Only luke-warm HW some mornings

[andyste1]

We have a standard boiler, and the HW is timed to come on for half an hour in the morning and half an hour early evening. This has always been more than enough, but since December there have been several days where the hot water has only been luke warm in the mornings. The only thing these days have in common is that they are mornings after a bath was run the night before. We've been running 2 or 3 baths a week for years without any problems, so not sure what's going on now.

It's also intermittent, as it's only happened a few times - the HW is fine on other mornings (even after a bath). And we haven't noticed problems with lack of HW in the evenings.

Initial gut feeling was the 3-port valve playing up, but I would expect the problem to be more frequent/random (and wouldn't explain why it only happens after a bath night). Thoughts?

Not sure if this is related, but for years we've had a problem with the hot bath tap flow slowing right down once the bath gets to around half full (the water coming out is still red hot). It's been particularly bad twice over the past week - it happened last night (down to a tiny trickle) and this morning was one of those luke warm ones.

 

[macackau]

When you next run a bath go in to to your loft and watch the large cold water tank and see what happens to the water level. It maybe that the float valve to the tank is on its way out and isn't allowing enough water in to the the tank when you run a bath so you are draining out water faster than it can be replaced. Potentially what's happening is you are draining out the cold water tank and it's only being replaced by a float valve that's trickling fresh water back in to it.

 

[jtsplumbing]

Its either 3 port valve is on way out or cylinder stat is faulty.