Vokera 20/80 RS Turbo - hot, cold, hot, do the dance with me

[stubertio]

I just love the name of that boiler!
We've just moved into a flat in Twickenham that has a Vokera installed which is playing up.
British Gas came for the annual service and said the boiler is beyond economical repair and it's time to replace it. Forgive me for being sceptical!
I have done some searching on these forums and have noticed that DP seems to be a Vokera guru, but unfortunately is based in Scotland!
The central heating appears to work fine when we tested it, with the radiators maintaining consistent temperature (weird?). The problem lies with the hot water. If I turn on a hot tap in the flat, the pressure is very good and VERY hot water (scalding) eventually comes out after about 30 seconds. It lasts for a couple of minutes at the most and then goes cold. After a minute or two it warms up again and repeats this cycle — you should see the 'hot cold' dance I do in the shower in the morning.
The strange thing is that we have tried adjusting the hot water temperature thermostat on the front of the unit, but the hot water still comes out at a very very hot temperature - for a few minutes at least.
I'm next to clueless when it comes to plumbing, but I like to think I'm a semi-practical person. Is there any chance that the thermostat or some kind of hot water sensor could be faulty, therefore the boiler is heating the water up to a ridiculous temperature, overexerting itself and then cooling down, rinse, lather & repeat?

I phoned Vokera, but they were also only interested in selling me a new boiler. They said they could come and have a look for £200, but there's no guarantee they would even be able to supply parts for it and if they were unable to fix it I'd still be charged!

Any advice greatly appreciated. We've had a quote from a plumber for a replacement boiler and it's not nice! He also said that there may be a problem with the gas supply as it's only 15mm. It's 22mm from the meter in the basement but then it goes into concrete and downsizes to 15mm SOMEWHERE. We've both looked everywhere but we couldn't see where it downsizes. Even the pipework for the cooker is all 15mm and then concreted into the floor.

Many thanks

Stuart

 

[Dan Robinson]

Sounds like a scaled/blocked heat exchanger/calorifier.

Plenty of posts about this on the forum.

 

[timmy74]

on this model i'd check the fan venturi 1st (clean it and the fan blades) before anything you will need a RGI to do this. as you dont notice the boiler closing down on heating but it shows up quicker on dhw as you are noticing the hw go cold then hot etc.

 

[scatmanjohn]

on this model i'd check the fan venturi 1st (clean it and the fan blades) before anything you will need a RGI to do this. as you dont notice the boiler closing down on heating but it shows up quicker on dhw as you are noticing the hw go cold then hot etc.

Here, Here

 

[Agile]

The temperature control system on this boiler is a little simple and needs some skill to set it up properly.

It needs to be carefully watched to identify the exact cause of going totally cold. It might be as simple as the DHW stat being faulty or needing heat transfer paste in the pockets.

Although probably not quite as simple as just dirt in the secondary HE that is probably at least partly involved.

Whilst DP is in Scotland he does sometimes answer his phone and Chris R is able to reach Twickers and can call DP for any advice.

Tony

 

[bob1960]

these boilers can be repaired but needs an experienced vokera engineer.all parts are still available.
not really a diy repair lots of electrically live parts behind case,always ensure a 3 amp fuse is fitted as resistor wires can come adrift and short to case.
resistor mounting will be red hot so dont burn your self.
possible defects for hot/cold water temps.
hot water stat gone out of calribation.
no gas during modulation.
but needs further investigation.
if you have a bg contract get them to repair it.
did bg service it ?.

 

[Dan Robinson]

if you have a bg contract get them to repair it.
did bg service it ?.

I haven't laughed so hard in weeks - thanks Bob.

