Pleae Help! heat exchanger seems ok as can blow through it

[jaymi]

Hi I have a ravenheat boiler. LS100. About 7 or 8 years old.

Anyway for a while it has had the overheat / lockout issue when the central heating is on. This seems to be the classic of put your hand on the flow pipe and after a few minutes it gets very very hot and the boiler shuts down. The pump was changed not that long ago.

I've had a couple of bits of advice (mainly get a new boiler) but that the problem itself is caused by the heat exchanger.

The system has been flushed and seemed pretty much ok.

So I disconnected the pipes to the heat exchanger (you can do this without entering the combustion chamber) and it is very easy to blow through them.

I did this for both heat exchangers, the main one and the dhw one. The water that came out of them was clean.

What I don't know is if something changes when the heat is on? What I mean is, is blowing through them when cold and there appearing to be no blockages a valid test? I have also blown through the pipes serving them and they seem fine using the blow test.

Thanks very much,
Jamie

 

[jaymi]

Anyone any ideas. Just really want to know, if you can blow through it easily if that means it is ok? Thanks.

 

[dcawkwell]

I would have said perhaps just a thermistor or diverter valve problem
if the system is clean.

 

[jaymi]

Thanks for the reply. I took off the diverter valve and could blow through it easily as well (in the direction the CH would flow). Oddly the thermisters were changed a while ago, when the problem first started, but has gradually got worse. I have also now checked all the pipes going to the HE, DHW HE and pump, again all seem clear. But as I have never had any experience of a heat exchanger that needs replacement I wasn't sure what the characteristics were? According to the plumber he thought as the outflow CH pipe gets very hot before it shuts down it must be a HE problem (as the pump is fairly new). But as it is a pretty expensive part to buy and have fitted I was unsure especially as you can blow through it VERY easily...We had a filter and radiator fitted a while ago and the bypass was moved slightly, I have noticed there is no double check valve on it, but not sure if there was originally either.

 

[oilhead]

Blowing through is not really a valid test as you are not measuring volume. If C/H flow pipe is getting hot, then it suggests a circlation problem which could be restriction in the heat exch. particularly if you are confident that the pump is good. Have you checked to make sure the isolation valves are fully open?

 

[jaymi]

Thanks for the reply.
Yes I wasn't sure about the blow test but there appears to be no resistance there. All the isolation valves are fully open. There is a fernox filter fitted that handily has an isolation valve above and below it. This is on the CH flow. Now if I turn the isolation valve below the flow off and leave the one above it on then open the filter obviously water comes out the filter. Then I switched on the filling loop and water can come out at what appears to be full mains pressure. This water must be going through the HE. In from filling loop, up CH return, through HE, out central heating flow, out filter. But again this is using the mains pressure in this test and not the CH pump. But water seems to flow thorough pretty steadily...I was hoping, either via blowing or putting water through to get some form of resistance as that is the picture I had in my head of a "blocked HE".

Is there anyway to test the flow rate? I thought about getting two buckets, one full of water, connecting some hose to the pipe work, connecting the pump (I have a connector that plugs into the wall to switch the pump on) and measuring how much water the pump is pumping. But this would not be at heat, would just be cold water.

The plumbing is pretty accessible so I was also considering taking a feed directly from the CH in and out and piping it to just one radiator that is next to the boiler to see if this makes the problem go away. The boiler was fitted by a corgi, but the bypass is some complicated arrangement that sits under the main radiator next to the boiler. But it worked fine for several years...

The actual symptom goes something like this:

Switch on CH.
All as normal.
Radiators start getting warm.
Flow pipe hot, Return pipe warm.
Then suddenly flow pipe gets very hot.
Burner Shutdown (after a few times of this, boiler lockout)
The return pipe is still just warm at this point.
The second the burner goes off, the pump overrun immediately cools the flow pipe.
About a month ago I put in some system cleaner and the problem disappeared completely for about 48 hours, then returned

Thanks very much.

 

[dcawkwell]

Thanks for the reply.
Yes I wasn't sure about the blow test but there appears to be no resistance there. All the isolation valves are fully open. There is a fernox filter fitted that handily has an isolation valve above and below it. This is on the CH flow. Now if I turn the isolation valve below the flow off and leave the one above it on then open the filter obviously water comes out the filter. Then I switched on the filling loop and water can come out at what appears to be full mains pressure. This water must be going through the HE. In from filling loop, up CH return, through HE, out central heating flow, out filter. But again this is using the mains pressure in this test and not the CH pump. But water seems to flow thorough pretty steadily...I was hoping, either via blowing or putting water through to get some form of resistance as that is the picture I had in my head of a "blocked HE".

Is there anyway to test the flow rate? I thought about getting two buckets, one full of water, connecting some hose to the pipe work, connecting the pump (I have a connector that plugs into the wall to switch the pump on) and measuring how much water the pump is pumping. But this would not be at heat, would just be cold water.

The plumbing is pretty accessible so I was also considering taking a feed directly from the CH in and out and piping it to just one radiator that is next to the boiler to see if this makes the problem go away. The boiler was fitted by a corgi, but the bypass is some complicated arrangement that sits under the main radiator next to the boiler. But it worked fine for several years...

The actual symptom goes something like this:

Switch on CH.
All as normal.
Radiators start getting warm.
Flow pipe hot, Return pipe warm.
Then suddenly flow pipe gets very hot.
Burner Shutdown (after a few times of this, boiler lockout)
The return pipe is still just warm at this point.
The second the burner goes off, the pump overrun immediately cools the flow pipe.
About a month ago I put in some system cleaner and the problem disappeared completely for about 48 hours, then returned

Thanks very much.

I would start by swapping out the pump head to verify the pump is actually ok. I would say it isn't. The fact that the radiators are starting to
get indicates circulation. Something is stopping the flow all of a sudden so
would say pump or a sensor problem.

 

[jaymi]

Firstly thanks for all the replies, they were really helpful and wife is so happy!
The pump was fine as it turned out (but worth checking), the problem, for anyone reading this is that the blow test it a complete waste of time (apart from maybe to check total blockage which would surely manifest itself differently). But by blowing through both HE I could honestly feel no resistance. But I had an old central heating pump so got some fernox descaler and rigged up a bucket, hose the HEs and the pump. The descaler turns from yellow to green when exhausted, it did this twice over, before staying yellow the third time. But more than the colour change I had to turn the pump down after about 5 mins due to the increased flow!!!

Put them back in the boiler and what a difference. All radiators piping hot within 15 minutes. Descaling was pretty easy but but the DHW HE is a bit of work to get to, so make sure you have plenty of new fiber washers and for the main HE you have to open the combustion chamber so there could be some GasSafe regs there.

So now, only one final little problem remains, maybe I should post it separately.

It seems nothing to do with overheat at all.

When you start the boiler first time it will ALWAYS run no problem, but when it goes off due to hot enough sometimes when it tries to restart the flame failure light will come on.

Now with a multi-meter it would appear the the main pcb does some checks then passes 240v to the overheat stat, which in turn, if ok, passes that on to the ignition board that then starts its sequence. When it goes wrong there is still 240v at the ignition board from the stat so seems nothing to do with that.

On failure the ignition board appears to do nothing, not even test the APS and no fan, then just flame failure light. When this happens you can't do anything. No button pushes, or turning off the electric makes any difference. the flame failure light will remain for about 20 minutes. Then there is a very audible click from the ignition board, at this point the flame failure light is still lit, but pushing it clears it and all works as normal again. Any ideas what the click might be?

Thanks again,
Jamie
(in lovely hot house!)