Hi. Here is a challenge for you to think about - just shows you the sort of mess you can into with plumbing by what looks like a simple omission - clue there if I am right. I have arrived at a friends , in very cold Scotland, - they are away and tried to turn on the central heating. Managed to turn the boiler, installed in March 2016, on and the pipes around the boiler started getting hot so thought I had cracked it. Hot water was flowing from the hot taps, condensing oil fired boiler, and low and behold the radiators where getting warm / hot but when I checked the stat it wasn't calling for CH yet the rads were on! How? Nothing I did with the stat made any difference so I thought I would look into it. Opened the boiler controls door - the CH pump light was off and nothing I could do with the stat made it come on, yet the rads were now glowing! The CH timer was on and the signal was reaching the boiler, the status of the stat didn't matter.... What is going on here?

System boiler and hot water tank, or combi boiler?

 

Motorised valve manually locked in the open position?
 

Frost stat on the oil boiler.

Some kick in at 7oC and others at ~10oC.

Is it an external boiler?

No electric value I can find, and not called for in the boiler installation notes I have read, combi boiler, and outside air is above 10, even here in the frozen north. I think the problem is a missing check value which is allowing the whole plant to get into a difficult place. My scenario is this.

 

The CH pump is disabled if the internal water volume temperature is below a set point. The boiler works to get this internal volume up to temperature but in doing so it starts a reverse convection around the CH backwards. I took a thermal image of the flow and return and the return was hotter than the flow by more than a few degrees! As the cold water flows back to the boiler from this reverse flow it cools the internal volume and so the boiler keeps trying to warm it up. This gets into a viscous spiral and the rads warm up and heat the house uncontrollably but the return water is always a fraction below the set point so the CH pump is never enabled. Does that make sense - its missing a check valve to prevent reverse convection.

 

Mike

I had a similar reverse convection flow problem in a previous house, the moral of that excercise was NEVER fit a combined return pipe!!!!

 

Have you got a model or picture of the insides of the boiler?  Is it a Grant by any chance?  I wire quite a few Grant boilers that my friendly plumber installs, and we had a problem with one that just kept on running. We called Grant in the end and it was a known fault with that model (it was a new model at the time)  the flow sensor switch had a sliding shuttle inside it, and at one end of it's travel a pip of plastic stuck out of the end.  the "fix" was file a bit off so it no longer stuck out of the end of the fitting.

 

The obvious question has to be has it ever worked properly since being installed?

 

Yes its a grant - and no it has never worked correctly as I understand it, we were here in the summer, just after installation, and although I took a brief look at it their plumber was coming to have a look so left it to him, sadly he never showed. Now we are back, on our own, and right now the return is hotter than the flow! Here is a pic of the beast:

 

Your picture is not showing.

 

If it has never worked properly, then it's plumbed wrong, wired wrong, or it may have that fault I had that I mentioned in which case you need to talk to Grant.

 

Why did the plumbtrician not fix the problem when he installed it?  No 1 rule, don't pay him until it is working, that will keep his attention to get him back. Now he seems disinterested.
 

@MikeSharp01

Any chance of that pic so we can round this off ? Curiosity is getting the better of me too   

cheers. 

Sorry Guys, driving home yesterday so not able to do any posting... Here is boiler, but I also have a thermal image showing return pipe hotter than flow! I am reasonably sure our friends have asked the plumber back but no luck yet, anyway they are home this evening and will get onto him in the morning.