Pump overrun is killing my heating!

[John.g]

EN12897.....temperature difference between water (65°C) and ambient (20°C) of 45K

The spec above is for the "VIH GB 260/2/S"

Total capacity litres 120 155 180 210 260 310
Actual capacity litres 119,2 149,1 176,6 211,2 250,3 298,5
Hot water capacity litres 104,0 136,7 166,4 203,9 247,5 270,5
Maximum supply pressure to pressure reducing valve MPa (bar) 1,2 (12)
Rated pressure of cylinder MPa (bar) 0,7 (7)
Maximum operating pressure of heating coil MPa (bar) 0,35 (3,5)
Operating pressure MPa (bar) 0,35 (3,5)
Pressure reducing valve MPa (bar) 0,35 (3,5)
Expansion relief valve MPa (bar) 0,6 (6)
Temperature and pressure relief valve °C,
MPa (bar)
90,
0,7 (7)
Charge pressure of hot water expansion vessel MPa (bar) 0,4 (4)
Maximum temperature of heating circuit °C 85
Maximum temperature of potable hot water °C 85
Standing heat loss kW/24 h 1,26 1,53 1,66 1,89 2,07 2,26
Heat up time according to EN 12897 mins 18 23 24 28 35 42
Recovery time (70% capacity) mins 16 18 17 20 25 30
Primary heat exchanger performance kW 18,6 18,8 22,0 22,6 22,3 20,4

 

[SJB060685]

Don't know what the EN standard is but assuming heating 250 litres from 20C to 65C with 22.3kw, 23.3LPM in 35 minutes gives, by calculation, a deltaT of 8C, rapid heat recovery systems don't care what the deltaT is they use the highest possible boiler temps with the greatest flow rates to achieve their object.

Can't attach files for some reason.

How are you arriving at those figures?
I'm right with you right up until the DT

 

[EvilDrPorkChop]

Interesting, the older UniStore is 22.3kw coil and the newer model is 16.6kw. They must have changed who manufactured the cylinders.

 

[John.g]

How are you arriving at those figures?
I'm right with you right up until the DT

I think you spotted my deliberate mistake SJB.

KW = LPM*deltaT*60/860 so deltaT = Kw*860/LPM/60 = 22.3*860/23.3/60 = 13.7C.

 

[SJB060685]

That was the figure I had !

 

[John.g]

@milleniumaire, have you noted the flow/return temps with HW heating on?, @EvilDrPorkChop says 80C flow temp, also I think the cylinder temp is set in the controller and not on the boiler front, maybe the boiler front acts as a max limiter. There is also reference to pump overrun in the controller manual which would seem to state that pump overrun will take place for a limited period once the cylinder reaches temperature (with the zone valve opened) and the "charging" pump running, there seems to be a 10 min overrun time judging by the 10 min delay between HW off @ 0700 and CH on @ 0710, whatever about the "charging" pump run status.

 

[milleniumaire]

@milleniumaire, have you noted the flow/return temps with HW heating on?, @EvilDrPorkChop says 80C flow temp, also I think the cylinder temp is set in the controller and not on the boiler front, maybe the boiler front acts as a max limiter. There is also reference to pump overrun in the controller manual which would seem to state that pump overrun will take place for a limited period once the cylinder reaches temperature (with the zone valve opened) and the "charging" pump running, there seems to be a 10 min overrun time judging by the 10 min delay between HW off @ 0700 and CH on @ 0710, whatever about the "charging" pump run status.

Hi guys, I did some checking of temps this morning as requested. The system (heating and hot water) started at 06:30am and I checked it at 07:10am. It was still in DHW mode, trying to heat up the cylinder.

The analogue pipe thermostats showed:
Boiler flow to LLH: 68
Boiler return from LLH: 63
LLH flow to system: 68
LLH return from system: 60

On the VRC 700, I could see the following temperatures shown:
DHW current flow temp: 79
System flow temp: 76
DHW Target flow temp: 90
Domestic host water: 65
Current temp: 63
Cyl. temp top: 73
Cyl. temp bottom: ---

In the DHW menu, I also noticed the following settings:
Max. cyl. charg. time: 60 min.
DHW req. anti cy time: 60 min.
Ch. pump overrun time: 5 min
Parallel cyl. charging: off

It would appear the reason I'm seeing the heating start to come on an hour after it has started is due to the "Max. cyl. charg. time" setting of 60 mins? So, even after an hour the cylinder isn't reaching 65 deg C, but then switches to heating mode due to this setting. Not sure of the impact of the other DHW settings.

