Central heating problem – major headache!

[easydoesit]

We have a Buderus 600 boiler on a y-plan system which replaced the original 1980s boiler. Since it was installed it would only operate on either HW only or CH+HW. If we switched it to CH only then the boiler came up with an error code indicating a circulation problem.

I’ve spoken to a heating engineer about this and he thinks this is most likely because the hot water return joins in the wrong place. All the pipes are mostly concealed so it’s not possible to see exactly what is going on. He says there should be nothing teed into the return run after the hot water. Have I understood this correctly?

Another question – there is a 15mm and 22mm pipe teed into the flow quite close to the boiler. I think these pipes are the cold feed and vent to the F&E tank. The original boiler installation had them close to the boiler but the engineer said they are normally joined in the airing cupboard. I’m sure it would have been easier to have them in the airing cupboard originally but since the boiler was replaced they have been left connected close to it. Does it matter whether the feed and vent pipes are close to the boiler or should they be closer to the pump?

Before I start pulling up floorboards can anybody tell me if I am on the right track?

 

[MANDATE]

I can't help with the pipe work, but if you have a 'Y' plan then you must have a 3 port mid position valve.
These valves incorporate micro switches, one when triggered allows power from the valves orange wire to fire the boiler for CH only.
When the valve is in the HW mode or the HW/CH mode the boiler is fired from the cylinder stat.
Also the valve operates over two halves. Power on the white wire drives the valve over the first half, then the power is dropped by a micro switch but picked up again from the grey wire providing it is 'live'
The grey wire is made live from the cylinder stat when HW is satisfied and the power diverted or if HW is not on then there is (or should be) a wire direct from the programmers HW OFF terminal.
As I understand it. There seems to be a lack of understanding by some and the wire from HW OFF was never fitted so valve was prevented from continuing beyond mid position.
So before lifting floorboards you need to ensure the the valve is wired correctly and it moves to the CH position and delivers the power out of the orange wire.

 

[Agile]

Surely you dont expect it to work if a wire is missing ?

Even if only two wires go to the cylinder stat then a clever person can use a relay to correct the circuitry

Tony

 

[easydoesit]

@mandate – thanks for the info on how the 3 port valve works. I’m going down the track of a pipe work problem so wanted to take wiring out of the troubleshooting as far as possible. What I did was remove the 3 port valve motor head and manually moved the valve spindle over to CH only. This still caused the boiler to error.

So, I spent most of yesterday pulling up the chipboard flooring, tracing all pipes between the airing cupboard and boiler. If I’ve got this right, the piping is completely bonkers. Basically I think the only return coming back to the boiler is from the HW circuit. The return from the CH circuit is teed into the flow from the boiler!!

It’s a miracle this has worked for 30 years and explains why I’ve never been too impressed with the new boiler we had installed 5 years ago.

Anyway, I’m meeting a plumber tonight to confirm what the problem is. I’m hopeful that it should be pretty straight forward to fix once and for all...

 

[easydoesit]

An update on the piping problem - our plumber has looked at this and agreed that the return from the CH circuit had been connected in to the flow from the boiler. It was a fairly easy fix to hop this over, to connect into the return from the HW cylinder coil where it should be. I'm stumped as to why it has worked at all for 30 years prior to this... but it explains why the heating was not very effective.

So now everything seems to be circulating okay but there's still a problem with the boiler (Buderus 600). When it starts up from cold the fan spins up but it does not fire. A fault code 9P is displayed (which is not in the manual!)

After a few cycles (fan spins up-error code-fan spins down-repeat) it does eventually fire but this takes a few minutes. Once it is actually running everything seems fine.

The wiring for CH only seems okay, when we select this from the programmer the 3 port valve moves over as expected. However another quirk is that when running on HW+CH, when the CH stat becomes satisfied the pump (Grundfos Alpha 2) seems to stall for a moment as the valve moves over to CH only. This then throws the boiler into its 9P fault code cycle again...

So the saga continues. I'll do more troubleshooting today to get a clearer view of when this 9P thing happens...

