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Discuss Unvented cylinder won't stop heating up when CH is turned on in the Central Heating Forum area at PlumbersForums.net

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I have recently moved into a house with the following heating system:
  • Traditional boiler (Worcester Greenstar Ri 24 with only one temperature dial and no pressure gauge)
  • Vented central heating (cold tank in loft)
  • Unvented hot water (Excelsior cylinder with pressure gauge plus white expansion vessel above)

The system is behaving strangely and I'd like an overview of the cause of the problem before calling a plumber out.

This is what I have observed:

  1. The good. When the boiler is set to "hot water only", everything seems fine. The pressure gauge by the cylinder goes from 2.5 bar to about 3 bar over the course of 30 minutes or so as the water is heated and settles at 3 bar. Increasing the flow temperature on the boiler doesn't cause any issues: the pressure in the cylinder doesn't go beyond 3 bar. I'm guessing this means the cylinder thermostat and expansion vessel are doing their jobs fine.

  2. The bad. However... When the boiler is set to "central heating only", things don't work as expected. The radiators work fine, but the hot water is still being heated. The pressure in the cylinder gradually increases from 2.5 bar to 4 bar (the end of the scale) over the course of a couple of hours, showing no signs of slowing down, at which point I shut off the heating for safety. Exactly the same happens when the boiler is set to "both central heating and hot water". I tried lowering the flow temperature of the boiler so the radiators weren't piping hot, but the issue persists.
Any explanations gratefully received.
Bluto
 
Ok, 1st you have an expansion issue because the pressure shouldn’t be going that high, and it’s set too high to begin with at 2.5 bar.
2nd - how do you know the water is still being heated?
3rd - sounds like a motorised valve failure.
 
What pressure is the cylinder designed for?, it will be written on a label attached to it?., that will dictate the E.vessel precharge and the cold water PRV settings.
 
Thank you both for your replies.
I don't think I can change the initial 2.5 bar of the cold water: it's not a Combi.
I can't see any label on the cylinder (access to it is awkward), but I have read on an online manual for it that the pressure vessel comes preset to 3.5 bar.

There are two Honeywell motorized valves in the cupboard housing the cylinder: both are 2-port valves.

Could the failure of one of these valves be consistent with the follow symptoms?

  • Hot water heated safely when only hot water called for
  • Hot water heated continuously to dangerous levels when central heating is called for (whether or not hot water is called for)
  • Radiators behave as expected at all times
 
Its unusual to have a pressure gauge on a HW cylinder or its E.vessel, if so, it should be a 0 to 10bar gauge, normally the precharge setting is 2.8 bar for the E.vessel and the PRV (pressure reducing valve) is set to 3.0 bar (cold), the minimum pressure with a stone cold cylinder will be 2.5/3.0 bar and if the whole cylinder is heated to 60C then a pressure of 3.8bar to 4.o bar is completely normal. the expansion relief valve is set to 6.0 bar and the T&PRV on the cylinder set to 7bar& 95C. I wouldnt be worried even if the pressure went up to 4.5 bar.
Obviously check the temperature of the water leaving the HW cylinder, a household thermometer will do this.
Check the setting of the cylinder thermostat strapped to the cylinder, it should be 60C.

With no heating on, either CH or HW, open a hot tap and run off say 5 litres of water, note the pressure, watch it for a hour or so and see if it increases.
 
Its unusual to have a pressure gauge on a HW cylinder or its E.vessel, if so, it should be a 0 to 10bar gauge, normally the precharge setting is 2.8 bar for the E.vessel and the PRV (pressure reducing valve) is set to 3.0 bar (cold), the minimum pressure with a stone cold cylinder will be 2.5/3.0 bar and if the whole cylinder is heated to 60C then a pressure of 3.8bar to 4.o bar is completely normal. the expansion relief valve is set to 6.0 bar and the T&PRV on the cylinder set to 7bar& 95C. I wouldnt be worried even if the pressure went up to 4.5 bar.
Obviously check the temperature of the water leaving the HW cylinder, a household thermometer will do this.
Check the setting of the cylinder thermostat strapped to the cylinder, it should be 60C.

With no heating on, either CH or HW, open a hot tap and run off say 5 litres of water, note the pressure, watch it for a hour or so and see if it increases.

Thanks for the detailed reply. After turning off demand for both CH and HW and releasing some hot water:
  • the pressure drops slightly (0.05 bar) as the water is running,
  • returns quickly to previous reading (i.e. back up by 0.05 bar) afterwards,
  • then very gradually drops over the next few hours to 2.5 bar.

I can see a 6-bar pressure relief valve and a 3-bar pressure reducing valve.
The pressure gauge isn't part of the cylinder itself, but connected to the mains pipework (i.e. not the heating coil circuit) nearby.
The cylinder stat appears to be set to 55 degrees.