Last night, I happened to catch the boiler showing S.7 (pump overrun). The external pump was off, but it showed the internal pump was on and there was a DHW demand. My son had recently had a shower. While I watched it, the S.7 changed to S.0 (no demand). I can't say how long it had been at S.7, but I did notice the radiators were luke warm, so suspect the DHW demand had been running for some time.

Right now, 3 hours after starting, the system is in Heat mode, DHW target flow temp is showing 0 and DHW current temp is showing 65, so clearly it has eventually reached the required hot water temperature.

As suggested, I think I will turn the boiler temperature down to 60 deg C and see what impact that has in the morning. Just need to figure out if it should be turned down using the boiler dial or the VRC 700. I'll probably do both, just to be sure!

Yesterday, I emailed a bunch of questions off to Vaillant, including details of how the Grundfos pump was connected/configured and how long they would expect it to take for the cylinder to heat up, with examples of the current settings in the hope they might see something that needs to be changed.

 

[ShaunCorbs]

It’s not getting rid of the heat that return should be under 50 if the cylinder is calling

 

[milleniumaire]

It’s not getting rid of the heat that return should be under 50 if the cylinder is calling

So, the only way to loose the heat is within the cylinder coil? Does that suggest the water in the cylinder is being kept at a fairly high temperature, even when not being heated i.e. during the night, but then why does it take so long to heat up in the morning if it's already reasonably warm/hot? If I could drag myself out of bed before 06:30am I could check on the "Start" cylinder temperature, before the boiler turns on. Maybe I'll give that a try tomorrow morning.

The cylinder is only about a meter from the boiler, so not much exposed pipework between them.

 

[John.g]

The deltaT through the secondary circuit is 8C (68-60), unfortunately we don't know the pump power, if we did then we could calculate very accurately what the flowrate is and thence the cylinder input, if the boiler Hx pressure loss was available then could get a fairly accurate picture as well. However I can't see the pump head being greater than 2.5M so the flowrate is at least IMO 25 LPM, with a 8C deltaT gives a cylinder input of at least 14kw, the cylinder top is showing 73C, if this sensor is also used for temperature control then why didn't it shut the zone valve at 65C?, maybe worth checking the zone valve operation, the few that I saw failing did so with the valve "1/2" open and with the end switch still closed which would mean that the cylinder would remain heating until the 1 hour period had elapsed, this assumes that the end switch is used for boiler control with this particular controller.
You also said "I think I will turn the boiler temperature down to 60 deg C and see what impact that has in the morning", maybe you mean the cylinder temperature?, if you turn that down to say 50C 0n the controller then IMO the cylinder should reach that temperature very well within the hour limit and the boiler should then switch to CH.

 

[ShaunCorbs]

So, the only way to loose the heat is within the cylinder coil? Does that suggest the water in the cylinder is being kept at a fairly high temperature, even when not being heated i.e. during the night, but then why does it take so long to heat up in the morning if it's already reasonably warm/hot? If I could drag myself out of bed before 06:30am I could check on the "Start" cylinder temperature, before the boiler turns on. Maybe I'll give that a try tomorrow morning.

The cylinder is only about a meter from the boiler, so not much exposed pipework between them.

I would check the cylinder temp before you go to bed and if it’s at 60 would turn hot water off and see how it runs the next morning

It might be upto temp and the boiler / controller is waiting for the temp to drop and after an hour it moves on to heating

 

[milleniumaire]

Just had a response from Vaillant technical support regarding one of my emails and their response is that I need to wire the external pump upto the VR 71 wiring centre! The exact opposite of what they told my heating engineer. I'm taking this with a pinch of salt as at the time I emailed them I wasn't aware that the value of d.27 was 5 and they have assumed it is 2 (apparently this is the default). Also, they are telling me that a value of 2 means the external pump would NOT work during DHW mode and so if using the VR 40 to connect up the pump, I would need to connect a SECOND external pump to relay 2 and set d.28 to 3. The fact that I'm seeing the external pump work in both DHW AND heating mode doesn't match what they are telling me, possibly due to the d.27 value of 5 (I've now asked them about this).

