Baxi 105e combi - CH works OK, no DHW

[Dragonfire1]

Hi,

I'm hoping someone here can help me, I'm ALMOST at the end of my tether!

I'm having problems with my 4 year old Baxi 105e (it's NOT the "instant" version) - the central heating works just fine, but, as of Christmas day (well, it WOULD be, wouldn't it!?), I have had no hot water.

When a HW tap is turned on, the HW green LED lights up (as it should), the pump starts running, but the burner does not ignite. After about 20 seconds or so, the "fault on pump or low system pressure" LED (the 60 degree LED) starts flashing. (The flashing LED goes out once the HW tap is turned off.) It doesn't seem to matter if the system is in CH/HW mode, or HW only mode.

I have reset the boiler several times, using both the reset switch, and by turning off the electricity supply at the wall. Resetting does not make any difference - still no hot water!

So far (mainly from other advice re 105e combis found on this site), I've done the following:
- checked the diaphragm and push rod assembly in the DHW valve (the rod moves out when there is a demand for HW, and activates the microswitch, as it should do) - the diaphragm looks OK (no obvious damage), and the rod appears straight, undamaged and not seized.
- checked and cleaned the connections on the PCB.

When I hand operate the DHW valve microswitch (with no water flow), the green HW demand LED lights up, and the burner ignites as it should. When the microswitch is operated by the pushrod from the DHW valve, the HW demand LED lights, but the burner does not ignite (not even when the microswitch is "hand-assisted" while the water is flowing). Not sure if this signifies a problem with the DHW valve, the microswitch or the PCB (or something else?)

FYI, the boiler is professionally serviced once a year.

Am I looking at a PCB failure, a problem with the diverter valve assembly, or something else that I haven't even considered?
I'm no expert in this, just a fairly competent mechanical engineer with a dislike of excessive bills for problems I can probably fix myself (although I'm not averse to calling out the experts when I'm out of my depth!). Any helpful advice, or pointers in the right direction, would be gratefully received!

Thanks in advance,
Helen

 

[namsag]

You say if you operate switch by hand demand light comes on and burner fires.

But if you allow pushrod to come out and do it demand light come on but wont fire even if you try and help.

Try taking microswitch off (1screw) turn hot tap on and then (being careful not to touch connections) activate switch manually.
If burner stays on it would indicate faulty switch

 

[ChrisR]

WHat's the system pressure gauge reading?
The diaphragm operated microswitch plunger you need to look at is the one for the pump - in the red nut:

 

[Agile]

The water flows round the boiler in a different route when the diverter valve is actuated by a DHW flow.

In your case the system flow switch ( red gland on Chris's picture ) will operate on CH but not on DHW.

The pump impeller may be partially blocked, the pump weak or the plate HE routing blocked and obstructing the flow. The latter would usually have been noticed as the DHW temperature fluctuating.

Tony

 

[Dragonfire1]

Many thanks for the speedy responses!

namsag,
With the microswitch away from the pushrod assembly, and the water flow on, the burner doesn't even attempt to fire when the switch is operated manually. It only fires when the switch is manually operated (i.e. with no water flow). I'm guessing that this means the switch is OK then...?

ChrisR,
The system pressure is about 1.5 bar. I only checked the water diaphragm (in the blue section of the pic), I didn't check the CH diaphragm, (under the red nut in the pic) as the CH seems to be working OK. But, if you think this is worth checking, I will do that and report back...

Tony,
How easy is it to check if the pump impeller is blocked? Or how to tell if the pump is weak? I'm willing to have a try at anything, but may need some more detailed instructions (before this week, I'd never had cause to even look inside the dratted thing!). I'm guessing it's not a blockage to the heat exchanger, as I've not noticed any fluctuations in the DHW temperature recently ... it just stopped working!

Many thanks,
Helen

 

[Agile]

If the system flow switch works on CH then dont touch it. they virtually never fail.

Drain and remove pump head and look for dirt blocking the eye of the impeller.

If none then IF you can do it safely, power the pump and do the finger test to see how much torque the pump is giving.

Tony

 

[Dragonfire1]

Thanks Tony, I'll remove the pump head and check that. Not sure about being able to SAFELY check the torque though - don't know if I'm THAT brave!

Thinking about it, the pump seems to operate just fine when running the CH (or does it not need to provide quite so much pressure for CH as for DHW?), and the flow rate from the HW taps seems normal - when I turn the HW tap on, the boiler is getting the signal for HW demand, and running the pump, and supplying a decent flow rate, just that the water's cold due to no ignition in the boiler! Just a thought...

I'll report back when I've checked the pump head out...

Cheers.
Helen

 

[Dragonfire1]

Hi,
Tried to remove the pump head this morning, unfortunately, it turns out that my hands are not strong enough to undo the hex head bolts that hold the head to the body! Think they've probably been overtightened a tad...!
Primary circuit refilled back to a little under 1.5bar (not sure what colour the water I drained from the circuit SHOULD have been after 4 years, but was a mucky brown colour, if there's any clue in that to diagnosing this fault) - still no hot water (not that I was expecting any!)...

Is there any other way I can prove or disprove that the pump is ACTUALLY doing what it's supposed to?

Looking at the various circuit diagrams in the installation manual, could it be a problem with the flow sensor at all? I find it a little weird that the burner will fire up if I activate the DHW valve microswitch with NO flow, but the burner will not fire up when the microswitch is activated by the valve pushrod...
(Not that I could find any way of checking this theory about the flow sensor, I could see no easy way to get to it!)

