I don't think you can just soak them, that would just be to build up absorbency really.
If the machine isn't doing very well, I would do the dishwasher tablet method. Even if you don't have a dishwasher (I don't), I think it's well worth buying just a cheap box of the tablets. I got the Tesco eco ones for £3-£4 I think and they're lasting forever! Then you just need a normal 60 degree wash plus a couple of extra rinses. You could soak them instead of the rinses but I don't think you'd need to.
If you prefer to use the 'full dose of detergent' method, you could use soaking instead of some of the rinses. Wash them on 60 degrees with the detergent, then do a few rinses. After that, soak them in a bath of water (no need for warm water, cold is fine) and swirl them around whenever you go near. Change the water every couple of hours and keep swirling until there is no scum/bubbley stuff on top of the water. Then rinse a few more times in the machine.
I do the dishwasher tablet method every 6 weeks or so, if I remember, as prevention. It's so quick and easy that I think it's worth doing cos the detergent method takes ages! If you want to use the detergent method, you could do it whenever you want really but I'd only do it when I thought problems might be starting. If you sniff the nappies while they're still wet (and when they're warm too is a good idea, like if you tumbe dry them for 10 mins) and they smell a bit musty, that's when I'd strip them.
With the dishwasher tablet method, I put everything in as it's ok for PUL and elastic and I do it straight after a wash so I don't bother to sort the load out. You would always want to put inserts in, plus liners and any fleece covers. PUL can go in but probably doesn't need to. Basically, anything that is vaguely absorbent or has wee passing through it (like liners) should be stripped.
It may be worth doing a service wash on your machine as well from the sound of things. I use dishwasher cleaner fluid for that - the bottles of liquid, where you put the whole bottle in the machine. Just put the bottle in your washing machine instead of the dishwasher, and use the hottest programme you have. That will clean out all the pipes as well. And wipe around the door seal with an anti-grease spray, and clean out the filters. It may give your machine a new lease of life!
You could do but I would still do at least a couple of machine rinses straight after the wash, then soak and keep swirling the water and changing it every hour or so, then another couple of machine rinses so you probably won't be any better off! Why not try a dishwasher tablet one instead?
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