 

[stubertio]

Thanks for the replies so far. I've contacted ChrisR and will hopefully hear back from him in the next day or so.
In the meantime, I have been studying the service manual and watching the boiler carefully.
I turned the shower on this evening and had hot water for about 5 minutes. I watched the temperature gauge (is it for the temperature of the water or the temperature of the boiler itself?) on the front of the boiler rise from rest (about 20 degrees) up to 60. Hot water continued to flow at a decent rate and temperature. All the while the flame being at medium/high strength. Then, all of a sudden the boiler seemed to cut out and the flame went back to a low flame (what it's normally like when there is no demand for hot water). After a few seconds, the shower went cold. I stood there for a few minutes waiting for it to warm up again. During this time the temperature gauge on the boiler stayed at 60 and the flame remained low. It then kicked in again with a stronger flame and the hot water returned.
Weird.

Fingers crossed, we'll have it repaired in the next few weeks!

Thanks
Stu

 

[DP]

Check if the burner downsizes during hot water demand.

Reduce water flow at hot tap. Set HW stat to max and then reduce setting while watching burner flame (not pilot flame). Also check temperature (should sit at around 70 for max setting of HW stat.

 

[bob1960]

seems like the plot continues.
ok this is what i can see fron the posters new post.
1.temp guage is only the boiler primary temp,not same as temp expected at an outlet ie tap/shower.
2.i hope boiler goes to lower flame (modulation) a click of the gas valve will be heard whilst it does that and another click when goes back to full flame.are the gas pressures correct.ask last bg engineer to clarify.
3.hot water flow rate should be 10.5 l/min raised by 30 oc.
4.have you got the correct shower valve,a thermostatic shower mixer is required.with equal pressures either side.
5.please tell us make of shower mixer you have.ie is it a flexi hose from bath mixer tap.
6.running same vokera now with an average wicks thermostatic shower mixer,boiler temp at 68 oc on setting 5 hot water thermostat on boiler.great supply of constant hot water for as long as we want.
7.aussie girlfriend in shower now.so its time to gooooooooooo.

 

[DP]

Bob do agree with what you post apart from (1). It IS the primary water that heats the secondary water (I agree), therefore temperature of primary water DURING hot water drawoff is a diagnostic tool to establish if heat is BEING transfered adequately during HW call.

Let us assume for a moment that HW HE is scaled up (very remote). Primary water temperature would be in the high 80s and modulation may not even get a chance to kick in.

To OP, what is the temperature when you ask for hot water?

 

[stubertio]

To OP, what is the temperature when you ask for hot water?

Hi there, DP

I've never seen the temperature gauge go over 62. This is when I've been 'fiddling' and have turned the DHW thermostat up to its highest setting. When the hot water turns cold (out of the shower or any tap), the temperature gauge doesn't really move that much like I'd expect it to (don't know why, I don't know how these things work!). For example, when the hot water tap goes cold, the temperature gauge could drop from 60 to 55 but hot water outlet pipe from the boiler goes almost freezing cold to the touch.

Thanks

Stu

 

[stubertio]

seems like the plot continues.

2.i hope boiler goes to lower flame (modulation) a click of the gas valve will be heard whilst it does that and another click when goes back to full flame.are the gas pressures correct.ask last bg engineer to clarify.

Yes, I hear and see it clicking in and hot from low to high flame. The BG annual service/inspection was carried out by the previous owners in March. I have the paperwork somewhere, but I don't think it mentions pressures anywhere on it.


4.have you got the correct shower valve,a thermostatic shower mixer is required.with equal pressures either side.

It's not just the shower, the taps go cold as well.

6.running same vokera now with an average wicks thermostatic shower mixer,boiler temp at 68 oc on setting 5 hot water thermostat on boiler.great supply of constant hot water for as long as we want.



7.aussie girlfriend in shower now.so its time to gooooooooooo.

What are you doing on here? Go go go !

Thanks

Stu


PS, does anyone have ChrisR's phone number? I've emailed, but perhaps he's not an email junky like me

 

[gas4you]

He hasn't been posting for a while, perhaps he is on holiday.

 

[bob1960]

never seen any serious scale problems in twickers with these boilers.
going to english pub now to drink irish stout,served by polish staff,then getting a mini cab whose driver will have gold teeth and then going to have kilburn kebabs.then back home with sweet smelling aussie girlfriend.
a world tour in an evening.