Ferroli system boiler CH comes on when HW called

[padairg]

Hi, would appreciate any thoughts on the below. I have an almost 10yr old Ferroli Modena 32S HE system boiler. (I am aware of their reputation! ) 2 weeks ago HW went lukewarm. When HW was called, the rads came on. The cylinder motorised valve has been replaced today, but it's not fixed the issue. On closer inspection of the boiler display, when I call for HW the boiler fires but the little radiator symbol starts flashing, which shows the boiler thinks it's putting the CH on. This despite the programmer showing HW light on, CH off and I can hear the new cylinder valve opening. In addition, the rads are now no longer heating up. The issue I then come up against, I think, is that the boiler heats up what it thinks it CH, but as the rads aren't heating, the return temp is high and the boiler quickly turns itself off with a d2 symbol.
I'm using Drayton Wiser system, but I dont think that's the issue as the motorised valve is opening when I call for HW. It's like the boiler is confusing the HW signal from the heat hub/programmer unit and thinking it's CH being called.
Anyone seen anything like this before? Many thanks

 

[fowlerboi]

With 2x two port valves the hot water being on will always use the radiator symbol on the boiler. Below is a wiring diagram, if you trace the boiler SL on the left side it goes into the same terminal as both heating and hot water valves. This explains why it just shows the radiator because it doesn't know the difference, its just the way the manufacturers set the boiler up.

Does you system have a bypass next to your 2 port valves? It will look like the photo below.

I've had it before where programmers lose their marbles and stop working. Using the wiring guide below you can stick a multimeter into the terminals and check you have voltage at the right places, I'll write an order to check in below. Alternatively order an st9400c from screwfix, wire it up like for like with your current one (they normally are like for like but don't just take my word on it) if it fixes it you're laughing if not take it back. You could get a direct replace of your one to be safe but I like the st9400c

There are also a lot of plumbers that are completely thrown by electrics so if you're handy with a multimeter just check what he's done today as 2 issues at once are insanely difficult, so eliminate the easy mistake first.


Just re-read what you put, it looks like you've got a non-conventional system. I still think your programmer is the issue but its not quite as simple to diagnose or change. I'm going to presume you've got the smart TRV's which shut the radiators?

1. Check live to programmer
2. Turn on hot water and check the live out from the programmer
3. Turn the cyl stat down and check there is no voltage at the live out from the cyl stat
4. max out the cyl stat and check to ensure there is voltage
5. Check the hot water valve has voltage to its permanent live (grey)
6. check the valve has voltage at the orange
7. Check the boiler is on and the pump
8. Check the heating valve isn't open at this stage
9. Then do exactly the same for the heating ensuring the hot water is off and the pump has had 5 mins or so to finish pump over run. (you'll hear it go off) Its much harder to trace the cables for the room stat but you can typically do that by finding the cable that comes from the programmer and the other that comes from the heating valve
10. Check the hot water valve isn't open

 

[padairg]

Hi Fowlerboi, thanks very much for your reply. I definitely have a non-conventional system...I don't have a cylinder stat for a start. I don't think I have a bypass but will have to check that later. I do have my old programmer though so yes I will rewire that in later and see if it makes a difference. I kept the old room thermostat wired and just turned it up high, so I can control that too. I do have some of their remote rad TRVs to create various zones, but not all rads have them.
You make an interesting point with the wiring diagram, I'd never noticed whether the HW display symbol ever flashes, but that wiring diagram kind of answers that. My next question would be... in theory I can set different temps for HW and CH on the boiler but if the boiler only ever thinks its heating, is the HW temp redundant?

 

[fowlerboi]

I don’t know your boiler but would expect it to be redundant yes. I believe its there to be able to convert to a combi if needed.

If you don't have a cylinder stat then what tells your hot water that it is up to temp?