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Finding a blockage in heating pipework

View the thread, titled "Finding a blockage in heating pipework" which is posted in Gaining Plumbing Experience on UK Plumbers Forums.

jaydebruyne

jaydebruyne

Plumbers Arms member
Plumber
Gas Engineer
I did a powerflush a few weeks back and when I was dumping the water the flow dropped immediately. So I rang tech support (Anton) for my pump and he said it's because there's a blockage in the pipework - I tested the pump itself which was fine.

It's a flat with ground and first floor. All rads get hot upstairs but not down. Pipework from the boiler run straight to ceiling/1st floor void and tees somewhere to feed the ground floor.

Based on upstairs rads getting hot I know the blockage isn't there.

So.. how do I go about pinpointing the blockage? I'm thinking taking up some flooring and running across the pipework with a FLIR .. any other suggestions?
 
If you can follow/touch the Pipework go around with a magnet

Or if you have flir walk the route should see roughly where the blockage is
 
Is that vented or sealed system?
Quite often it's the T junctions or somewhere close to the boiler.
I had it few times though.
 
When the flow dropped during the dump stage, did you try dumping in reverse and if so did it make any difference ?

Where all the rads open at the stage you were at or were you dumping through one rad at a time ?

And as above (jts) are the downstairs individual drops or are the downstairs fed from one drop ?

Are there two heating zones or one ?

Was the water particularly soiled?
 
Last Plumber said:
When the flow dropped during the dump stage, did you try dumping in reverse and if so did it make any difference ?

Where all the rads open at the stage you were at or were you dumping through one rad at a time ?

And as above (jts) are the downstairs individual drops or are the downstairs fed from one drop ?

Are there two heating zones or one ?

Was the water particularly soiled?
Click to expand...
Tried both ways and no difference.
I think all but one we're closed at first then I opened them all - but I can't remember really tbh
Single zone
No idea how many drops as pipework is concealed
Water wasn't that bad no
 
Zone valves or combi with one open zone from Boiler?

Where were you flushing from, meaning did you remove a pump, connect under the appliance?

Was the flush passing through the Boiler or was it system only ?
 
Last Plumber said:
Zone valves or combi with one open zone from Boiler?

Where were you flushing from, meaning did you remove a pump, connect under the appliance?

Was the flush passing through the Boiler or was it system only ?
Click to expand...
Combi with open zone from boiler
Disconnected system filter on return under boiler and pumped through the valves
Was passing through boiler
 
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
 
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.
 
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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.
 
Last Plumber said:
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
Click to expand...
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?
 
jaydebruyne said:
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?
Click to expand...


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.
 
Last Plumber said:
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.
Click to expand...
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...
 
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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.
 
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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.
 
Chuck said:
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.
Click to expand...

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

Last Plumber said:
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.
Click to expand...

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

----

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..
 
jaydebruyne said:
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

----

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..
Click to expand...


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.
 

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