Vaillant eco Tec plus

I would think that heating water is the heavier load, 24 or 28 kW rather than the five to one kilowatts when heating say in Spring of Fall...

Thomas Prufer

Return sensor on the hot water ?

a partially blocked or scaled up DHW heat exchanger can cause this issue

Did the boiler engineer fit a limebeater to the cold water supply to said boiler?

S.

P.S did the boiler engineer powerflush the rads and fit a Magnaclean before commissioning the boiler?

very important to prevent the iron oxide (magnetite) from finding its way from the radiators to the DHW heat exchanger and blocking it.

PLus you need corrosion inhibitor installed at each annual service.

Only if you have a hot water tank, not on a combi boiler supplying hot water.

It's recommended that a power flush is performed before a new boiler is installed. If it running a hot water cylinder a magnetic filter should have also been fitted to meet the Vaillant 10 year guarantee conditions.

The 4/8 being a high power output system boiler?

Nope very little! (at least assuming those are the exact phrases used)

DHW recovery is usually a fairly high load for a shortish period of time. Compared with CH in the warmer months, which is more of a gentle trickle of heat for a long period.

Also flow/return temperatures have nothing to do with flow / return sensors - which detect flow not temperature.

Do you have the fault codes that were displayed?

Possibly... :-)