So, currently, they are telling me I either have to wire the pump upto the VR 71 or get a second pump, which I believe is overkill for my system. I'm still waiting for answers to a number of questions I asked yesterday.

Based on feedback so far I think I need to investigate the DHW 2 port value to make sure it is functioning correctly. If this isn't fully opening then even if the pump is running I guess the flow wouldn't go anywhere, but then I would assume the cylinder water would never get hot, which it clearly is (eventually).

Reducing the cylinder temperature setting may help the situation, so I've now reduced it to 60 deg C and I will monitor the impact in the morning and also try to determine what the start temperature of the cylinder is before it starts being heated.

I'm still on the fence about the pump wiring/configuration, but wouldn't be surprised if there was potentially an issue, even though the pump always seems to come on when the gas burner is on i.e. when there is demand from one of the zones/DHW.

As far as pump overrun goes, I'm not really seeing that as much as I expected to, so although that is the subject line for my post, it may be a red herring! I still can't help thinking it would be better if the external pump was on during pump overrun, providing at least one of the zone valves was open. Maybe this doesn't happen as pump overrun tends to happen when the heating demand has been met and it is cooling down the water temperature in the boiler to protect the heat exchanger, so to continue to pump hot water around the system might increase room/cylinder temperatures beyond what was desired? Just a thought.

 

[John.g]

I think post #23 suggests that the zone valve is kept open during the pump overrun and because of the LLH should entail the secondary pump running also?.

Have you ever watched the system when there is a call for hot water normally at any time when the cylinder temperature falls below its SP minus its hysteresis to see if it just reheats the cylinder and then stops or changes back to CH?.

 

[milleniumaire]

I think post #23 suggests that the zone valve is kept open during the pump overrun and because of the LLH should entail the secondary pump running also?.

Have you ever watched the system when there is a call for hot water normally at any time when the cylinder temperature falls below its SP minus its hysteresis to see if it just reheats the cylinder and then stops or changes back to CH?.

I've just tried to replicate what you have asked.

I did all this on the VRC 700.

System Status was showing Standby, so no demand from any of the 3 heating zones or from the DWH.

Increased desired temp for radiator zone to 22, current temp was showing 21. System Status quickly changed to Heating mode and the zone valve was shown as open.

Increased DHW temp to from 60 to 65, cylinder temp was showing as 59. System Status quickly changed to DHW mode and cyl. charging pump changed from off to on. Checked the external pump and LED showing. Checked Honeywell zone valve and no resistance in level suggesting it was definitely open.

Decreased DHW temp from 65 to 59, cylinder temp still showing 59. system Status changed back to Heating mode and valve shown as open. Pump wasn't on and boiler showed S.0 (no demand), although radiator zone valve had no resistance, so definitely on. Checked the VRC 700 and it was showing desired temp was 22 and current temp was 21.5, so possibly the heating curve was causing it not to demand heat (this happens a lot).

So, from what I can see, everything I expected to happen did happen exactly as it should have happened. DHW took priority over heating and then switched back to heating.

I'm not exactly sure what you meant by "SP minus its hysteresis". Is "hysteresis" a typo? I assume there's some flexibility before the cylinder calls for heat and this is what you mean by "hysteresis"?

 

[John.g]

That's right, the cut in temperature is the SP (or cut out temperature ) minus the hysteresis, which might normally be 5C, but is settable in some controllers. A Gas boiler SP setting works some what differently because you don't want the boiler burner to cut out once it reaches its setpoint as it can generally modulate down to maintain this SP temperature, when the heating load is less than the boiler minimum output, the boiler temperature will start climbing and the burner will cut out at the SP+5C, it will then re fire when the temperature falls to SP-5C.

Did the boiler go to overrun after your test, above?.

 

[milleniumaire]

Did the boiler go to overrun after your test, above?.

No, I didn't notice it going into pump overrun, but I didn't leave it in those states for very long. I was actually surprised how responsive it was to me changing the various temperatures.

