Vented hot water problem

[Andy]

Hi everyone,
Recently my central heating system has developed a problem. When the system is on and just calling for hot water the boiler lights and runs for 1 to 2 mins and gradually starts kettling before cutting out. Once it has cooled a bit it cycles and does the same again.
If it is calling for heating it runs just fine and the radiators get nice and warm. The system has a gravity fed hot water tank in the roof and pumped rads.
I think there must be a blockage or air in the gravity feed to the hot water tank stopping the flow. Does anyone have any experience of a problem like this?
For the moment I have left the whole system turned off, relying on the immersion heater, but I'm going to need it soon with autumn coming.
Thanks for any advice,
Andy

 

[ShaunCorbs]

TBH I would be looking to convert the hot water to pumped as well

 

[snowhead]

Check, or take a photo of, the side of the cylinder where the pipes from the boiler connect, there maybe an air bleed on the upper one.

 

[fixitflav]

Hi everyone,
Recently my central heating system has developed a problem. When the system is on and just calling for hot water the boiler lights and runs for 1 to 2 mins and gradually starts kettling before cutting out. Once it has cooled a bit it cycles and does the same again.
If it is calling for heating it runs just fine and the radiators get nice and warm. The system has a gravity fed hot water tank in the roof and pumped rads.
I think there must be a blockage or air in the gravity feed to the hot water tank stopping the flow. Does anyone have any experience of a problem like this?
For the moment I have left the whole system turned off, relying on the immersion heater, but I'm going to need it soon with autumn coming.
Thanks for any advice,
Andy

Does it cut out on the control-stat or the limit-stat? The limit-stat usually needs manual reset, but if it is cutting out on that, the control-stat could be the problem. In HW only mode the heat transfer into the cylinder will be less than boiler output, so if the control-stat doesn't stop it the water gets too hot and kettling starts. In CH mode it's OK because the rads dissipate the heat so it doesn't reach control-stat setting before it stops on the room-stat.

 

[Andy]

Thank you for the replies.
I've had another look tonight. There is no air bleed on the water cylinder where the boiler connects, but that is where the vent take off is, so I guess that acts as an air bleed for the coil?
The boiler cuts out on the control stat. If I set the boiler temp control to low and it trips I can get it to restart by turning the temp control, so I assume the control stat is OK?

 

[fixitflav]

Thank you for the replies.
I've had another look tonight. There is no air bleed on the water cylinder where the boiler connects, but that is where the vent take off is, so I guess that acts as an air bleed for the coil?
The boiler cuts out on the control stat. If I set the boiler temp control to low and it trips I can get it to restart by turning the temp control, so I assume the control stat is OK?

It still sounds as if the heat isn't being dissipated when in HW mode, though why the problem has only just come up is a puzzle.
You didn't confirm whether the HW primary is gravity or pumped, you said "The system has a gravity fed hot water tank in the roof and pumped rads" but that tank is on the secondary (taps) side.
Are the pipes very hot before the kettling starts? If so you might get away with turning the control-stat down a touch. Unlikely to affect the CH performance, as I said in #4.
If it is gravity HW my guess it's an old system, maybe there's sludge build-up in the pipes.

 

[Andy]

Thanks fixitflav, the system was left idle for a bit in the hot weather, so maybe that wasn't a good thing, and that is why it has started playing up.
Sorry I wasn't clear, only the rads are pumped, the hot water primary is gravity. Because it cuts out so quickly when set to HW only, the tap water doesn't get heated much, it gets a bit warmer in CH mode, but not really enough.
It is an old system and I think you're right there is sludge build up, although it was cleaned and flushed 18 months ago. Think it's time to do it again.
Thanks to everyone for the help.
Andy

 

[JCplumb]

Last job I had with those symptoms had 2 issues.
Blocked pipework going into the cylinder coil and a broken 2 port valve.
System had been run for at least a year with the broken port valve as they said HW only setting still heated rads, so the least used route for the water ended up blocking over that time.
After replacing the 2 port the boiler would kettle and cut out on HW only setting.
The blockage was around the air separator, the area where the expansion pipe is teed into the pipework, that's a common area for a blockage that could give the symptoms you describe.
You can test the pipework for blockages with a strong magnet, any pull at all shows a blockage. If it is blocked there the best solution is to cut out that section of pipework out and replace it.
Would be worth getting a magnetic filter installed while the work's being done, and make sure the system is inhibited.

 

[fixitflav]

Thanks fixitflav, the system was left idle for a bit in the hot weather, so maybe that wasn't a good thing, and that is why it has started playing up.
Sorry I wasn't clear, only the rads are pumped, the hot water primary is gravity. Because it cuts out so quickly when set to HW only, the tap water doesn't get heated much, it gets a bit warmer in CH mode, but not really enough.
It is an old system and I think you're right there is sludge build up, although it was cleaned and flushed 18 months ago. Think it's time to do it again.
Thanks to everyone for the help.
Andy

With gravity HW, you can only have HW only, or HW + CH, not CH only. Wiring diagram below (I forgot to label the switches in the timer, but it's self-explanatory).

But what you could do is rewire it as the dotted line. Then the boiler only fires when the pump is running, ie only in CH mode. That would get rid of the kettling. You would need to use the immersion for hot water, but you've been doing that anyway.

 

[Andy]

OK, thanks for that, I'll give it some thought. Not sure how often we use CH and don't need hot water. I guess it's always been like that so we never thought about it, we always seem to need hot water ...

 

[fixitflav]

OK, thanks for that, I'll give it some thought. Not sure how often we use CH and don't need hot water. I guess it's always been like that so we never thought about it, we always seem to need hot water .

Agreed, you would have to use the immersion for hot water. I assume there is no cylinder stat, or any automatic valves (in the cylinder circuit)?
As it is, any time the whole system is available on the timer, if the room-stat is satisfied the pump doesn't run, so kettling will happen. The revised wiring avoids that.

 

[fixitflav]

Agreed, you would have to use the immersion for hot water. I assume there is no cylinder stat, or any automatic valves (in the cylinder circuit)?
As it is, any time the whole system is available on the timer, if the room-stat is satisfied the pump doesn't run, so kettling will happen. The revised wiring avoids that.

To add to that - if you do this mod I would also close a valve in the gravity circuit, otherwise you could get some flow through the cylinder from the pump, cooling it before things get up to temperature. So you'd be partly heating the house with electric, which you don't want.