 

[Agile]

Is the pump wired back to the pump connection at the boiler so that it provides over run?

Tony

 

[MANDATE]

When running in the HW/CH mode the boiler gets its power from the cylinder stat. When HW is satisfied cylinder stat cuts the power off and boiler /pump stops. The power is diverted to the grey wire to move valve to CH only. During this movement a micro switch is triggered and allows power out from valves orange wire to relight boiler/pump.
The stopping and restarting of boiler happens fairly quickly so you just have not noticed it before.

 

[easydoesit]

Tony - I don't believe the pump is wired back to the boiler. The service manual for the boiler says "The Pump over-run time switch is not applicable to this boiler" so I guess it doesn't need overrun.

Mandate - from what you are saying it sounds like the cut over to CH only is happening as expected, maybe this is not a problem at all.

So the only problem I have is this 9P fault on the boiler. It happens when starting up (whether the system is cold or warm) and whenever the system switches to CH only. When it does this it can cycle round for a minute or so before firing up, or up to 10-15 minutes when I get sick and then turn it off.

Interesting you say the boiler gets its power from the cylinder stat Mandate. I was talking to another heating engineer the other day and he said he had a similar problem on a Buderus boiler. In his case replacing the UBA board made no difference and it turned out that replacing the room stat solved the problem. I couldn't see how this would make a difference (but then I have no experience in this field!) Anyway I thought it was worth a go so I have replaced the original room stat which has made no difference.

I wonder if this boiler problem could be due to a faulty cylinder stat? Perhaps if there's a dodgy connection and it is not getting a clean 240v? The cylinder stat looks original, so up to 30 years old.

 

[easydoesit]

Tony - I don't believe the pump is wired back to the boiler. The service manual for the boiler says "The Pump over-run time switch is not applicable to this boiler" so I guess it doesn't need overrun.

Mandate - from what you are saying it sounds like the cut over to CH only is happening as expected, maybe this is not a problem at all.

So the only problem I have is this 9P fault on the boiler. It happens when starting up (whether the system is cold or warm) and whenever the system switches to CH only. When it does this it can cycle round for a minute or so before firing up, or up to 10-15 minutes when I get sick and then turn it off.

Interesting you say the boiler gets its power from the cylinder stat Mandate. I was talking to another heating engineer the other day and he said he had a similar problem on a Buderus boiler. In his case replacing the UBA board made no difference and it turned out that replacing the room stat solved the problem. I couldn't see how this would make a difference (but then I have no experience in this field!) Anyway I thought it was worth a go so I have replaced the original room stat which has made no difference.

I wonder if this boiler problem could be due to a faulty cylinder stat? Perhaps if there's a dodgy connection and it is not getting a clean 240v? The cylinder stat looks original, so up to 30 years old.

 

[MANDATE]

The room stat is a one way switch. It provides power to the white wire of valve in order to move valve from HW only (rest position).
If the CH/HW mode is achieved, then valve must be in mid position and therefore room stat is functioning OK.

The cylinder stat is a two way switch. It either provides power to boiler or (when HW satisfied ) power to grey wire in order to move valve from mid position to CH only.

There are two ways for the grey to receive power.
One is as above, diverted through the cylinder stat when HW is satisfied.
The other is direct from programmers HW OFF terminal. This needed if HW is not on and cylinder stat is not in use.

Some of the valve problems
(a)Valve does not move. Could be faulty motor or power not getting to motor (programmer, room stat, micro switch1). Or valve spindle stiff.
(b) Valve moves to mid position but no further. Could be no power on the grey wire or faulty micro switch 1. Or valve spindle stiff

(c) Valve moves to CH position, but boiler does not receive power.
Most likely micro switch 2.

Also note: When The CH becomes satisfied (when in CH only mode) the room stat cuts off the power to the white wire, this cuts off the power out from the orange wire so boiler stops.
The grey wire however is still powered up, so the valve does not drop back. It stays at CH only waiting till room stat switches power back on again.