Diagnosis help - Worcester Bosch Combi boiler

[frod]

Hi, I came across this forum in a Google search and I was impressed with the expertise offered with diagnosing issues with combi boilers, hopefully you can help me.

We are losing faith with our landlord's chosen heating engineering firm - they have now been out three times and seem unable to offer an appropriate solution to our issue.

Since around Easter, our combi boiler does not reliably provide hot water, so as you can imagine, shower time is a bit like russian roulette. Additionally, the pressure in the system drops quite alarmingly when we are trying to coax hot water out of it, as much as a bar in a couple of minutes. There does not appear to be any leaks in or around the radiators to explain it, and it definitely drops during attempts to use the hot water. There has always been a very slow degradation in pressure loss but now it is a lot worse. I try to top it up so that it doesn't get dangerously low; I've never seen what occurs when it does get too low (although if the pressure gauge is accurate I think I maybe should have done... another cause for concern?).

Our third engineer visit has diagnosed a faulty thermistor, would that explain the above issue? It doesn't seem likely to me, I can imagine it resolving the hot water problem but not the dropping pressure problem. I'd really appreciate any insight as it is hard enough to get them out in the first place, so when they go away again without resolving it, it just means another couple of weeks of cold showers

 

[frod]

I should mention that when it doesn't offer hot water, the burner LED does not come on, although the unit itself is doing the various clicks and whirrs that suggests it is trying to do something.

 

[Dan Robinson]

Any chance of telling us what the model is?

 

[frod]

Any chance of telling us what the model is?

It's definitely a Greenstar, I think maybe the 24i Junior (from a quick google image search, so obviously not certain) as we only live in a small 2 bed flat.

(I can be more accurate this evening when I get home).

 

[McCombi]

If the system water pressure is too low, as a safety precaution the boiler won't operate.
When you top it up, does it work ok?
When you say the pressure drops when you run water, does it go back up again when you stop running water and revert to heating mode?

And no, a thermistor fault has nothing to do with pressure problems, although possible it may have been faulty.

 

[frod]

Just to confirm the exact model, it is a Greenstar 28i.

If the system water pressure is too low, as a safety precaution the boiler won't operate.
When you top it up, does it work ok?
When you say the pressure drops when you run water, does it go back up again when you stop running water and revert to heating mode?

And no, a thermistor fault has nothing to do with pressure problems, although possible it may have been faulty.

Actually in our case, the reverse seems to be true - if I top it up, we're more likely to get temperature fluctuations. It takes me operating the kitchen tap on and off that drops the pressure down to approximately 0.5 bar before it becomes more reliable at providing a constant temperature.

I've never actually seen it refuse to fire up at any pressure level. We do have what seems to be perfectly functional central heating (although I suppose this could also be suffering from temperature fluctuations, but I don't think we'd know about it).

The pressure does not go back up when we stop using hot water, at least not noticeably.

 

[frod]

Well, the thermistor change appears to have resolve the variable temperature issue, he topped the pressure up and it seems to have lost a little already but it's probably too early to tell, I'll have to keep an eye on it.

 

[adp240]

likely the leaking is in the water-to-water heating exchanger.
when hot water is running, water leaks from primary flow to DHW and the pressure drops to 0.5 Bar.
Once hot water tap is closed, the boiler will be filled via the leaking hole to main cold water pressure.

 

[frod]

I spoke to soon regarding the temperature, it's gone variable again sadly

I'll suggest that comment about where the leak might be to the engineer who is due again ( ) this week.

 

[picasso]

Ask the engineer to check the plate heat exchanger (blocked?) and the expansion vessel (flat?) I cant see it being a hole in the heat exchanger as the system would pressurise to what ever the mains pressure is.

 

[frod]

ok, will do, thanks.

 

[frod]

Here is a link to a video of what the pressure dial on the boiler does while the shower is in use. The shower was going hot/cold during the course of this video.

In this example, there is no heating on (I turned it off via disabling all the timed switches on the timer dial the night before).

The video is 92MB in size (sorry if it's sideways and slow to download, I don't know how quick dropbox is for this) and only gets interesting at the 1 minute mark:

https://dl.dropboxusercontent.com/u/583609/VID_20130605_081937.mp4

Any thoughts? Doesn't seem right to me!

Still waiting for the engineer to arrive (but looking increasingly unlikely!).