 

[milleniumaire]

Just to give an update after setting the hot water temp to 60 deg C yesterday.

I dragged myself out of bed this morning before 6:30am when the heating/DHW is due to start and checked the current cylinder temperature on the VRC 700. It showed 44 deg C, so since I last checked at 11:00pm last night, which is when the heating/DHW turns off, it has lost 16 deg C overnight.

By 07:05am I noticed the radiators were starting to warm up, suggesting the cylinder had been heated from 44 to 60 deg C in around 30 minutes. It seems strange that to increase the temp by another 5 deg C, to 65, it would take more than another 30 minutes (based on the hot water demand running for an hour each morning prior to this morning, before heating started).

 

[John.g]

I think you have two temp measurements, one on the cylinder top and one that shows the target temp? which I assume is where the reading for control is taken, yesterday you had the top reading 73C and the target 65C, what are the corresponding readings now.
Also if you heated 250 litres of water from 44 to 60C in 30 minutes then the cylinder input was 9.3 kw, less than half of its rating, did you notice the secondary flow/return temps, if they were similar to yesterday at 8c then it means that the circulation rate was ~ 17 LPM which sounds reasonable for that ups3 pump, if the flow temp was 75/80 deg then it points to a fouled "coil", I think you have "tank in tank" cylinder heating.
Do you live in a hard water area?.

If your boiler can only modulate to 12kw then it should have cycled also.

 

[milleniumaire]

Unfortunately, I didn't go to the boiler only the controller, so don't have any other details and now the system has been in heating mode since around 07:00am, so nothing will reflect the DHW period between 06:00 and 07:00.
I'll do the check again in the morning and try to get to the boiler before the heat mode starts.
We don't live in a hard water area. Limescale is not an issue for us.

 

[John.g]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

 

[EvilDrPorkChop]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

It isn’t that great, mine loses 7oc over night on a 170L that’s located in a well insulted garage and pipes lagged.
The reintroduction of cold water just by running a hot tap will lower the temp, and depending where the probe is located then this will pick up that change pretty quick.

 

[milleniumaire]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

It isn’t that great, mine loses 7oc over night on a 170L that’s located in a well insulted garage and pipes lagged.
The reintroduction of cold water just by running a hot tap will lower the temp, and depending where the probe is located then this will pick up that change pretty quick.

It appears my 20 year old son (currently back from uni) did have a shower at 11:30pm last night, so that explains some of the heat loss from the cylinder. He's now aware that I'm monitoring the hot water so won't do it again tonight!

During our conversation he also mentioned he's had to turn up his shower dial to almost maximum setting to get hot water, which ties in with what we are also doing with our shower.

This got me thinking as surely a 60 deg C water temperature in the cylinder shouldn't have that big an impact on the heat delivered to the shower, and don't forget it has been set to 65 deg C until yesterday! The required increase in shower settings seems to have coincided with the CH upgrade, but I don't really understand what could have changed. As it's an unvented cylinder, I did wonder if the water pressure may have had an effect, but I don't see why that would have changed! There is a pressure reducing valve near the cylinder.

So, as well as the cylinder heating issue, the hot water appears to be not as hot as it used to be by the time it gets to the showers!

 

[EvilDrPorkChop]

It appears my 20 year old son (currently back from uni) did have a shower at 11:30pm last night, so that explains some of the heat loss from the cylinder. He's now aware that I'm monitoring the hot water so won't do it again tonight!

During our conversation he also mentioned he's had to turn up his shower dial to almost maximum setting to get hot water, which ties in with what we are also doing with our shower.

This got me thinking as surely a 60 deg C water temperature in the cylinder shouldn't have that big an impact on the heat delivered to the shower, and don't forget it has been set to 65 deg C until yesterday! The required increase in shower settings seems to have coincided with the CH upgrade, but I don't really understand what could have changed. As it's an unvented cylinder, I did wonder if the water pressure may have had an effect, but I don't see why that would have changed! There is a pressure reducing valve near the cylinder.

So, as well as the cylinder heating issue, the hot water appears to be not as hot as it used to be by the time it gets to the showers!

You’ve probably got a shower back feeding or mixer tap. Had the same issue before.

