Boiler still heating water despite immersion having recently heated the water

[UKMO]

Hi. I would be very grateful for any help please.
I have a hot water cylinder that is heated primarily by a gas boiler. I also have an immersion heater in the cylinder attached to a timer.

I have solar panels and a 10kwh storage battery and so usually have a lot of surplus and/or saved electricity and so I usually have the immersion come on at night and then again early afternoon (say, 2pm ish) with the intention to not often need the gas boiler to come on to heat the water because I can do it with free solar/stored energy. The gas boiler is on a timer so that it will kick in to heat the water at 4.30pm-5.30pm either if the water in the cylinder needs the temperature topping up or if, for any reason, i haven't used the immersion earlier that day/previous night.

On recent occasions when i have heated the cylinder overnight and then switched it on again in the early afternoon (in the afternoon the cylinder is usually hot enough so that the immersion only needs to be on for a short time - approx 1 hour is usually enough. 2 hours is the absolute maximum needed) to get the water back up to temperature (the temperature dial at the bottom of the cylinder is set to 65 degrees C). I am finding that the gas boiler still kicks in at 4.30pm despite the immersion having heated the water up to temperature only an hour before and is staying on for the hour that it is set for.

My query is why is the gas boiler still kicking in to heat the water? I had the immersion on overnight to ensure the water was up to temperature, so that would have gone off around 4am. Then at around 2pm the immersion has been switched on again to ensure the water is still at the maximum temperature. Usually the immersion switches itself off about 2.30pm because the water only needed a brief boost to get it hot again. So then at 4.30pm I would either expect the boiler to NOT kick in for hot water or, if it did, to only kick in very briefly to get the (already hot) water up to the maximum temperature.

Could someone please help me understand what is happening here and if there are any adjustments i need to make. Is the boiler possibly sensing that the water in the cylinder is below a temperature that it is aiming for and therefore is still trying to heat the already hot water up even more? If so, how do i adjust that?

In addition, i was trying to work out was going on and so I switched the boiler off for the day and put the immersion on overnight (went off around 4am) and then switched the immersion on for an hour again at around lunchtime. So all the while the gas boiler is completely switched off and only the immersion has been used to heat the water (and the cylinder has got very hot after each time - indicating it has been heating up as expected). Yet that evening when having a shower there was only limited hot water for a short while and then the water started to get noticeably cooler. Not cold, but lukewarm. I would have expected the water to still be pretty hot from about 6 hours previously.

The cylinder and boiler are all pretty new (about 4 years old). The house was built in 2019 and there have been no indications of any problems with the boiler, cylinder or immersion. And the cylinder is well insulated. I watch the timer for the immersion and the energy readings on my PV / solar app when the immersion is on and when it switches off and it seems to be operating fine. So I am just curious about the boiler still kicking in when the water is seemingly hot and also why the water is only lukewarm after a few hours when the boiler is off yet the immersion has been on during the day.

Any help would be greatly appreciated. Many thanks

 

[cross thread]

Cylinder stat set at 65 degrees ! Lower it to 50 if it’s near the bottom of the cylinder

 

[ianmcd]

post a pic of your cylinder showing position of the thermostat and the immersion heater(s)

 

[SpecialK]

As above if the immersion is at the top it will only heat the top of the tank.

 

[UKMO]

Hi all. Thanks so much for the replies. I have turned the dial at the bottom down to 50 degrees as suggested and I also attach photos of the cylinder. Any further guidance is hugely appreciated

 

[JohnD]

The immersion heater is at the bottom of the cylinder, which should be ideal for your purposes. But you say the amount of hot water is small.

I wonder if there is something draining heat or hot water.

How far is the bathroom from the cylinder?

Experimentally, turn on the immersion heater, with the boiler off, and watch the meter to see how long it runs for before its own thermostat turns it off. A 3kW immersion heater heats water at the rate of about 1 litre per minute (easy to remember) so if your cylinder is, for example, 150 litres, the immersion heater should run for about 150 minutes. If no hot water is being used it will remain hot enough for a bath for several days because the insulation is very good. While it is turned off, feel all the pipes to see if any of them are hot or warm. The pipe coming out of the cylinder will be hot in the vertical section, but should be cool after a couple of feet. The pump should not be running.

 

[Mottie]

I have the same thermostat on my hot water cylinder for the gas heated water and 45° at the bottom of the cylinder would give me VERY hot water so I have mine set at just above 40°.

