Baxi combi boiler DHW fine but won't fire when heating needed

[paltibine]

Boiler is a Baxi combi 133HE+ and has been having a few problems.
The current problem is that when the thermostat calls for heat, the valve on that circuit opens and after a few minutes the main CH pump starts to run. The pump runs for almost exactly a minute without the boiler firing up and then the pump stops. After about 3 minutes this is repeated. So for CH it never actually fires up just runs the pump for one minute out of every 4.

Hot water works absolutely fine.

Before it started behaving like this it was seemingly randomly coming up with the LED code for dry fire every week or 2. Sometimes the reset button would bring it back and other times it needed to be turned off and on again. It never does this in the summer when only the hot water is used.

The engineer came to look at it for the dry fire issue but it didn't do it when he was there so he listed about half the parts in the boiler and said it could be any of them.
The pressure is fine - in the middle of the green segment.

Any ideas folks?

Thanks for your time to read!

 

[terryplumb]

What valve opens ,and where is the " main central heating pump" actually located ?
The pump would be inside a combi boiler ??

 

[paltibine]

Yes the pump is inside the boiler but you can still tell when it's running or not.
The valve that opens is whichever one of the two way valves controls the zone that is calling for heat - or sometimes both when they both are below set temperature.

System has 2 zones, one for main rads and other goes to a manifold for underfloor heating in the extension. I didn't go into that as I didn't think it was relevant as the problem is the same whichever zone calls for heat or when they both do.

 

[terryplumb]

Sounds like the boilers diverter valve is stuck in domestic hot water mode....gonna need an engineer to visit .

 

[paltibine]

That sounds likely - that way the hot water would be fine but the CH wouldn't work.
If the control board switched it to CH but it stayed on HW then there would be flow through the CH pipework but the heat exchanger would be heating the HW instead of the CH. Would that cause the main pump to stop after a minute?

Does the internal valve have a manual override like the external ones do?

Engineer was coming back this week anyway with a new flow switch as a possible fix for the dry fire issue but could that have been caused by the diverter valve being a bit sticky perhaps?

 

[paltibine]

Weirdly it seems OK this morning.
I left it doing the 1 minute on 3 minutes off last night expecting to wake to a freezing house but the heating was on when I woke up and it's been running fine all day.

 

[DP]

Page 30 of the manual
Fault you have should be easy to locate fact that boiler does not function as required.
There is no need to list a number of parts which get changed to locate the fault. Should be other way round- locate the part that is stopping the progress of boiler function and then change it.

Honeywell motor head moves from heating to hot water position. I would be looking at the cartridge to see if it restores when motor head is in heating mode. Also would be looking at the CH flow sensors operating to indicate there is flow present else boiler will not fire.

The engineer came to look at it for the dry fire issue but it didn't do it when he was there so he listed about half the parts in the boiler and said it could be any of them.

Boiler will not dry fire as water flow operates a pedal switch which allows operate sequence to proceed. You have also said there is healthy pressure in the boiler. Have also said hot water is ok which means pump, burner and fan are healthy

Make sure the guy that looks at that boiler knows his marbles as the burner is eye watering ly expensive, the heat exchanger base area often needs proper service as do the case seals. Boiler known to display red lockout light. Not the best out of Baxi factory

When callying for someone to look at the boiler, ask if he or she is familiar with fault locating on that boiler. Large proportion of heating ‘engineers’ rely on words of others to change parts in order to try and repair Tell them what the boiler is not doing and what is. That will get rid of the part changers.

Final note.
Consider replacing that boiler

 

[paltibine]

I hadn't posted an update here before because the guy didn't come back to fit the part and the boiler has been working fine since my last post.

Now it seems to have failed completely - it comes up dry fire at the point in the cycle where the burner should fire up. I think I can hear the main circulation pump running and the burner fan running. I hear some solenoids clicking but any time the boiler should fire, it just comes up with the dry fire indication.
This happens when either hot water or central heating is called for so nothing is working.
The guy is coming back tomorrow for another look.....

Any suggestions folks?

 

[DP]

A boiler that fails like that is easy to fix. Intermittent defects are the hardest to locate

 

[CBW]

You have got pressure in the boiler I presume?

 

[paltibine]

Yes, plenty of pressure.
It failed on Saturday and the system was drained down on Friday but it worked fine Friday night and Saturday morning.

 

[DP]

Is the flow switch operating?

 

[paltibine]

The flow switch seems to be the problem. I turned on the hot tap and then tapped the flow switch with the head of a screwdriver and the boiler fired up and burst into life! It must have been stuck or possibly had some debris in it.
I guess it might need replacing if that was the problem!

It's very nice to hear it firing away again!

 

[DP]

The flow switch seems to be the problem. I turned on the hot tap and then tapped the flow switch with the head of a screwdriver and the boiler fired up and burst into life! It must have been stuck or possibly had some debris in it.
I guess it might need replacing if that was the problem!

It's very nice to hear it firing away again!

That could be a dangerous move
If that switch is stuck in operated mode and boiler is short of water, you could end up with boiler going on fire

 

[paltibine]

It's not like I shorted the switch out or anything, I just tapped it and I assume it then found it's proper position. The switch will now be replaced very soon anyway!
I guess if the switch can stick closed it could stick open too but do you think a tap with a screwdriver handle would make that more likely!

Thanks for the warning though, I'll keep an eye on it....