Central Heating not working

[jspss]

Cental heating not circulating.
My system is about 30 years old its has for most of it’s life had fernox inhibitor but perhaps only two lots over this time scale. No other problems in this time scale.
It is a Y plan system pump flow to 3 port valve (diverter) extra 2 port zone valve in the flow to control upstairs rads via separate room stat.
I noticed about a week ago that the upstairs rads where only getting warm and ground floor cold. I shut off the first floor by closing the first floor zone valve then some of the ground floor rads got warm ie those nearest to the flow pipe.
The domestic HW flow and return are working OK.
The system is 22mm main runs with 15mm off the rads.
The system is DHW priority.
I have drained the system, changed the pump for a new one, stripped down the 3 port valve (working ok) purged the system of air but still no circulation on rads circuits upstairs or down.
I notice if the 3 port valve is held on the latch ie mid position with the DHW calling for heat with the upstairs zone valve open the upstairs rads get hot, but if I then set the system to CH the 22mm flow pipes to up and down circuits get hot but only until the boiler stat shut off the boiler, the nearest rads to the flow main pipe are luck warm but go cold after the boiler shuts down. Boiler only cycles when the water in the flow / return and in it cools.
ANY Ideas please.
Don’t take this as the answer just a thought, could it be magnetite (iron oxide, black water) blockage , soft water area no lime scale, if so do 22mm pipes generally totally block with magnetite. I ask this because it looks like the return 22mm pipes from the up and down circuits are the problem. HOW do I get it out?
Just one point when draining the system down the run off water was clear clean water!

 

[Slugbabydotcom]

Between your diverter valve and the flow connection to the cylinder there is an air vent. Let the air out of it!

 

[jspss]

Hi Thanks there is an air vent at the cylinder it is an auto-vent !
Please any other ideas.
jspss

 

[Slugbabydotcom]

Does it have a Valve cap something like you would see on a car tyre and is it loose or screwed tight ?

 

[jspss]

Hi Thanks
Yes the black plastic screw is out, vent open.
jspss

 

[Slugbabydotcom]

if the 3 port valve is held on the latch ie mid position with the DHW calling for heat with the upstairs zone valve open the upstairs rads get hot

I've got a feeling that the problem is something to do with the diverter valve so lets try and eliminate that possibilty first.
When there is a call for heating the white wire should become live and the valve should move to the CH position when it does so there is a microswitch inside that makes the orange wire become live which then operates the boiler.
Check that is whats happening and post back.

 

[meldrew's_mate]

Jspss, you can answer your own question about magnetite. When you drained the system was the water black & gungie, or clear & crystal-like? I suspect clear; my dad lives in your area and has had 8mm microbore with no treatment for 25 years, only one pump replacement after 21 years ("they don't make them like they used to..." he said)

Was the old pump sludged, again probably not, so discount your primaries being blocked with sludge.

I notice if the 3 port valve is held on the latch ie mid position with the DHW calling for heat with the upstairs zone valve open the upstairs rads get hot

A bit ambiguous, do you mean at that point BOTH up and down work? If so Sluggie has cracked it, check-out the 3 port.

 

[jspss]

Thanks for you help.
Problem solved, it was sludge in the 15mm return pipes from ground and first floor curcuits where they common, just before they connect to the 22mm main return pipe.
Was able to get the Fernnox DS 40 to ciculate around just one rad at a time, now the entire system is working OK.
jspsss