Flow temp vs power rating

[John Smith Esq.]

Hi all,

I have my greenstar 25i combi set to 9.1KW and 53C flow temp. With the outside temperatures falling I want to know what would have the biggest effect on warmth indoors:

- increasing the rating to say 10.5kw
- increasing the flow temperature by 1 or 2 degrees
- increasing the amount of time the heating is on

However, I want to change the one that will increase warmth in the most efficient and economical way. I assume the last option will be effective but most costly. Which would win out of the first two?

There isn't much I can do to improve other factors like insulation.

Thanks

 

[Johntheo5]

Hi all,

I have my greenstar 25i combi set to 9.1KW and 53C flow temp. With the outside temperatures falling I want to know what would have the biggest effect on warmth indoors:

- increasing the rating to say 10.5kw
- increasing the flow temperature by 1 or 2 degrees
- increasing the amount of time the heating is on

However, I want to change the one that will increase warmth in the most efficient and economical way. I assume the last option will be effective but most costly. Which would win out of the first two?

There isn't much I can do to improve other factors like insulation.

Thanks

Increasing the rating will have no effect except that its present rating is stopping boiler flow temperature from reaching its set point so forget about that as a economy measure, it should be set to the max rating of your radiators, maybe something like 15kw or so, the boiler will use the exact amount of fuel necessary to satisfy the flow temperature SP set by you, no more and no less.

Adjusting the flow temperature is the most convenient and easiest way of getting the optimum performance.

For every 1 DegC rise or fall in the OAT (outside air temperature) then the Boiler flow temperature should be decreased or increased by around 2 DegC which will result in ~ 4% increase/decrease in the power required or if you want to increase the room temperature (with no change in OAT) then use the same method, if, say you wish to increase the room temperature by 2 DegC then increase the boiler flow temp by 4C which will result in ~ a 8% increase in power demand.
Slight correction, for every 1 DegC increase/decrease in required room temp, increase/decrease the boiler flowtemp by 3 DegC, the extra power will still be the same + or - 4%.

 

[John Smith Esq.]

Increasing the rating will have no effect except that its present rating is stopping boiler flow temperature from reaching its set point so forget about that as a economy measure, it should be set to the max rating of your radiators, maybe something like 15kw or so, the boiler will use the exact amount of fuel necessary to satisfy the flow temperature SP set by you, no more and no less.

Adjusting the flow temperature is the most convenient and easiest way of getting the optimum performance.

For every 1 DegC rise or fall in the OAT (outside air temperature) then the Boiler flow temperature should be decreased or increased by around 2 DegC which will result in ~ 4% increase/decrease in the power required or if you want to increase the room temperature (with no change in OAT) then use the same method, if, say you wish to increase the room temperature by 2 DegC then increase the boiler flow temp by 4C which will result in ~ a 8% increase in power demand.
Slight correction, for every 1 DegC increase/decrease in required room temp, increase/decrease the boiler flowtemp by 3 DegC, the extra power will still be the same + or - 4%.

Thanks for this, very helpful and interesting.

Just to say, the boiler does reach the set flow temperature with the rating set to 9.1KW. Perhaps slower than might otherwise be the case. Presumably it saves electricity if not gas to have the rating lower?

I have increased the rating to 12KW. I have 9 radiators in this three bed bungalow, 7 of those are doubles, the other two are a single and a towel one in bathroom. Some of the pipes on the attic do have to travel a way through to reach two of the further away radiators. I gather 12KW is a bit more aligned with the situation, and still only half what the boiler is capable of.

I've also increased the flow temp to 54 for now. I'm finding the house does get up to 20C within the three 2hr blocks I have it running for each day but will have to stop being so tight if needs required more heat. Also, I followed your advice and increased the anti fast temperature to -14. It does seem to result in less stop-start operation by the boiler - plus I assume the average flow temp is less overall since it isn't constantly overshooting the set flow temp. So less gas used and more condensing.

On a separate note, after I cleaned out the magnetic filter the other day and lost about 4L of water (which I obviously replaced) would you now add more inhibitor? The trouble is, I don't know what inhibitor was put in last year when two radiators were replaced. Some people say brands don't matter, others say they shouldn't be mixed. I was going to get Fernox Express and add it through the filter. If it is necessary to drain the system, would I get away with just doing half to at least reduce the existing chemicals by that amount, or do I have to drain the lot? I'd be doing this through the filter using a bucket. I realise several trips to the drain would be necessary.

