remains pretty much spotless for the rest of the day.

Thank you for your help with this!

Cheers,
Matt.

[MattW]

Could it be that the slight bit of dirt is just in suspension while pool/spa is in use and then when all quiets down again, it resettles back to the floor?

That is entirely possible, but what's confusing me is the difference in the amount of dirt that settles based on whether the hot tub was running by itself for an hour or so before switching over to allowing the water to flow into the pool.

To say I'm a little perplexed is an understatement.