Water not getting hot - boiler cycling

[SBDIY]

I've had this problem for over a year and am struggling to solve it. I've had 2 plumbers look at it and also had the boiler serviced and yet the problem persists.

When there is a demand for CH and HW my boiler operates as it should, fires up and gets up to the set operating temperature and continues like that as long as there is a demand. When there is only a demand for HW the boiler fires up but quickly reaches the set point and trips out without any regulation of the burner. It continues like this as long as there is a demand for hot water but the water takes a long time to get hot on account of the boiler coming on for only about 20 seconds at a time and then going off for 3 or 4 minutes. I've changed just about every part on the system (tank thermostat, pump, diverter valve and header unit). I've run sludge remover through the system and drained it out yet the problem persists. I'm getting to the limit of my ability and the next thing I'm thinking of doing is checking the thermistors in the boiler. Then again it was serviced by the manufacturer (Gloworm) only 2 weeks ago and given a clean bill of health. It seems to me that the hot water is not escaping from the boiler fast enough and so the boiler shuts down to protect itself. It was working perfectly fine for the past 20 years until a year ago and then suddenly just started doing this after I drained it to fit a new rad. I have drained it many timed before and never had a problem. I don't think its air as I've bled the system and it runs very quietly apart from this problem.

I really hope someone can help me solve this.

 

[PlumbBobBob]

hi sbdiy welcome to the forums. so this only happens when you have hot water on? not when you have both hot water and heating. i think there must be a blockage somewhere on the system that is not common with the heating. the only place i can think of is the coil in the cylinder. with the heating and hot water both on can you check the temperature of the feed and return pipes for the coil and see what the temps are like.

 

[steadyon]

It could well be that in draining it sludge / debris from the system, or from the feed and expansion (header) tank, has got into the system and lodged somewhere in the DHW circuit. Try removing the diverter valve and poking some net curtain wire (or similar) through the pipes. If you meet a lot of resistance then renew the appropriate pipes. Sludge remover won't remove heavy encrustations or solid debris, and, if you have a blockage, running sludge remover through the system probably won't have got to the affected parts as they have little or no circulation.

 

[SBDIY]

Hello Steve, thanks for the reply.

It does only happen when there is a demand for HW only but when I run it with a demand for HW and CH at the same time I get hot water ok. The feed and return pipes to the cylinder get hot as they should do, feed hotter than the return. The tank was replaced in 2005 when I got a new boiler. The problem would suggest a blockage of some sort but I do get HW when I run the CH at the same time.

 

[PlumbBobBob]

hm i'll put my thinking cap on then lol

 

[lame plumber]

big old air lock in the coil, no water movement so your boiler is cycling. drain down and fill via a drain off cock on the ground floor to push air thro with motorised valves locked open manually for a first effort

 

[PlumbBobBob]

lame he's getting hot water when heating is on as well just not when by it self. could be a problem with the diverter, is it fitted round the right way? A towards heating and B towards cylinder.

 

[SafeGasInstall]

there is not a problem just boiler riches temperature too quick as cylinder can not absorb the heat quick enough ...
what is the set temp at boiler ? stop spending your money on stuff you dont need to change

 

[SafeGasInstall]

also visit from a heating engineer will sort things out for you , and it will be cheaper

 

[mache]

Hi, First of all let me say I am not a plumber and I am a new member (today). I am however an engineer who has been involved in fault tracing. You mention that you get hot water when CH and DHW requested so there appears to be no blockage in the DWH heating circuit. Restriction possibly.
When DHW requested only then ther appears to be no flow or extremly restricted.
A point was made regarding the diverter valve causing an issue. Possible incorrect positioning ??

What is the situation when CH only selected. Is the flow only to the radiators or is ther bleed though the cylinder coil as well. i.e can you feel hot water being pumped through the return pipe from the cylinder. If you get that on CH demand only it would reinforce that the diverter valve is faulty/misspositioned. As to fix or replace if this is the case....you need to ask a plumber about that :smilewinkgrin:
Regards
Martin