Pump overrun is killing my heating!

[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.

 

[milleniumaire]

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

Okay, thanks, that actually makes me feel a little better - knowing its likely to be a config issue rather than a fault somewhere. As it's a boiler setting, I guess this parameter must have always had that value and it isn't something that would change with the upgrade of controller from VRC 430 to VRC 700.

I haven't changed this setting as this is one of the many questions I've put to Vaillant technical support (showing them what the current settings are and asking them to advise on the best values). Unfortunately, I haven't yet received a response.

 

[EvilDrPorkChop]

Okay, thanks, that actually makes me feel a little better - knowing its likely to be a config issue rather than a fault somewhere. As it's a boiler setting, I guess this parameter must have always had that value and it isn't something that would change with the upgrade of controller from VRC 430 to VRC 700.

I haven't changed this setting as this is one of the many questions I've put to Vaillant technical support (showing them what the current settings are and asking them to advise on the best values). Unfortunately, I haven't yet received a response.

Personally, if I was configing your system. I'd set -
HW Target Temp on 55oc
D.20 - 60oc
D.72 - 120
D.75 - 45 mins
D.77 to 22kw
D.78 to 80oc

D.19 - 3 (Always Speed 2)

 

[John.g]

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.

Back to basics for me.
Is there a clue in the label re UPS3 connection to VR 71 as they suggest, above.
TITLE 12-37kW, Low Loss Header, DHW (Secondary Circulation), 1 UFH Zone (3rd Party), 2 Radiator Zones (Wired or Wireless VRC 700, VR91)

Why not connect a indicating light between L/N in R4 where it says DHW Secondary Circulation and observe it, if it always lights up and goes out with boiler circ pump operation in both CH & HW mode , then maybe that's where the UPS3 should be connected?.
Also if boiler circ pump on and R4 "on" with UPS3 off then maybe another clue that it should be connected in to R4.

 

[John.g]

Don't think R4 is correct, that seems allocated to a cylinder secondary recirc pump.

 

[milleniumaire]

Back to basics for me.
Is there a clue in the label re UPS3 connection to VR 71 as they suggest, above.
TITLE 12-37kW, Low Loss Header, DHW (Secondary Circulation), 1 UFH Zone (3rd Party), 2 Radiator Zones (Wired or Wireless VRC 700, VR91)

Why not connect a indicating light between L/N in R4 where it says DHW Secondary Circulation and observe it, if it always lights up and goes out with boiler circ pump operation in both CH & HW mode , then maybe that's where the UPS3 should be connected?.
Also if boiler circ pump on and R4 "on" with UPS3 off then maybe another clue that it should be connected in to R4.

Hi John, I understand what you are suggesting, but as things stand I/the engineer have received different advice from different Vaillant support people, so I need to understand why they are making these different suggestions, hopefully once they respond to my email.

What they actually told me in the last email was:

"I would advise to remove the VR40 and wire the single system pump as shown in the example diagram below with a permanent 230V supply to the grey wires and use the oranges from all zone valves wired to the pump live."

So, they are essentially wanting to turn the external pump on and off whenever any of the zone valves open or close, which I believe is what the engineer was originally considering doing, but then got confused looking at the schema diagram they had supplied (see attached) and called Vaillant, who told him he needed to use the VR 40, which he then ordered and installed the following day!

It's an interesting thought about using R4 for the external pump, however in the schema diagram it does appear to be associating R4 with the cylinder, so my gut feeling is it is likely to only operate in DHW mode, but as you say it may be worthwhile testing it, but I'll wait until I have more clarity from Vaillant. Maybe they'll suggest yet another way of wiring up the pump

EDIT: Just seen your post from 11:36 last night, so it appears we agree about R4.