Greenstar Heatslave Oil Boiler Overheating and Pump Running 24/7

[scottwhittaker]

Hi

We have an old 2006! Greenstar Heatslave internal combi boiler and we've had a few strange things happen recently.

Firstly about 2 weeks ago I heard the radiators ticking at about 6:15 and wondered why the heating was on, I went to check the thermostat, and the thermostat was not calling for heat. It was set to 17 degrees and the house was 20. I went to the boiler, and it was off, but the CH flow pipe was red hot. I checked a couple of radiators and the closest one to the boiler was warm.

I thought it was odd, but the thermostat only has one setting and it’s at 6:00 so I figured it might have been a glitch. However, it normally comes on earlier to get the house to temperature FOR 6:00, not actually start at 6:00 but just thought 'we'll see how it goes'

Then on Wednesday night the local electric board switched off power to the houses for an hour to fix an issue at the transponder or whatever it’s called.
It seems, since then (or VERY close to that time) the boiler hasn’t been working properly.

I noticed on Thursday that the boiler was humming away quietly so figured it had just gone off as it has a water storage tank, so it does come on to keep that warm.
I then noticed it was still doing it on Friday and had a deeper look and found that it was the pump that was humming away. So, I thought it might be stuck.
I removed the bleed screw and found the propeller to be spinning though.
The boiler at this point would not fire up when heating or hot water was requested. It just sat there with the pump humming away.
I turned it off at the switch to have a think, let the pump stop and hopefully reset anything overnight.
I turned it back on in the morning and it was the same. Just humming.

Then I noticed the high limit stat had been triggered. As soon as I pressed this to reset it, the boiler fired up.

However, I checked it later that day and it was back to how it was. Humming with the high limit stat triggered again.
Ive also noticed the pressure goes above 3 bar.

The pump appears to be working fine. The diverter valve and actuator motor are only a year old.

I reset the stat and manually moved the diverter over to heating and the flow pipe got warm very quick so I’m pretty sure the pump is fine and I presume this means the heat exchanger isn't blocked?

I'm starting to lean towards maybe the hot water thermostat but has anyone got anything else they think might be worth checking?

I'm thinking if I manually put the diverter valve to heating and restart the boiler, I would either get the same outcome which would mean its unlikely (just) the hot water thermostat OR the boiler would fire and then turn on and off as per normal which I think would confirm the hot water thermostat?

 

[CountryFan]

Sounds like a potential motorised valve and or switch live issue.

 

[scottwhittaker]

Hi we dont have any zone valves on the pipework, or do you mean within the boiler?

 

[scottwhittaker]

Ive manually put the diverter valve to heating and run the boiler which seems to run fine, flow & return pipes get hot and warm as normal. However the dials on the front panel when turned, they click but they dont shut the boiler off. Normally when I turn the CH dial down it would shut the boiler off until the water temperate dropped but its not doing this. Pretty sure the HW dial was the same as well.

Is there one stat that would respond to these two dials or are they separate. Cant see both having failed together so then possibly PCB board?

 

[CountryFan]

I would imagine both separate. I'm not au fait with this boiler. Could be an ntc at fault. Maybe get an engineer in?

 

[Exedon]

Have you tried following the fault finding flow chart in instalation manual?
That boiler could be old enough to have relays rather than a full pcb board.

 

[scottwhittaker]

Yeah it's not the best fault finding chart. The pcb is part code 87186852610, well that's the newer board but looks identical.

 

[Johntheo5]

Does that boiler have just one circ pump + a diverter valve for CH/DHW or does it have another DHW circ pump?. The heat store on most combi oil boilers is kept hot and will have its own temperature sensor which will call on the boiler to fire up and start the main circ pump, the water is then circulated through the main heat exchanger and through one side of the DHW plate heat exchanger?, don't know if the primary water target temperature is then fixed at a fairly high setting at say 75/80C and then reverts to its CH target temp when demand for DHY ceases. I think there are at least two hi limit stats, one on the DHW store and one in the flue, imagine there is another one on the main heat exchanger. Can yo do without DHW for say 24 hrs or so and see if the same faults are flagged.

 

[scottwhittaker]

Hi I literally found another thread where you were helping someone and seemed to be on the right lines for our issue...

https://www.diynot.com/diy/threads/...boiler-combi-thermostat-cut-out.627133/page-2

Theres one pump and a diverter valve.
Iv let it run on just heating and also just hot water but the dials in the front don't seem to do anything, they click but they don't turn the boiler off. It just runs until the high level stat is triggered but I turn it off before then.

 

[Johntheo5]

Is it the "normal" hi level stat that is triggered and what temperature is it reaching when its approaching its trip temperature?
Schematic below from a Grant Combi with one circ pump + Diverter valve, which I imagine is similar to yours, so if the store is kept preheated wonder what temperature probe? as the store is only primary water and not a DHW store and looks like its just used as well when the CH is on.

Apologies, its not I think as the heating flow and return come from the main heat exchanger

Edit: If not electrical problem maybe worth checking those non return valves for failure/leakage.

 

[scottwhittaker]

Yes its the normal high level stat, not sure on the temperature but its very hot and the pressure is very high, seen it at 3.5 bar and then when it cools its generally lost some pressure. There is a CH and DHW thermostat also. The store is kept preheated but I think its just used for the HW.

 

[Johntheo5]

it will lift the PRV at 3.5bar (3.0bar set pressure) so some water will be lost each time.

I would certainly examine those/any check valves. Could also be a pump problem, what make/model is it?

What happens if you try to run the DHW with a numer ot HW taps opened?.

Can't think of anything more just now.

 

[scottwhittaker]

We'd have hot water at the taps and if I manually move the diverter to heating it warms the radiators. Haven't run it on either for too long as like I say the control knobs aren't doing anything besides clicking so whatever is causing that I feel is responsible for the boiler just running and running and hitting the high level stat.

Pump is a Bosch/Grundfos. 87161082460 is the part code.

 

[Johntheo5]

Just back from delivering neighbours to the Airport.

I notice the Diverter Valve Ports A & B go respectively to DHW & CH, and CH, Port B, is the default position, see 2 below, if so, why do you have to.... "manually move the diverter to heating it warms the radiators" ?

 

[scottwhittaker]

I think A is default (DHW)?
I manually moved it to B and locked it in place to test the overheat issue on CH.

The boiler is switched off currently at the spur as iv reset the overheat stat so if I didn't turn it off the following happens...

As it been off say all night, if I turn it on at the spur, the boiler by default notices the water tank inside it is cold and starts to heat the water. Therefore the thermostat must be ok?

It never stops until it hits the high limit at which point it triggers the stat and then the pump keeps running to dissipate the heat. It just sits there humming away for days. It's like it's not being told that the heat has gone and therefore the pump can stop.

I moved it to B (CH) to test the high limit stat and the control dials which I've found don't turn the boiler off when they're turned down as they normally would.

I'm feeling it's not limited to the CH or DHW side.