Central Heating Question - Why wont it work?

[LondonStone]

Hi all,
I have a problem with my central heating system and would be grateful if anyone could tell me what might cause it.

Let me describe the problem and my system in as much detail as I can.

Basically when I turn on my central heating the motorized valve opens and the pump starts and the boiler fires up. But then after a minute or so, it all stops. Then it wont restart again until I leave it for about 30 minutes.

My hot water is working perfectly, and if I open the CH motorized valve manually while the hot water is circulating, every radiator in the house warms up as it should. So there doesn't seem to be a problem with the flow or the pump.

I have a sealed system, with a Potterton gas boiler.
There are two Honeywell motorized valves, one for the hot water, one for the central heating. Both of these are fully working. (a plumber replaced the motors in both earlier this year)
The system is driven by a Honeywell pump, which is set on number 2 speed.
It runs 6 radiators upstairs and 6 radiators downstairs.
I have bled all the radiators, and did discover a lot of air in one, probably caused by some recent decorating work.
I let this air out and re-pressurised the system to 1 bar at the sealed header tank.
I have tried with all the house's radiator valves fully opened and fully closed, but get the same result.
I tested the electrical connection at the downstairs room thermostat and this was working properly.
The electronic timer/control panel for HW and CH seems to be working as it should as does the Potterton boiler.

Any suggestions?
I'd like to have an idea of where the problem lies before I call in a plummer.
I thought I had quite a good understanding of how my central heating system worked, but this has really got me scratching my head.

If you need any more info to narrow down the problem, please let me know.

Thanks in advance.

 

[ChrisR]

When

it all stops.

what exactly is happening?

(For a start what boiler is it?)
Is the boiler fully hot (turn boiler stat - does it click, though some won't)
Could it be the boiler has overheated and switched off on the overheat stat (noisy, kettling sounds?)
Then check the SWitched Live connection to the boiler - is it Live?
Is there a Bypass in the system?

Once in "stopped" mode, immediately after premature shutdown, does the boiler fire up if you call for HW?

 

[LondonStone]

Hi Chris,

When it all stops..... The boiler just halts as usual. There is no need to press the restart switch underneath it. The boiler stat does click.
It stops exactly like the heating reached the temp on the room stat. But this is turned upto max.

The Pump stops and the motorised valve for the CH shuts.
The boiler is fully hot.

It is possible to start the HW immediately after this has happened, both with the central heating valve shut or manually opened. Which makes me wonder if it is overheating, why doesn't the HW overheat too?

When the boiler starts for the CH, it gives exactly the same noises as when it starts for the hot water. A noisy as normal, no unusual noises.

I am at work right now, I'll post the boiler model as soon as the wife gets home and can tell me, and I'll test that live connection later.

I haven't heard of a bypass before, so don't know if I have one or not.

Thanks for the help so far.

 

[Agile]

Its pretty likely that either the CH motor valve is not opening at all or the demand switch is not properly making contact or an electrical connection is high resistance.

One test is to "feel" the lever on the CH motor valve to see if it opens when CH is called for. The next test would be to see if the orange wire out of it is independently live.

It would be better to call a boiler engineer rather than a plumber but before you do so ensure he is competent with fault finding on controls, not all are!

Better still if he offers a No fix-No fee, that weeds out the cowboys!

Tony Glazier

 

[ChrisR]

Odd that the switch in the CH valve should "make" for about a minute then break again.
You do have 2 x 2 port valves not 1 x 3 port?

 

[LondonStone]

The Boiler is a Potterton 80e.
And both valves are 2 port, not 3.

I have 'felt' the lever on the CH valve as it is switched on, and it slowly glides to the right hand side (opened position) Which I guess means that it's working.
Is it possible for the lever to move over there with the valve still being shut? That would certainly explain the problem, but I didn't think it was possible.

If the CH valve is manually opened, and then I turn on the CH, after about one minute the valve closes itself. To me this indicates the CH valve is working properly. But not why it cuts out after 1 min.

I have tested that there is a live feed going into the CH motor valve and there is.

As I mentioned before, the motor for this valve was quite recently replaced. Could that have caused this problem? Wrong motor part? damaged valve? bad wiring?

Could resistance within the motorised valve wiring cause the valve to open for a minute and then shut?

If my memory serves, the wires are connected to the motor with a small terminal block, which is not wrapped in electrical tape. I thought this a bit dangerous for electricity near water!

 

[ChrisR]

So, live on motor but motor not motoring open. Das wrong bro!

Could be a connection inside the valve powerhead breaking when itgets warm. Worth a careful look in there. If you can remove the head and wires, clip a resistance meter on the brown/blue wires and wiggle the connections, you might find it.

By the way the lever on the valve only pushes the valve part way open, NOT enough to operate the switch. If you push it with gusto the momentum takes it to where you can hear the switch click shut, then open as it comes back.

 

[slippyr4]

If my memory serves, the wires are connected to the motor with a small terminal block, which is not wrapped in electrical tape. I thought this a bit dangerous for electricity near water!

Tape would not meet the requirements of the wiring regulations- the connector should be in some kind of enclosure.

 

[LondonStone]

I'm only guessing that the motor isn't motoring because that would explain the problem. Actually it seems to be motoring fine as far as my limited knowledge goes.

