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Open vented system, No HW, CH works fine, BAXI 80HE boiler shows Dry Fire after every restart

Discuss Open vented system, No HW, CH works fine, BAXI 80HE boiler shows Dry Fire after every restart in the UK Plumbers Forums area at PlumbersForums.net

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Hi,

I have an open vented system with Cylinder and F&E tank in the loft. I have noticed for about 2 weeks now that the hot water has been gradually getting less hot and currently is only luke warm. I tried raising the termostat on the cylinder which does not seem to take any effect.

I do not think (although I am fairly new to this type of system) that the problem is the 3 port diverter valve as when i crank up the termostats on the radiators and boiler kicks in I can feel the pipe is hot on both side of the A and B piping. The Grundfos || pump which is located below the diverter valve also seem to be working fine. What I noticed however is the fact that even though the pipe that feeds the coil in the cylinder is very hot the return pipe from the coil seems to be cold (this only happens when I am blasting the radiators as with CH disabled they never get even luke warm). What I also noticed is the BAXI boiler going into Dry Fire mode in any other setting apart from when I turn the heating on via valves located on the radiators. I have also a Drayton thermostat downstairs that has CH and HW constantly on (a toddler in the house).

If I understand the situation the problem is most likely either an airlock or a blockage inside the coil. Am I right? I probably have an automatic air vent installed as you can see on the attached photo but is there a possibility it got stuck and there is an air lock somewhere near or inside the coil that I would be able to bleed? Also I was checking if a magnet attracts in the flow and return sections of the piping and I could not see this happen.

Another issue I have is the immersion heater which worked for a while and now completely stopped working too but that may be a different problem altogether.
 

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There is almost certainly a vent on the 15mm pipe that rises between the 3 port valve and the cylinder.
It may not be an automatic air vent, but that's where the problem will be.

Might look like any one of these, manual air vents,

Air vent collage.jpg
 
There is almost certainly a vent on the 15mm pipe that rises between the 3 port valve and the cylinder.
It may not be an automatic air vent, but that's where the problem will be.

Might look like any one of these, manual air vents,

View attachment 94444
Hi @snowhead. Thanks for your post. From what I have checked just now it looks like the top left one. I can see online now it is a manual air vent - thanks. How do I bleed it?
 

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Just LOOSEN the screw cap and let the air out till it's water, assuming there is air at that point.
The caps usually have a sealing washer in which sometimes sticks to the vent body stopping both air and water coming out, you may need to fully remove the cap to free the washer.
There won't be much pressure as it's almost at the height of the feed tank.
 
Just LOOSEN the screw cap and let the air out till it's water, assuming there is air at that point.
The caps usually have a sealing washer in which sometimes sticks to the vent body stopping both air and water coming out, you may need to fully remove the cap to free the washer.
There won't be much pressure as it's almost at the height of the feed tank.
Thanks for the advice @snowhead. I did unscrew the cap and some air escaped after which water showed up on the outside. I then quickly closed the cap. As the boiler still shows dry fire I switched it off and on (the boiler goes to dry fire unless I turn up the rads). Once I turned up the rads they warmed up but the return pipe got only marginally warmer if anything (it is still luke warm and miles apart comparing how it used to be before this started). As soon as I turn off the rads the boiler goes back to dry fire. The Drayton thermostat downstairs has HW and CH put on constant ON.

Would you have any other ideas what can I try or should I call for someone? Could it be the blocked pipes which comes to mind next?

I would like to first check all I can as last time I had problems a plumber told me power flushing the system could fix any sludge problems that cause blockages and wanted to charge me £600-£700 for that. He could not guarantee, however, that this would be the fix.
 
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Yes I managed to fix it now. I believe I release some trapped air from the manual air vent few times that definitely helped unblock the HW flow.

I did a lot of tests and turned out apart from 3 port valve being slight worn (failing to operate properly on the CCIII settings) there was nothing fundamentally wrong with my system.

One other thing I did was to change the circulation pump from CCIII to CCII setting.

Currently it all works fine on CCII (pump shows around 18-19W), 20C on room stat, CH and HW ON on programmer, cylinder stat reset to 0C and back to 60C and boiler temperature gauge on a high setting. I am waiting few days to verify the boiler does not go into DRY FIRE any more (which was caused by all TRVs being turned down and room stat set to 22C so no flow to the boiler) but so far so good.
 

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