central heating only comes on with h water, stumped engineer

[bonkersbill]

All but 1 rad are wide open on both sides, i will open this to try but it is under my plasma tv, i dont know where the switch/jumper is to change it, there is nothing i can see on the front.

 

[algas]

Take prog off its backplate on back should be switch or jumper marked P or G wants to be P sounds like pump not kicking in when boiler fires for c/h

 

[bonkersbill]

Took programer off and found no switch or anything marked with p or g or anything else for that matter. Ran CH with all rads fully open, pump does come on but boiler still turns off after about 30 seconds, i have noticed that you can hear the water boiling before it shuts down and it does not do this with HW on aswell. I also turned up the Knob on the fire to full when CH and HW on together and this does not make the pipes from the boiler rattle like it does with CH only. This has to be a circulation problem when HW is not selected, i am going to flush the rads and change the 3 port valve just to rule these out, though i dont think it is the valve.

 

[MANDATE]

Hi Bill! So lowering the resistance of the radiators made no difference.
I would have thought at least the time period of 30secs would have increased.
I know we have almost ruled out the valve, but can I take you back to the position where you turned the spindle 180 degrees.
Can you confirm the result when with the head off the spindle and set to Ch
and you turned spindle 180 degrees.
I got the impression that the heating system then worked ok, suggesting the only thing preventing it working correctly was the spindle position relative to the ports.
Just trying to think what it proves or disproves if the system can be run like this for a few minutes.

 

[bonkersbill]

Position CH 180 degrees lets hot water rush through the CH which reaches 1st rad, the boiler does not start up and the grundfoss got too hot to touch, i have noticed it occasionally get hot but this was red hot.

 

[jackthom]

Bill, I got the impression earlier that the radiators all get nice and hot when both heating and hot water have been on for a while, is that correct ?

If so this makes me think the path through them can't be all that restricted, otherwise the heated water would simply take the path of least resistance circulating around those 28mm pipes and through the hot water tank coil.

 

[MANDATE]

To recap then.
With head off spindle and head set to CH only.
Spindle set also to CH only, boiler lights but cuts out after 30secs.
With spindle turned full 180 degrees, boiler still lights but cuts out after 30 secs.

Other than the above there is no problem when both HW and CH are set and both are in demand. What happens then when HW becomes satisfied? The valve is then supposed to move to CH only. Do I assume the problem also happens in this situation?

Lowering the resistance at radiators shows no change.
As the system functions when HW is also in demand, the difference is what?
The HW port is now closed and CH port fully open (or supposed to be)
The CH circuit now has to accommodate the total water flow.
OK so we have the narrowing of the pipes.(maybe can't cope with the extra water)
Also there is no by-pass (which is considered to be desirable)
Then there is the pump (which is not capable of dealing with the increased resistance, but can cope providing HW port is open).
From the above I think firstly examination of pump is easiest to deal with and costs nothing unless new pump is required.
Leaving 'no by-pass valve' and 'narrowing of pipes'
Do you think that's a fair description of the problem as we know it?

 

[bonkersbill]

Hi, HW and CH on system works perfect, rads get hot reasonably quick.

CH only, boiler cuts out after 30 secs to 1 minute,
CH only, spindle at 180 degrees from CH pos, latest test had boiler running for 8 minutes before shut down with thermo turned up to full, red hot water went through the valve but only slightly warmed 1st rad, i did not hear the water boiling at the boiler before shut down as i do when CH in normal pos is selected.
CH on, valve set to 180 deg from mid warms rads slowly, same when HW is also selected.
When HW is on, then turned off, CH still on, boiler shuts down 30 secs later.

i'm not absolutely sure about the bypass as i did not get a full description as to where i would find it, from where and to does it go?

Other than that your description Mandate appears to be correct, how can we check the pump to rule this out?

 

[Eyebrowes80]

CHeck the resistance of the pump with a multimeter. Turn power off disconnect the wires and put your probes on the live and neutral pins see what the ohms reading is.

 

[MANDATE]

Hi, (1) HW and CH on system works perfect, rads get hot reasonably quick.

(2)When HW is on, then turned off, CH still on, boiler shuts down 30 secs later.

I'm a bit puzzled with statement (1)"system works perfect"
When HW and CH are on the valve takes up mid position so both systems get share of water, But it can't stay like that for long. As soon as HW becomes satisfied the valve is then moved to CH only position which is still in demand. So with your problem, the problem should still be there when this occurs.
This would be the same as statement (2)

Regarding the by pass. If present it would be a pipe running from the "flow" to the "return" with a valve fitted which could adjusted to control the flow across. The purpose being to prevent water reaching temperature to operate boiler stat. It would lower the resistance(if adjusted correctly), very much like opening the HW port.
My thinking regarding the pump, is that the inlet and outlet orifaces may be reduced in size making it less efficient and considering it costs nothing to remove it for examination. (Make sure you have new washers to hand)

 

[bonkersbill]

Hi Mandate when both on works until you turn off hot water or when its satisfied it turns HW of then boiler shuts down, i'm pretty sure there is no by pass. i will look at the pump tommorrow

 

[stardanny]

About these 28mm pipes, did it used to be gravity primary with pumped heating that been changed to fully pumped?

Is the radiators return pipe goes straight to boiler or via 28mm return?

These back boiler use injector tee fitted on gravity return with heating return tee in to help with gravity moving when heating is on.

Maybe the injector is nearly blocked?

Just a thought....

Dan.

 

[bonkersbill]

Hi Dan, i could not tell you what the system may have had before as this was fitted when i moved in, there are no Ts into the 28mm upstairs, the 2 15mm pipes from upstairs rads go down the same conduit as 28mm pipes to the down stairs rads, i think i will soon have to get someone to service my boiler to see if theres any thing wrong there but i'm still getting suggestions to look elsewhere, the problem is the 4 engineers i've had are all stumped and have not been prepared to say whats wrong before ripping my house apart, for now i will check all accesable options 1st. Thanks for your input as this may triggure a new line of tests.

 

[MANDATE]

Hi Bill, Are you any nearer to resolving your problem?

 

[stardanny]

Hi, Bill.

A photo of pipework and cylinder in airing cupboard would be nice. I might tell if it used to be gravity hot water / pumped heating. Also a photo of pipework where it goes through wall to boiler, outside at side of chimney breast.

Dan.