Hi Windmill I am guessing you have a Phalaenopsis Orchid (moth orchid)? Spritz wrote a blog about caring for these might be worth your looking it up?

Leaves going yellow sounds like lack of water to me. Spritz's blog was called Awkward Orchids.

Although the older leaves will naturally die off. If they're the "first" leaves and the rest are indeed healthy, it may well be nothing to worry about.
This happens sooner on Dendrobium than Phalaenopsis. Very normal on the old pseudobulbs from the Oncidium alliance. A photo would help, as would the type of orchid in question....

I'd have a couple more questions - is it warm enough - Moth Orchids need about 18 - 20 degrees in the daytime, and mustn't be shut behind curtains at night! Also - does it get enough light? They don't want direct sun on them, but do need bright light.

The leaves may be discarding if they are the lowest ones(moth orchid). Follow Spritze's advice. Then, you have rain water. Fill a jug, let it warm up indoors. Then pour it through the compost and follow up with a touch of liquid fertiliser. If it has flowers, that will be it for now, if not, then it may start a new leaf, root or flower shoot or all three. Worked like a charm for me on my collection this year......expecting lots of flowers for Christmas.

I don't get too wound up about rainwater - if I don't have clean, reasonably fresh rainwater available (which hasn't been too hard this year, has it), I just use boiled tap water.

how often do you water and is it allowed to drain so that it doesn't sit in stagnant water?