Central heating pump sizing

[Reddwarf4ever]

As you can adjust the output of the boiler work out your rad kw and add them all together that’s what you set your boiler out put to

I can’t actually adjust the output, only limit the maximum.....can’t forever keep adjusting it, which is what I am doing now.....have to find a website to help me calculate the rad KW consumption....unless you know of one ?

 

[Reddwarf4ever]

I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.

Will do as you suggest.....it’s all driving me mental......had really hoped the new pump would solve my problems......I turned on hot water, while the boiler was firing steadily, the firing increased slightly, due to the cylinder demand.....left it alone, but after a while it started cutting out on over temperature.....and went into its usual cycle.....the cylinder temperature was increasing, had hoped it would be ok.....now turned off the hot water and the boiler is firing ok....it’s at 7KW 68oC inlet 43oC, bit only because it has been cycling for an hour or more....so for some reason it doesn’t like heating and hot water on together.....I can’t restrict the inlet flow to the cylinder as if on HW only, the boiler would have, I assume an insufficient flow......

the boiler does have an option for a separate HW control, just needs a valiant module, can’t find any info on that....have emailed vailiant....so if I can get some sort of stability in the day with the heating ( miracles do happen ) then can allow the HW to heat overnight with the higher temperature set for HW.....

thanks for all the continued support.......my Wife says my life has been taken over trying to get this boiler sorted...

😎😎😎

 

[ShaunCorbs]

I can’t actually adjust the output, only limit the maximum..can’t forever keep adjusting it, which is what I am doing now..have to find a website to help me calculate the rad KW consumption....unless you know of one ?

screwfix book find your existing rads roughly they list the kw output on there website

and yes you need to set the maximum to this value

I would stagger hot water times roughly an hour earlier than heating in the morning

 

[Reddwarf4ever]

I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.

Well I increased the KW to 15 or the boiler wouldn’t be able to increase firing, set output to 75oC climbed to 78oC and cut out......reduced KW to 7 as before changed outlet to 68oC and it cut out at 71oC.....even at only 7KW.....😩 waiting for it to ‘ settle down ‘ again

 

[Reddwarf4ever]

screwfix book find your existing rads roughly they list the kw output on there website

and yes you need to set the maximum to this value

I would stagger hot water times roughly an hour earlier than heating in the morning

Thanks will have a look.....going to run hot water 4am to 7am.....when the heating is off.....but will need to change outlet temperature to 75oC for the night and back to 68oC for the day....

 

[ShaunCorbs]

Well I increased the KW to 15 or the boiler wouldn’t be able to increase firing, set output to 75oC climbed to 78oC and cut out...reduced KW to 7 as before changed outlet to 68oC and it cut out at 71oC..even at only 7KW.***.😩 waiting for it to ‘ settle down ‘ again

this is normal if the system is hot

 

[Reddwarf4ever]

Well as expected the boiler ‘settled down’ again and is currently set at 6KW outlet temperature 68oC inlet 52-53oC......going to leave this alone and see how it goes tomorrow, will see if it heats up the hot water tonight, but suspect I will need to change the output temperature to around 75oC.....

don’t think it’s raising the house temperature much, but comfortable, hopefully long term if I change all the single rads to double, it will give the boiler more to work on.

 

[ShaunCorbs]

The output temp doesn’t have that great effect on room temp unless your running the room temp at your flow temp

 

[Reddwarf4ever]

The output temp doesn’t have that great effect on room temp unless your running the room temp at your flow temp

Sorry don’t understand that.......

 

[John.g]

I can’t actually adjust the output, only limit the maximum..can’t forever keep adjusting it, which is what I am doing now..have to find a website to help me calculate the rad KW consumption....unless you know of one ?

Well as expected the boiler ‘settled down’ again and is currently set at 6KW outlet temperature 68oC inlet 52-53oC...going to leave this alone and see how it goes tomorrow, will see if it heats up the hot water tonight, but suspect I will need to change the output temperature to around 75oC..

don’t think it’s raising the house temperature much, but comfortable, hopefully long term if I change all the single rads to double, it will give the boiler more to work on.

You said in one post that you have to run on CP3 to heat up all the rads and would suggest that you run in this mode for the time being at any rate.
What mode is giving you the above numbers?.

There is clearly something wrong with the boiler as you shouldn't have to keep chasing the output up/down to get the boiler to run, IMO you should be able to leave it at max output, I can understand setting the CH output to say 20kw on a 36kw combi with a heating load of 18kw, otherwise not.
You might consider getting a fixed price (if they provide this type of service) from vaillant to fix this boiler.

 

[Reddwarf4ever]

You said in one post that you have to run on CP3 to heat up all the rads and would suggest that you run in this mode for the time being at any rate.
What mode is giving you the above numbers?.

There is clearly something wrong with the boiler as you shouldn't have to keep chasing the output up/down to get the boiler to run, IMO you should be able to leave it at max output, I can understand setting the CH output to say 20kw on a 36kw combi with a heating load of 18kw, otherwise not.
You might consider getting a fixed price (if they provide this type of service) from vaillant to fix this boiler.

Complained about this from dat one, but said nothing wrong, put me off Vailant I can tell you....using CP1 with the above figures......

 

[John.g]

Complained about this from dat one, but said nothing wrong, put me off Vailant I can tell you....using CP1 with the above figures.***.

