Baxi Duotec combi boiler - hot water problem

[Driggly]

I've a problem with hot water supply from my combi boiler as above.

When a hot water tap is opened, water starts to flow, the boiler fires up and the boiler temp display goes up slowly to around 60 degrees, all as usual, and the water gets warm. Then, the boiler temp starts to rise very fast, there's a rattling noise from the boiler and at 95 degrees the burner cuts out. While this is happening the water goes cool again. This cycles for as long as the tap is on.

I've checked the central heating works normally, and the system pressure is normal. There are no error codes.

This might have been caused by recent work on the CH circuit. A couple of weeks ago an old soldered joint to a radiator began to leak. I turned the boiler off and partly drained the system to stop it leaking, then called my plumber to repair the joint. In the day or so before the plumber could attend he suggested regaining hot water by isolating the CH pipework and refilling/re-pressurising the boiler (no CH demand right now!). The first occurrence of this DHW fault was just before the plumber arrived, but once the system was refilled everything worked OK. Then a couple of days ago it happened again, once, and today it is happening every time. Said plumber isn't answering his phone - probably on holiday!

Is there any chance that this problem is user-serviceable? I don't want to lose DHW till the plumber comes back.

 

[powell30]

Usually replacing the cartridge in the diverter valve sorts it (depends on whether you have integral bypass via plate heat ex or bypass pipe tho) if not you need to change the whole div valve. Could be blocked plate heat ex or the div valve motor head also but my money is on the DV

 

[gazthepottertonengineer]

Sorry i disagree this is always down to crap (Dirt) in system which blocks up the Plate Heat Exchanger ! This cn be cleaned out or ideally replaced! and then clean your system and put a magnaclean or similar in or it will do it again in 6 months

 

[Baxi-Remeha]

plate heat x blocked. replace/thouroughly clean heat x, flush system and fit filter.

 

[Driggly]

I get the crap in system diagnosis - though the system was flushed and cleaned when the boiler was installed 2 years ago, and the water I drained when fixing the leak was clean and clear and smelling of inhibitor (that doesn't mean the heat exchanger wasn't collecting it all up for me).

I think the diverter valve is diverting at least one way because the CH runs fine, and the boiler wouldn't overheat if it was not operating at all, so a blocked heat exchanger could be it, however why was the DHW fine either side of the initial two incidents? I'd expect low flow or low temp as a precursor to a complete block depending on how the temp is regulated.

I'll get it serviced properly I think. Don't see dismantling the heat exchanger as a DIY job, the service manual instructions start with "remove the gas valve"

As a supplementary, its a 33Kw boiler but the hot water flow rate has never been good. Measuring now at the bath taps I get about 18 litres/min from the cold tap and 7.5 from the hot. That's pretty much what I've always had, but I think I should get around 12-15 litres/min from this size boiler. Does the low flow indicate another problem?

 

[gazthepottertonengineer]

The low flow could be caused by the cold water inlet filter being blocked so get that checked! For referance it is possible to change DHW Heat exchanger without removing gas valve but it is a bit tight! and i would recomend getting an engineer in !

 

[Baxi-Remeha]

boiler should produce approx 12ltr/min at 35 degree rise. the flow goes through a flow restrictor inside the inlet valve. however the pipe work from the boiler to tap, and water presure can also have a bearing on this.

the blockage in the plate heat x will be on the primary side, it will be caused by magnatite from the water reacting with the radiators. this disolves in the water then solidifies in the main heat x when the water gets hot. this then flakes off due to the heat x being stainless steel. it then travels round and gets stuck in the plate, which in turn slows the primary circulation down through the boiler so the boiler cant dispurse the heat properly and it over heats.

cleaning a plate out would not normaly be covered by a service, but is only a 15 min job to replace.

hope this helps