 

[Johntheo5]

Don't know if you can mix chemicals or not, I wouldn't be too worried about adding just 4L of water alone to say 65L of system water, say 6 or 7%.

 

[John Smith Esq.]

Increasing the rating will have no effect except that its present rating is stopping boiler flow temperature from reaching its set point so forget about that as a economy measure, it should be set to the max rating of your radiators, maybe something like 15kw or so, the boiler will use the exact amount of fuel necessary to satisfy the flow temperature SP set by you, no more and no less.

Adjusting the flow temperature is the most convenient and easiest way of getting the optimum performance.

For every 1 DegC rise or fall in the OAT (outside air temperature) then the Boiler flow temperature should be decreased or increased by around 2 DegC which will result in ~ 4% increase/decrease in the power required or if you want to increase the room temperature (with no change in OAT) then use the same method, if, say you wish to increase the room temperature by 2 DegC then increase the boiler flow temp by 4C which will result in ~ a 8% increase in power demand.
Slight correction, for every 1 DegC increase/decrease in required room temp, increase/decrease the boiler flowtemp by 3 DegC, the extra power will still be the same + or - 4%.

Hi mate,

You've previously said:

"It will stay running at 53C providing the heat load (rad TRVs throttling in) stays higher than the minimum output of the boiler, once the heat load falls below the boiler minimum output then the return and flow will start rising until the flowtemp rises to SP+5C, (53+5), 58C, the burner will then trip, the circ pump will continue to run, when the anticycle time has elapsed and providing the flowtemp is < SP-5C."

What causes it to trip at flow temp +5? Is that a setting that can be changed or is it built in?

Right now, I have flow temp on 57C. Earlier, as it was burning gas, it reached 60C and then tripped. The circ pump continued to run without gas burning for about 5 mins and the temperature dropped to 48C. It then refired.

Settings are:

CH rating 14.2KW
Flow temp 57C
Pump head 2 (low to medium)
Anti fast time 5 mins
Anti fast flow temp -12

I assume it is better to spend less time over the set flow temp. How do I make it trip sooner? It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?

Thanks

 

[Johntheo5]

Settings are:

CH rating 14.2KW
Flow temp 57C
Pump head 2 (low to medium)
Anti fast time 5 mins
Anti fast flow temp -12

I assume it is better to spend less time over the set flow temp. How do I make it trip sooner? It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?

Thanks

Settings are:

CH rating 14.2KW
Flow temp 57C
Pump head 2 (low to medium)
Anti fast time 5 mins
Anti fast flow temp -12

I assume it is better to spend less time over the set flow temp. How do I make it trip sooner? It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?


[/QUOTE]
SP+5C is a fairly prudent setting IMO, because, one, you don't want the temperature to exceed SP+5C on refiring, its quite difficult to get this spot on by just adjusting the anticycle time or the temperature that the boilrer refires at, Anti fast flow temp -12, in your case, two, is that when the boiler is firing normally, hovering around its SP temperature and if you had a CH zone scheduled to shut down then the boiler flowtemp will rise by a few degrees or more before the boiler modulates down to return it to its SP setting.

"It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?"

I'm very surprised to hear that the burner will refire once the anicycle time has elapsed even if the flowtemp hasn't reached SP-12C, what's the point of having it then?, all boilers that havn't this feature will not refire when the anticycle time has elapsed unless the flowtemp is SP-5C, maybe this is whats happening with yours but it certainly shouldn't IMO.
I'm also surprised that its refiring once the SP-12C is reached without the anticycle time having elapsed, you can easily test this, give the boiler a very long anticycle time of say 10 minutes and see does it refire well before this at SP-12C. If its behaving in this manner then give Greenstar a ring.

 

[John Smith Esq.]

Settings are:

CH rating 14.2KW
Flow temp 57C
Pump head 2 (low to medium)
Anti fast time 5 mins
Anti fast flow temp -12

I assume it is better to spend less time over the set flow temp. How do I make it trip sooner? It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?

