Mistral Oil Combi High Limit Stat Tripping

[Skids]

Mistral Oil Combi about 3 years old High Limit Stat tripping
about every 3 days when the heating (under floor) is being used.

Pump on position2
Boiler pressure 0.5 to 1.5 bar

Parts replaced so far:-

Heating stat
High limit stat (just in case!)

There’s no differential bypass valve fitted. Could this be the problem?

 

[Boilerman2]

Try turning down the Centrtal heating stat a bit, are you sure it's not overheating while in hot water mode??

 

[Skids]

Central heating stat on the lowest setting.
On a very odd occasion it has tripped out on hot water only. But it trips on a regular basis on heating and water.

 

[oilhead]

What output is the boiler? They come factory set mid range. Try reducing output. The high limit trips because the boiler is not clearing the water it is heating. UFH can take very little when in recirc mode. Alternatively try fitting a pump overrun stat

 

[Skids]

Thanks oilhead for your reply.


The Boiler output is rated at 70 to 90 BTU/hr x 1000. You suggest reducing the output, how is this done ?

There is an integral pump overrun stat fitted which seems to be working ok.

As mentioned in my first post, there’s no differential bypass valve fitted to the system. As you say the UFH takes very little hot water on recirc so presumably if there is only one UFH loop open the boiler cannot dissipate enough heat, therefore it overheats and the high limit stat trips. Could my theory be correct ?

 

[oilhead]

If your UFH loop is in re-circ, then does the boiler pump to a dead head? If so, then a by-pass would help.

 

[Skids]

Ok oilhead, I think I need to fit a bypass.

Thanks for your help.