Airlock in primary hot water circuit/Power flushing old gravity fed indirect system

[big_al]

Hi, I've got a very old Glow-worm 92/100 boiler which is gravity fed indirect (it must be 30-40 years old. Having had a leak from an old galvenised F&E tank for a year finally got around to replacing it (and not a moment too soon as it had gone literally paper thin). Refilled primary and no HW but CH working ok - heat soak radiator in bathroom (no TRV) got hot but pipes up to cylinder didn't so gravity not working. Thought either airlock or blockage as there was a bit of sludge went down the fill pipe when draining the F&E tank. Tried draining and refilling slowly and bled rads as it filled. Still no improvement, same as before. Tried connecting rising mains (5 bar) to hot water vent and water welled up into F&E tank. Tried connecting rising mains to cold fill pipe in F&E tank and water came out of hot vent into F&E tank. Tried alternating above a few times (attempt at sort of home flushing). The above rising mains flush was doen with system full and drain tap next to boiler closed. Got impression that amount of water going in was more than coming out into F&E so was hoping might have got rid of air lock and it had filled up a bit more. Fired up and now I've still got CH working but no HW. However, the HW return at boiler gets hot and HW feed at boiler remains cold - the soak radiator in bathroom appears to get hot but from the return side now rather than the feed (Up until a week ago it was the other way around). I think I might have got a blockage further down the HW gravity system (possibly in the boiler or between boiler and soak radiator?) Boiler thermostat appears to be working ok and cutting boiler out at certain temperature. Don't want to boil system due to age. Don't want to spend a lot of money as everything is due to be completely replaced in a few months (boiler, tanks, rads, and pipework).

Probably should mention that there is no cylinder stat (just what looks like a pressure regulator valve on flow input to cylinder), only CH has a pump and there are 4 pipes to boiler (2 for HW and 2 for CH). Can't see any other valves in the system.

I know these airlock/blockage problems can be buggers to sort. Any ideas? Is it worth trying a flushing chemical, trying the rising mains again on hot vent and/or F&E cold water fill at same time with drain tap next to boiler open, or even with whole system drained down first....?

 

[GPSNewcastle]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

The boiler surely only has two pipes? One flow and one return

 

[PlumbingSucks]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

Went to look at a system yesterday with pretty much the same problem but on the heating. Down stairs radiators are fine but upstairs the pipework it completely empty of water! Took cold feed of f&e tank to see if it was blocked but was fine. I'm guessing a blockage or severe air lock. Going back when i have more time to fanny around and figure it out. Was going to flush with mains pressure water as well. You should cap the vent or feed off when filling with mains so water doesn't come out of the pipe your not filling into. Allways the risk of causing a leak with the increased pressure on old systems tho be aware. Happened to me before thats why my name is plumbing sucks, theres allways a can of worms to be opened!!

 

[big_al]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

The boiler surely only has two pipes? One flow and one return

No, it definitely has 4 pipes - what looks like a flow and return on the LHS (believed gravity HW circuit) and a flow and return on RHS (CH circuit as this one is pumped). I think the practice of using two pipes and a diverter came after this - said it was an old one - and well past it's sell by date!

 

[big_al]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

Went to look at a system yesterday with pretty much the same problem but on the heating. Down stairs radiators are fine but upstairs the pipework it completely empty of water! Took cold feed of f&e tank to see if it was blocked but was fine. I'm guessing a blockage or severe air lock. Going back when i have more time to fanny around and figure it out. Was going to flush with mains pressure water as well. You should cap the vent or feed off when filling with mains so water doesn't come out of the pipe your not filling into. Allways the risk of causing a leak with the increased pressure on old systems tho be aware. Happened to me before thats why my name is plumbing sucks, theres allways a can of worms to be opened!!

I thought CH systems were less likely to block/air lock (except for electrolytic build up) as pump keeps things moving and it is gravity systems that are much more likely to have problems because the flow is much slower.

I've seen someone else mention capping off the vent whilst pressuring water into F&E feed and vice-versa but don't understand why as all you are doing then is pressurising the system rather than moving the water around, isn't it? My logical thinking (I may well be wrong as I just do a bit of DIY and not plumbing normally) is that leaving it uncapped will get the maximum movement of water around the system and increase likelihood of moving a blockage or air lock? I'm happy to try capping as the missus is giving us a bit of grief about having no HW!

 

[justlead1]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

Although it can be dangerous. removing the boiler stat with pump off and let it boil for a second or two often does the trick on gravity F&R. Good Luck

 

[PlumbingSucks]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

should of said put a hose on somewhere and drain the water the same time as flushing to move the water through the system

 

[big_al]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

Bingo - sort of!

Managed to clear the gravity F&R by putting rising mains through F&E cold feed and then trying same through F&E vent. Shifted airlock and now got HW and CH!! I think there was also a blockage which has moved into the sink/soak radiator as this doesn't get as hot as quickly as it used to but at least that should be easy to take off and flush with mains cold outside. Alternatively, I've got a spare rad in the attic the same size that I took off a year ago but it was left uncapped - will this have corroded by now as it was exposed to the air or is it worth a try (would try and flush cold mains through it before use anyway). Any thoughts on reusing an old rad?

 

[justlead1]

Re: Airlock in primary hot water circuit/Power flushing old gravity fed indirect syst

Others may have different views, but providing rads are on a closed circuit corrosion is not a major issue.