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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3792
Experience:  Heating and Controls engineer
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I have no hot water on my baxi combi instant 105e ,have

Customer Question

I have no hot water on my baxi combi instant 105e ,have replaced the 3 way valve the motorised head the the dhw diaphram and the microswitches as the leaky dhw diaphram had ruined one , also checked temp sensors and they are fine , the boiler dosen't call for pre heat , burner kicks in when you run the hot tap but only get cold water, all neons light steadily untli it hits 80 then the burner shuts off, plumber friend lent me a pcb and does the same , also have swapped the plate heat exchanger, old one was quite dirty but not terrible, now head sctratching
Submitted: 8 months ago.
Category: HVAC
Expert:  Andrew Smith replied 8 months ago.
Does you boiler have the small preheat tank?
Customer: replied 8 months ago.
Hi Andrew, yes it does, I noticed yesterday that the pressure is rising more than usual when on heating so my next port of call was going to be to drain the system and then check the expansion tank and re pressurise if necessary and check it is holding ok
Expert:  Andrew Smith replied 8 months ago.
Ok.
Doesn't the instant have a thermal store as a preheat tank?
Customer: replied 8 months ago.
it does this is where the preheat is not kicking inso no store and when the boiler runs on DHW all valves move etc but the boiler hits over heat after approx 30 seconds and you get no hot water
Expert:  Andrew Smith replied 8 months ago.
The temperature of the preheat store should control the boiler, have you checked the thermistor on this?
Customer: replied 8 months ago.
Yep checked and replaced both CH and DHW thermistors, the top half of the plate exchanger is hot, all micro switches are clicking as and hen they should and the motor on top of the 3 way valve is doing what it should, have replaced the three way valve othewise I would guess that is the cause as has the classic symptoms of hot water going back up the heaing flow pipes but not coming thru the DHW pipe like happens when the diverter is stuck, , have checked and cleaned the filters on the inlet and outlet pipe and replaced the venturi in the inlet manifold, it just seems the flow on DHW is not taking the ha away from boiler and so gives an overheat lock out , when you turn the hot water on there is some rushing noise and a bit of gurgling
Expert:  Andrew Smith replied 8 months ago.
the heat is going into the plate, but there's no transfer?
Does the hot water get warm/hot?
Customer: replied 8 months ago.
You get coldt hen luke warm for a couple of seconds then cold then the after about 20 seconds temp neons hit 80 and the flame trips
Expert:  Andrew Smith replied 8 months ago.
the first slug of warm is what is sat in the boiler when you turn the tap on.
It sounds like the divertor valve as no water is being transferred into the plate heat exchanger.
Can you run the tap and then hold the flow and return pipes and see if they heat at all?
Customer: replied 8 months ago.
You don't whilst the tap is running but a soon as you switch it off you get heat thru the heating send side
Customer: replied 8 months ago.
the top of the 3 way valve gets very hot too
Expert:  Andrew Smith replied 8 months ago.
Ok, so it shuts correctly to push the heat into the heating to stop it overheating, but it us not opening fully to the plate.
You say it is new?
Front, back or both?
Customer: replied 8 months ago.
on the pic the bits with red dots on are new, but not sure about the bit that attches to the heat plate exchanger where the DHW actuator is, so both micro switches and actuators the motorised head, the 3 way valve all replaced nothing on the inlet /pump side apart from the pump head and the venturi and filter also plate exchanger is new
Customer: replied 8 months ago.
Actually just realised all the bit on the left is part of the same valve.i
Expert:  Andrew Smith replied 8 months ago.
you replaced the WHOLE brass block?
Customer: replied 8 months ago.
the DHW actuator and microswitch that sits on top of it
Customer: replied 8 months ago.
I notice there is a small bolt/bleed sir as asked with an arrow on pic, is this just for training down the valve or is it an air bleed screw ?
Just clutching at straws here !
Customer: replied 8 months ago.
Just double checked and as you said the only time the DHW feed pipe is warm is when the heating is on, when you run the tap you get that initial slug of hot water then if you keep your hand on it you can feel the pipe go cold then the burners cut out
Expert:  Andrew Smith replied 8 months ago.
You are still not getting full diversion to the plate so either not working correctly or the motor does not activate the pin correctly
Customer: replied 8 months ago.
That what I thought but have a spare motor head and it does not work with either, if i remove the motto head and run the boiler the pin moves in and out depending on heat demand or water,I've just tried leaving the motor off and manually operating the valve and still no joy
Expert:  Andrew Smith replied 8 months ago.
Was it a refurbished unit?
Expert:  Andrew Smith replied 8 months ago.
I assume the pump is running the correct way?
Is it the flow that gets hot on the heating?
Customer: replied 8 months ago.
No brand new from Wolsey/parts centre an the DHW actuator also, I couldn't be bothered to change the diaphram so swapped the whole thing, my mate fitted it who is a plumber and gas safe etc but he was scrathing his head Baxi did gave their usual advise to switch the PCB which we've done although I couldnt see from the symptoms what this would cure as firing sequence etc all fine , flame modulates depending on flow or demand on CH the only thing not working is call for preheat on the DHW but I would have thought that if you ignore that it shouldnt effect the main demnd for hot watr as buners pump etc all working as they should
Customer: replied 8 months ago.
Yep flow side?left side of boiler on CH this side is hottest as you would expect and the return on the right is coolest, it was the same before we change the pump head, the only reason we changed it was as the old one felt gritty and didnt spin that well even after cleaning so were not sure if it was working effectively new head 3 speed and as recomeneded by baxi set on speed 3
Expert:  Andrew Smith replied 8 months ago.
That's fine.
Just going through options as some pumps are designed to spin the opposite direction.
Customer: replied 8 months ago.
No prob, if it was my car over heating I would have gone thermostat then air lock then pump
Expert:  Andrew Smith replied 8 months ago.
Are you able to manually manipulate the pin as far as the motor head should?
Customer: replied 8 months ago.
Yep already done that , mentioned a while back , ran the boiler with the motor electrically attached and held the pin down for a while whilst running the tap then released it and continued no difference
Customer: replied 8 months ago.
The only part not changed on the left is the pressure differential assembly that attaches to the bypass loop behind the heat exchanger as below pic have replaced the from part the hydraulic sensor with the pin that operates the micro switch.
Is there another valve inside there tat could be blocked ?
Expert:  Andrew Smith replied 8 months ago.
Is this not the hot water part?
Do the two small pipes connect to the hot and cold pipework and puck up the differential?
Customer: replied 8 months ago.
yep differential is fine the pin moves in and out as it should and the microswitch sends its signal all works fine there.
The front piece where the diaphram is for the flow sensor has ben replaced but not the back part, and the actual 3 way divertor valve with the the ch sensor on has been replaced this bit below
Expert:  Andrew Smith replied 8 months ago.
😉
Nice wheels?
Customer: replied 8 months ago.
Ha ha sorry
sent the wrong pic !
More interesting than boiler parts but not quite as importatnt to my other half who hasnt had a hot shower at home for a week Bits with ticks on replaced bit with cross not
Expert:  Andrew Smith replied 8 months ago.
The first picture is the bit I would think.
Can you confirm where all 3 ports are piped to so I can confirm this on the functional diagram.
Expert:  Andrew Smith replied 8 months ago.
Domestic Hot Water Flow Priority Assembly
Part no.22 page 8 of the manual
Expert:  Andrew Smith replied 8 months ago.
Baxi Combi Instant 80e & 105e
PDFhttps://www.baxi.co.uk › baxi-combi › b...
Customer: replied 8 months ago.
Thanks I have the manual permanently open at the mo !
Yep part 22, they show as whole assembly but just the sensor part I ticked has been changed along with part 21, 18,23
Customer: replied 8 months ago.
It'd be good if they showed the correct flow of water thru the boiler somewhere in the manual!
Expert:  Andrew Smith replied 8 months ago.
This vslve diverts for the preheat also.
You said this did not work either?
Customer: replied 8 months ago.
Correct preheat not working at all even after reset where it should kick in after 25 seconds, I've been trying to figure out what it is that calls for preheat , a temp sensor etc ?
Expert:  Andrew Smith replied 8 months ago.
The preheat temp must be controlled by the central heating thermistor as it us the only one that side of the plate.
Customer: replied 8 months ago.
That's what I thought , but no fault showing on the thermistor and have swapped it , maybe I'll plug in a spare one but just into the loom not the pipe work and see if it then triggers, I get no flashing neon either on the water symbol which I should when the pre heat is functioning
Expert:  Andrew Smith replied 8 months ago.
It's as if the PCB isn't switching to preheat mode.
But you've already checked that - perhaps the wiring loom for the thermistor if your spare doesn't trigger It?
Customer: replied 8 months ago.
but I would presume that once you demand hot water properly the preheat mode is over ridden, to be honest I don't really care about the pre heat mode as long as its not affecting anything else, I just checked the inlet pipe and the outlet side of the plate heat exchanger whilst running the hot tap and they both felt as hot as each other , I presume you would expect a drop across the exchanger as the cold feed takes the heat away to the tap ?
which still makes me suspect a blockage somewhere , I'm assuming tho that the cold water feed flows thru the plate heat exchanger no matter what to the hot tap ?
Expert:  Andrew Smith replied 8 months ago.

