Low hot water pressure upstairs

We had recently noticed low hot water pressure upstairs in the stand-up shower and two tubs. In the one tub (jacuzzi tub) we had previously noticed white, soft, easily disintegrating to the touch scales, and assumed it was calcium build-up. The two factors combined, I flushed the hot water heater per the manufacturer's instructions, and got a lot of nice gunk out (no sediment or discoloration, just the calcium). Water pressure out of the drain valve improved as I flushed. When that cleared, I started up the heater again.

This helped the upstairs water pressure (temperature is fine) but it's still not back to what is was a few days ago.

Is there something else I should check? The house is a staggered 4-level house to access to pipes is a bit goofier than a normal story house.

Also, how far open should the cold water supply valve be to the hot water heater? Could it not being turned open far enough contribute to the resulting hot water pressure?

Have you been having less and less hot water? It sure sounds like a diptube is coming apart.

I have enough hot water. I don't take long showers by any means, but never had a problem with having enough hot water. Just, the problem now is a loss of pressure. I get hot water, just by volume there's less water.

The water heater was made in October 1996. Enough references online suggest the dip tube problem was addressed in March 1996, but that doesn't tell me if manufacturers still used the bad ones in their inventory past that date.

If you have a pressure balancing valve and the spool is stuck and not able to move due to crud this may be the problem...

Given the date of manufacture, there is a real possibility that the dip tube has failed. Easy enough to check and replace if necessary, so I'd have a look at it.

If you are able turn off the water valve to your water heater and drain out some water and remove the nipple on the cold supply. put your finger in the cold inlet and remove the plastic diptube. If it breaking apart replace it with a new one.

Depending on the brand and model, some valves have inlet filters...the scale you saw may only be part of what was caught on the filter. Same on the showerhead...it could be clogged up with deposits. If they are tub/showers, is the flow to the tub good? If it is, and the shower is weak, look for a clogged up head.

The way to tell if the deposits are minerals or plastic from the dip tube is to place some in a cup of vinegar overnight. If it is plastic, it will still be there in its entirety. If it is typical mineral deposits (usually calcium), it will either all or mostly dissolve overnight (depends on the strength of the vinegar and how much you put in).

Take a look see at this link...
http://www.rustylayton.com/htmlxtra/diptube.html