The HW side of things behaves perfectly when there is no call for CH: the cylinder stat kicks in when the temperature dial on the boiler is on maximum, and the hot water doesn't get ridiculously hot. The pressure gauge on the cylinder gets to a maximum of 3 to 3.1 bar or so. Does this suggest that the 2-port valve for HW is actually working properly in this case (i.e. it isn't stuck open), as are the expansion vessel and cylinder stat?

But as soon as there is any call for CH, the cylinder stat seems to be ignored whether or not HW has been called for. After a couple of hours, the hot water is noticeably hotter than before (I don't have a thermometer, but can't leave my hand under the tap at all) and the pressure gauge has passed 4 bar at the end of the scale. The radiators are heating correctly, however, in line with the temperature dial on the boiler. Wouldn't this suggest that the 2-port valve for CH is also working properly in this case, as is the CH stat? But there is some interaction with the HW...

Could it be that the individual components are fine but wrongly connected somehow? Maybe:
  • incorrect pipe work (e.g. bypass in the S-plan is in the wrong place)?
  • or faulty wiring in the S-plan junction box?
 
So far, so good, everything normal to my next text.
Thanks for the detailed reply. After turning off demand for both CH and HW and releasing some hot water:
  • the pressure drops slightly (0.05 bar) as the water is running,
  • returns quickly to previous reading (i.e. back up by 0.05 bar) afterwards,
  • then very gradually drops over the next few hours to 2.5 bar.

I can see a 6-bar pressure relief valve and a 3-bar pressure reducing valve.
The pressure gauge isn't part of the cylinder itself, but connected to the mains pipework (i.e. not the heating coil circuit) nearby.
The cylinder stat appears to be set to 55 degrees.

The HW side of things behaves perfectly when there is no call for CH: the cylinder stat kicks in when the temperature dial on the boiler is on maximum, and the hot water doesn't get ridiculously hot. The pressure gauge on the cylinder gets to a maximum of 3 to 3.1 bar or so. Does this suggest that the 2-port valve for HW is actually working properly in this case (i.e. it isn't stuck open), as are the expansion vessel and cylinder stat?


The > 4 bar pressure does indicate that the cylinder is still heating,
suggest the following.
CH off, HW ON . Note the cylinder stat setting (normally 60C), turn the stat up until the Hw 2 port valve opens, turn the stat back down until it closes.
CH ON, HW OFF. Turn the stat up and see if the Hw 2 port valve opens, if it does, turn the stat back down until (if) it closes.
Post back.

But as soon as there is any call for CH, the cylinder stat seems to be ignored whether or not HW has been called for. After a couple of hours, the hot water is noticeably hotter than before (I don't have a thermometer, but can't leave my hand under the tap at all) and the pressure gauge has passed 4 bar at the end of the scale. The radiators are heating correctly, however, in line with the temperature dial on the boiler. Wouldn't this suggest that the 2-port valve for CH is also working properly in this case, as is the CH stat? But there is some interaction with the HW...

Could it be that the individual components are fine but wrongly connected somehow? Maybe:
  • incorrect pipe work (e.g. bypass in the S-plan is in the wrong place)?
  • or faulty wiring in the S-plan junction box?
 
Easy way to test with the heating system cold feel the cylinder 2 port valve ( eg the side that the cylinder connects to) now turn the heating on just heating does this start to warm up you may have to wait 5-10 mins you will be able to feel a difference if the port valve is working correctly if it’s the same temp as the heating port valve it’s passing
 
Thank you both.

John.g: the cylinder thermostat dial doesn't seem to want to turn. There's the tiniest bit of movement (1 or 2 degrees in either direction), but I don't dare force the dial in case the plastic snaps. It looks like a Fabdec 951879a. I'm not sure what the black button next to the dial is for.

ShaunCorbs: yes, the pipework just after both 2 port valves gets hot when only the CH is turned on from cold.
 
Port valve passing need replacing
 
Looks that way all right but a bit strange that with HW only selected that the cylinder temperature doesn't continue rise, if the zone valve is passing it can only mean that the HW programmed on time is spot on and shuts the boiler down or the CH comes on shortly afterwards.
Might be worth extending the HW on time and letting the CH off until/if the cylinder continues heating.

Probably no good though because boiler will shut down anyway once SP temp is reached on HW only.
 
Last edited:
Thanks again for your help. Having got hold of a thermometer, I was wrong about the water being hotter than before when the pressure reading was higher.

I have since discovered that the pressure gauge is attached to a tiny pressure reducing valve between the cylinder and a shower I don't often use. The gauge is just after this valve and before the shower. There was some trapped air in this part of the pipe which was causing havoc. I think the trapped air was being heated due to the pipe in question being close to a very hot central heating pipe under the floor somewhere. The expanding air may have increased the pressure in that section of the pipe where the gauge was. The pressure reducing valve was stuck closed, not allowing any backflow when other hot water taps were turned on. Letting hot water out of that shower seems to have displaced the trapped air and sorted things for now.
 

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