 

[Johntheo5]

On recent occasions when i have heated the cylinder overnight and then switched it on again in the early afternoon (in the afternoon the cylinder is usually hot enough so that the immersion only needs to be on for a short time - approx 1 hour is usually enough. 2 hours is the absolute maximum needed) to get the water back up to temperature (the temperature dial at the bottom of the cylinder is set to 65 degrees C). I am finding that the gas boiler still kicks in at 4.30pm despite the immersion having heated the water up to temperature only an hour before and is staying on for the hour that it is set for.

If you reheat any size cylinder for 1 hour with a 3kw immersion and assuming the whole cylinder at 65C in the morning then it will have reheated 47L of water from mains at say 10C, this equals to a usage (or leakage) of 86L at 40C, or 172L if reheated for 2 hours, does this sound like your normal usage in the morning??,

 

[UKMO]

Thanks again for all the replies. It is greatly appreciated. I have followed the advice and done some more experimenting this morning but got the same result again. To clarify, I ran the immersion overnight to heat the tank up and that worked fine. Then, just to be sure that the water was up to temperature I switched the immersion on again this morning. I heard it kicking in and also felt the tank and pipes were very hot. The immersion was only on for a short while (maybe less than 20 minutes because the water was already hot from being heated last night). I then set the gas boiler timer to come on a few minutes later and would have expected the boiler to recognise that the hot water cylinder is already full of hot water and not need to come on. But the boiler did come on at the set time and was on for maybe 20 minutes or so. I had set the boiler timer for 1 hour to see how long it would stay heating the water. The boiler itself went off after approx 20 minutes and stopped firing at that point. However, the thermostat still has the tap symbol showing. Why is this? Is this just indicating that the water heat timer is still active? Or is it indicating that water heating is in progress? Sorry for all the questions. I am at a loss to understand why the gas boiler would need to come on at all if the water has just been heated up fully by the immersion. I've attached a photo below of the boiler timer/thermostat where it has a tap symbol showing now that the boiler has gone off.

And to answer an earlier question, there are 3 bathrooms upstairs, one is immediately next to the cylinder (literally the other side of a plasterboard wall) and the other 2 are less than approx 5 or so metres away. And there are no signs of anything draining the cylinder.

 

[Johntheo5]

I think the tap signal shows for the whole programmed time, the cylinder stat determines how long the boiler actually fires for, the display may show a flame symbol for this when the boiler is actually firing.
What is the boiler flow temperature set to, if its set below the cylinder stat set point then the boiler will keep firing on/off just to keep itself warm as the cylinder stat will never be satisfied, however as you have now set it to 50C then the boiler set point would have to be as low as 50C or lower. Does your boilrer use outside te mperature compensation for its CH??.

 

[UKMO]

I think the tap signal shows for the whole programmed time, the cylinder stat determines how long the boiler actually fires for, the display may show a flame symbol for this when the boiler is actually firing.
What is the boiler flow temperature set to, if its set below the cylinder stat set point then the boiler will keep firing on/off just to keep itself warm as the cylinder stat will never be satisfied, however as you have now set it to 50C then the boiler set point would have to be as low as 50C or lower. Does your boilrer use outside te mperature compensation for its CH??.

Many thabks for the input. Much appreciated. The clarification about the tap symbol is very helpful. I had misunderstood that to mean that the water was actually being heated. But it seems to just indicate that the timer period for hot water is still active. Even though the boiler is off. So that part is all clear and understood and seems to be working ok.

The boiler flow is set to the economy setting which when I turn the dial indicates 68 degrees. Which seems high for an ‘E’ setting?
But it is set above the cylinder stat set point.

And I don’t believe the boiler has any link to outside temperature.

 

[Johntheo5]

A boiler temp of 68C with a former cylinder stat setting of 65C may have resulted in the boiler continuing to fire on/off because that cylinder stat seems to be located just above the coil return. It is probably best not to set the cylinder stat higher than 58C/60C.
To check if the flame signal is present when the boiler fires just briefly turn up the cylinder stat setting to max with the boiler heating period programmed on.

 

[UKMO]

Super helpful Johntheo5. Thankyou. Please can i just clarify the last sentence and what i need to do and what i am looking for when i do it please?

 

[Johntheo5]

I think you have reduced the cylinder stat setting to 50C now?, if so, increase it to is max of 65C and see if boiler fires up with a flame signal as well as the rad symbol on the front, what make/model of boiler?

 

[UKMO]

OK. It's an Ideal Logic Heat H24. To run this test should i have the hot water timer set so that it should be calling for water or timer off?