 

[John.g]

It could be be that you havn't a balanced hot and cold shower supply, someone here will explain what the problem may be. You should have no problem getting a hot shower with hot water at 60C and a showering temp of 40C, if mains water is presently say 7C then the shower should mix 6.2 litres of hot water with 3.8 litres of cold water to give 10 litres (or LPM) of 40 deg water which a thermostatic shower should mix perfectly. (or the same,manually).

 

[milleniumaire]

You’ve probably got a shower back feeding or mixer tap. Had the same issue before.

Both showers are Mira Excel rear fed, thermostatic valves and have been working brilliantly for many years.

It could be be that you havn't a balanced hot and cold shower supply, someone here will explain what the problem may be. You should have no problem getting a hot shower with hot water at 60C and a showering temp of 40C, if mains water is presently say 7C then the shower should mix 6.2 litres of hot water with 3.8 litres of cold water to give 10 litres (or LPM) of 40 deg water which a thermostatic shower should mix perfectly. (or the same,manually).

The cold water supply into the house is reduced to 3.5 bar, the same as the hot water feed and this hasn't changed. I'll Google balancing, but haven't had to do that before (except for the radiators ).

Don't get me wrong, it's possible to get nice hot showers in both showers, but the temperature setting knob has had to be turned up quite a bit.

 

[ShaunCorbs]

Easy way to test turn mains cold water off at the cylinder open a hot tap does it stop

yes no back feed
No there’s a back feed somewhere

 

[milleniumaire]

Easy way to test turn mains cold water off at the cylinder open a hot tap does it stop

yes no back feed
No there’s a back feed somewhere

A few weeks ago before Xmas I was fitting a new Mira Platinum digital shower in our refurbished bathroom and had to turn off the mains cold supply. I then ran the hot and cold taps until they stopped. It took longer for the hot taps to stop, but they did eventually. That seems to match the test and suggests there isn't a cold water back feed?

 

[John.g]

Not at the mains, shut the cold water only at the HW Cylinder, then open a HW tap is the suggestion, I think.

 

[EvilDrPorkChop]

There's three things it could be;

1 - Something back feeding - shower or mixer tap.
2 - Your incoming PRV on the cold main has failed and letting too much pressure through.
3 - Turning up your cylinder to 65oc has knackered the cartridges in on the two older Mira Excels (Had this before on other makes) Max input on showers is 65oc and it's probable your HW was nearer 70oc. The easiest way to check is whether your Digital shower is still outputting the same temp on the same setting.

I'd be going with option 3 tbh if everything else seems OK.

 

[John.g]

Thermostatic control should give very tight control, I have one on my HW cylinder set to 60C, and it controls to within a few degrees of this from a HW cylinder temp of 85C (summer solar) to 65C from oil fired boiler.

 

[EvilDrPorkChop]

Thermostatic control should give very tight control, I have one on my HW cylinder set to 60C, and it controls to within a few degrees of this from a HW cylinder temp of 85C (summer solar) to 65C from oil fired boiler.

I was more meaning the HW at the Top of the cylinder is more likely to be nearer 70oc, rather than the 65oc at the bottom of the cylinder where the probe is located.

 

[milleniumaire]

There's three things it could be;

1 - Something back feeding - shower or mixer tap.
2 - Your incoming PRV on the cold main has failed and letting too much pressure through.
3 - Turning up your cylinder to 65oc has knackered the cartridges in on the two older Mira Excels (Had this before on other makes) Max input on showers is 65oc and it's probable your HW was nearer 70oc. The easiest way to check is whether your Digital shower is still outputting the same temp on the same setting.

I'd be going with option 3 tbh if everything else seems OK.

After your post and assuming the worst, I've just spent some time testing both Mira showers. There's a definite range of cold to hot when turning the temperature control, so they appear to be working fine, it's just that the "hot" setting is now around 9 when it used to be 5.

It reminded me that it's possible to change these, so out with the screwdriver and I took the front of the dial off and adjusted it, so now it is in position 5 at the expected shower temperature rather than at 8. Both showers temperature settings now seem to be around where they used to be!

Unfortunately, the new digital shower hasn't been used yet (apart from testing) as we are still awaiting the new shower enclosure, which won't be in stock until February.

