Potterton combi 80 kettling and excessive venting

Hi, I've got a Potterton combi 80 which is playing up at the moment.

Central heating is fine, but it's starting to lockout on the DHW side of things, ok when the water is turned on but when taps closed, it kettles? (lots of banging) and vents a lot of steam from the auto vent and then locks out.

It's been getting progressively worse over the last few weeks.

I've cleaned the flow switch which has a habit of sticking up with scale (I'm in London so pretty hard water here) it was serviced just less than a year ago by BG (that was fun) and they replaced the diverter and the pump, I've also replaced the diverter again about a month ago.

In my limited knowledge I'm thinking it's scale in the heat exchanger on the DHW side, am I right in thinking this is an engineer job as it's part of the gas train and involves removing the burner?

I have a local Gas Safe chap coming to take a look in the next few days but wanted to get your opinions as I've never used him before and you never know!

Thanks in advance.

Will.

Your symptoms are classic case of a sticking on DHW demand switch!

I really dont understand why you have replaced the diverter valve. Unless your boiler is very badly sludged up there is no reason to replace it after just a year!

The design of the heat exchanger on that model does not cause it to get scaled up under any norman conditions.

London water may not be soft but is not significantly hard to cause problems with properly set up boilers.

Tony

Hi Tony thanks for the reply, I said my knowledge was limited! you're right it was sticking again, I'd cleaned some limescale of the pin a couple of weeks ago and it moved ok again but I think the rubber channel it moves in has swollen as it was really hard to move it when I just checked. Presumably worse when it expands when in situ.

New switch on the way!

Regarding the diverters it's always chewed through them approx one every year, they gradually start failing by the rads getting warm when you run the HW to the point where they stick open, no idea why as there is no appreciable sludge in the system and it's always had inhibitor in it, regularly topped up/flushed, various engineers have said they are prone to failure so I assumed that to be the case.

Many thanks for your help

I have never encountered that problem with a diverter valve on that boiler. I suspect another issue was at work if water really was clean. Even if the water looks clear there can be deposits formed on components.

The operating pin on the DHW diaphragm unit needs to be lubricated with silicone grease before irs put back in place. The operating gap of about 0.7mm needs to be set fairly accurately too.

Tony

I'd used a silicone spray on it but it was really grabbing it when I just checked although more spray has freed it up again hopefully ok until the new one arrives, I've never changed it in the 9 years we've lived here, just cleaned it a couple of times when it started playing up so probably ion it's last legs and well overdue due for a replacement.

Interesting about the diverter though I'll mention it to the engineer next time we get it serviced.

Best regards

Will.