Combi boiler fault

I have a fault with a Baxi HE 133 combination boiler. I have a gas boiler engineer

round to inspect it. He suggests that the boiler requires a new PCB electronic

board. But I would like a second opinion, or as many informed opinions as I can

get before forking out £200+ to have this done. I'll try and give as many details

on the symptoms I can.

There are two symptoms:

1. After 10-20 minutes of operation the boiler shuts down and the indicator lights

tell me that the boiler is "overheating". This is the light panel combination as

according to the boiler manual. The engineer went through the checks in the manual

for this fault and they were ok.

2. The water pressure is regularly dropping both when the boiler is fired up and

when the boiler is idle e.g. during the night.

However, these issues are intermittent. If I close off most of the thermostats on

the radiators and just leave 3 rads on then the boiler will operate perfectly well

for up to 6 hours providing hot rads and does not appear to lose water pressure. I

then open up more radiators and the overheating/drop in water pressure problems

re-occur eventually.

I get the impression one of two things are happening:

1. Either the PCB and sensors are correct and the boiler really is over heating in

which case insufficient water is entering the heat exchanger. Which could be a

sticky or clogged valve?

2. Or the PCB and sensors are faulty and the boiler is not overheating and the

electronics are giving a false reading. But if this is the case why is the water

pressure dropping?

This is a link to the boiler manual

http://www.gasapplianceguide.co.uk/Baxi_HE_Plus_Range.pdf

Overheat lockout image from the manual uploaded.

Get the pressure issure sorted first. Its either being lost on the system rad valves/under the floor or more likely through the PRV and is dripping outside.

Ok, Rob that sounds how it feels to me.

I have NO leaking rads, all the pipework is above the concrete floor and there is no sign of water in my house. Water being pushed out through the PRV (pressure release valve I assume) is more likely but a little tricky to check since the pipe for this does not extend outside but enters an old cast iron sewarage pipe used by the downstairs bathroom. I can get to look at it but have so far not seen any flow of water being released. Is it also posisble that the water may be escaping through the flu itself and if so do you what I would be looking for i.e. lots of steam coming out.

I currently have only 4 rads open and the boiler is behaving itself. If I open up more rads then I expect the symptoms to reappear. Could this be because opening more rads generates an increase in the the internal water pressure in the exchanger and the expansion tank?

You could have a couple of problems here, it sounds like your expansion vessel is almost flat, it can cope with the expansion of a couple of rads but not the whole system, secondly depending on how many times you have topped up the system could be sludged or a failing pump, has your engineer got a thermometer ? easy enough to tell if the boiler is over heating or the pcb is at fault.

When the engineer took the combustion chamber door off, was there a lot of pinkish powder in the bottom? This is the first silver door - not the one with electrodes in it. If so the seals are all that needs replacing re your cutting out after 20 minutes problem.

The expansion vessel is another problem, which may be easily fixed with a foot pump and a new PRV.

Hi Beerlover

I did not watch the engineer take the combustion chamber door off and so cannot say. If I let the boiler cool down, turn it off at the power supply, is it safe for me to inspect this myself. The boiler was serviced in March when the combustion chamber was given a good clean but whether the door and seal were resealed properly??

It would be no surprise if this seal has failed. The seal was replaced 3 times before 4 years ago and the engineer commented that the bottom left screw on the combustion box was a little tricky ( I have read the same thing on another web site ). I did tell the engineer this.

The engineer claimed he was a Worcester expert not a Baxi expert and I think he maybe confusing known symptoms on Worcesters for similar symptoms on this Baxi which is why he assumes it is a PCB problem. If I get another engineer in I'll try and get one who has more experience with Baxi's.

I had a pressure drop problem with one boiler once. Turned out the compression fitting where the CH returns to the boiler was just letting a small amount of water out. When the CH was on the leak was evaporating and couldn't be seen. Check all compression fittings where the CH comes out and returns to the boiler when it's cold.

It shouldn't be a problem, make sure the mains is off first IIRC the fan is 325V DC - that will hurt.

If there is powder there go no further, you will need a RGI for the repair.

Hi John

The piping enters the boiler from underneath and are visible but show no signs of leakage. All of the other piping (or is it tubing) are soldered joints.