It makes a quiet whirring noise when I switch on CH and the lever goes loose.
Then when it switches off it makes a louder whirring noise and the lever gets pushed back to one side and goes stiff. ooh er!

I don't quite understand what you mean about the lever not opening the valve fully. Do you mean if I put it on manually, the valve will not shut properly afterwards? What, even when the valve motor shuts it off?


I'll check the connections properly tonight, but I don't own a resistance meter. It does sound probable that wiring is overheating somewhere, and as this is the only bit of kit that the HW doesn't use, it's either that or something in the rad pipes.
Would it work if I opened up the valve casing, and checked the electric feeds to and from the motor with a neon screwdriver?
Correct me if I'm wrong, but I shouldn't get a live in, unless the CH is on.
If I'm getting this, but the valve isn't opening, then it's a motor problem, right?

Or could it be a blockage in the pipes somewhere? and the boiler or pump switch off as a safety feature? But when the hot water is on too the water can flow freely around the HW pipes and so flow is not restricted.

I'd have thought if the pipes were blocked none of the rads would get hot.
Any easy way of checking?

Sorry for the plethora of questions, and thanks so much for all the help so far.

 

[ChrisR]

I don't quite understand what you mean about the lever not opening the valve fully.

The motor opens the valve, and the spring closes it when the motor has no volts on it. When the valve is fully open , the switch operated to turn the boiler on.

The lever is only there to open the valve for filling etc. It doesn't need to be fully open for that. Sometimes when the motor opens the valve it will unhook the lever from the hooky bit.

Do you mean if I put it on manually, the valve will not shut properly afterwards? What, even when the valve motor shuts it off?

It might, it might stay opn on Man. When new they do that.

Would it work if I opened up the valve casing, and checked the electric feeds to and from the motor with a neon screwdriver?
Correct me if I'm wrong, but I shouldn't get a live in, unless the CH is on.
If I'm getting this, but the valve isn't opening, then it's a motor problem, right?

Yes to all that. But if the valve shuts while the live is still there, and the motor we know isn't dud, then the connections are faulty.

 

[LondonStone]

Tape would not meet the requirements of the wiring regulations- the connector should be in some kind of enclosure.

It's housed inside the box of the motorised valve casing.
But that's made of metal! another good conductor of electricity.
Just seemed a bit risky to me. But guess it's just as safe as a light switch or any other electrical connection.


Thanks for that quick answer Chris. I think I've got it figured how it works now.
I never realied it was the valve that sent the siganl to the boiler.


Thanks guys!

 

[LondonStone]

Yes to all that. But if the valve shuts while the live is still there, and the motor we know isn't dud, then the connections are faulty.

Right, I tested the motorised valve. I get an elecrical current on the brown wire when the CH is turned on. But don't get anything at all on the blue wire. Is that right?

When the system halts the current to the brown wire is no longer there. So the valve is shutting when the current stops. So I'm doubting it's a electrical connection problem now.

Could it be a problem with the switch inside the motorised valve? Would a new valve unit sort the problem do you think?

I'd be grateful for a quick reply as I'm off work today and for once have the time to try and sort this out.

Thanks

 

[meldrew's_mate]

When you say the pipes are hot, are they HOT at the pump, or are you saying the boiler casing is warm/hot. From your description I assume it's a Profile 80e, so it probably won't bang and crash in the first minute, even if the pump has stopped.

You say the "connections" are inside the metal valve head. If this is a Honeywell valve the motor connections are crimped. Could the recent "change of valve head" been a change of sync motor only?

If this is the case it would point strongly to the microswitch in the valve head breaking down, they fizz for a bit, then dry-up and go high impedance = heating stops.

HOWEVER this would NOT make the valve shut, nor take the power off the sync motor so it doesn't fit your symptoms.

Have you checked the connections at the programmer and/or the room thermostat? These would interrupt the motor drive signal. Suppose you had a poor electrical connection here; when on the (small) current going through would heat the connection, and it MAY move enough to break itself. Listen for fizzing, or have the circuit properly checked.

Finally check the programmer terminals are secure, then check the wiring by (detail given for std 6 wire block, not Potterton EP type) temporarily joining the live feed to terminal 4 using a short pice of wire. This will turn on the heating only, and if this works it points to the programmer being broken.

 

[LondonStone]

Thanks,
Yes it's profile 80e boiler. And the pipes are hot at the pump, the boiler casing too and the rads close to the pump start to warm up.

I can hear the pump circulating the water, and there are no unusual sounds from the boiler.

Yes the recent changes were only the syncro motors, as the old motors stopped working.

I took out the microswitch and motor from the valve, both are working as they should. As you say, I'd still have a live feed and open valve if it was the microswitch.

I've checked the connections at the room thermostat, but not the programmer yet, will test that next. Are these the only two places that can interrupt the valve circuit? Be nice if I could narrow it down to one or t'other.

 

[meldrew's_mate]

Good, do the prog link test to eliminate/confirm programmer fault. Sometimes the internal relay contacts get dirty, usually its
's a dry/cracked solder joint on the programmer pcb.

Don't forget to turn off the power while you connect !!

You're getting close