kingfisher MF RSL 50 lockout

[Danny Jones]

Hi All,
I have potterton kingfisher MF RSL 50 freestanding boiler. The boiler operates normally for about 20 minutes and then the lockout (RED neon) comes on. The pump is still running when the lockout happens. You have to wait for about 10-12 minutes before pressing the reset button, then the green light comes on; flame is re-ignited and the boiler works fine for the next 15-20 minutes. I have gas engineer change both the thermistor (located on heat exchange) and thermostat (on the flue pipe). The problem still persists. Is it due to faulty PCB? or some other problem. As mentioned I have called the gas engineer twice. One advised me to change the central heating pump. The other told me that the heater fan inside the boiler might be faulty: fan seems to be working fine until the lock out happens.

 

[stem]

Looks like the boiler is overheating, which may be as much to do with the system it is connected to than just the boiler. It suggests to me that maybe the hot water it generates is not getting away from the boiler fast enough. Has anything changed recently? TRV's fitted to radiators. Has the balancing of the system been changed? Has any manual by-pass valve been adjusted? Have you recently turned the thermostat up to a setting higher than normal? Is the system sludged up preventing water circulation?

Or perhaps the boiler's thermostat is not switching the burner off when it should. When the boiler is operating does the burner cut in and out, or does it run continually until the lockout comes on?

Try running it with the boiler thermostat on the lowest setting, does it still lockout now?

 

[Danny Jones]

The boiler cuts in and out when operating (green light is on). I tried running the boiler thermostat on lowest setting, it still locks out. Is it to do with central heating pump (circulation). Problem persists both with Central heating as well as Hot water. TRVs not fitted on radiators. All the radiators have been bled and no air in them. Is it due to central heating pump gone weak? Or is it the PCB?

 

[Agile]

A simple test would be measure the flow and return temperatures on the boiler pipes.

Should be a difference of about 11 C on that model.

The thermistor element is placed into a recess which should be filled with thermal grease. Very few engineers seem to carry that and use it when required.

Tony

 

[stem]

Problem persists both with Central heating as well as Hot water.

Yours is quite an older boiler so it may have gravity hot water (circulated naturally instead of via the pump) If you do, then the pump won't be operating when 'hot water only' is selected anyway.

Or is it the PCB?

Can't say that I have ever known a PCB failure cause this fault. But my experience with this particular model of boiler is zero.

There are sensors that are connected to the PCB to control various functions such as the firing of the boiler, or triggering the overheat, I would suspect them first, but Agile's test should confirm if the boiler is actually overheating or not, or if it's a false alarm.

 

[Steelmasons]

The thermistor element is placed into a recess which should be filled with thermal grease. Very few engineers seem to carry that and use it when required.

Tony

Edit..

 

[Agile]

Edit..

???

 

[Steelmasons]

???

Tony , I was going to give you the heads up about the thread in the CC regarding the heat sink paste.
Up to now the poll has revealed that @ least 90% of gas engineers use heat sink compound....the statistics seem an awful lot different to yours.

 

[Agile]

That is not a poll but a request for comments. By its nature it will attract comments from those who use thermal paste. Out of all the 1000 registered GEs on that site there are only about 12 who have replied to say they have it.

But those 1000 there have been described by trainers at Vokera etc. to be much more knowledgeable than the average GS!

 

[Steelmasons]

But those 1000 there have been described by trainers at Vokera etc. to be much more knowledgeable than the average GS!

I know a few gas installers who have never heard of DIYnot let alone the Combustion Chamber , these guys would wipe the floor with most of the CC members , you don't even come close to their knowledge regarding this industry Tony.
What the hell has Vokera got to do with it???
In saying that I hold VC (David) in high regard.

 

[Danny Jones]

Hi Tony,
Heat sink paste has been used on the thermistor ( located on heat exchange unit). I checked the flow and return pipes temperature difference which is more than 11 C. The hot water tank and pump are located on the top floor wherease the boiler itself is freestanding on the ground floor. Pump runs when the boiler is on irrespective of it being CH or HW. When the boiler locks out the pump is still running (previous gas engineer said that it is running to dissipate/remove the heat from the boiler). Is it that the central heating pump has gone weak or something else.

 

[Agile]

Hi Tony,

Heat sink paste has been used on the thermistor ( located on heat exchange unit). I checked the flow and return pipes temperature difference which is more than 11 C.

That is not very scientific!

What is the "more than 11 C "?

What did you use to measure it?

What is the flow temperature from the boiler when it lockouts?

Tony

 

[Danny Jones]

Temperature was measured using a digital thermometer. Temperature difference just after lockout between the feed and return pipes was 18 degrees C. Flow temperature was 78 degrees C.

 

[Agile]

I don't know why you are measuring the flow and return after the boiler has locked out and the burner is off.

If it locks out at 78 C then that is too low for the overheat condition.

Your boiler is designed for a differential of about 11 C and if it is 18 C then that indicates a reduced flow rate and the pump and or balancing need checking.

What kind of digital thermometer! Infra red or K type thermocouple?

Tony

 

[Danny Jones]

Hi Tony,

Since I changed the pump (new grundfos 15-58/60), when I turn the control knob of boiler clockwise to higher setting and just operate the Central Heating and not the Hot Water, boiler does not lock out anymore. However when I put on both ( CH and HW ), the boiler locks out after 30 minutes or so; radiators are not warm enough neither is hot water (only luke warm water coming from taps). When I put only Hot water ON, boiler again locks out after 30 minutes. Called another engineer today who told me that I have sludge in the system (there goes £85 + VAT on call out charges). Now he has advised me to do a Powerflush of the system (Is it true?). He, as you said, did check the flow and return pipes temperature and said that flow pipe temperature is lot higher and boiler is overheating. It is not circulating hot water properly. Question is - mine is an open vent system (tank in the loft apart from hot water cylinder). Is powerflush good enough? Will it solve the problem?