Finding a blockage in heating pipework

[Last Plumber]

I'm thinking that if the water was not that bad, it is more likely something changed to restrict flow.
e.g. if diverter moved across it would restrict the path for the water you were shifting around the system. I'm not saying that's the answer but it does sound more like a valve changing than a blockage to me.

It can be a blockage of course and you should not rule that out but i'm trying to help you think outside the box

 

[Chuck]

You may be able to learn something useful by starting with the system 'cold', all upstairs radiators off, circulating pump set low. Then use your thermal imager to figure out the sequence in which the radiators come on when the heating comes on.

The idea is the the blockage will be in the flow or return section immediately downstream from the last radiator to light up. Not a sure-fire solution, of course, but relatively easy and non-destructive.

 

[Vee]

If it once was a open vented system, especially gravity fed hot water, look for blockage where the old cold feed would have joined the system. Generally at ground floor level near where the boiler used to be. Or any position where the cold feed joined the flow. And for the complete joker, if an old anti gravity valve left in the system has finally collapsed, the position of which is your guess on the central heating return back to the old boiler position. Good luck.

 

[jaydebruyne]

I'm thinking that if the water was not that bad, it is more likely something changed to restrict flow.
e.g. if diverter moved across it would restrict the path for the water you were shifting around the system. I'm not saying that's the answer but it does sound more like a valve changing than a blockage to me.

It can be a blockage of course and you should not rule that out but i'm trying to help you think outside the box

UPDATE:
I've just changed the pump as I wanted to rule this out.
The return pipe above the pump going back into the main hex is hot, flow from hex and to system is fine. Flow from system going back into boiler is barely like warm.
Funnily enough the plate hex is also hot whic leads me to think the diverter could be at fault.
When I run the dhw it gets stupidly hot and then cools to probably 40c and stays that way but the flow pipe underneath boiler gets hot also which kind of confirms that it's the diverter and also the plate hex could also have a blockage possible but best to change the diverter first.
What you reckon?

 

[Last Plumber]

UPDATE:
I've just changed the pump as I wanted to rule this out.
The return pipe above the pump going back into the main hex is hot, flow from hex and to system is fine. Flow from system going back into boiler is barely like warm.
Funnily enough the plate hex is also hot whic leads me to think the diverter could be at fault.
When I run the dhw it gets stupidly hot and then cools to probably 40c and stays that way but the flow pipe underneath boiler gets hot also which kind of confirms that it's the diverter and also the plate hex could also have a blockage possible but best to change the diverter first.
What you reckon?

I think, it sounds like that was the problem during flushing like I said earlier.

Those machines use a higher flow rate and pressure than a CH pump/Circulator.

I am not saying that the Flush has damaged the valve but it could be a possibility.

I would replace it.

It is only my opinion of course and others may well disagree but I would always avoid flushing through the boiler unless it's an older one with a cast iron heat ex.

In combi/modern boilers there are too many small water ways and delicate component parts that are at risk from a high volume, crud filled flushing process.

 

[jaydebruyne]

I think, it sounds like that was the problem during flushing like I said earlier.

Those machines use a higher flow rate and pressure than a CH pump/Circulator.

I am not saying that the Flush has damaged the valve but it could be a possibility.

I would replace it.

It is only my opinion of course and others may well disagree but I would always avoid flushing through the boiler unless it's an older one with a cast iron heat ex.

In combi/modern boilers there are too many small water ways and delicate component parts that are at risk from a high volume, crud filled flushing process.

Just changed the diverter, cleaned out the manifold with the filter on the flow and removed the filter and still the same issue. It's circulating through the boiler via the bypass. In DHW mode it ramps up then back down to 40c where it stays.
In heating mode it ramps up to 78c and then burner goes out, cools down and reignites to bring it back up to temp and cycles through this constantly.

EDIT:
Just spoke to Vaillant, they got me to do a few things and shout out the S codes and they've determined the blockage is def inside the boiler.
I'm gonna check the thermistors on the flow/return when it cools down but he reckons it could be the main hex??? If it were the main hex why would it go through the bypass? Surely it would go through the heating circuit still, no? I know Vaillants are a bit strange...

 

[Chuck]

You said in your first post that the upstairs rads get hot but not the downstairs ones. The boiler is short cycling, presumably because the blockage in the downstairs loop is causing the bypass to open (or it has stuck open). To me these suggest a blockage external to the boiler, specifically in the circuit that serves the downstairs rads. You haven't mentioned the pump sounding noisy (due to pumping against the unbypassed impedance of a blocked HEX) or kettling/banging coming from the boiler, which I'd expect if you have a blocked HEX.

 

[Last Plumber]

Is this the Vaillant with Black rubber hoses?
If it is, Take them off and look inside !
They corrode and decompose to the point were they will block the flow. I have come across what you're describing a few times. If you flushed through the Boiler, it could have removed that black residue from inside those tubes.

 

[jaydebruyne]

You said in your first post that the upstairs rads get hot but not the downstairs ones. The boiler is short cycling, presumably because the blockage in the downstairs loop is causing the bypass to open (or it has stuck open). To me these suggest a blockage external to the boiler, specifically in the circuit that serves the downstairs rads. You haven't mentioned the pump sounding noisy (due to pumping against the unbypassed impedance of a blocked HEX) or kettling/banging coming from the boiler, which I'd expect if you have a blocked HEX.

No kettling, no banging, no noisy pump. I didn't think a blockage in the main hex would cause the bypass to open either.

Is this the Vaillant with Black rubber hoses?
If it is, Take them off and look inside !
They corrode and decompose to the point were they will block the flow. I have come across what you're describing a few times. If you flushed through the Boiler, it could have removed that black residue from inside those tubes.

No its 2 copper pipes for flow/return inside the boiler

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Isn't the bypass part of the pump housing on these boilers? I might be wrong! As I changed the entire housing for the pump too..

 

[Last Plumber]

No kettling, no banging, no noisy pump. I didn't think a blockage in the main hex would cause the bypass to open either.



No its 2 copper pipes for flow/return inside the boiler

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Isn't the bypass part of the pump housing on these boilers? I might be wrong! As I changed the entire housing for the pump too..

There's a small 12-13mm pipe that connects the brass (flow filter housing) block across the back to the bypass screw, which is facing you and is situated to the left of the pump as you look.

 

[jaydebruyne]

There's a small 12-13mm pipe that connects the brass (flow filter housing) block across the back to the bypass screw, which is facing you and is situated to the left of the pump as you look.

Ok .. i think tomorrow I'm going to flush the system and bypass the boiler to see if the blockage is in the pipework or the boiler. At least then I'll know for sure.

 

[jaydebruyne]

There's a small 12-13mm pipe that connects the brass (flow filter housing) block across the back to the bypass screw, which is facing you and is situated to the left of the pump as you look.

To the left of the pump is the diverter which has a brass screw, is that what you mean?

 

[jaydebruyne]

Blockage is def in pipework. Just tried to flush system bypassing the boiler and getting hardly any flow at all. up comes the floor!

 

[Last Plumber]

To the left of the pump is the diverter which has a brass screw, is that what you mean?

Yes, Long thread and circlip, Normally set around the halfway point

 

[jaydebruyne]

Yes, Long thread and circlip, Normally set around the halfway point

Just tried to flush system bypassing boiler, now have no flow at all so blockage is now worse than before. Boiler is fine in DHW but just goes round bypass on heating. What's the best way now of finding the blockage? lol literally exposing entire circuit and checking pipework with a magnet?