damon
New Member
Posts: 3 
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Post by damon on Mar 17, 2013 11:08:44 GMT -5
Hi all. Been fixing various faults on my 155x over the years. My central heating works fine, hot water fine but if central heating running I loose hot water. If I turn c heating stat down and wait 15 min the hot water returns. I did think the blending valve was faulty but surely this would effect hw performance all the time? Many thanks Damon
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Post by Mike the Boilerman on Mar 18, 2013 10:50:47 GMT -5
Damon,
This is a fault I've heard mentioned but I've never actually seen it in the flesh. It sounds a perplexing fault but should be quite easy to diagnose for someone actually in front of the boiler. First thing to check is whether the DHW pump is running while you're getting no hot water. I suspect it isn't. Can you check this and report back please?
Thanks,
Mike
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Post by northpole on Mar 18, 2013 10:52:43 GMT -5
Damon,
You probably know this already, but when the central heating starts the contents of the hot water tank are fed into the radiator circuit, which will probably be much colder. Hence if you try to run the hot tap for the first 10-15 minutes until the tank contents have returned to working temperature, the water will be much cooler than normal as the heat exchanger has less heat available to transfer.
Could this be the cause of the problem you describe?
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damon
New Member
Posts: 3
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Post by damon on Mar 18, 2013 11:47:06 GMT -5
Hi and thanks for the advice so far. Yes the dhw pump runs fine. It doesn't seem to matter if the ch has been running for ages. It seems like the boiler maybe struggling to heat the dhw at the same time as ch. There is an external heat exchanger on side of boiler, is this how the dhw is heated or is there a coil inside the ch tank? Many thanks Damon.
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Post by Mike the Boilerman on Mar 20, 2013 14:58:34 GMT -5
The DHW is heated in that external plate heat exchanger. Feel the temperature of the water entering it during DHW demand. I bet it is not especially hot. This can happen when the CH load is higher than the power of the boiler, preventing the thermal store from ever recovering to it's proper temperature when the CH is ON.
Mike
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Post by steve on Apr 3, 2013 11:34:52 GMT -5
hi
I have the exact same problem.
I have been told it may be the external heat exchanger as these get furred up causing restriction and this will be worth changing. My heat exchanger gets very hot and is too hot to touch when drawing water - sign of being blocked ?.
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Post by Mike the Boilerman on Apr 5, 2013 3:13:08 GMT -5
Steve,
I suspect you have a different problem really. Does your hot water work fine when the heating is OFF, like Damon's?
Scale in the DHE causes cool water regardless of whether the heating is ON or not, but it's a common problem.
Mike
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Post by steve on Apr 5, 2013 5:03:14 GMT -5
Mike
Thanks for prompt reply and all other other info on your site.
Hot water is ok when heating is off.
Also forgot to mention on forum that the top left (item 2) limit thermostat trips out and needs resetting quite regularly - Are both these problems connected?
Any common problems that may cause these?
Regards
Steve
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Post by Ian on Apr 5, 2013 12:53:14 GMT -5
What is the differnce between the limit thermostat (item 2) and the overheat thermostat (item 4)?
Thanks
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damon
New Member
Posts: 3
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Post by damon on Apr 7, 2013 4:42:33 GMT -5
Thanks for lots of useful info. I am thinking with 11 rads and dhw demand in the recent cold weather it is to much for the boiler. I have told everyone to use the hot bath tap at half open and seems fine now. Great site ! Cheers
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Post by Mike the Boilerman on Apr 8, 2013 14:14:35 GMT -5
Thanks for lots of useful info. I am thinking with 11 rads and dhw demand in the recent cold weather it is to much for the boiler. I have told everyone to use the hot bath tap at half open and seems fine now. Great site ! Cheers Hmmm that sounds as though you might have a scale in the heat exchanger problem then. If it gets slowly worse as time passes, then this will be the problem. Mike |
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Post by Mike the Boilerman on Apr 8, 2013 14:15:59 GMT -5
What is the differnce between the limit thermostat (item 2) and the overheat thermostat (item 4)? Thanks Hardly any difference. Their sensors go into different pockets in the thermal store but the function is virtually identical. Mike |
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Post by LukeBrzeczek on Sept 16, 2019 5:57:27 GMT -5
Just wanted to share my experience and add to the information here as it may help others. Forgive me if in my explanation I state some obvious points but I am trying to help everyone, not just those who are experienced.
