Baxi Boiler tripping out

[Slaydeuk]

Wow. You guys are keeping me on my toes. I thought I had it cracked, but nothing is happening as I expected. The boiler is still running (11:20) and hasn't tripped yet. I have an open mind about the pump. I thought that, as it was clearly pumping the water, all was well. I now accept that maybe it isn't.

I don't have any instuments to check water temperature. Using the touch method, however, what I can say is that I can hardly bear to touch the flow pipe, whereas I can keep a finger on the return for about a second or so. There is clearly a significant difference in temperatures, but it seems to me to be about what I would have expected (bearing in mind that I know nothing about boilers but I am a retired engineer with a lot of general practical experience).

As regards the pump, it may be my imagination, but it could be humming a bit louder that it used to. I wouldn't put a lot of weight on this though as I am not in the habit of sticking my head in the landing cupboard to listen to the pump. Looking at the external lever, I noted that it was set on three. however, when I turned off the power and stopped the pump to remove the cover, I discovered that the connection to the speed rheostat had sheared off. I managed to turn it using a small screwdriver and I cranked it backwards and forwards a few times to maybe clean up the contact with the rheostat. On switching the power back on, the boiler power light came on, but it didn't fire up. The pump sounded just the same and I left it on speed setting three. After a while, the boiler fired up and has just now shut down, having satisfied the room stat. I have just turned up the room stat and the boiler has fired up and the pump started. In other words, the boiler is behaving itself and the system is working normally. I would rather there be a specific fault which could be rectified. It's a bit worrying as I am going on a cruise to the Canaries on Monday and, while taking the sun in Lanzarote, I don't want to be thinking about my house freezing up back home. Any further thoughts?

 

[Agile]

I said that the pump would normally be used on setting "2" !

But your problem is most common when first starting from cold and that's when you should concentrate your attempts to find the fault.

When on heating how long can you keep your hand firmly on the top of the rads if touching the water containing part?

Tony

 

[Slaydeuk]

I left the pump on setting three, which I presume is the fastest speed, on the assumption that the fastest throughput of water will result in the lowest temperature rise through the boiler. The boiler ran perfectly today from startup this morning until late afternoon, when it tripped on overheat. Since then, I have started it several times, but it trips again.

You advise attempting to find the fault on first starting from cold, but I don't know what to check. The pump is always running (unless the boiler shuts down when the HW and CH stats are satisfied) and there is no way I can tell if it is not pumping satisfactorily. What I have done is check the continuity through the black stat on the flow (Safety Thermostat) immediately after an overheat lockout and found zero resistance. As the manual says there should be continuity when the flow is less than 60 C, I presume that when it is well over that, as it must have been, there should be no continuity. I also checked the red thermistor at the same time and found a resistance of about 1 ohm. I don't know what this means as I am not clear on how each of these stats work.

As regards radiator temperature, I have just touched one and I can't bear to leave my hand on it at all, it is that hot. The boiler has also locked out at this time. Bit surprising to me as the radiator has a thermostatic valve, although set to 4, which is a bit high.

How does all that grab you?

 

[Slaydeuk]

Hi All

Thought I would post a final (I hope) message on this problem as it might be of interest to others. I have been away on holiday for a while and, to my surprise, the boiler was still working on my return. I had left it on twice a day with the HW off. However, on returning the system to its normal settings, it started locking out again.

In view of the emphasis placed by some on boiler flow and return temperatures, I bought an infrared gun. It didn't tell me a lot, except that the difference between flow and return temperatues varied between 11 and, I think, 15 degrees. I have actually thrown away the paper on which I recorded a variety of temperatures and I am going off memory. One of the reading I took was of the fan chamber cover, as the cutout switch on the fan was a possible cause of the lockout. The casing got pretty hot, but I was never sure what to believe, as I discovered that an infrared reading was not necessarily accurate, depending on the emissivity of the surface. Until I realised that the infrared spot was somewhat below the actual measuring area, I was taking a reading from a white label, which gave a much higher result than off the galvanised metal cover plate.

Anyway, enough of this prattle, I concluded that a possible cause was overtemperature in the fan compartment due to a leaking combustion chamber seal, which I knew was rather brittle and may not have been properly located on the cover plate. So I fitted a new seal. Didn't help, the boiler still tripped.

Then I realised that the boiler seemed to work OK when it was on CH only (I have HW on twice a day, coming on at 4:30 pm) It always seemed to trip when CH and HW were both on. This led me to look at the Honeywell motorised valve in the pipe from the three-way valve to the HW tank. It seemed to be very sticky and difficult to move from auto to manual. So I replaced the head. Hey Presto! The boiler was fixed. Since then it has performed perfectly. I am still not really any wiser as to the actual cause of the trips, but I am sure there are those out there who will know exactly why. If so, I would appreciate a few words in reply out of interest and for future reference.

 

[stem]

Something still doesn't sound right. A three port valve cannot be closed. It has a movable ball that swings to cover either the hot water port, heating port, or is in the middle and both are open. Wherever it is, at least one port is open so wouldn't normally restrict the flow enough to cause the boiler to overheat.

 

[Slaydeuk]

Slight misunderstanding. There is nothing wrong with the three-way vave (I had to replace the head on that last year). The problem appears to have been on the Honeywell valve in the line from the HW side of the three-way valve to the HW tank. I'm not sure why this is there at all. I believe it to be moved by spring to the closed position when the tank reaches the set temperature. There are only two wires out of the five to the valve head which are used and one of these, according to the wiring diagram, should be connected to the tank thermostat (I have not actually checked this). I am presuming that the valve is normally closed by spring and is motored open when the tank stat is calling for heat. This being the case, if the valve was stuck in the closed position and the room stat satisfied, the three-way valve would be over in the HW tank position and the water would go nowhere. The pump would be working and the boiler fired and overheating the water. However, if this were the case, I would have thought the boiler would be shut down by the thermistor controlling the boiler outlet temperature. All very puzzling to me. All I know is that now the boiler and the system are working properly, radiators and hot water are heated and the boiler shuts down and power to the boiler is cut off if the room and tank thermostats are both satisfied.

Does that make sense to you? You may have gathered that this is not my field of expertise.