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.....

 

[John.g]

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.

You mentioned increasing the rad sizes so I came up with the following numbers based on that., I based them on doubling the rad outputs but can calculate it for any increase you want.
Based on your actual results it seems that you have a a relatively small installation with total rad output of ~ 9 kw.
If you look at the 9kw base below (top row) you can see that your actual output is 6.73kw with a return of 52C. If you were to increase (second row) the flow rate (CP2/CP3) to 0.63M3/hr to keep the boiler happy then by reducing the flow temp to 60C you will still get a return of 52C BUT a reduced output of 5.87kw, you would againthen need to up the flow temp on HW.

If you now doubled the rad outputs to 18kw, again with a happy boiler flowrate of ~ 0.6M3/hr, you can see the effect of different flow temps on rad output etc, if you select a flow temp of 60C then you will get a return of 44C and a output of 10kw, you would need to up the flow temp on HW.
If you select a flow temp of 72C then you will get a return of 52C and 14.35kw output and so on.

Table Below might be of some help.

​	​	​	Flow​	Return​	DeltaT​	Boiler​	Boiler​	Boiler​
​	​	​	Temp​	Temp​	Temp​	Output​	FlowRate​	FlowRate​
​	Deg rad​	Output​	Deg.C​	Deg.C​	Deg.C​	KW​	LPM​	M3/hr​
9KW Base​	40​	75%​	68​	52​	16​	6.73​	6.03​	0.36​
​	36​	65%​	60​	52​	8​	5.87​	10.52​	0.63​
​	​	​	​	​	​	​	​	​
18KW Base​	32​	56%​	60​	44​	16​	10.08​	9.03​	0.54​
​	38​	70%​	68​	48​	20​	12.60​	9.03​	0.54​
​	42​	80%​	72​	52​	20​	14.35​	10.28​	0.62​

 

[Reddwarf4ever]

You mentioned increasing the rad sizes so I came up with the following numbers based on that., I based them on doubling the rad outputs but can calculate it for any increase you want.
Based on your actual results it seems that you have a a relatively small installation with total rad output of ~ 9 kw.
If you look at the 9kw base below (top row) you can see that your actual output is 6.73kw with a return of 52C. If you were to increase (second row) the flow rate (CP2/CP3) to 0.63M3/hr to keep the boiler happy then by reducing the flow temp to 60C you will still get a return of 52C BUT a reduced output of 5.87kw, you would againthen need to up the flow temp on HW.

If you now doubled the rad outputs to 18kw, again with a happy boiler flowrate of ~ 0.6M3/hr, you can see the effect of different flow temps on rad output etc, if you select a flow temp of 60C then you will get a return of 44C and a output of 10kw, you would need to up the flow temp on HW.
If you select a flow temp of 72C then you will get a return of 52C and 14.35kw output and so on.

Table Below might be of some help.

​	​	​	Flow​	Return​	DeltaT​	Boiler​	Boiler​	Boiler​
​	​	​	Temp​	Temp​	Temp​	Output​	FlowRate​	FlowRate​
​	Deg rad​	Output​	Deg.C​	Deg.C​	Deg.C​	KW​	LPM​	M3/hr​
9KW Base​	40​	75%​	68​	52​	16​	6.73​	6.03​	0.36​
​	36​	65%​	60​	52​	8​	5.87​	10.52​	0.63​
​	​	​	​	​	​	​	​	​
18KW Base​	32​	56%​	60​	44​	16​	10.08​	9.03​	0.54​
​	38​	70%​	68​	48​	20​	12.60​	9.03​	0.54​
​	42​	80%​	72​	52​	20​	14.35​	10.28​	0.62​