I also recently experienced a loss in hot water however, I was able to get hot water as long as the central heating was up and running. Turning up the stat and running the central heating for a hot shower or sink full worked but was no solution but it did make it manageable while waiting for parts. After reading here I was able to narrow the problem down to:
A) Blender valve malfunction
B) Heat Exchanger calcification/build up as I have fairly hard water
C) Flow switch malfunction
Initially when opening the hot tap you would get a small surge of about 20-30 seconds of hot then it would run cold, that was it. I assumed this was the latent heat left in the exchanger and that was it. With that I left the tap running out, albeit cold. Checked the boiler with the cover off and the lower right circulation pump was not running. Obviously this would mean with no water circulation, no hot supply to the heat exchanger, no thermal exchange across the plates of the heat exchanger and so no hot water. To confirm if you are not certain if the pump is running you can carefully feel the pipes either side of the pump unit.
This lead me to believe that there was insufficient flow across the flow switch for the switch to make contact and send the demand signal to the circulation pump. However, as mentioned, there is a second demand for this pump when the central heating is running which enabled me to still receive hot water. With that, as I had read that the blender valve can fail or stick I demanded heating to run the circulation pump, I then also ran a hot tap and while running the hot tap, once confirmed hot at the tap, felt the outlet from the blender valve as I exercised the blender/adjuster screw full travel from fuel open to full closed. The valve appeared to be functioning and adjusted the water temperature accordingly. While it may of appeared to have functioned normally it may still have been restricting flow but at this time it was now bottom of my three causes.
My attention turned to the heat exchanger as a small drip from an old 'o'-ring on the flow switch had dripped on to the small, inlet pipe mounted ionising water softener module (a small white unit, fed from the internal boiler supply). If this had failed then it was likely my exchanger had an excessive build up. I removed, cleaned and checked this ionising unit and it was functioning correctly, power to it and on/off LED functioning so it was also likely that the exchanger was fine, but this didn't rule it out.
I also gave some thought to flow switch as this was not in great condition and I have heard, not only of the 'o'-ring seals in these perishing but the housing itself being prone to cracking. Also due to the loss of hot water being a more or less an instantaneous problem not a gradual build up I was turning more away from the exchanger and to the switch. I had heard some experiencing a gradual loss of water to almost a trickle. I had not experienced this, the blender valve appeared working and I had not had the need to adjust up the blender valve to compensate for the lack of flow worsening through the exchanger over time as you may expect.
I decided to order a new flow switch and relay, while not 100% convinced it was the fault, thinking I would replace it due to it's condition and that I would also descale the heat exchanger, then if that was still not working I would replace the blender valve being the most expensive part.
At this point I would like to mention and part of the reason for this post be warned. If you try to search for a flow switch online there are a few sellers out there who will offer this switch for around £25-£70. This is for the switch section only and not the included relay in that circuit. Furthermore, you cannot just wire this new switch into the old relay because the original is a resin sealed unit and you have no chance. So for the sake of being a proper repair I sourced an original part from a reputable plumbing centre at around £170.
I wanted to replace the switch first, turned off the boiler, shut off the boiler mains water inlet and drained down using hot water taps downstairs (my boiler being positioned upstairs). I removed the old switch and checked the paddle which moved freely. At this point, and of course where I could have checked this prior to ordering a new part, I turned the boiler back on, hot water only and functioned the switch, no demand for the circulation pump was seen. As mentioned my choice for replacing the switch was also due to it's condition. I turned the boiler back off and wired in the new switch and relay. Prior to installing the switch I turned the boiler back on and checked the switches function and sure enough the circulation pump kicked in and so this was my problem solved. Of course I then reinstalled the switch, turned the mains water back on, bled the system and fired up as normal. Upon checking the system all is now functioning as it should without having to remove and descale the heat exchanger.
Hope this may help
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