Help me understand my Vaillant boiler!

[PeteCUK]

Hi All,
The house we moved in to a year ago has a Vaillant Ecotec 428 fitted and I'm trying to get my head around how it works. This is my first time as a home owner and never lived in a property in which I've had access to the boiler until now, so trying to get my knowledge to where it should be!

The boiler runs through a UPS 15-60 pump and 3x CH zones (one of which is UFH with a separate UPS 25-60) plus the HW cylinder. I have the boiler HW temp set to 60 deg and the CH temp set to 75 deg (as I'm pretty sure was recommended in the manual or elsewhere). What is confusing me a little is the way the boiler is reacting in relation to the set temps.
The weather has (thankfully) been glorious as of late so the CH heating has been off (ie set low enough through our Tado that there is never any call for heat) yet when the Hortsman progammer (which all the Tado thermostats run through) has the HW channel ON in the morning and evening the boiler LCD displays both the radiator symbol (heating mode) and the tap (hot water supply). Why is the heating mode active when there is no call for heat from the CH side of the system?
What's more, when the HW is on, the boiler constantly goes through short cycles of heating up (heating mode) to 75 deg probably takes about 30 seconds with all the CH zone valves closed, flame turning off, heating then entering an anti-cycling mode (flashing radiator) until it gets down to around 70 deg (probably take 2 minutes) before repeating. Why is the boiler heating to the CH temp with no call for heat? Why is it not just maintaining the HW temperature? Should it he cycling on and off so short and repeatedly?

Thanks in advance for any help/advice you can provide!

 

[Aquarius]

It’s possible that one of your zone valves has a stuck microswitch or a burnt out sychron motor. Do any radiators heat up when the hot water is on? Does this happen with no demands?

 

[PeteCUK]

It’s possible that one of your zone valves has a stuck microswitch or a burnt out sychron motor. Do any radiators heat up when the hot water is on? Does this happen with no demands?

Only radiators that heat up without demand are the heated towel rails upstairs but they branch off before the valves to always provide a flow from what I can tell, so to be expected?
When the HW channel isn't ON then the boiler is just idle, so seems to only be an issue when the HW programmer is asking the boiler to be on. In terms of correct operation, am I right in thinking that the radiator symbol should only be on when there is a call for heat from the thermostats and only the HW tap symbol on when the only demand is from the HW channel??

 

[Last Plumber]

It sounds as though you do not have Vaillant controls?
If you have not then the DHW (domestic hot water) control temperature is not relevant. The only temp you can set is the boiler temp. This temp is the same for the DHW coil/cylinder as well as the rads. It will appear as though you can set the DHW temp but you cannot.
The issue with it firing when the valves are closed could be one of a number of things but it doesn't sound right. I would advise you to get a heating engineer in to look at it.

 

[PeteCUK]

It sounds as though you do not have Vaillant controls?
If you have not then the DHW (domestic hot water) control temperature is not relevant. The only temp you can set is the boiler temp. This temp is the same for the DHW coil/cylinder as well as the rads. It will appear as though you can set the DHW temp but you cannot.
The issue with it firing when the valves are closed could be one of a number of things but it doesn't sound right. I would advise you to get a heating engineer in to look at it.

Ah perfect, wondered if that may be the case - I have none of the additional Vaillant controls, so the radiator heating indicator will be on for when the boiler is firing for both CH and DHW. Thanks, very useful bit of info! I will probably turn it down towards 60 then over the summer period when we aren't using the rads.
If think that must mean I effectively don't have an issue with it firing when the valves are closed, in that the programmer will be asking for the hot water cylinder valve to open and for the boiler to fire - I guess the issue is lack of awareness on my behalf that CH rad indicator is in effect the indicator for a call for heat from anywhere within the system, including the DHW?

How about the short burn cycles repeating every few minutes, is that an issue??

