Grant Combi MkII - Overheating after using hot water

[Agile]

Yes, It often helps to use a little thermal paste ( or silicone grease ) to ensure the test probe is reading the actual temperature.

 

[Boilerman2]

Yuo will need a Wire thermocouple for the thermometer so that you can push it into the 2 pockets (where the Thermostat phials go)

The Boiler stat should stop at 82'c (adjusted by the knob on the boiler (control stat)) the store stat should delay firing of the burner on Hotwater only operation untill the the bottom pocket has reached 75'c the store stat is hidden inside the boiler control panel, you have to undow a couple of crosshead screws to release the stat, and use a fine bladed screwdriver to adfjust as necessary.
As I said this can be a very fiddly and frustrating task to get right,
after setting the boiler control to 82'c leave the boiler to settle for 10-15 mins -the boiler temperature will have reached 90-100'c by then (residual heat) then open the hot tal and watch the temperature at the bottom pocket start to fall, when it reaches 75'c the burner should fire up.

 

[6666nick]

Yuo will need a Wire thermocouple for the thermometer so that you can push it into the 2 pockets (where the Thermostat phials go)

The Boiler stat should stop at 82'c (adjusted by the knob on the boiler (control stat)) the store stat should delay firing of the burner on Hotwater only operation untill the the bottom pocket has reached 75'c the store stat is hidden inside the boiler control panel, you have to undow a couple of crosshead screws to release the stat, and use a fine bladed screwdriver to adfjust as necessary.
As I said this can be a very fiddly and frustrating task to get right,
after setting the boiler control to 82'c leave the boiler to settle for 10-15 mins -the boiler temperature will have reached 90-100'c by then (residual heat) then open the hot tal and watch the temperature at the bottom pocket start to fall, when it reaches 75'c the burner should fire up.

Boilerman2 - Thanks for the info , i'll give this a go tonight and see how i get on , i'll let you know how it goes!

 

[6666nick]

Boilerman2 - Just re-reading your last post , silly question but where is the control stat? its not the user adjustable one on the front panel is it? (i thought this was just to adjust the heating temperture) thats what the label says). - thanks

 

[6666nick]

Thanks for your advice , i checked the temps as you suggested and found the DHW stat was calling for the burner to fire when the temp on the lower pocket was around 84C, i found the DHW stat and adjusted it down , it now calls for the burner at 75-76C (tried it a few times and the temp varies but its normally in that bracket) the burner will continue to fire until the temp in the lower pocket hits around 87C , leaving the DHW running and the burner comes back it again at a lower pocket temp around 75-76C.

I've now run the procedure several times , i've removed the HW demand and i'm now measuring the temp in the upper pocket , after approx 20mins its sitting around 105C , would you say this is approx correct?.

I see the Overtemp phial also reads off this upper pocket , do you know what the over temp trigger point is??

Thanks
Nick

 

[Boilerman2]

Well done! you have got the store stat sussed now you need to adjust the dial stat on the boiler panel (the one that you say is the "heating control") to stop the burner at 82-84'c this will give a final temprature of no more than 100'c. (105'c is a bit too high, as the overheat stat will operate at 112'c) get this last issue sorted and all should be tickedy boo!

 

[markgowdy]

i found the DHW stat and adjusted it down , it now calls for the burner at 75-76C

Hi, I believe I am having a similar problem. The burner fires immediately when the hot water tap is turned on (I suspect that shouldn't happen). And it overheats when the shower has been running for about 10 minutes (maybe less).

I can't seem to find where the Store Thermostat adjustment controller so I can turn it down a little. I can see the wire running from the lower phial, up into the control box area (just below the power block area).

I tried to access that (removed the 2 screws at each side, and one on the bottom), but it was almost impossible to see what was going on in there. Am I looking in the right place?

Thanks,
Mark

UPDATE 1: Found this picture online, I presume this is what I am looking for. Primary Store Thermostat

UPDATE 2: Found it. It is in the centre of that control box area, held on by two screws accessed from the front panel (between the mode switch and the pressure gauge). The adjuster is hidden, so it has to be taken out to adjust it. I adjusted it down, and the boiler no longer fires up immediately when the hot water is turned on. It comes on a minute or 2 later. I don't have a way to measure the temperature in the lower part of the boiler, so gotta do this by guesswork (i.e. if it overheats again, I will turn it down a little further).

UPDATE 3: Success !! Had a nice long warm shower this morning, and when I checked the boiler afterwards it looked happy. The pressure was nice and stable.