Well, that's one reason for having to limit it to 6kw as the flowrate on CP1 is only 6.7 LPM and with the stated deltaT of 16C gives a boiler output of 7.5KW. All gas boilers fire up initially with a output of ~ 70% = 12.6kw on your boiler (limiting output has no effect on fire up, for stable ignition ) so a flowrate of 6.7 LPM and 12.6kw will result in a deltaT of 27C, very close to the deltaT limit of 30C, also, a higher flow will result in slower flow temperature rise which will help to prevent the boiler reaching (SP+3C).
CP3 at 10.5 LPM will only result in a deltaT of 17C on fire up and IMO the pump should always be run at this flowrate and should certainly allieviate some of your problems, must bear in mind that most system boilers with internal circ pumps will generally run with a flowrate at least as high as this on a 18kw boiler.
Once your HW cylinder is heated up in the morning you can then reduce the boiler flow temp to give ~ 50C return temperature.
You may also find now on CP3 that the boiler will supply both CH and HW without tripping.

 

[John.g]

A few other things have come to mind, if, which I think you have, a open vented system, you might check that there is no pump over through the vent above the F&E (feed and expansion) tank, on CP3 or higher, and also obviously that it contains water.
If you have a mid position diverter valve then the water can recirculate on pump overrun.
the default overrun time is 5 mins, you might reduce this to 2 /3 mins and the default anti cycling time is 20 mins, assume you have reduced this to something like 3 mins. The table below shows anti cycling times but afraid I can't interpret it, any ideas?.

 

[Reddwarf4ever]

A few other things have come to mind, if, which I think you have, a open vented system, you might check that there is no pump over through the vent above the F&E (feed and expansion) tank, on CP3 or higher, and also obviously that it contains water.
If you have a mid position diverter valve then the water can recirculate on pump overrun.
the default overrun time is 5 mins, you might reduce this to 2 /3 mins and the default anti cycling time is 20 mins, assume you have reduced this to something like 3 mins. The table below shows anti cycling times but afraid I can't interpret it, any ideas?.

A few other things have come to mind, if, which I think you have, a open vented system, you might check that there is no pump over through the vent above the F&E (feed and expansion) tank, on CP3 or higher, and also obviously that it contains water.
If you have a mid position diverter valve then the water can recirculate on pump overrun.
the default overrun time is 5 mins, you might reduce this to 2 /3 mins and the default anti cycling time is 20 mins, assume you have reduced this to something like 3 mins. The table below shows anti cycling times but afraid I can't interpret it, any ideas?.

A few other things have come to mind, if, which I think you have, a open vented system, you might check that there is no pump over through the vent above the F&E (feed and expansion) tank, on CP3 or higher, and also obviously that it contains water.
If you have a mid position diverter valve then the water can recirculate on pump overrun.
the default overrun time is 5 mins, you might reduce this to 2 /3 mins and the default anti cycling time is 20 mins, assume you have reduced this to something like 3 mins. The table below shows anti cycling times but afraid I can't interpret it, any ideas?.

View attachment 47252

I couldn’t make any sense out of this table either 🤯 yes it’s open vented & the feed & expansion are joined together...yes I have a 3 port diverted valve.....will give this a few days to stabilise.....then try CP2......this morning the heating came on at 68oC with an inlet of 53oC set at 7KW, modulating lovely, all rads hot 🔥 if I change the two lounge rads from single to double, that may reduce the inlet temperature 2-3oC or will allow me to run at 70oC......which will be better for HW heating in the night....although as an experiment I turned on the HW this morning, after showers & the boiler temperature initially took a dive and the inlet temperature dropped to 45oC, the temperature gauge at the stat has risen from 25oC to 39oC so it is heating....boiler has now re established at 68oC with an inlet of 51oC so its modulating ok and heat still being transferred to the HW cylinder.....🤞 the cylinder reaches at least 44oC ( prefer 48oC )which gives 55oC at the top.....looking good so far....🙏 anti cycling and over run now reduced to 3 minutes thanks for explaining the 70% fire rate for initial startup, that helps in my understanding

View attachment 47252

A few other things have come to mind, if, which I think you have, a open vented system, you might check that there is no pump over through the vent above the F&E (feed and expansion) tank, on CP3 or higher, and also obviously that it contains water.
If you have a mid position diverter valve then the water can recirculate on pump overrun.
the default overrun time is 5 mins, you might reduce this to 2 /3 mins and the default anti cycling time is 20 mins, assume you have reduced this to something like 3 mins. The table below shows anti cycling times but afraid I can't interpret it, any ideas?.

View attachment 47252

 

[John.g]

On CP1??.
When you first installed the Pump you ran on full speed and your were able to up the rating to (where it should be) 18kw, CP3 should, more than likely achieve the same and just maybe CP2.

I know Vaillant arn't very helpful but i would ask them why the flow temperature hysteresis of 3C is so low and if it can be adjusted.

 

[Reddwarf4ever]

On CP1??.
When you first installed the Pump you ran on full speed and your were able to up the rating to (where it should be) 18kw, CP3 should, more than likely achieve the same and just maybe CP2.

I know Vaillant arn't very helpful but i would ask them why the flow temperature hysteresis of 3C is so low and if it can be adjusted.

Have emailed valiant about the hysteresis ( a new word I have learnt 👍 ) as everything is running nicely at CP1, would like to keep it there for at least a week, so I know I have somewhere to go back to 🤞 also at CP2 and especially CP3 the return temperature was rather high, non condensing. But will try next week.

 

[Reddwarf4ever]

Just got an SF.H flashing, found a post from kevindgas, 2011 with how to reset by pressing + & - together, well it worked 👍 couldn’t find reference to this error anywhere else on the internet, states it’s a Service Code, should I worry ??? Thanks

 

[ShaunCorbs]

Due for a service ?

 

[Ben-gee]

There’s a lot to be said for boilers that don’t give any information to end users...😈....💣

 

[Reddwarf4ever]

Maybe, but it’s good to know what the problem is.....