Vaillant boiler taking ages to get to target temperature

[John.g]

Thats interesting because (assuming a room(s) temp at 20C) that your rads are effectively "T36" which emit 65.2% of a T50 so, your rads output, 35.5*65.2%, 22.1kw which is probably why the LLH gave similar outputs with a boiler target temp of 75C but secondary temp of ~ 65C and your d0 setting of 20kw was sufficient to allow the boiler to reach its target temp of 75C.

If you didn't have that LLH to hand one wonders was one required in the first place?.

 

[b21playa]

Thats interesting because (assuming a room(s) temp at 20C) that your rads are effectively "T36" which emit 65.2% of a T50 so, your rads output, 35.5*65.2%, 22.1kw which is probably why the LLH gave similar outputs with a boiler target temp of 75C but secondary temp of ~ 65C and your d0 setting of 20kw was sufficient to allow the boiler to reach its target temp of 75C.

If you didn't have that LLH to hand one wonders was one required in the first place?.

Hi, thanks for that.

Just to clarify where has the 35.5 come from?

Thing one of the main reasons was the loss of head in the hex of the Vaillant and needing additional pump to circulate especially to loft rooms.

I may give it until Wednesday (my next free day) to put the LLH back in and see how the next few days go. Of course one zone at a time set at 20kw

 

[John.g]

That should read 33.5kw, "Just checked all rads, based on deltaT50 values upstairs circuit 14.8kw and downstairs 18.7kw" so your total rad output was ~ 21.8kw? (33.5*65.2%).
I obviously see your point re head loss.

 

[b21playa]

That should read 33.5kw, "Just checked all rads, based on deltaT50 values upstairs circuit 14.8kw and downstairs 18.7kw" so your total rad output was ~ 21.8kw? (33.5*65.2%).
I obviously see your point re head loss.

Thanks for clearing that up, makes sense and adds up.

Maybe that’s where the 24kw boiler made sense? I need to change the boiler at some point as something has dropped against it and damaged the casing

 

[John.g]

Well, all we know for sure is that the installed radiator output (T50) is 33.5kw, whether this is actually required at something like -10C or so is unknown except a heat loss was carried out, even if 24kw only is required and the rads are oversized, good, if weather compensation was installed which would give maximum boiler efficiency and even without it, you can manually set the boiler target temperature.

 

[b21playa]

Thanks for clearing that up, makes sense and adds up.

Well, all we know for sure is that the installed radiator output (T50) is 33.5kw, whether this is actually required at something like -10C or so is unknown except a heat loss was carried out, even if 24kw only is required and the rads are oversized, good, if weather compensation was installed which would give maximum boiler efficiency and even without it, you can manually set the boiler target temperature.

Rads are definitely oversized, the reason was to run a lower flow temp but that hasn’t happened yet….i have family members who really feel the cold (old age). No weather compensation fitted, just running hive units but have been thinking of upgrading controls but not sure which to go for. I’ve seen either Vaillant which seems expensive and can not work out exactly what I require or the other option was nest using open therm module

 

[John.g]

Did you ever actually measure the secondary flow temperature?, if the secondary pump which looks like a UPS3 is on PP2? setting then that can result in a very low actual head which may not have been sufficient for some rads.

 

[b21playa]

Did you ever actually measure the secondary flow temperature?, if the secondary pump which looks like a UPS3 is on PP2? setting then that can result in a very low actual head which may not have been sufficient for some rads.

I did at some point but can not remember the value unfortunately. Yes you are correct about the pump model and setting, I took that from the manual that it was recommended to be on PP2 but that was just for 2-pipe system. What would be the ideal setting from your experience?

 

[John.g]

I did at some point but can not remember the value unfortunately. Yes you are correct about the pump model and setting, I took that from the manual that it was recommended to be on PP2 but that was just for 2-pipe system. What would be the ideal setting wfrom your experience?

I've never even seen a LLH but think I have a fair idea of how they work, if you need the secondary flowtemp to be the same as the primary (boiler) flowtemp then the secondary flowrate has to equal to or less than the primary flowrate so a constant pressure or constant curve (speed) may be more appropriate but there's no point in overthinking it, the temperature gauge will give the best advise IMO.

 

[b21playa]

I've never even seen a LLH but think I have a fair idea of how they work, if you need the secondary flowtemp to be the same as the primary (boiler) flowtemp then the secondary flowrate has to equal to or less than the primary flowrate so a constant pressure or constant curve (speed) may be more appropriate but there's no point in overthinking it, the temperature gauge will give the best advise IMO.

Thanks John, much appreciated. Once I’ve fitted the LLH and get temp reading I’ll post back