Grant Combi 90/70 overheating

[Special]

Hello
I have an original (may be Mark1) Grant Combi 90 oil fired boiler that is overheating my hot water and central heating.
Recently I have changed the Auto bleed valve, the tank filler loop non return valve, the vessel and the thermostat.
Just to repeat I have changed the control thermostat.

The boiler, to my surprise, has two thermocouples in the recess in the tank at the rear of the boiler.
One goes to the control thermostat at the front of the control panel.
the second goes to a "valve" at the top of the boiler with an adjuster.

However my question is why is the water too hot and uncontrollable.

is it to do with the valve thing?

 

[simb82]

Primary store stat changed?
Hidden behind panel, try adjusting it down slightly with a small screw driver. It'll be painted in a fixed position.

expansion vessel fexi hose blocked?

 

[Special]

Thanks for the quick reply.

Primary store thermostat, if we are talking about the same thing it is connected to a 15mm pipe on one end only and has an adjuster on the top that I am fearful of turning because everything else starts to leak as soon as I do.

The adjuster is conical and red ?

The expansion vessel has been changed due to pressure being out of control and is now good so I assume the tube must be good.

 

[oilhead]

The primary store stat is located in the control panel and has a capillary leading to the heatstore on the right hand side behindthe diverter valve and DHW heat exchanger. This overides the control thermostat.

 

[Special]

I've tried turning down the conical adjuster that has the thermocouple attached to it only to fin it is closed.
Opening it slightly and i hear water start to run, so I have closed it again

Anyone any ideas, the boiler is still producing very hot water that the control thermostat can not seem to control

 

[Terrywookfit]

Sounds like you're turning the prv.
Does the red conical thingy have 2.5 marked on it?

 

[oilhead]

Simb82 and I have suggested what the problem may be. Look at the heatstore and follow the capillary into the control panel. If you do not understand what we are saying, then leave alone and consult a professional.

 

[Special]

sorry for late reply

Terrywookfit thanks, i cant see the 2.5 but difficult to access as previous owner has built boiler into the kitchen units/worktop. I think you are correct though. 2.5 would be the psi release pressure i assume?

oilhead ; thank you but the thermocouple capillary doesn't go to the control panel and goes behind the panel to the aforementioned valve.

I read some info on the Mk2 grant combi and it says it supplies water from 75deg to 85 deg which seems bonkers to me. Could it be that this boiler will not supply just warm water?

thanks again

 

[oilhead]

All the grants supply DHW via primary water from a heatstore controlled by a fixed thermostat. The capillary that comes from the pressure release valve is the feed to the pressure gauge. You need to open the control panel and on a Mk1 there is a wiring diagram which will show you how it works. With regard to your hot water supply, you can only get warm water if you 'overload' the heat exchanger and look to exceed the design flow rate, but this is a crude way. If you need to limit an outlet, then consider fitting a blending valve at that outlet.

 

[Special]

Thanks Oilhead I will have a look for the diagram, maybe I've done something wrong tracing the capilaries.

Are you saying (i'm new to oil fired) that the water will always be very hot from a standard grant boler

 

[Andy4444]

If it's a Combi 90 Mark II then it should have a blending valve.

Is your problem with hot water from the taps being too hot?

Have you tried adjusting the temperature of the blending valve? Try turning it down. On Grant Vortex's the valve is factory set at 50c.

I had one recently that had a blocked cold water inlet filter so no amount of adjustment cooled the hot water temperature.

 

[Special]

Hi there awowen, I think your on to it. I have had look at the diagram fro the Mk2 and although mine is a mark one I think it is very similar.

My wife has medical appointment this week but will get back to boiler asap and check.

I can't help but think you have the answer though