So, with the water temp now set to 60 deg C and the shower temp gauges adjusted, things should be back to normal, assuming the hot water takes less than 30 minutes to heat up in the morning, allowing at least 30 mins to start warming up the house (boiler is timed to come on an hour before we get up).

Interestingly (or not), after testing the showers, I noticed the system was in DHW mode (not surprisingly), but the boiler was showing status S.53 (Appliance is within the waiting period of the operation block function due to water shortage (VL-RL spread too big). I haven't noticed this one before. Everything still seemed to be working as normal (external and boiler pump on, cylinder valve open, gas burner on).

Went back to the boiler 10 minutes later and it was showing S.31 (Central heating thermostat knob turned off or no heat demand by the eBUS control unit). Again, not seen this before! At this point nothing is on, but that's not surprising as according to the VRC 700 the system is in standby as there's no demand for heating or hot water. Ah well, I'll assume all is well as this is obviously a "status" code, not an "error" code.

 

[John.g]

In my case the solar coil is in the bottom of the cylinder so the whole cylinder heats to up to 85C at times before cylinder & collector protection operates.
I noticed the OPs was 73C at the top and 65C at the bottom. With a normal gas/oil fired cylinder coil with the flow from top to bottom one might expect to see the temperature higher in the cylinder top than at the sensing point, I have a good few PT1000 sensors in dry pockets on my cylinder but I've always found it strange that the cylinder heats up almost completely uniformly from top to bottom with only a few degrees difference, say 3C max. If I start running off hot water at a few LPM with boiler still firing then the top is ~ 5C higher than the other sensors. coil flow temp is 75C.

 

[EvilDrPorkChop]

After your post and assuming the worst, I've just spent some time testing both Mira showers. There's a definite range of cold to hot when turning the temperature control, so they appear to be working fine, it's just that the "hot" setting is now around 9 when it used to be 5.

It reminded me that it's possible to change these, so out with the screwdriver and I took the front of the dial off and adjusted it, so now it is in position 5 at the expected shower temperature rather than at 8. Both showers temperature settings now seem to be around where they used to be!

Unfortunately, the new digital shower hasn't been used yet (apart from testing) as we are still awaiting the new shower enclosure, which won't be in stock until February.

So, with the water temp now set to 60 deg C and the shower temp gauges adjusted, things should be back to normal, assuming the hot water takes less than 30 minutes to heat up in the morning, allowing at least 30 mins to start warming up the house (boiler is timed to come on an hour before we get up).

Interestingly (or not), after testing the showers, I noticed the system was in DHW mode (not surprisingly), but the boiler was showing status S.53 (Appliance is within the waiting period of the operation block function due to water shortage (VL-RL spread too big). I haven't noticed this one before. Everything still seemed to be working as normal (external and boiler pump on, cylinder valve open, gas burner on).

Went back to the boiler 10 minutes later and it was showing S.31 (Central heating thermostat knob turned off or no heat demand by the eBUS control unit). Again, not seen this before! At this point nothing is on, but that's not surprising as according to the VRC 700 the system is in standby as there's no demand for heating or hot water. Ah well, I'll assume all is well as this is obviously a "status" code, not an "error" code.

Your showing S.53 because of the output still being 37kw, have you rated this down to 22kw on D. Setting on the PCB?

 

[EvilDrPorkChop]

In my case the solar coil is in the bottom of the cylinder so the whole cylinder heats to up to 85C at times before cylinder & collector protection operates.
I noticed the OPs was 73C at the top and 65C at the bottom. With a normal gas/oil fired cylinder coil with the flow from top to bottom one might expect to see the temperature higher in the cylinder top than at the sensing point, I have a good few PT1000 sensors in dry pockets on my cylinder but I've always found it strange that the cylinder heats up almost completely uniformly from top to bottom with only a few degrees difference, say 3C max. If I start running off hot water at a few LPM with boiler still firing then the top is ~ 5C higher than the other sensors. coil flow temp is 75C.

Mine heats up fairly uniform too, however mine is only a 170l. I find that when you get over 200l you get the difference, the top being hotter, really you should probably need a destrat pump. I also find you find it worse with a higher flow temp, like on HW priority. This is where a circulating return, tee'd into the cylinder Inlet works quite well.