Bosch Boiler Issue

Martynt74 · 8 Oct 2024 at 11:17

Hi All,

Our boiler in this rental has been working great for 4 months, however on Sunday we had no hot water. I went to check the boiler and noticed it wasn't firing. The pressure has stayed steady at 1.5bar.

I tried the reset buttons (+ and - together), and it restarted and the Temp dropped from 55ish down to 26c, suggesting the controller was stuck and not acknowledging the current temperature. However it then fired into life and worked it's way up to 55c again along with distributing hot water. However that was short lived.
Since then it's been quite erratic and seems to hold some hot water but then that goes quickly and the boiler continues to not fire. I've tried the reset but that seems to have stopped resetting it. I can put it in Chimney Sweep mode which says 100% and fires the boiler up but not sure what that does!

Boiler is a Bosch GC4300iW, fed by an above ground gas tank. I initially wondered if it was an issue with that, but it's 80% full and the fact the boiler fires during the chimney sweep mode suggests it's not the culprit.

964rs · 8 Oct 2024 at 12:01

If its a rental its the landlords problem surely or is this not the case in Spain

Martynt74 · 8 Oct 2024 at 13:06

964rs said:
If its a rental its the landlords problem surely or is this not the case in Spain

Oh yeah, it would be. I just thought it might have been an easy fix and something i could just sort myself. From previous chats with the landlord, she prefers me to sort it out and then just refunds me the cash and it's really hard getting someone out quickly.

NvidiaDan · 8 Oct 2024 at 14:26

Assuming that's a combi model. Are you saying the heating is working fine? And it's not displaying any error codes?

Martynt74 · 8 Oct 2024 at 14:30

NvidiaDan said:
Assuming that's a combi model. Are you saying the heating is working fine? And it's not displaying any error codes?

Yeah, no errors at all. The display just shows 49.6c which i assume is the currently temperature of the boiler, and when i switched the heating to 30c on the thermostat it fired up straight away.

It feels like it should be an easy solution, but there's so few buttons on the unit itself it's hard to really know. Especially since the "Reset" function does nothing, although i'd have thought that would mirror the power off/on anyway.

NvidiaDan · 8 Oct 2024 at 14:35

So it doesn't even attempt to fire on hot water? Or fires and then quickly stops?

I'd be checking all the usual suspects. Flow switch, diverter and plate heat exchanger in that order

Martynt74 · 8 Oct 2024 at 15:23

NvidiaDan said:
So it doesn't even attempt to fire on hot water? Or fires and then quickly stops?

I'd be checking all the usual suspects. Flow switch, diverter and plate heat exchanger in that order

No never seems to even try. Occasionally when it's been powered off and back on, there's hot water for a tiny bit, and then it resorts to cold again.

The above all sound out of my knowledge realm so i'll defer to experts!

LuckyBenski · Friday at 23:56

Sounds like flow switch. Does the boiler have a mode where it basically preheats the exchanger periodically? This would explain the brief bit of warm but otherwise cold water.

Basically the sensor that detects you running the hot tap, and tells the boiler to fire and also to pump water around the exchanger loop, isn't working. So the boiler doesn't fire up but also doesn't circulate the heat through the exchanger. I'm guessing when you fire up the CH, the pump runs and a radiator gets warm?

Edit: I used a magnet to trick the sensor and that worked, so I used to fit the magnet while I ran a bath/sink then remove it. This was for a week or so til I could get and fit a replacement turbine sensor (it wasn't the "sensor" that failed but the turbine that gets spun by the hot water flow, and triggers said sensor).