Vokera Excell 80SP Intermittent hot water

[retiarius]

Hi, got a really annoying problem that hopefully has a simple solution.

I've got a Vokera Excell 80SP, no idea of age, it came with the house

We have excellent water pressure and the boiler is on the same level as all the plumbing.

When turning on the shower, or any other hot tap the boiler will fire up, sometimes after a good minute or so delay and hotwater will run unfortunately after a further minute or less the water will run cold for a minute or so (ice cold as if straight from the cold tap) and then eventually the water will run hot again, this yo-yo'ing is reasonably consistent, although on other ocassion i can run the shower with just the hot tap fully open and with the dhw temp at max and get constant hotwater (although this is almost to hot to bare).

I'm leaning toward the thermistor.... but hey i'm no expert...

Almost forgot, the problem gets worse if we try to use the cold tap to reduce the temperature (runs hot cold more often)

 

[Agile]

Probably the diverter valve failing to register the hot water demand.

Its a common fault on that model!

The flow sensing part is subjected to too much heat and that causes it to fail prematurely, usually every 1-3 years.

We have to carry the spare parts needed for the repair all the time as its such a frequent problem for our clients.

Tony

 

[Gasman1015]

Replace the diaphragm, its a 10 min job.

 

[retiarius]

Wow, thanks for the quick reply.

Are the parts easy to come by as i noticed from the Vokera site that this model has been discontinued.

Having just scraped up enough funds to finally get a property a huge repair bill is something i could do with out, I'm pretty capable mechanically and would probably like to do the work myself....

 

[ChrisR]

Very easy.
Diaphragm alone is 6882 (~£5) or the kit for the whole hydraulic section, T0019 (about £25). No longer have to look these up!
Daunting job when you first look at it, but nothing actually hard.
Manual from Vok or one of us if you put your email in your profile.

 

[retiarius]

Thanks for the advice, I've just ordered the service kit from Godfreys Online, £27.92 delivered and already got the service manual from Vokera.... I'll let you all know how i get on.

 

[mcmarshon]

You should also try cleaning the venturi tubes, stops the APS from bouncing and cutting the gas supply. Another common fault on these boilers !

 

[weareleeds]

My money is on dirty fan. As you state blr is going hot & cold watch main burner with hot demand if it goes from high gas to off instead of low gas then fan is dirty so you will need to clean it thoroughly or replace it & the gasket

 

[retiarius]

My money is on dirty fan. As you state blr is going hot & cold watch main burner with hot demand if it goes from high gas to off instead of low gas then fan is dirty so you will need to clean it thoroughly or replace it & the gasket

The service kit did not resolve the issue which slowly got worse. I cleaned the fan as best i could and the tubes for the differential pressure switch, i assume this is the venturi switch, which got the boiler working better.

The boiler is still unable to run at low gas and simply switches from high to off as the water runs, this results in the water temperature varying a great deal during use (not so fun for showers) but nowhere as bad as before.

Should i look to replace the fan and differential switch now?

 

[scatmanjohn]

When you say that you cleaned the fan and venturi tubes, what did you do exactly.

It can ONLY be done by removing the fan, the venturi is in the fan outlet.

 

[gas4you]

Thanks for the advice, I've just ordered the service kit from Godfreys Online, £27.92 delivered and already got the service manual from Vokera.... I'll let you all know how i get on.

Godfreys online Wow the last of the professional heating outlets

 

[nickso]

My money is on dirty fan. As you state blr is going hot & cold watch main burner with hot demand if it goes from high gas to off instead of low gas then fan is dirty so you will need to clean it thoroughly or replace it & the gasket

The service kit did not resolve the issue which slowly got worse. I cleaned the fan as best i could and the tubes for the differential pressure switch, i assume this is the venturi switch, which got the boiler working better.

The boiler is still unable to run at low gas and simply switches from high to off as the water runs, this results in the water temperature varying a great deal during use (not so fun for showers) but nowhere as bad as before.

Should i look to replace the fan and differential switch now?

properly clean fan blades and venturi. if that doesnt solve it then its the air pressure switch.

 

[DP]

You might as well as start replaceing parts one by one. You will (hopefully) be able to resolve the problem.

If you are hoping to repair the boiler the way way it should be done, you need to give a little more information as sketchy info leads to wild answers (these may well be correct when the engineer is in front of the boiler). Look at the flowchart in the manual. This will help to pinpoint the fault.

A boiler needs to be serviced regularly. Dirty fan on yours suggests otherwise. Another recomendation would be to service the boiler as suggested in the manual as sluggish operation might itself lead to incorrect operation.

 

[retiarius]

You might as well as start replaceing parts one by one. You will (hopefully) be able to resolve the problem.

If you are hoping to repair the boiler the way way it should be done, you need to give a little more information as sketchy info leads to wild answers (these may well be correct when the engineer is in front of the boiler). Look at the flowchart in the manual. This will help to pinpoint the fault.

A boiler needs to be serviced regularly. Dirty fan on yours suggests otherwise. Another recomendation would be to service the boiler as suggested in the manual as sluggish operation might itself lead to incorrect operation.

I tried running through the flow chart but it is only checking electrical points not mechanical, which are all functioning correctly... at this stage i'll probably look to replace the fan and pressure switch. What i need is someway of finding the reason for the burner not being able to run in low flame mode (the troubleshooting guide only covers on or off)

I removed the fan and having cleaned it and the tubes in the flue and the ones running to the pressure switch.of all loose debri, the only problem i appear to have now is the boiler refuses to run in low gas mode. i'll try removing the modulator coil to force low gas mode on saturday. I can here the pressure switch activating the relay in the burner when i disconnect and reconnect the tubes to the pressure switch.

 

[ChrisR]

Don't try to remove the mod coil.
Disconnecting/reconnecting it should make the flames go hi lo not on off. Quite a common problem and a corgi job to fix.
You can check the voltages the mod coil is getting.
Sludge in your heat exchanger is also a strong possibility causing hot /cold showering.
Do a search, plenty here.