Immersion heater now providing less hot water.

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My hot water is supplied by a 1500 x 450 direct cylinder (210 litres capacity). It has two immersion heaters; the upper (boost, which is generally switched off) and the lower which runs for about 3-4 hours overnight. This has worked well for about 15 years (albeit the lower heating element needed replacing a year ago) giving plenty of hot water from just overnight heating of the lower element.

In recent months (about 8 or 9 months since the new element was fitted), I've noticed that the thermostat started to cut out occasionally and needed resetting - annoying because this resulted in waking up to no hot water. It's a 7" RDT for horizontal installation only, from Heatrae Sadia. I replaced it with a new one but even this one cuts out occasionally (i.e. once every few weeks). It is has also been noticeable over the last month or so (both with thermo #1 and the new one) that the water is not as hot as previously, thinking back over the 15 years, and consequently we run out of hot water much sooner meaning we sometimes have to resort to the boost immersion. I whacked the thermostat setting right up on the lower immersion but it doesn't seem to have made any difference - neither with the old one nor the new one.

As I said, the lower immersion heater was replaced about a year ago. I don't know if this is a clue.

I wouldn't have thought there would be any furring up because we have a water softener (installed more than 15 years ago) and in fact when the element was replaced a year ago, the old one looked clean (if that's the right word - at least there was no sign of furring).

And just to complete the picture, the CH is on a totally separate system.

I'd welcome any advice. Am I unlucky with a my thermostats? Should I get another one? (this would be the third) Could the immersion heater be faulty? Could the cylinder by faulty?

spiro

Two questions - how old is the cylinder and do you live in a hard water area?
If the answer to the 2nd question is yes and the 1st one years ago it's possible the bottom is slowly filling with scale.

splinternet

spiro wrote: »

Two questions - how old is the cylinder and do you live in a hard water area?
If the answer to the 2nd question is yes and the 1st one years ago it's possible the bottom is slowly filling with scale.

Hi Spiro, it's about 15 years old. Yes, we're in a hard water area but have a water softener