Boiler Not Heating the Hot Water Properly

[Johntheo5]

Your stat is OK, the click is happening at the ~ present cylinder temperature.
At least its heating up now in 45 minutes but this means that the coil (assuming no HW leakage) is only outputting ~ 3.5/4.0kw, coil may be getting scaled up but it still wont require any extra energy to heat up the water. (this is why the coil flow & return feel more or less the same temperature)
if you like, just switch on the HW just now, take a photo of your gas meter and exactly 3 minutes later, take another photo, we can then see what the coil output is, within reason.

 

[WhyMyBoiler]

Your stat is OK, the click is happening at the ~ present cylinder temperature.

That is good to know... One thing that did occur to me but sounds as though it may not be the case is that, in my initial post I mentioned the following:

The boiler is set to come on for twenty minutes each morning to heat the water. Usually, this works as expected and I hear the boiler come on and the pipes knock for around two minutes (which I assume is the pipes expanding slightly, they've always done that). Then everything settles down, the boiler runs quietly and I have piping hot water.

Recently instead of the pipes just making a knocking sound for two minutes, they now do it every 15 seconds so it's almost as though the boiler is kicking in, stopping and starting repeatedly even though that's not actually what's happening, i.e., the boiler remains on for the full 20 minutes.

Thinking about that made me wonder whether in fact, the hot water cylinder thinks the hot water has reached the correct temperature and so is constantly telling the boiler it's up to temperature so the boiler is switching off but then five seconds later sending another message to say, actually it's not up to temperature so it kicks back in again. I don't know if that would explain the unusual knocking pattern in the pipes while they are expanding which it only does for the first two minutes when everything is working correctly.

It sounds as though it is constantly starting and then stopping again over the course of the 20 minutes rather than running continually over the full 20-minute duration and so subsequently it never gets above around 45 degrees.

I don't know if that makes any sense but it almost 'sounds' as though it is coming on for around 15 seconds then switching off because it's getting a false temperature reading but then coming back on again and repeating throughout the 20 minutes.

Having said that though, as far as I'm aware the boiler isn't switching off, in as much as the second green light that appears on the front when the boiler kicks in remains on for the full 20 minutes, at least I think it does, it certainly did when I put it on for 45 minutes, though it did switch off after around probably 35 to 40 minutes, I assume because the water in the cylinder was up to temperature.

iI you like, just switch on the HW just now, take a photo of your gas meter and exactly 3 minutes later, take another photo, we can then see what the coil output is, within reason.

The gas usage on the smart meter is consistent each morning...

I've taken readings over a 3-minute time span while the HW was on but I'm not sure the figures make a lot of sense as they bear no resemblance to my monthly gas usage which is on average 14 to 15 m3 (though that does include some brief cooking on a gas hob) but if I've used 0.134 m3 over three minutes if you divide by three to give you one minute and multiply by 20 to give you 20 minutes and multiply that by 31 for the days in the month that gives a figure of 27.69 m3, almost double the actual usage.

After 3 Minutes - 01317.000 m3
Before HW on - 01316.866 m3

Total Usage = 0.134 m3

The Volume Correction value is 1.02264
The Calorific Value is 39.9
The Conversion from Joules is 3.6

 

[Johntheo5]

That equals a boiler output of 28.9kw!!, boiler running flat out, can you repeat it but then after the boiler fires up, wait for ~ 5 minutes until the boiler modulates down, I presume this boiler does modulate?
Then repeat that 3 minute measurement. You will/should hear it modulating down.

 

[WhyMyBoiler]

That equals a boiler output of 28.9kwk!!, boiler running flat out, can you repeat it but then after the boiler fires up, wait for ~ 5 minutes until the boiler modulates down, I presume this boiler does modulate?

Yes, the boiler does apparently modulate... Okay, so that seems to have provided a more sensible reading...

After 3 Minutes - 01317.223 m3
Before HW on - 01316.160 m3

Total Usage = 0.063 m3

 

[Johntheo5]

Yes, the boiler does apparently modulate... Okay, so that seems to have provided a more sensible reading...

After 3 Minutes - 01317.223 m3
Before HW on - 01316.160 m3

Total Usage = 0.063 m3

1317.223-1316.16 = 1.063m3?? it read 1317.223 after the last test.
Don't start taking readings until the boiler has modulated down, ie first reading after its modulated, second reading 3 minutes later.

 

[WhyMyBoiler]

Apologies, that was a typo by me, it should have read...

