hi chip, a few questions to clarify. I don't know anything specifically about your heat pump, this is just general stuff.

when you refer to reheating the cylinder after a small draw-off of water, is that the immersion coming on? or is it the heatpump doing it with water from the heat pump going through the cylinder coil? I ask because you refer to "piping hot water" which might be an indicator of an immersion. if its the immersion, does your heat pump control the immersion - i.e. is the immersion wired through the heat pump in some way? or is the immersion wired independently? if its the immersion and its wired seperately, you just need to look at what temperature that is set to and when its coming on. It could perhaps be on all the time at a high temperature from what you've said.

if its heat pump coil: what does the heat pump controller say that the temperature of the cylinder is when the cylinder heating comes on? is there a target upper and lower temperature set in the heatpump controller? are there timers set in the heatpump controller as to when it should maintain this temperature and when it should not? does it have settings to say whether it does it using the immersion, the heat pump, one after the other, or both ? you refer to turning off timers, where are these ? where, in height terms in your cylinder , are the immersion, the sensor that informs the heat pump about the cylinder temperature, and the hot input and cold output to/from the heat pump to the coil in the cylinder? doesn't need to be to the exact mm, just rough.

I'd be very skeptical of someone saying they didn't change anything if the issue started right after they were there doing things. they must have done something.

@iancalderbank many thanks for the reply and input. In answer to the points you raised;

No, it isn’t heating up via the immersion. The heat pump kicks in and it is reheated that way. When I say “piping hot” I meant that the water is still more than hot enough for additional showers etc without moving the blender tap. The immersion is on a separate switch / fuse spur and is always switched off. The timers are set in the Vaillant VRC wall controller that is linked to the system

The control is set to a target water temp of 49C. This is reached once the timed program has done its stuff. The problem I then find is after we’ve both showered the cylinder temp shows as approx 15C and the heat pump comes on to reheat the water (despite the fact you could easily have another 4 or 5 showers with the water already more than hot enough and the system not being timed to come on). As previously mentioned, Vaillant are insisting this is correct procedure, that the tank will always reheat once it drops below 35C regardless of whether it is timed to come on. However, as I also mentioned, this has never happened in the past and, in fact, after a few showers today it’s currently at 25C and it’s not attempted to reheat. So that kinda disproves that argument. Likewise when we’ve gone away (or I’ve forgotten to set the timer) I’ve found the cylinder temp at around 12C (stone cold) which again shows it doesn’t always reheat below 35C. 

The service engineer insists he hasn’t changed anything (I’m not a 100% sure that the issue didn’t start prior to the service) and Vaillant have checked and confirmed the settings are all set correctly (ie nothing has been tweaked) 

The sensor is set fairly low on the cylinder (approx 1/3 from the bottom) so it will obviously always read much cooler as the water inlet is also situated towards the bottom. 

Having now looked at the going ons / cylinder temps more closely I have also noticed another issue. After the anti legionnaires cycle, the water temp is still only ever 49C (ie doesn’t go higher than the normal daily target). I’ve even set it to run the anti legionnaires daily and it won’t rise higher than 49C. I’m wondering if this has always been the case and, if so, is quite alarming

@chip

I think you will find that on most system, the immersion heater is used for the final stage during the legionella cycle to achieve the required 60C. This is normally controlled automatically by the heat pump controller, via an external electrical contactor, to switch the immersion heater on and off.

If your system is not wired to control the immersion heater then this will not be possible, particularly if the immersion is permanently switch off.

@chip as derek says, the immersion needs to come on , one way or the other, to get over 60C. if its not setup to do this under the control of the heat pump, you need to do it separately. usual method is once a week during the night via an immersion timer. sounds like you've never had this.

as for your hot water, it sounds like 2 showers is using up the bottom 1/3 of the tank, thus the stat at 1/3 up shows cold. but the top 2/3 is still hot water.

(sorry, just reread). If vaillant support are saying it comes on if temp is less than 35, regardless of whether the DHW "reheating is allowed" timer is active or not, then I can't help you with that. that implies that the system does what it wants to in that situation and you can't influence it. There may be vaillant owners on here who can address this question first hand. I can't think of any other way to approach it.

and/or 1/3 up sounds a bit low - where is that in relationship to the input and output of the heat pump into the cylinder coil? Is there another higher up pocket?

@chip I have a Vaillant and have been trying to get mine to do what yours is now doing, as every morning my heat pump is having to heat the water up from about 20 degrees to 51, I have now managed to set it so it reheats the DHW when temp drops to 41, this was done in basic settings and I altered the DHW from auto to day. So I suggest you see if yours is set to day and if so alter it to auto, this means it will only heat the DHW at the times you have set