Boiler overheating when first firing up in the morning

[JasperW]

Hi,

A strange fault has developed over the last week or so in our Worcester Bosch 40CDi (conventional) based central heating/hot water system.

Several times over the last week or so there has been load banging coming from the boiler when it first fires up in the morning. I was able to see what was happening yesterday and the boiler temperature was hitting the high 90s and which point steam was obviously being produced causing the banging. At which point the boiler went into some sort of recovery mode and after a couple of minutes fired up again and everything then works normally for the rest of the day. The hot water produced by the boiler is of the correct temperature, I have checked all the radiators (which heat up normally) and there is no air in the system that I can detect and the header tanks are full.

We had the central heating pump changed about 5 years ago, and about two years ago I changed one of the motors on the 2 motorised valves we had in the system.

There is obviously a problem with water getting to the boiler which corrects itself on the second attempt at firing up. This makes my think that something is sticking or getting jammed which is then starting to work on the second attempt.

Is there anything in the boiler which could be causing this (i.e. do I need to get Worcester Bosch out to fix it) or is it more likely to be a pump or valve sticking in the airing cupboard?

Any thoughts on what it might be or fault finding techniques I could try?

 

[Agile]

You mention two valves which indicates an S plan.

To allow pump over run there should be an auto bypass fitted to an S plan. Do you have one?

Quite why this has just started to be a problem is not clear. Perhaps one of the valves is being slow to open?

You could try to see if the valves are indeed opening before the boiler starts. This would normally happen because the valves are usually wired to only bring on the boiler when they are open using the internal switch.

You could, depending on the type, lock the heating valve open before starting the heating.

The pump might be slow to start or only turn slowly for a while. A listening stick on the pump might help to check that.

Tony

 

[JasperW]

I have had a look at the valves and they both seem to be operating normally. In fact the one that I recently changed is operating the hot water side. The pump does over run after the CH/HW is switched off and there does appear to be some sort of bypass pipes in the system (The pipework was fitted in the late 80, we replaced the boiler in 2006).

Though of course the problem does appear to be intermittent so perhaps it is not surprising that everything is working normally.

I am assuming the problem is likely to be outside the boiler as I assume the boiler relies on the CH pump to supply water to it

 

[Agile]

When the boiler starts up is one of the valves open?

The bypass would need to be on either of the 15 mm pipes at the left or right of the photo.

What is the nature of the bypass?

 

[JasperW]

The bypass pipes disappear under the floor I am assuming they connect to the heated towel rail in the bathroom but that is about as much as I know.

When I looked at the system just now the value opened as soon as the HW system came on, but again, the system is working perfectly during the day.

I might try locking the valve open tonight and see if that stops the problem.

Do you think my hunch that the problem is not the boiler is likely to be correct?

Thanks BTW for your help.

 

[Phill Peck]

What setting is your pump on?

 

[JasperW]

The pump is on the maximum setting, when I first had it fitted I did try the auto settings but I found the rads were not getting hot enough.

 

[Phill Peck]

The only thing that springs to mind is the pump sticking.l but without being there I couldn't be sure.

But unlike tony I won't take a random stab in the dark as to what it is.

Is this the symbol?

Bottom line is the water isn't moving/not moving fast enough on first start up.

If it runs normally the rest of the time your best bet would be to get an experienced engineer there and make sure the heating/hw doesn't cone in before their arrival so they have the best chance of finding the fault.

 

[JasperW]

Hi Phill

I don't remember if I saw that symbol or not, I did see a U1, U2, U3 symbol which I think means over 100C, it then basically reset itself automatically and when it fired up again (with me doing anything) everything was fine.

What I will do tonight is switch the system off before I go to bed then switch it on manually when I get up and check that the valves and pump start up normally. If everything seems to work and I still get a problem then I will assume it is the boiler and get Worcester Bosch out to look at it. If it looks like it is the pump or valve then I'll get my local heating engineer to sort it.

With boiler problems I have found that the Worcester Bosch engineers are much quicker at getting problems fixed since they have a full range of spares in their vans, but they are not so helpful if the problem is somewhere else in the system. Hence why I am trying to fault find.

The pump sticking when cold does seem a likely cause to my (non expert) mind.

 

[Agile]

But unlike tony I won't take a random stab in the dark as to what it is.

I did not take a random stab in the dark but instead mentioned the two most likely causes.


Anything else would be most unlikely indeed. But rare faults do happen. But by definition only rarely!


I can only think of one rare fault that I have ever encountered ( on three occasions )! That was only identified when a boiler was removed and returned to the manufacturer.

And another unexpected fault caused by a design malfunction. That did take me a while ( two hours ) to find it as I had been lazy and had not followed the correct testing procedure and the odd operation was completely unexpected.

 

[JasperW]

Thinking this through (again with the assumption that the fault is not the boiler) even if one of the motorised valves was sticking shut the pump (if it is working properly) should be able to supply enough water to the boiler to stop it over heading via the bypass. If however the pump is sticking when cold then no water will flow and I assume will rise in temperature very fast.

 

[Agile]

As I said in my first post, the normal way a system is wired relies on the switch in the motor valve to create a demand for heat on the boiler. But motor valves can be faulty!

It really needs you to be able to see if you think the pump is running as soon as there is a call for heat.

You cannot assume that a bypass based on a towel rail will give adequate flow rate to prevent the boiler from overheating. Although usually it probably would do.

Tony

 

[Phill Peck]

Bearing in mind the boiler works perfectly on 2nd start up the valves are obviously opening.

If the valve/s had seized and the end switch was made the pump would run constantly.

I would say either the pump is faulty. (Fit the head the correct way up this time please)
Or there is an air issue that settles over night at a high point clears after a few minutes of the pump running.

I wasn't going to guess any further but I thought it best to offer sensible advice!

 

[JasperW]

I have just fired up the system manually and the LEDs in the pump flash madly and the pump does not spin, after a short time I hear the boiler begin to bang. If I switch the system off then the pump continues to flash madly for a short time then turns itself off. When I restart it the pump non longer flashes and starts up (what I assume) is normally with a gradual increase in pump power over a short period (@ 20 seconds).

This is supposed to be a smart pump which can adjust its power to the demands of the system but I found it just left the rads too cold and we use in in the manual mode which rather defeats the object of the pump. I am guessing that the processor that controls it is not firing up correctly now and that either the electronics or most probably the whole pump need to be changed.

I remember the difficulty the heating engineer had in changing the last pump so I think I will get a professional to do this. I will however check with the manufacturer do see if their is a cheaper option of just changing the electronics which I would quite happily do.

As always any advice from the wise gratefully accepted.

 

[DP]

I can only think of one rare fault that I have ever encountered ( on three occasions )! That was only identified when a boiler was removed and returned to the manufacturer.

Very interesting Tony
What was the rare fault
Did you identify this rare fault before the boiler was returned, did the makers inform you what this rare fault was, or was it you who told the manufacturers what this rare fault was
How can something be rare if it has occurred three times