Yes, even if you turn the boiler off, there is a still a hydraulic path through the boiler for the cold water.

With the boiler on then the water picks up heat from the plate heat exchanger as long as the boiler is heating the primary side of it.

Customer: replied 8 months ago.
Primary exchanger making a lot of rushing/gurgling noises when on hot water demand but all was working fine last week before the diverter went , had noticed a slightly lower flow at ehe shower over the last month but put that down to colder mains supply being winter and the fact that Severn Trent have been reducing pressures in the area I've been told, I checked my cold water flow rate with a litre jug and a stop watch and it's approx 8 litres per minute where it used to be over ten and as much as 12 in the past
Expert:  Andrew Smith replied 8 months ago.
Lower flow rate means hotter water (or should do)
But gurgling means boiling water and no temperature control.
Customer: replied 8 months ago.
Or not getting the hot water away from the primary exchanger quickly enough too I guess ?
The only temp control I can see is the DHW temp sensor and that is in the pipe that is not getting hot, only other way I can see of temp control is the hot water temp control know which if I turn whilst hot water is being demanded modulates the burner
Customer: replied 8 months ago.
I still keep coming back to the back part of the DHW pressure differential assembly, I am still not clear if the rear part of this has any control over flow?
Found this on ebay where you can see some internals which look blockable !
Same part number , you can see inside are springs etc if you zoom in
Could this be the culprit ?
https://www.ebay.co.uk/itm/POTTERTON-PERFORMA-28i-24iHE-28iHE-PRESSURE-DIFFERENTIAL-248728/331978892810?hash=item4d4b7be60a:m:mA7xfTqC7qbFbqwOdSqP4Jw
Expert:  Andrew Smith replied 8 months ago.
Preheat goes through this part, so I can imagine it us important for flow through the plate instead if turned off
Customer: replied 8 months ago.
Tried that still no joy, new boiler time I think and flog the working bits off this one on eBay
Expert:  Andrew Smith replied 8 months ago.
Sorry, just that sort of problem