How can I fix my ideal isar he24?

Alternatively the excessive PREHEAT could be a sign that the PLATE HEAT EXCHANGER SEALS 171031 are "melted" by running the DHW

Or the DIVERTOR VALVE 174200 is sticking in the mid position heating the radiators whenever running DHW

You would need a gas engineer to check both if you are absolutely sure you don't have dripping DHW tap/pipework

Q
September 2015

You have a drip somewhere on the DHW pipework or taps

If the drip is not obvious then I would strongly suspect mixer taps especially shower mixers

It has been known for the paddle that separates hot & cold in mixer taps to fail causing cold mains to back up into the boiler via the DHW pipework & cause OVERACTIVE PREHEAT

there are 2 solutions

1 replacing defective mixer taps (once identified)
Or
2 fitting a non return valve on DHW outlet immediately coming out from boiler

Q
September 2015

Thanks, but we have no dripping taps or pipe work. This only happens after the hot water has been used for a long time. When I stop using the hot what the "t" comes on and heats for a minute or so as it should, but about five minutes after that the "t" comes on again and heats for another 10 minutes. It's this extra ten minutes which is bothering me. 😖

Leah
September 2015

I think you are referring to the PREHEAT function on the ideal isar symbolized by the letter "t" appearing on the display
if the boiler is not being used PREHEAT normally fires every hour (lasts approx 1 minute)

If you have DRIPPING DHW TAPS/PIPEWORK then the PREHEAT will be firing CONSTANTLY

check to see if your DHW taps are DRIPPING

Q
September 2015