Firstly I need to warn you I'm not a plumber BUT.....
This will only work if you have a standard water supply to your property, which I've read that you may not have. Here goes anyway.
I had the low pressure problem when I first connected my 200GPD RO-Man. It just dribbled out and I could get nothing to register on my p.s.i. gauge. The problem is solved if you can identify which of your house pipes are supplied FROM your loft tank and which pipes are on their way TO supply the loft tank.
The supply TO my loft tank is rated at between 50 and 60 p.s.i. but the water FROM the loft tank is gravity fed only. My stopc o c k (forum software...duh!) (where the mains water comes in to the property) is located under my kitchen sink. I just traced where the pipes led to and fixed the supply to the RO Unit at the closest convenient point where I wanted to make water. I know this not be as easy as I have made it sound.
Now I get 25 litres of water in approx 2 hours or 300 litres in 24 hours.
Whilst reading the instructions to my RO-Man I also noticed that with low pressure you will get different TDS reading in comparison to normal pressure, so your water may not be as good as you think. Always double check with a handheld TDS meter.
Also, RO-Man does a water pressure gauge at £9.50 + delivery
http://www.ro-man.com/shop/product_info.php?cPath=28&products_id=76Hope this helps. If your property is supplied from a well, brook or a stream then this post is useless
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I'm 4 weeks into WFP so I'm no expert. I'm forum-trained.