Ariston storage combi overheats on DHW only (with pictures)

[petewoods]

My boiler is an Ariston Genus storage gas combi (will find exact model when I get home tonight).

Just talking about it being set on DHW to begin with.

Originally the gas burner would not stay lit for more than 5-10 seconds, so the thing had been set on a really high temperature for it to get hot at all. I believe that it was going off the electic part of the unit to keep the water hot and running it all the time! Which is why my electricity bill has been so insane. I think it's been like this for quite a while.

The plumber that came round figured out that the oxygen sensor in the burner was broken, so that was replaced and now the burner works ( ).

So now if I leave the unit on central heating only, things are all fine. However as soon as I put it on DHW, things aren't so good.

Depending on the temperature setting the burner either stays more or less constantly on or completely off. This results in it rapidly overheating or cold water respectively.

I've had the thermostat in the small hot water tank replaced - and every other sensor in the thing (including the overheat sensor).

I think the plumber has run out of ideas. Does anyone have any here? Thanks for any advice!

 

[Agile]

I have never heard of an oxygen sensor in an Ariston boiler!

Nor have I encountered a stoprage combi with an electric immersion heater element.

Always a first time though!

Tony

 

[petewoods]

Hmm. It may have been called a pressure sensor then?

It may also not have been an electric element, but the boiler seemed able to keep water hot without the burner coming on.

Obviously I am far from an expert about boilers, but do have a good understanding of mechanics.

 

[petewoods]

I take it nobody has any ideas then?

Could it be a broken logic board?

Could the flow through the heat exchanger be blocked?

 

[petewoods]

Okay, given that the guy who tried to repair the boiler was replacing the thermostat inside the tank (but that hadn't helped things) I figured I'd check to see if the connectors on it were corroded or anything. I sanded them down a bit and now the thermostat seems to be more sensible. i.e. it doesn't always think that it's at the same temperature now.

What happens now is that the overheat comes on even when the tank is cold. This happens within 10 seconds of switching to DHW from CH.

It'll stay on happy without overheating on CH but goes off very quickly on DHW.

Here's a picture:

Untitled

• petewoods
• 5 Apr 2008
• 1

If I feel the temperature at the point where the arrow is with it on CH only it's quite warm, but not unbearably so. As soon as I switch to DHW it gets really hot an then the overheat comes on.

This suggests to me that maybe the water coming out of the heat exchanger isn't able to flow properly when it's set to DHW. A blockage somewhere?