After 3 Minutes - 01317.223 m3
Before HW on - 01317.160 m3

Total Usage = 0.063 m3

 

[Johntheo5]

But you had 1317.223 at the end of the last test so if you left the boiler run for say 5 minutes to settle down then the reading for the start of the new test must read more than this

 

[WhyMyBoiler]

But you had 1317.223 at the end of the last test so if you left the boiler run for say 5 minutes to settle down then the reading for the start of the new test must read more than this

It does... at the end of the first test the meter reading was 01317.000 m3 and at the start of the second test, (after letting the boiler run for five minutes prior to taking any readings) the meter reading was 01317.160 m3

First Test Running for Three Minutes

End Reading - 01317.000 m3
Start Reading - 01316.866 m3

Total Usage = 0.134 m3
Second Test After Letting the Boiler Run for Five Minutes and Then Taking Readings Over a Three Minute Timeframe
End Reading - 01317.223 m3
Start Reading - 01317.160 m3

Total Usage = 0.063 m3

 

[Johntheo5]

OK then, thanks, consumption 0.063*60/3*39.9/3.6, say 13.97kw, assuming a very minimum 78% boiler efficiency then the boiler output was 10.9kw.
If 120L of water is heated from say a worst case of 15C to 60c then this will require, 120*(60-15)/860, 6.28kw. time to heat = 6.28/10.9*60, 34.5, say 35 mins. you say the cylinder temp was 45C so heat input required, 120*(45-15)/860, 4.19kw, time to heat = 4.19/10.9*60, 23 mins. Based on those numbers then it does appear that 35/40 minutes is required to reheat the full 120L from 15C to 60C, if you were getting 60C after 20 minutes then the only logical reason IMO is that the whole 120L are/were not used, if 50L at 60C remained in the cylinder at the start of each reheat period then the boiler will reheat the bottom 70L from 15C to 60C in 20 minutes.

 

[WhyMyBoiler]

Many thanks for your detailed calculations, I suppose my questions would be:

Why would this have suddenly changed over the last few weeks when for the past ten years, running the boiler for 20 minutes has produced piping hot water?

When this issue first started happening, i.e., the hot water temperature from the taps gradually reduced until it was very noticeably cooler than 60 degrees. It was suggested by my plumber to MagnaCleanse the system which he did and for the first couple of weeks after doing so running the boiler for 20 minutes once each morning was once again producing piping hot water but then it slowly started reverting back to the 45 degrees it's providing again now.

I'm absolutely not disputing your calculations since they make a great deal of sense, even though I appreciate the figures provided are not very scientific but fundamentally nothing has changed over the past ten years apart from the hot water cylinder being changed to a much more efficient one (the old one was a copper tank with thin insulation wrapped around it, the new one is a stainless steel tank with about an inch of fixed insulation covering the entire tank, both tanks are the same volume. Even prior to the cylinder upgrade the old inefficient tank was providing piping hot water based on a 20-minute hot water cycle from the boiler.

That's what makes no sense to me along with the change in the way it sounds as though the water is being heated with a seemingly constant on/off effect in the pipes, knocking for 15 seconds then stopping for 5 seconds and repeating for the entire 20 minutes.

When I was getting piping hot water, as mentioned earlier the knocking of pipes which I understand to be the pipes expanding as they heat up lasted for 2 minutes when the boiler kicked in and then everything was silent for the remaining 18 minutes.

 

[Johntheo5]

If the boiler is cycling on/off which I think it is (knocking for 15 seconds then stopping for 5 seconds and repeating for the entire 20 minutes.) then this means that its minimum output is greater than the cylinder coil demand, you would have to monitor this especially as the hot water nears around 50C as the coil output decreases with increasing cylinder temperature, could be boiler circ pump degradation, other than that I really dont know but it wouldn't bother me unduly if it only involves just increasing the programmed time.

What is the minimum output of your boiler?.

 

[WhyMyBoiler]

Okay, I can certainly run the boiler for an extended length of time and see what impact that has overall...

By the boiler circ pump are you referring to the external pump (shown in the earlier photo) or is that a different pump inside the boiler itself? I only ask because the external pump shown in the earlier photo was replaced about a year ago as well.

I do live in a very hard water area so I can imagine that it's quite likely this could cause issues over time though I assume one reason for the MagnaCleanse was to flush the system out.

From the symptoms described so far, is there anything at all that would lead you to believe there could be an issue with the heat exchanger in the boiler or an issue with any other component in the boiler itself?

I can't recall which setting the external pump is on, from memory it has three speed settings and I think it's on the lowest, would it make any difference switching it to the next speed setting?

Many thanks for your patience and the time you've taken to answer all my questions...

 

[Johntheo5]

The hard water should have no effect on your boiler (or cylinder heating coil) internals except you have a leak and its continually being replaced with fresh (hard) water. It could cause a build up on the heating coil externally which will impede heat transfar but this is generally a slow process. You could increase the pump speed but if you have a open vented system with a feed&expansion tank, ensure that its not pumping over through the vent as this will certainly lead to sludge/corrosion.

 

[WhyMyBoiler]

Just a quick update...

It occurred to me that before the Magnacleanse, the thermostat on the HW cylinder was set at 60 degrees. For some reason, after the Magnacleanse my plumber set it to 65 degrees.

I set it back to 60 degrees a couple of days ago and lo and behold it 'appears' (touch wood) as though normal service is resumed. There is no continual knocking in the pipes for the full 20 minutes the boiler is running so it no longer sounds as though it is starting, stopping then starting again. I hear knocking in the pipes for around two minutes (as before) as they expand then silence for the remaining 18 minutes and after 20 minutes I have piping hot water again.

I'm curious as to whether this makes any sense...