Thanks that’s defo given me something to think about and work on....currently I a, using CP1 @7KW inlet 53oC & flow temperature 66oC the house is slowly heating up, I set the HW to come on 2 hours before the heating goes off at 10pm and 3 hours in the morning when the heating goes on at 7am....this achieved 47oC at the cylinder stat, which I know is around 60oC at the outlet, unfortunately although the boiler modulates really well, every so often, 1-2 hours it cuts out, I can only assume ( don’t suppose these boilers have an event log ), that it’s over temperature maybe it climbs to 69oC if a TRV changes up or down, I suppose if it had a 5oC tolerance instead of 3oC maybe it would manage.....when it cuts out, I set the flow temperature to 85oC ( max ) when the gas firing starts reducing again, the temperature drops to 60-65oC & i then set the flow temperature back to 66oC and it starts modulating again.....will try the other ideas soon.....

thanks

 

[John.g]

If the TRVs are of the manual type, like mine, they should give pretty good modulating control as against on/off even though they have a very small hysteresis (in Air) of 1 to 2C, also when setting up I find that even a difference of 0.2/0.3 on the index makes a big difference to the room temperature, all of mine (8) are set to between 1.8 and 2.2 to give ~ 18C to 22C room temperature and with a boiler flow temp of ~70C (OF boiler, cutin/cutout 75C/65C) quite regularly give return temps as low as 40C, oil fired boilers have no problems in dealing with very high deltaTs and even though they run flat out all the time and are constantly cycling, the 20/25 litre heat exchanger acts as a buffer so no cut on flow temp cut out on firing up.
I have read of some on/off and other control problems with the programmable TRVs.

 

[John.g]

Thanks that’s defo given me something to think about and work on....currently I a, using CP1 @7KW inlet 53oC & flow temperature 66oC the house is slowly heating up, I set the HW to come on 2 hours before the heating goes off at 10pm and 3 hours in the morning when the heating goes on at 7am....this achieved 47oC at the cylinder stat, which I know is around 60oC at the outlet, unfortunately although the boiler modulates really well, every so often, 1-2 hours it cuts out, I can only assume ( don’t suppose these boilers have an event log ), that it’s over temperature maybe it climbs to 69oC if a TRV changes up or down, I suppose if it had a 5oC tolerance instead of 3oC maybe it would manage..when it cuts out, I set the flow temperature to 85oC ( max ) when the gas firing starts reducing again, the temperature drops to 60-65oC & i then set the flow temperature back to 66oC and it starts modulating again..will try the other ideas soon.***.

thanks

Even though I would be surprised if any of the TRVs are doing much controlling with the house not fully heated, it is possible, and if so, to help avoiding this annoying 2 hourly cut out you might consider changing to PP2, this, by my calculation, should give you very close to the present performance on CP1 but may well help to smoothen out the flowrate if these TRVs are inclined to hunt.

 

[Reddwarf4ever]

Thanks John, currently I am running the Boiler at 66oC, @7KW with a typical return temperature of 52-53oC. It’s modulating very well & the house does get up to 22.5oC ( I actually set the room stat to 23.5oC during 7am to 10am & and the boiler goes off, which allows the rads to cool down and improve the return temperature when the boiler restarts) I set the heating to come on at 7am which goes off at 10pm. Have set Hot water to come on at 7am to 10am & 8pm to 10pm this manages to keep the Hot water at a good temperature, also it heats the water while the heating is on & in condensing mode. If I increase the pump pressure / flow and have to lower the flow temperature, I will have problems with Hot water heating. Don’t have separate controls and settings for heating and hot water and would be ***bersome to keep changing the boiler flow temperature.

I will try PP2 and see how that performs

 

[Reddwarf4ever]

Thanks John, currently I am running the Boiler at 66oC, @7KW with a typical return temperature of 52-53oC. It’s modulating very well & the house does get up to 22.5oC ( I actually set the room stat to 23.5oC during 7am to 10am & and the boiler goes off, which allows the rads to cool down and improve the return temperature when the boiler restarts) I set the heating to come on at 7am which goes off at 10pm. Have set Hot water to come on at 7am to 10am & 8pm to 10pm this manages to keep the Hot water at a good temperature, also it heats the water while the heating is on & in condensing mode. If I increase the pump pressure / flow and have to lower the flow temperature, I will have problems with Hot water heating. Don’t have separate controls and settings for heating and hot water and would be ***bersome to keep changing the boiler flow temperature.

