System boiler, hot water issues

[sbis]

Hi, we have a Valiant system boiler that has over the summer heated the hot water without an issue. Now when running at the same time as the radiators, the hot water isn’t getting warm.

If I run the hot water before the heating, the tank heats up and no issues.

The motorised valve for the water works normally but doesn’t call for heat and the boiler doesn’t fire/pump start running. This is the same if I boost via Hive or manually move the lever on the motorised valve.

I’ve only ever had combis. Assume there’s no diverter valve as it’s a heat only boiler.

Stat on the cylinder clicks when I turn the dial, but haven’t tested it further.

Showing code f72 when hot water on. Can I swap the flow/return thermistors myself? Could it be something else?

Trusted local boiler company won’t take it on as it’s 7 years old so need me a new local engineer.

Thanks

[aeromech3]

It sounds a little muddled correct me if wrong, DHW alone is good, but with CH only the boiler does not fire?
The code is no use without the model number and age !
Motorised 3 way valve are at rest for DHW, your statements are a contradiction, you need to be clearer, either the boiler fires for DHW only or Not?
In the manual position the DHW and CH might work but the micro switches are rather a law unto themselves at this position.
Does the valve move fully to CH only (what make model of valve?).

[sbis]

Thanks - you’re right, that was a bit of a jumbled mess, let me try again.

Boiler: Vaillant ecoTEC plus 428. Age 7-10 years.

System: There’s two valves. One two-way for the HW (Tower), another for the CH (Drayton). Both operate normally.

Original issue: no hot water/tepid when HW run at the same as CH. Improved by running hot water for longer and before CH scheduled to come on and no problems for last fortnight.

Yesterday, no HW but CH operating normally. With CH off, boosted hot water via Hive, motorised valve moves but pump does not start and boiler does not light. Operate HW valve manually, no different. Check boiler and it’s showing code f.72. Cycling boiler power did nothing. Reset boiler by holding flame button on the front panel of boiler and code disappears. HW boosts as normal and heated properly. What I haven’t been able to check is if the CH would operate when the f.72 code was showing as reset before thinking to try CH alone, but assume not as appeared locked out.

Overnight, CH has kicked in when ambient room temperature fell below 18c. Water is tepid at best so assume the programmed HW at 4am didn’t work.

Boiler otherwise operates normally, modulates and isn’t showing a code currently.

Here’s the set up from 12 months ago - only change is I swapped the original Honeywell valve for a Tower when it failed:

IMG_4764.jpeg (260.36 KiB) Viewed 754 times

Thanks for your help