SP+5C is a fairly prudent setting IMO, because, one, you don't want the temperature to exceed SP+5C on refiring, its quite difficult to get this spot on by just adjusting the anticycle time or the temperature that the boilrer refires at, Anti fast flow temp -12, in your case, two, is that when the boiler is firing normally, hovering around its SP temperature and if you had a CH zone scheduled to shut down then the boiler flowtemp will rise by a few degrees or more before the boiler modulates down to return it to its SP setting.

"It seems that the Anti fast time and flow temps, which ever one is hit first causes it to refire. It isn't getting down to minus 12 within the 5 mins so is it worth extending the antibfast time?"

I'm very surprised to hear that the burner will refire once the anicycle time has elapsed even if the flowtemp hasn't reached SP-12C, what's the point of having it then?, all boilers that havn't this feature will not refire when the anticycle time has elapsed unless the flowtemp is SP-5C, maybe this is whats happening with yours but it certainly shouldn't IMO.
I'm also surprised that its refiring once the SP-12C is reached without the anticycle time having elapsed, you can easily test this, give the boiler a very long anticycle time of say 10 minutes and see does it refire well before this at SP-12C. If its behaving in this manner then give Greenstar a ring.
[/QUOTE]

So what triggers the antifast cycle? Did you say flow temp +5 causes it? But on mine earlier, it tripped at 60C.

 

[Johntheo5]

So what triggers the antifast cycle? Did you say flow temp +5 causes it? But on mine earlier, it tripped at 60C.

Well, it shouldn't except there is a settable trip, have a look in the MIs, I have seen a few complaints from Vaillant owners of trips at SP+3C.

 

[John Smith Esq.]

Well, it shouldn't except there is a settable trip, have a look in the MIs, I have seen a few complaints from Vaillant owners of trips at SP+3C.

If TRVs are rejecting hot water from the boiler, isn't it inevitable that the flow temperature at the boiler will overshoot, so the trip is supposed to stop stop burning gas and cycle the hot water for a bit? Or are you saying this shouldn't be happening?

 

[Johntheo5]

Yes, and it is happening?, but shouldn't until SP+5C?

 

[John Smith Esq.]

It is happening yes ' it over shoots flow temp then stops and cycles for a while (5 mins probably, as in the anti fast time). When I say over shoots, it doesn't happen quickly like 10 seconds or whatever. It will be on the set flow temp for a bit, then a degree higher for a bit and so on. I don't know why it would be SP+5 as I'm not aware of any setting for that, unless you mean that it is a built in fixed setting? I don't see any advantage to it being +5 rather +3 to be honest. Could it be that it is SP+5C and there's a timed factor as well, like 5 mins?

Could it be to do with the pump head setting? It is set on 2 out of 4, but there is also an automatic option.

 

[Johntheo5]

First of all, there has to be a dT between the flowtemp SP and the burner trip otherwise the burner would trip every time the flowtemp reaches its SP, as stated previously SP+5C is used in the majority of boilers (built in) as it gives a bit of head room for the shutting off of zones etc and allows the boiler to refire without exceeding the SP bu too much, 2 to3C is typical, the reason that it creeps up is that the heat demand is falling very slowly below the minimum output of the boiler and eventually will reach SP+5C and burner trip, I certainly wouldn't be happy with a burner trip of SP+3C and would consider it a boiler fault.

 

[John Smith Esq.]

First of all, there has to be a dT between the flowtemp SP and the burner trip otherwise the burner would trip every time the flowtemp reaches its SP, as stated previously SP+5C is used in the majority of boilers (built in) as it gives a bit of head room for the shutting off of zones etc and allows the boiler to refire without exceeding the SP bu too much, 2 to3C is typical, the reason that it creeps up is that the heat demand is falling very slowly below the minimum output of the boiler and eventually will reach SP+5C and burner trip, I certainly wouldn't be happy with a burner trip of SP+3C and would consider it a boiler fault.

I just did a test and it tripped at 61C (flow temp is 57) but wad probably near 62 when it tripped.

Will lowering the pump setting from 2 to 1 reduce the return flow temp (water in radiators for longer)? Trying to decide what the best setting is...

I just noticed after it tripped that it never fell below about 53C. It cycled without burning gas for a little bit. At the end of the 7 mins I now have it set to for anti fast time, it started again. So it seems to be a case of anti fast ends when either the time or temp settings are reached, whichever happens first. I will try an anti fast time of more like 10 or 12 mins. But the temperature on the boiler wasn't falling very fast.