 

[John.g]

I may be interpreting this incorrectly but you seem to think that you are controlling the boiler water temperature to obtain your desired CH or HW temperatures?.
You should be able to select (programme) either CH on or HW on or both CH+HW on together or none on. You should then be able to select your desired CH and HW temperatures, with CH you are changing the desired room or rooms temperature room stat(s) temperatures somewhere between say 18C & 25C, the room stat(s) when satisfied will then shut off the appropriate motorized (zone) valve, if all zones are satisfied then the radiator symbol may still remain (if selected) on.
The HW desired temperature is obtained my changing the cylinder water temperature set point temperature and a motorised valve, traditionally this was achieved by having a thermostat (cylinder stat) strapped to the cylinder and you manually changed the setpoint on this stat, the Tado I assume should change this remotely. (Normally set to 60C)
The boiler water temperature may be changed at the boiler panel or maybe with the remote control? and once set is rarely changed.
Re short burn cycles, this can happen in the HW case if the heat emitted by the cylinder coil is less than the minimum modulated output of the boiler, for example if the minimum boiler output is 5kw and the cylinder coil is only emitting say 3kw (especially when the hot water is near its set point) then the boiler will cycle frequently but the biggest problem which seems to be common to most gas fired boilers is that to achieve proper/safe ignition the burner fires up at ~ 75% output and by the time it has modulated down to minimum the boiler temperature has exceed its setpoint by ~ 5C and cuts out the burner which then has to complete a anti rapid fast cycle time of maybe 2 to 5 minutes before it refires on reaching its SP temperature.

 

[PeteCUK]

I may be interpreting this incorrectly but you seem to think that you are controlling the boiler water temperature to obtain your desired CH or HW temperatures?.
You should be able to select (programme) either CH on or HW on or both CH+HW on together or none on. You should then be able to select your desired CH and HW temperatures, with CH you are changing the desired room or rooms temperature room stat(s) temperatures somewhere between say 18C & 25C, the room stat(s) when satisfied will then shut off the appropriate motorized (zone) valve, if all zones are satisfied then the radiator symbol may still remain (if selected) on.
The HW desired temperature is obtained my changing the cylinder water temperature set point temperature and a motorised valve, traditionally this was achieved by having a thermostat (cylinder stat) strapped to the cylinder and you manually changed the setpoint on this stat, the Tado I assume should change this remotely.
The boiler water temperature may be changed at the boiler panel or maybe with the remote control? and once set is rarely changed.
Re short burn cycles, this can happen in the HW case if the heat emitted by the cylinder coil is less than the minimum modulated output of the boiler, for example if the minimum boiler output is 5kw and the cylinder coil is only emitting say 3kw (especially when the hot water is near its set point) then the boiler will cycle frequently but the biggest problem which seems to be common to most gas fired boilers is that to achieve proper/safe ignition the burner fires up at ~ 75% output and by the time it has modulated down to minimum the boiler temperature has exceed its setpoint by ~ 5C and cuts out the burner which then has to complete a anti rapid fast cycle time of maybe 2 to 5 minutes before it refires on reaching its SP temperature.

Thanks for your reply John, I'm not 100% clear what you mean however. My understanding now from the earlier post from Last Plumber is that although I have separate CH and DHW rotary temperature setting knobs on the front of my specific boiler, the DHW one is redundant without the additional vaillant controls (I see where it suggests this in the manual now) so the CH knob is in effect setting the boiler temp (outflow?) for the whole boiler, regardless of if the call for heat is generated by the DHW via the cylinder/programmer or the Tado thermostats that control the rads. Is that correct? Obviously the actual room/cylinder temps are controlled by their individual settings, with radiators heating up quicker and hotter (so end call for heat quicker) with a setting of 75 deg on the boiler compared to if I set a lower temp like 60 deg.
My logic behind setting the boiler temp down to 60 deg over summer is that I don't need the rads to heat up quickly (or indeed at all) and 60 deg should be plenty to get the water temp up to a decent amount without wasting too much energy - or is that logic flawed in that it will take a set number of KW to warm up the hot water cylinder no matter what the temp is set at, all it will change is the speed at which it warms up?
As to your second point, I think that is what my problem is. Someone elsewhere has suggested I de-rate the boiler down from 28 kw as that is very excessive for what the two of us need for how we use the house (we don't yet have the three kids the previous owners had!) - would that help with the short/micro cycles or will the boiler always start up at 75% of max output and ignore the derated output?
Unfortunately I haven't yet found an electrician who is interested in wiring up the HW to the Tado, so that, rather annoyingly, is still controlled by a white thermostat on the side of the cylinder as far as I'm aware - I've not touched it since we moved in!