I will try PP2 and see how that performs

For some reason the pagelayout using safari has got reallybad, is in Columns tried Firefox and ok, wonder if can be fixed? Looks like all the adverts causing it

 

[John.g]

I just go back to new posts to display the full page or something like that.

 

[Reddwarf4ever]

Oddly enough, the webpages are displaying ok today open safari, must depend on which adverts are running the DAB 3 evosta pump I received did NOT have the foam insulation shroud that I saw in a YouTube installation video and assumed it would be included any thoughts ?

 

[Reddwarf4ever]

Just had a reply from DAB, despite it showing this pump with the insulating shroud, all over their website, apparently it’s not on the model I bought, even though it’s a DAB 3 and can’t be bought separately.....although another online retailer said I can buy it for £29 + VAT plus £4 delivery, quite expensive for a piece of foam will use my hessian wrap from the old pump, not so pretty but effective.....don’t see why DAB couldn’t just send me one out......

 

[Reddwarf4ever]

I tried PP2, but the boiler seemed to be cutting out more, not sure if fluctuations upsetting it, as it only has a +3oC Flow to Actual tolerance, maybe this is the reason, still waiting for Vailant to respond about if it should be +5oC trying CP2 at the moment, has been behaving very well on CP1, but one radiator only barely gets warm, it’s in a downstairs extension room, maybe it’s a plumbing problem but on CP3 it certainly got hot....

thanks

 

[John.g]

The reason the rad is not circulating is probably that CP1 is limited to a 1.8M head which realistically is a bit low, CP2 at 3M should be a good compromise to get circulation, theoretically, at least, the flow rate should increase from 0.4 m3.hr to 0.52 M3/hr and you may be able to nurse another few kw from the boiler without cut out. CP3 at 4.5M is probably the best choice, hydraulically, you may loose a few % in boiler efficiency but maybe a small price to pay for comfortable heating until Vaillant respond.

 

[Reddwarf4ever]

The reason the rad is not circulating is probably that CP1 is limited to a 1.8M head which realistically is a bit low, CP2 at 3M should be a good compromise to get circulation, theoretically, at least, the flow rate should increase from 0.4 m3.hr to 0.52 M3/hr and you may be able to nurse another few kw from the boiler without cut out. CP3 at 4.5M is probably the best choice, hydraulically, you may loose a few % in boiler efficiency but maybe a small price to pay for comfortable heating until Vaillant respond.

Thank you CP2 did work reasonably well, the ‘ problem rad ‘ began to warm up. I then switched to CP3 and the ‘ problem rad ‘ is now very hot, can still keep my hand on it just....the other rads are really hot, the one next to my Wife’s chair is too hot too touch, with the Boiler flow set at 64oC however the return temperature has now increased to 56oC Might leave it at CP2, where the return was 51-53oC the ‘ problem Rad ‘ is not so important, as it’s in the ‘ ‘game room’ not used very much. Probably need a CP 2 1/2 , since increasing the flow from CP1 to CP2 and more recently CP3 the boiler is modulating a lot better, hasn’t cut out at all yet. Also the Hot water heated from 7am-11am & 8pm to 10pm is working very well, with CP1 & 2, Will be a different story in the Summer months when it will be Hot Water only.....

 

[ShaunCorbs]

Have you tried balancing the rads more with more pressure you will need to close the lockshields more

 

[Reddwarf4ever]

Have you tried balancing the rads more with more pressure you will need to close the lockshields more

only three can be adjusted, the other 9 all have TRVs....will have a look, thanks

 

[ShaunCorbs]

only three can be adjusted, the other 9 all have TRVs....will have a look, thanks

the other side should be a lockshield valve you can adjust that