Gravity Hot water pipes cold from boiler - blocked pipes?

I have an old (1980s) pumped heating and gravity hot water system from I believe a Netaheat boiler. Gravity hot water pipe work is all 28mm as is the indirect cylinder. This was all working fine until...

I replaces some radiators upstairs this week after partially draining the system. These all went on fine and after refilling the system all the radiators were lovely and warm. However the hot water cylinder went cold and wasn't being reheated. Feeling around the pipe work the flow and return from the cylinder were pretty much cold, from the boiler hot water flow was hot and the return merely warm (central heating flow and return were hot and nearly hot as to be expected). I figured some sort of air lock and run the boiler on full with heating pump off, but no joy.

Last night I decided to flush some water through the system to see if this would shift it. So fed the hose pipe into the feed pipe from the expansion tank, and lots of water started coming out the expansion pipe, so ran this for a good few minutes flushing out lots of black sludge etc (and down the over flow). Than for good effect run the hosepipe down the expansion pipe for a decent while.

Turned the system back on and now radiators heat up then stop and still no hotwater at the cylinder. Feeling the pipes from the boiler, both the hotwater (supply and return) pipes are now cold from the boiler. The boiler gets hot, starts to fizzle/kettle and then the thermostat shuts it down for a while. When it cools it starts up again, this then repeats.

I guess when refilling the system originally some rubbish from the expansion tank got into the pipe work and caused a blockage. As there hot water flow and return pipes are both cold I expect this blockage would be somewhere very close to boiler, could it even be in the heat exchanger? I would suspect it is also on the flow side as if on the return side there should still be hot water from the flow via the central heating return.

What is my next course of action?

1) Flush the system properly through the drain off valve blocking feed/expansion pipes and alternating between the two. The drain cock isn't work at the moment so I will replace it and will just use straight pipe to flush it so should get a decent flow rate.
2) Is it work back filling from the drain off pipe?
3) Start looking for the blockage. As I suspect it is near the boiler should I cut into the 28mm pipe near the boiler or undo the compression joint between the boiler and 28mm pipe?

Sorry for War and Peace, I read the forums daily and know you guys (and girls) can help.

Kris

 

Feed and and expansion pipes for the boiler go straight past the hot water cylinder. I turned off all but two radiator valves before flushing.

Also going to replace the header tank for one with a lid this weekend as well.

 

LL I'm confused, I have given it a good flush through the expansion pipe to the header tank. Would have two rads partially on have made such a difference? Besides as the pipes from boiler are cold, wouldn't any air near the boiler naturally move up the expansion pipe?

 

Doh, of course, so it just went through coil in the cylinder and back out again.

So flush through the feed/expansion (blocking the other) and out the drain cock right?

 

So I have now checked all pipe work between the boiler and cylinder, and replaced a few bits. The pipe work is now clean as a whistle.Flushed the coil which did have some grit in it, but seems clear now.

Still no hot water.

With hot water only called (CH pump off - gravity hot water) the boiler runs for a few minutes starts to fizzle, then cools down. Checking the pipework from the boiler:
CH flow pipe soon gets hot.
CH return gets warm
HW return gets warm
HW flow stays cold

I've tried flushing throught the boiler, both from header tank and mains pressure on drain point but still no luck.

What should I be looking for next?

 

Thanks you for your help so far.

I've taken the flow and return pipes out of the system and thoroughly cleaned or replaced them. Also flushed the cylinder through and removed a bit of grit. So I know the pipes are clean and cylinder coil is clear, this is why I think the problem is now in the boiler.

There is a mud flap/hole on the Netaheat heat exchanger, but I am rather loathe to go messing with that, after 30+ years the chances of getting it sealed again might be slim.

 

No injector Tee.

The Netaheat has 4 connections (+ gas).
1 x 3/4" -> 22mm CH flow coming out the side at the top of the heat exchanger - pipe feels hot
1 x 3/4" -> 22mm CH return to the side at the bottom of the heat exchanger - pipe feels warm/hot
1 x 1" -> 28mm HW flow from the top of the heat exchanger up a pipe inside the boiler and out the side at the top of the boiler - pipe stays cold
1 x 1" -> 28mm HW return to the side at the bottom of the heat exchanger (just above the CH return) - pipe feels a little warm (but not as the CH return).

Because of the internal pipe and bend for HW flow something could be obstructing that but letting CH flow out fine.

Kris

 

Tom P - The whole pipe run from the boiler to cylinder to expansion tank rises. There is only around 2m of 'horizontal' run and even that slopes upwards. There is plenty of water in the feed and expansion tank (which was replaced last week) however the problem started after a drain down the week before that.

DIY - surely you mean feed expansion tank as hot water tank is separate. I will try your suggestion of blowing down the expansion pipe though. I have previously connected it to mains pressure and flushed mains through the feed pipe (and out the expansion pipe).