DHW temp fluctuation and boiler lockout

Hi Everyone,

I have a Viessmann Vitodens 100-WB1B 26Kw combi boiler in a 2 bed flat with 1 shower. Hot water in taps and shower used to fluctuate and was never quite hot. In Jan of this year it triggered a F2, so I had a plumber in who said the DHW heat exchanger was blocked likely due to sludge in the CH side. So I got it replaced and had a mag filter and lime scale reducer installed at the same time. Things were ok after that.

Then last week, the hot water temp started fluctuating again, where it would get hot in the shower for 2 min then drop to lukewarm for a min and then back up. I went over to the boiler and could see it firing at 65C then going up to 85C. At that point it would shut off but the temp on the screen kept going up to 104C. It would then drop to 65C and refire. And this morning it finally caused the F2 lockout. Engineer came by and said 2 hrs later the internal temp in the heat exchanger is still 117C.

My mains pressure is 4 bar, and I had the boiler set to 1.7 bar. Could it be the DHW heat exchanger is gone again this quickly? Or an issue with the pump or PCB? Am wondering if its worth fixing or do I need to just take it on the chin and look at getting the boiler replaced? I suspect its 7-8 years old (previous owner didn't leave any documentation).

Thanks

What conclusion did the engineer come to ? How does boiler perform for central heating ?

He said the PCB is gone along with the pump. They will cost 800 pounds to replace as apparently Viessmann parts are quite expensive. So it's looking like I'm going to have to shell out for a new boiler

How does it perform for central heating ??

CH works fine although its always taken a while to get to set temperature. And I had to turn the heat dial all the way to max otherwise it wouldn't even reach a 1C rise in 90 min

1. I'd question how any boiler can have a heat exchanger internal temperature of 117 degrees two hours after the boiler stopped running. It would have had to have run dry while still burning gas.
2. If it performs OK for central heating, it would suggest to me that the problem is solely in the hot water side. Most likely culprit is a blocked secondary heat exchanger. Remember this has the central heating water circulating on one side, so if that water is sludgy, it could block quite easily.
3. If its 7 or 8 years old and has been poorly maintained, then maybe replacement is the economical option. Otherwise, perhaps a different engineer?
4. Don't know if Viessmann do it, but some manufacturers offer a fixed price service. Might be worth finding out if they do, what is the cost and do they include parts.

Thank you for your comments above. I agree with your assessment that a boiler having an internal temp of 117C is odd since I powered it down immediately after lockout. The engineer was from HomeServe and I don't think he knew how to work with a Viessmann as I saw him googling for instructions, call a mate and then tell me 10 min later it was conked and just leave.

I then got another person I trusted in for a second opinion (he wasn't available until a few days later so I opted for HomeServe first). He said there was gas (Hydrogen and Oxygen) trapped at the top of the HE. He was able to get the lockout fixed by draining via some plastic tube inside the boiler. He then started the CH and dialled down the DHW temp. It started working and has been ok for the lats week. He said he'd need to strip it down as he suspected there was blockage somewhere in the DHW loop so the problem would reoccur in a few weeks/months. I think the previous owner likely never got any servicing done over the 7-8 years he had the boiler and I saw sludge in the CH when replacing some rads. So as you said, that's probably getting into the secondary heat exchanger and gunking it up.

A power flush is probably needed but at 500 pounds, I don't know if that will be the end of this saga or just another stop-gap fix before I land up shelling out 2,000 GBP for a replacement. At this point I've spent about 800 quid with call outs, DHW heat exchanger replacement so am a little wary. So as you say might be economical to just bite the bullet.

The second engineer is recommending Alpha Evoke 33Kw as it has a stainless steel exchanger and good quality internal parts. I have not heard of this brand and online reviews seem mixed, so wondering whether to go with the safer choice of WB or Vaillant or something like Ideal Logic Plus. I trust this guy as he is old school and not wedded to any brand - but reliability is a long term game and he only fixed the Alphas at another site a few months back.

I didn't know about the fixed price service - thank you for that. I will certainly look into it.

1. Presumably you have 4 to 6 radiators for a two bedroom flat. At a VERY rough rule of thumb that's between 6 and 9 kW of heating requirement, perhaps 11 or 12 while first heating up from cold. . That's not much of a load for a 33kW boiler, so there is a risk that it will keep turning itself on and off. Not an absolute no no but not as efficient as running a boiler which can be closely matched to the heating requirement.
2. It's normal to size a combi boiler on the hot water requirements. Broadly, more kW = more or hotter water. However, you need to make sure the boiler can turn itself down (modulate) to a low enough level to sustain the central heating load without turning itself off. The Alpha Evoke 33 will modulate down to 4.9 kW, so should suit in this respect.
3. You could consider an alternative to a power flush, and that is to take the radiators off one at a time and flush each one through with mains pressure water in both directions while agitating (hitting with a non-marking hammer) it. Needs to be done outside really, so not always easy in a flat. After all done, install a filter such as the Adey Magnaclean Pro on the return pipe close to the boiler, add a cleanser such as Sentinel X400 or X800, refill and run the system for a while. Drain the system to get rid of the cleaner and flush it through with mains pressure water, then add inhibitor such as Sentinel X100 (through the Magnaclean). Check and clean the Magnaclean element every few hours at first, then once a day, once a week and so on, ending up with once every 6 to 12 months.
4. Might (but unlikely) to clear the secondary heat exchanger, so you might need to get that changed. I'd be a bit surprised if you needed a new PCB, but its possible. I'd suggest changing the secondary heat exchanger first, and if that works fine. If not, perhaps then would be the time to consider a new boiler. At least it would be installed onto a clean system!

I have 8 radiators with a total output of 26K BTU @ Delta 50 so the boiler should be more than able to handle that as you said. I was targeting a 30KW boiler replacement so that the flow rate for hot water would be 12 l/m and capable of handling a bath in case I add one in the near future. 26KW gives me about 9 l/m I think and that too has never been scorching hot (probably gets to 45-50C at the hottest).

Will keep your point @ boiler being capable of modulating down to sustain CH in mind if I do land up going for a new boiler as I did have concerns about the boiler switching on/off quite a bit in case its oversized for the property. All my rads were replaced just a few months ago when I moved into the property. I didn't know at the time about the sludge in the CH as it was my first purchase. Would you still recommend taking each one off to flush in that case, or is the sludge more likely to be in the flow return in and out of each rad and thus needs to be cleaned differently?

I did have the secondary HE changed 3 months back but am assuming 3 months of operation could have clogged it up again. And at that time I got a mag filter put in as well. Haven't checked it to see if its collected a fair bit as its not the Magnaclean one but a metal one similar to the spirovent.

Thanks again for all your inputs - really appreciate it.

As the plate heatexchanger was replaced in Jan it is possible but unlikely to be that, it is not the PCB , it might be the pump but most likely to be blocked hoses,have a look at this fixed price offer, Viessmann used to do a fixed price offer but not sure if they still do.

https://sse.co.uk/home-services/boiler-repair

I love the idiot's opinion of " hydrogen & oxygen trapped..."
We call that air here.

kidgreen61 said:

" hydrogen & oxygen trapped..."
We call that air here.

Click to expand...

Explosive "air" to be precise