hot water problem

I have the normal graivty fed gas boiler with header tank in roof for central heating but am having continual problems with the hot water function. I have checked the zone valve, the surface cylinder thermostat, the electrics and the pump but can find no fault there. What I am noticing is that the flow into the coil in the cyclinder (domestic hot water) gets hot after a while but the return pipe to the boiler is usually cool and rarely gets hot. I suspect that there is a blockage (maybe partial) in the coil in the cyclinder or perhaps in the return pipe. Does anyone know if there is a simple way to check this and then to eradicate the problem? Thank you.

  Quoted Text Here  
What system is it?
http://content.honeywell.com/uk/homes/systems.htm
Adam

Thank you, it is nearest to the S Plan schematic, both central heating and domestic water systems are gravity fed.
  Quoted Text Here  

Stewart wrote:
  Quoted Text Here  
If it's gravity fed, what's the pump for?
Andy

Andy, the only way to run a gravity fed hot water system without a pump is to have large diameter (30mm or more) pipes set to flow with runs; that is with the pipes rising from the boiler on a gradient to the radiators and then again back. With small bore and micro bore pipes it is not possible to achieve enough flow so the system needs a pump. It pumps the water from the boiler to the radiators and also to the coil in the hot water cylinder. It is this flow to the hot water cylinder that is giving me the bother.
  Quoted Text Here  

Stewart wrote:
  Quoted Text Here  
I agree completely. A gravity fed system relies on density difference between the hot and cold water to move it around the system; this doesn't generate much pressure, so not much flow, and you need big pipes.
A pumped system, on the other hand, uses smaller bore pipes and a pump to move the water.
You appear to have a pumped system, not a gravity fed one. Or you are using a different definition of gravity fed to me.
Sadly this doesn't help us to work out what's wrong with it!
Andy

  Quoted Text Here  
An air lock is a possiblity. Often there is a bleed valve high up next to the cylinder. If not then with the CH off and HW on try running the pump at full speed to see if you can shift any air.
Adam

Andy, my apologies; obviously I have a pumped system but I thought that by using the term gravity fed I would differentitate from a combi system. I have a traditional boiler, pump, 2 zone valves, associated timer etc all fed from cistern (this is what I meant by gravity) in the roof space. I have bled the system, there is still evidence of hot water on the input side of the coil but no heat in pipe coming out. I think there has to be a partial blockage so next wednesday i intend to drain down the whole system, flush through with a cleaning fluid and then recharge. Hopefully that will clear all up.
  Quoted Text Here  

Stewart wrote:
  Quoted Text Here  
No apology required; but accepted anyway.
  Quoted Text Here  
There are also systems that use a pressure vessel instead of a header tank. And I imagine there are various mixtures of sealed/open, pumped/convection, conventional/combi/heatstore etc. that we could talk about for ages (for no good reason...)
<snip
  Quoted Text Here  
Some of the cleaning stuff needs to sit in the system for a while - aweek, IIRC. Good luck, and keep us posted!
Andy

  Quoted Text Here  
Some years ago I had a similar problem to this, the central heating worked OK but one day the domestic hot water no longer got hot. The hot water is heated by a gravity loop, with 28mm pipes, no pump, so I guessed, after eliminating the zone valve and tank stat, that there was a blockage in the coil in the hot water tank. With the boiler off, I turned off the valves on a downstairs radiator, removed the Rad, made an adaptor from an old radiator tail, and copper pipe and connected the non-thermostatic valve of the removed Rad to mains pressure water via a hosepipe. This Rad valve was turned back on and the mains pressure water tap turned on by an assistant while I checked the flow of water upwards into the header tank. Initially nothing happened, then, with a gurgle, lots of water flowed rapidly into the header tank carrying with it black particles. The problem is that the flow rate into the tank is far higher than the overflow can cope with so there is the potential for a flood in the loft. That's why you have an assistant and shout a lot ! I pumped the dirty water in the tank into buckets and carried it downstairs. This was repeated several time until the return water ran clear, the valves were closed and the boiler fired back again, the flow to and from the hot water tank was restored. Job done ! At the time this system did not have seperate feed and expansion pipes between the hot water tank and the header tank, back in the '60s they sometimes did this to save on copper pipe. With seperate pipes you can inject mains pressure water into the header tank expansion pipe and watch it come up the flow pipe, and even vice versa. Flushing in both directions is supposed to be beneficial. Good luck !
Ian.

I drained down the system on wednesday and put in Sentinel sludge remover, left it until friday then flushed out severeal times. Heating and hot water are at the moment working perfectly; there must have been a slight build up of sludge or blockage in the coil. thanks to all for advice.

see my previous post On Wednesday I drained the system and filled it with sentinel sludge remover; left it working until friday when I drained it all again and gave it a thorough flush. Now filled with clean water and all is working well. If that continues then I shall add sentinel inhibitor.