Grant Combi MkII - Overheating after using hot water

[6666nick]

Hi All ,
My first post so please be gentle!.

I have a Grant Combi mkII which has been giving me an over heating problem after using hot water recently , prime example is i have a shower in the morning before work hot water is fine , by the time i've dressed and come down stairs the boiler has gone in to 'Overheat' , eventually once the boiler has cooled i can use the reset and the boiler will work fine , this does not happen every time the hot water is used.

This has been happening for few weeks now and on the advice of 'a mate' i changed the expansion vessil which i thought had cured it as it had not over heated until this morning when it did it again after i had a shower.

Any suggestions please?? - i'm thinking either partially blocked heat exchanger or faulty overheat stat maybe ??

Thanks.

 

[Agile]

I expect that your boiler has a pump over run facility.

Can you check for that in the book and see if its actually operating?

Tony

 

[dcawkwell]

Blocked plate heat exchanger or piping.
Boiler will overheat when supplying hot water for a longer period.

 

[6666nick]

Thanks for the replys , the manual states the following in method of opperation re. HW
"When the hot tap is closed and the diverter valve flow
switches sense that hot water is no longer required, if
the timer switch is set to either TIMED or
CONSTANT, the boiler will return to the central
heating mode, with the pump and burner running. If
the switch is set to HOT WATER ONLY, the pump is
turned off but the burner will continue to run for a
short period until the water in the boiler reaches the
required temperature ready for another hot water
operation."
Based on this it seems there is no run on of the pump.

Re. the possible blocked heat exchanger i presume its literally a case of removing it and flushing it through with something??

 

[Agile]

Usually a blocked plate would be obvious because of poor DHW performance.

Can you measure the resistance of the temp sensors?

Can you measure the water in and out temperatures when set for a flow rate equalling the boiler spec?

Tony

 

[6666nick]

Certainly no performance issues with the DHW , its a constant good temperture when in use , its purely after it has been used that the problem arises , did it again this morning after my shower.

Can you offer some guidance on how to check the resistance of the sensors please , i do have a multimeter with Ohm's setting.

Also not quite sure what you mean by "Can you measure the water in and out temperatures when set for a flow rate equalling the boiler spec?".
In and out of where? , i have a temperature meter with pipe clamp if required.

Is it possible that the water/boiler temp set point is too high? is it adjustable? , i know the heating temp is adjustable via the panel front control but obviously that won't effect the DHW , also there is a TMV/Blending valve on the DHW within the boiler which is adjustable but i'm thinking that will only alter the water temp leaving the boiler, just thinking if there was an adjustment for the actual boiler temp perhaps its too high and too close to the trigger point of the over temp sensor?.

Sorry i'm not a boiler tech so please forgive me if i ask obvious questions.

Really appreciate the help

 

[dcawkwell]

Certainly no performance issues with the DHW , its a constant good temperture when in use , its purely after it has been used that the problem arises , did it again this morning after my shower.

Can you offer some guidance on how to check the resistance of the sensors please , i do have a multimeter with Ohm's setting.

Also not quite sure what you mean by "Can you measure the water in and out temperatures when set for a flow rate equalling the boiler spec?".
In and out of where? , i have a temperature meter with pipe clamp if required.

Is it possible that the water/boiler temp set point is too high? is it adjustable? , i know the heating temp is adjustable via the panel front control but obviously that won't effect the DHW , also there is a TMV/Blending valve on the DHW within the boiler which is adjustable but i'm thinking that will only alter the water temp leaving the boiler, just thinking if there was an adjustment for the actual boiler temp perhaps its too high and too close to the trigger point of the over temp sensor?.

Sorry i'm not a boiler tech so please forgive me if i ask obvious questions.

Really appreciate the help

Been here done this.
When the piping and plate heat exchanger are in good order
they will drag more heat out of the boiler than it can supply.
However when there are problems the boiler will overheat
as the store doesn't get to temperature before the main heat exchanger
overheats.

 

[6666nick]

Any advice on how to check the heat exchanger and clean it out? , i presume it needs to be removed and then flushed with some kind of cleaner ???

 

[oilhead]

Your boiler does not have thermistors, Tony is not very 'au fait' with oil boilers.
Make sure the thermostat phials are fully in the pocket on the heatstore. Possible that the DHW thermostat is out of calibration. Its set temperarture is very close to the limit. If you have a flow out of the hot tap approaching that out of your cold tap, then I doubt you have heat exchanger problems.

 

[6666nick]

Regarding the phials - heres a thing that i've always thought is odd , there is a phial sat exposed to the air in a sprung clip just above the expansion vessil , is this correct ? what is it sensing as all i can see it is sensing is the ambient air temperture inside the boiler door. ( is it perhaps regarding frost protection in some way? )

 

[Terrywookfit]

That will be the Fire valve phial.

 

[Boilerman2]

Phone Grant UK and ask them for details of a local approved service Engineer in your locallity

The MkII combi has a boiler Control Stat and a Store Stat BOTH of whcih should be set to the required temperature using an electronic thermometer, the boiler stat should stop the boiler at 82'c +/- 2'c (measured at the top stat pocket)

When running a hot water tap, the Store stat should prevent the burner firing untill the store has dropped to 75'c +/- 2'c (measured at the bottom stat pocket)

it is a little tricky to get the settings exactly right.
the other issue is the burner firing rate assuming you have a Combi 90 the Nozzle should be a 0.75-60'EH at a Fuel pressure of 9.5 Bar

 

[Agile]

Yes, oil boilers do seem to lag technology somewhat.

But most of the latest seem to have thermistors!

Most of these o/h problems need contact thermomenters to monitor the operating temperatures.

 

[dcawkwell]

Yes, oil boilers do seem to lag technology somewhat.

But most of the latest seem to have thermistors!

Most of these o/h problems need contact thermomenters to monitor the operating temperatures.

No oil boilers had been leading the way.
Oil boilers have been using electronic ignition and efficiencies in the 80s
while gas boilers where still using pilot lights and thermocouples and had efficiencies in the 60s. Archie kid oil condensing boiler design dates back
to 1959.

 

[6666nick]

Phone Grant UK and ask them for details of a local approved service Engineer in your locallity

The MkII combi has a boiler Control Stat and a Store Stat BOTH of whcih should be set to the required temperature using an electronic thermometer, the boiler stat should stop the boiler at 82'c +/- 2'c (measured at the top stat pocket)

When running a hot water tap, the Store stat should prevent the burner firing untill the store has dropped to 75'c +/- 2'c (measured at the bottom stat pocket)

it is a little tricky to get the settings exactly right.
the other issue is the burner firing rate assuming you have a Combi 90 the Nozzle should be a 0.75-60'EH at a Fuel pressure of 9.5 Bar

Boilerman2 - I have an electronic thermometer with various types of probes etc that i can borrow from work as i'm an electrical engineer at a hospital , i presume from your comments these temp set points are adjustable?? can you advise where ? , do i presume its a 'mechanical' adjustment on a stat body rather than an electronic adjustment on a control board??