Going on nothing more than instinct and a little logic now, I'm still suspecting either the PCB or the diverter valve assembly ... is there any other way of verifying or disproving either of these theories? (BEFORE going to the expense of replacing either of them, of course!)

Any other hints/pointers would be greatly appreciated!

Cheers,
Helen

PS Sorry about the long post, as you can probably appreciate, it's getting a little frustrating now...!

 

[Agile]

Hi,

Going on nothing more than instinct and a little logic now, I'm still suspecting either the PCB or the diverter valve assembly ... is there any other way of verifying or disproving either of these theories? (BEFORE going to the expense of replacing either of them, of course!)

Sorry Helen but I have told you what you need to check! Your uninformed speculation is just wasting your time!

I fix boilers and I dont expect that you do!

If you want to get anywhere then you would be better off taking a professional's advice! My advice is based on engineering and not instinct. Humans have no instincts to repair boilers.

To others, isn't it wonderful female logic that if you cannot get the pump head off to check it then you then decide its OK and that it must be something else like the PCB or the diverter!!!

Tony

 

[gas4you]

Ah Tony, you know all the right words to use with a lady, you old romantic

 

[Dragonfire1]

Tony,
You are entirely correct ... I don't fix boilers, I'm just a lowly mechanical design engineer! And I greatly appreciate all the advice you are giving. (But please don't assume that just because I can't get the pump head off that I have used female logic and eliminated that as being the problem ... I know it doesn't work that way!)
I am going to call in some outside help (a friend with stronger hands than mine!) to assist me with removing the pump head, and will let you know later what the outcome is...
Regards,
Helen

 

[Agile]

Good ! We are waiting with bated breaths!

IF the pump is not blocked AND is turning with plenty of torque then while its drained take off the plate HE as that will be blocked.

Your mucky water is a strong pointer to a blocked pump and/or plate HE.

In fact the plate HE will be somewhat blocked anyway but its first necessary to check out the pump.

I am sure you design your mechanical things based on engineering design principles rather than instinct !!!

By the way you could always apply a mechanical lever to your allen key to increase the torque !

Weak female heating engineers buy nice "T" handle allen keys ! There really are a few females about too! WE saw one at Ravenheat !

Tony

 

[Dragonfire1]

Pump head finally removed, courtesy of a friend with hands stronger than mine... and even HE had difficulty turning the screws, WITH the aid of the mechanical leverage on the allen key (beat you to that, I had already tried with the extra leverage, and, yes I do happen to own a set of "T" allen keys...).
There was a small amount of scale around the very edge of the seal (not overly surprising due to the hard water around here), and a few very small loose black deposits at the centre of the impeller. All visible deposits cleaned off. Pump also appeared to pass the torque test when powered up separately.

System put back together and refilled (yes, I know I haven't yet removed the plate HE, but following one of those sound engineering principles of doing one step at a time), and ... on DHW demand, the burner fired! All good, I thought ... until I tried it again half an hour later, and ... no HW again!

Following your previous advice, which now indicates that the plate HE is blocked (still not discounting that the pump may also be faulty anyway), I will try to remove the plate HE tomorrow morning (any tips on how to extract it would be appreciated, along with what to look for once it's out); if that's not going to happen, I'm going to call in the experts to come and sort it out for me.

So, after you've said "I told you so" about the pump (which I never actually disputed might be causing the problem, and I bow to your greater expertise in these matters!), I would appreciate a couple of sensible answers.

Re the plate HE, if it's as "easy" to remove as the installation guide suggests, I should have no problem removing it. Is it fairly easy to clear any blockages, or am I looking at the probably simpler but more costly option of replacing it?
Is there any way of cleaning the pump head any further, or is it just a case of replacing it? (It looks like a sealed unit to me, so I think I can guess the answer to that one!)
If replacement is necessary, do I have to purchase the entire pump (as suggested by the parts manual), or can I just buy a replacement pump head? Not sure if it's worth it, as I haven't a clue how much the head on its own costs, but, if so, can you please suggest where from...? (I've already found somewhere that can supply the entire pump at a fairly reasonable-looking cost).
I would also appreciate if, as a professional, you (or anyone else...) could give me an indication of parts and labour costs (I'm not after exact prices, just an idea so I have an idea of what I'm looking at spending out on this).

Many thanks for all your help so far,
Helen

PS: Please don't presuppose that I am weak because I have the XX chromosome, it's actually a little more medically complicated than that... Let's just say I know my limits...

 

[gas4you]

Some on here advocate cleaning of the plate he, but to do properly this needs time and the use of an acid based cleaner, something like DS40 or DS3 depending which side you are cleaning.

I personally always replace with a new part.

The choice is yours

 

[ChrisR]

The plate hex on these isn't too bad at all to get out. In fact thinner hands would be useful when replacing it! If you can't get a good grip to replace it, some improvising with sticky tape can work. Watch the 4 O rings don't come out of their seats, and some silicone grease on them helps reseal. Clean scale etc off.
Cleaning heat exchangers takes 20 minutes or so once they're off, if you've done it before.
I've not had trouble cleaning them, but do sometimes swap for a new one or one I've cleaned out previously. Yours isn't toooo expensive, I think.

While the water's drained, unscrew and clean/grease the red gland I pointed out above, and check the pin is clean. They can go sticky.

Does this link work for you?
Here