Vaillant boiler taking ages to get to target temperature

[b21playa]

Hi all

Wondering if you could help with this strange issue.

I have a Vaillant ecotec 637 (set to 20kw part load) which was connected via a LLH to my system of 3 zones; DHW and 2 radiator zones. The gas usage was high and didn’t feel the boiler was modulating, so yesterday I decided to remove the LLH and just connect directly to the boiler. I noticed today with the target temp of 75, the boiler was sat around 60 after an hour and even after a few hours it’s only just about got the 70degC. Also when initial heating called for the boiler modulates down to a small flame and takes a while for it to ramp up.

This is strange as with the LLH I never had this issue and easily got the target temp and stayed there. Now I know most likely I’ll need to refit the header, but wondered why the boiler doesn’t ramp up straight away?

Thanks in advance

 

[b21playa]

Just to add the ground floor has 10 rads and the upstairs circuit (1st and loft) has 8 rads. If both zones on, the boiler now won’t ever get to set temp and will sit around 57degc

 

[b21playa]

Just checked all rads, based on deltaT50 values upstairs circuit 14.8kw and downstairs 18.7kw. We run the heating at 70-75degC and have a very uninsulated house - needs new windows and loft insulation

 

[gmartine]

Is there a reason why boiler d0 set to partial load of 20kw when you've got a combined heating load of well over 30kw?

 

[b21playa]

Is there a reason why boiler d0 set to partial load of 20kw when you've got a combined heating load of well over 30kw?

Hi. Most of the time only 1 zone is on. With temps dropping have started to use both radiator zones.

 

[Brambles]

When you removed the LLH how have you reconfigured the pumps?

If configured incorrectly (pumping arrangement) it is probable that the boiler performance response is being limited by the temperature differential (high) across the heat exchanger

 

[b21playa]

When you removed the LLH how have you reconfigured the pumps?

I have removed the additional pump on the secondary side and just using boiler pump

 

[John.g]

If any one zone or/and a number of zones are > than 20kw then the boiler will hardly reach target temp?.
The reason it was with the LLH was possibly because the secondary (rad) flow was greater than the primary flow and was diluting ( by recirculating) the boier flow to give reduced rad inlet flows = reduced rad output.
what are d.40&d.41 values now?. You will have to increase d.00, there was a good reason for installing that LLH, wonder why you thought it was causing excessive gas consumption.

 

[John.g]

I have removed the additional pump on the secondary side and just using boiler pump

Is the LLH removed and the boiler flow/return connected as in no LLH?.

 

[b21playa]

If any one zone or/and a number of zones are > than 20kw then the boiler will hardly reach target temp?.
The reason it was with the LLH was possibly because the secondary (rad) flow was greater than the primary flow and was diluting ( by recirculating) the boier flow to give reduced rad inlet flows = reduced rad output.
what are d.40&d.41 values now?. You will have to increase d.00, there was a good reason for installing that LLH, wonder why you thought it was causing excessive gas consumption.

D40/41 is 57/44.

Maybe I just put the LLH back in

 

[b21playa]

Is the LLH removed and the boiler flow/return connected as in no LLH?.

Yes LLH totally removed and piped as if it was never there, pictures added

 

[John.g]

Why not just increase d.00 to say 30kw??.

 

[b21playa]

Why not just increase d.00 to say 30kw??.

Downstairs circuit is say 19kw and main one used, thought 20kw would be sufficient. I was trying to reduce gas usage, wouldn’t this just increase it. Could this be the reason for slow ramp up?

Also looking at the picture, is the LLH connected right way around?

 

[John.g]

It will only increase it to the level required to give the rated output of the rads, if you increased d.00 to 30kw and reduced the boiler target temp to 57/60C then you will burn the same amount of gas as just now but you are limiting the rad output which may be fine as you will get better condensing, of course if you put another zone in service then obviously higher fuel consumption but you can't have it both ways.

 

[John.g]

Also looking at the picture, is the LLH connected right way around?

I think so, yes.

 

[b21playa]

I think so, yes.

Thank you. I see your point about increasing to 30kw and changing set point, but it will mean output to rads will be less which I don’t want. I think I’ll stick the LLH back in later. That way I know it heats up quicker and the secondary pump has a better head rate.

I was told previously that my boiler was oversized and would of been better with a 624 with d.0 set to auto

 

[John.g]

You have a total rad demand (2 zones on) of 33.5kw so how can a 624 service both?.
Before you reinstall that LLH, can you put the 2 zones in service with d.0 at 30kw and just take d.40&d.41.

Also remember that (probably) originally, the secondary flowrate was set up for both zones so, when one zone only in service, then the flow temperature to these rads will be greately reduced leading to reduced output but that is a "feature" of LLHs, it would be useful to install even a clamp on thermometer on the secondary flow pipe at the LLH, you can quickly determine the pump settings for both one/two zones to give optimum performance/economy.

Didn't you install this about a year ago??.

 

[b21playa]

You have a total rad demand (2 zones on) of 33.5kw so how can a 624 service both?.
Before you reinstall that LLH, can you put the 2 zones in service with d.0 at 30kw and just take d.40&d.41.

Also remember that (probably) originally, the secondary flowrate was set up for both zones so, when one zone only in service, then the flow temperature to these rads will be greately reduced leading to reduced output but that is a "feature" of LLHs, it would be useful to install even a clamp on thermometer on the secondary flow pipe at the LLH, you can quickly determine the pump settings for both one/two zones to give optimum performance/economy.

Didn't you install this about a year ago??.

Yes I can try that, how long shall I run it before getting d.40/d.41

I have a clamp on thermometer I can check with once reinstalled.

Yes was installed last year, since then I’ve had additional rads added/replaced

 

[John.g]

W w w w w w,tw,ww ww,r,

Yes I can try that, how long shall I run it before getting d.40/d.41

I have a clamp on thermometer I can check with once reinstalled.

Yes was installed last year, since then I’ve had additional rads added/replaced

Run them until the flow temp is say 65C.

 

[b21playa]

W w w w w w,tw,ww ww,r,
Run them until the flow temp is say 65C.

Finally got to 65, it was 65/47

 

[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

 

[ShaunCorbs]

You do realise that extra 15 degrees is wasted energy/ warming the return up so not required

Does the boiler pump manage the load eg heats all of the rads etc ?

 

[b21playa]

You do realise that extra 15 degrees is wasted energy/ warming the return up so not required

Does the boiler pump manage the load eg heats all of the rads etc ?

Is 60degC the ideal flow temp?

I need to check all rads when just on boiler pump, I had checked earlier downstairs and a few in the 1st floor but not all of them and not the loft rads

 

[ShaunCorbs]

Well ideally your return needs to be below 50 for condensing boilers to be happy and with sensor mad vaillants this is a must also with your split as it won’t reach max burn / temps unless the split and temp rise is within spec (split is the difference between flow and return ideally 20dc)

 

[b21playa]

Well ideally your return needs to be below 50 for condensing boilers to be happy and with sensor mad vaillants this is a must also with your split as it won’t reach max burn / temps unless the split and temp rise is within spec (split is the difference between flow and return ideally 20dc)

Thanks. I’ve turned down flow to 65dC and will see how that goes.

Do you have any thoughts on replacing hive controls with one that can modulate the boiler better and set different flow temps for hw/ch?

 

[ShaunCorbs]

Thanks. I’ve turned down flow to 65dC and will see how that goes.

Do you have any thoughts on replacing hive controls with one that can modulate the boiler better and set different flow temps for hw/ch?

There brilliant and priority hot water and wc heating but you will have cooler rads then stock temp etc