5 minutes ago, Nickfromwales said:

All very interesting and plausible responses to the OP's question, time to now throw an additional spanner into the works.

The conversation seems to have excluded the dreaded F&E tank, and the heat and evaporative losses thereof. Mono-speed pumps need somewhere to runoff their excess pump potential, so when the circuit closes there is unwanted migration to the F&E tank.

For this reason, and to gain a modulating pump (and some uplift in 'tech'), I would always go system vs HO.

I literally would ONLY ever go HO if it is a failed unit and needs a retrofit on a tight budget.

That's me screwed then replaced a tired and over spec'd HO Glow worm for a Viessmann HO that wasn't cheap

Although to answer the points about F&E - I had a temp probe on the F&E tank because I was interested in if it was contribution to loft humidity due to pump over - It's not - probably because my system runs very open with no TRV intervention in the heating seasons

I do intend to go sealed on the heating circuit side and ditch the F&E tank but there is some pipework replacement I need to do first so that's a job for 2027

Just found this post from someone 16 years ago asking the very same question... only to then spot it was me!! 😂

https://www.diynot.com/diy/threads/recommendations-for-eventual-replacement-of-an-ideal-icos.241090/

I did say about 'procrastinating over options' in my opening post here but that's taking the biscuit...

Edited 8 hours ago8 hr by MJNewton

36 minutes ago, MJNewton said:

Just found this post from someone 16 years ago asking the very same question... only to then spot it was me!! 😂

You may well have saved yourself the cost of a whole boiler in those 16 years! While I'm sure we all hope we beat the stats I believe the life expectancy of a modern boiler is about 10 years.

7 hours ago, -rick- said:

life expectancy of a modern boiler is about 10 years

Suspect that says as much about the installer and the use of smart thermostats in every room. Boiler just short cycles it's whole life