Central heating pump sizing

[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