HW takes ages to heat up

[padairg]

Hello, my HW is taking longer than expected to heat up and it's getting worse. I can go 10-40 degs after 1.5 hours of only heating HW, whereas I should be going 10-60 degs in 30-40 mins by my maths. Don't even mention trying to heat HW and CH at the same time. 30 mins with the immersion adds 10 degs. Last night the tap temperature on full hot mix was 50 degs after a minute of running and by the time there was 6 inches of water in the bath, the bath temp was 34 degs and water from the tap was 30 degs, still on full hot. I estimate 100l of water used, so half the cylinder. IS that normal? Perhaps the horizontal nature of the cylinder means the mains cold water incoming dilutes the hot, as there is no high/low hot/cold separation.
Almost 10 years old, CH is fine, system boiler, Ferroli 27kwh, JAB 200l horizontal cyclinder, pressure is fine, cylinder stat seems fine as I can hear it click on/off if I shift it lower (normally set 60 degs).
2 port motorised HW valve has been replaced as I thought that might be the issue originally. I can feel the pipes are hot either side of the valve when it's open and boiler is outputting at 65 degs.
It has been getting worse over time, i.e. taking longer to heat the HW.
Any ideas? Could it be either a build up in the closed system/coil, or as we live in hard water London, a build up of limescale on the outside of the coil possibly. Are either of those likely in a 10 year old system and would they affect to such an extent?
Can't decide whether to just suck up the higher gas bills or investigate/drain and inspect the coil/powerflush/replace cylinder.
Cheers

 

[Chuck]

Top of my list would be a build up of scale on the tank's coil impairing heat exchange.

A low-risk way to check is as follows: With the central heating off, use gas meter readings to measure the power / heat being supplied to the cylinder coil when heating hot water. Also record measurements of the flow and return temperatures at the cylinder during the test. Compare results with cylinder manufacturers's spec.

Alternatively, if it is a vented cylinder, take out the immersion heater and take a look with a torch and endoscope. If it's an unvented (i.e. pressurised) cylinder you need to get a plumber who is qualified to work on pressurised systems to do this for you.

 

[padairg]

Many thanks, I have a probe thermometer so will strap that to the pipes and see if there is enough heat loss either side of the cyclinder. Cheers.