We have a problem at the moment with the hot water supply. We can get hot water with taps open slightly, but as soon as you ask for more water, IE open two hot taps or open up a tap fully, the pump cuts off, which results in no hot water.
If you wait a min or so, then you can get hot water again (only low pressure).
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.
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