central heating flush issue

[coolideas]

I have been called in to sort a friend's heating system. They were constantly bleeding some of the radiators! The plumbing is a bit of a nightmare, done by the owner but cut and changed over the years. Originally there was a backboiler heating the system, this was removed and an oil boiler added outside. I moved the tank in the loft to a higher position, didn't seem to make it better. I decided to flush the system with a desludger. I opened the drain at the boiler and got about 30seconds of black then clear water! Then nothing. So I opened the cold supply to boiler from rads, an initial flow for 30secs then nothing! What can I do now?

 

[SJB060685]

Everytime air is sucked into the system black iron oxide (sludge) will form, as I'm sure you're aware of. From what you've said I'd say the system needs a proper power flush. Also what is the orientation of Vent, cold fill and pump?

 

[coolideas]

Everytime air is sucked into the system black iron oxide (sludge) will form, as I'm sure you're aware of. From what you've said I'd say the system needs a proper power flush. Also what is the orientation of Vent, cold fill and pump?

Thanks for the reply. I have bought a sludge clear additive to go in the system as I'm sure that is an issue. I first wanted to flush the system through with clean water before running it with the additive, but with no water coming out the pipes, there is obviously an air lock or sludge lock! Can I rig up a circulating pump (so no pressure just flow) and pump clean water from the boiler hot pipe to flush out through boiler return? Should the F&E tank be 3/4" pipe to system?
Thanks
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Also what is the orientation of Vent, cold fill and pump?
I noticed the owner had replaced an AAV on the cylinder coil outlet with a vent pipe to F&E tank, so there was no vent pipe originally. The F&E tank in loft only has a 1/2" pipe going into a 3)4" pipe which feeds a pipe to the boiler. The circulation pump is sitting vertically next to the old fireplace on the boiler supply, but there is a 't' off before it to a plastic 1/2" pipe (?).

 

[SJB060685]

This sounds I right mess and I'm struggling to picture it. When I say orientation I mean where are these pipes connecting into the system. It should be vent then cold fill (150mm apart) and then pump, this provides a positive pressure throughout the system. 15mm feed and expansion pipe is fine for boilers up to 25KW so my OFTEC book says, obviously tank capacity is determined by system volume. If you could supply a drawing of system layout and vent and cold fill entry point I could picture it better.
Rigging up your own system to try and remove blockages will be a pain. You can hire the machines on a daily rate which will do the job properly. We only ever hired the machine as we didn't do too much of it to warrant purchasing one.
What is that plastic pipe feeding by pump as well and how close to boiler?

 

[coolideas]

Ok I spent 4 hours trying to work out the system today and flush. I managed to rig up a supply from the mains and flush through from the boiler pipes, plenty black coming out! There is no knowing how far round the system this went though, it may have just gone through the cylinder coil and back. What I did discover was that the kitchen tap dripped when i flushed the heating system, bare in mind that the kitchen tap is not connected to this system!!! but a separate heating system for the extension of the house.
After firing up the boiler again all three rads downstairs got roasting but nothing upstairs. I could hear the pump alternating in pitch and figured the pump was not pumping effectively especially as the pipe above the rad take off (going to cylinder coil) did not get hot!
See diagram for the best I can figure out without ripping up floorboards etc.

Cheers
Dave

 

[SJB060685]

I've had a look at diagram, well done. From that it looks as though the pump is pumping straight over through vent, each time this happens air will be sucked in causing sludge to accumulate. I must add when flushing an open vented system the cold feed and vent should be blanked off. You say the pump was making an unnatural sound, a pump when spinning acts as a magnet, drawing in and accumulating the magnetic sludge, I suspect this is clogged up, working enough to pump over through a path of minimal resistance but not enough to get heat to up stairs rads which could offer a lot of resistance or further blockages. Sludge to start off with will gather at bottom of rads, further building up over time. You could stick a strong magnet against the flow and return of upstairs rads to see if there is sludge there, remember copper cant be magnetized but steel obviously is, so put magnet against copper pipes, if it sticks you know theres a large build up of sludge. I once had a system with 22mm pipe having an internal bore of 2 or 3mm the sludge was that bad, piped similar to this picture.
Personally I would recommend the whole system be reconfigured to prevent this venting over and properly power flushed. Sealed system will do away with the worry of venting over and allow positioning of the pump where it is, it needs to be fully pumped, complete thermostatic control with independent zones. As it stands, from what your diagram says matters wont improve

 

[coolideas]

Thanks for your reply. Unfortunately the guy has chopped and changed stuff over the years it is very difficult to know where some of the pipes are, like water coming out of kitchen sink tap in earlier post. Why is there a 1/2" pipe T'd off from below pump to just before cylinder? Looking at it again I might have it wrong, it may go into the return from cylinder coil, sorry.
I removed the pump, no sludge inside. I had to remove a length of pipe with pump attached as the pump valves were so rusted up to pump body I couldn't get the free. Will try some heat on them tomorrow.
Found some interesting 'faults' as I was inspecting the system. The drain at boiler was not tightened up, one bleed valve was loose on a radiator (rust line down side suggested its been going for a while!), a valve on the fill line was closed!
I don't really want to change it to sealed system due to not knowing the full system layout under the concrete floors etc. Whats the best thing to change this layout for the better?

Many thanks
Dave

 

[SJB060685]

With a drain off open, or not completely closed and a bleed valve the same you'll be drawing air, all that mounts up. I cant say why the tap is dripping because I genuinely don't know. You need to change the pipework so it's not venting over and have independent zones with full thermostatic control

 

[coolideas]

Ok so if I move the expansion pipe to exit from the bottom of hot water from boiler before the pump, I can use the 1/2" that is already there and connect to 1/2" coming off the intake to coil in cylinder. Then put an AAV there instead?

 

[SJB060685]

You need to re pipe vent position to come off before the pump, as it is its pumping straight over and drawing in air, power flush or any flush is pointless until that is changed. Zone valves and thermostats should be installed as well.

 

[John.g]

Ok so if I move the expansion pipe to exit from the bottom of hot water from boiler before the pump, I can use the 1/2" that is already there and connect to 1/2" coming off the intake to coil in cylinder. Then put an AAV there instead?

The "normal" method now I think is to have the pump on the boiler Flow side and from the boiler and as post #5 stated It should be vent then cold fill (150mm apart) and then pump, don't know if this will work as well where the pump is installed on the boiler return but if its been done this way before then? although I would think that the vent woks best on the hot, flow side.
I know you don't like the idea of a sealed system but there are lots of houses around me that were converted from oil to gas and Bord Gais installed all of them as "semi-sealed", a lot more forgiving than a fully sealed system. All you need to do is install a swing check NRV on a horizontal piece of the cold feed, install a E.vessel preferably as close to the pump suction as possible, and pre pressurise it to 0.3 bar, assuming a 12 litre E.vessel and volume contents of even 100 litres then the max pressure with everything heated up will not exceed 0.75bar to 0.9 bar which any system should have no problem with as the gravity head in a vented systen is ~ 0.3/0.4 bar anyway.

You could also consider a combined vent and cold feed (like mine, from new) but you would need to install a 3/4 ins vent for this.