Worcester Bosch System Boiler - Best pump settings?

[Johntheo5]

Now that makes me wonder if this is related.

I’ve noticed this boiler gets to target flow of 70c in about 25 mins. But when it gets there, the burner goes out for about 1minute during which time the flow temp rapidly goes down to maybe 50 or so. Then the burner comes back on at a much reduced output then gradually creeps back to target temp again over the next half hour, then stays there.

I’m assuming that’s due to the constant pump still running when the burners gone out for that minute. So I’m guessing option 1 would change that?

Should it be even doing that though? It’s always struck me as quite inefficient that it does that. Why cant it just stay at temp and modulate down?

That could be due to target temp overshoot, if the boiler temp exceeds the target temp by +5C then the burner will trip and the pump will continue to run, the burner will refire when the anticycle time has elapsed providing the boiler temp is 5C (or more) below the target temp, it shouldn't do this as long as the heat demand is > than the boiler minimum output but range rating (reducing) the boiler output can help prevent this.

 

[Johntheo5]

I am sure I am right about the meaning of residual head. But I don't trust the info. on this in the Worcester Bosch manual. I think you are correct that the graph is simply showing normal pump head. Probably for a standard UPM3 rather than the more powerful version I posted.

There's no doubt in Vokera's case that Residual head is the head available after boiler HEX losses etc, see two graphs below. The second was provided by @vulcancontinental a few years ago in another thread and states that this too is also the residual head which indicates that the boiler HEX resistance etc is ~ 1.6M at 20LPM as the pump head should be ~ 5.0M at this flowrate and the graph shows a residual head of 3.4M.

 

[GeddyMortgage]

That could be due to target temp overshoot, if the boiler temp exceeds the target temp by +5C then the burner will trip and the pump will continue to run, the burner will refire when the anticycle time has elapsed providing the boiler temp is 5C (or more) below the target temp, it shouldn't do this as long as the heat demand is > than the boiler minimum output but range rating (reducing) the boiler output can help prevent this.

I did limit the boiler output to 90% for a while but it still did this.

Is there another way I can stop it from happening? I.e i can see on the boiler settings I can set both; the flow target temp, AND there's a separate option for setting MAX flow temp. (currently set to 80). If I set that to the same as the target flow temp (70) should it circumvent this issue by way of a lower ceiling and not allowing the flow temp to exceed target?

Also to my question above, if I set the pump to be relative to heat output. If it does still cut the burner out, wouldnt that then mean its not constantly pumping for that period, and therefore better? Or is that just a silly statement.

 

[Johntheo5]

I did limit the boiler output to 90% for a while but it still did this.

Is there another way I can stop it from happening? I.e i can see on the boiler settings I can set both; the flow target temp, AND there's a separate option for setting MAX flow temp. (currently set to 80). If I set that to the same as the target flow temp (70) should it circumvent this issue by way of a lower ceiling and not allowing the flow temp to exceed target?

That option, 5-A1 only limits the adjustment range, if you set that to say 60C then you can't increase the target flow temp above 60C, it doesn't/can't prevent the temp exceeding its target temp.
There is a option 3-b3... it says, on the left that it is the dT that switches off and on the burner but on the right it seems to suggest that it only affects the switch on, IF it does affect the burner switch off, suggest changing that to -8C, it should help prevent the burner trip, first check that its set to its default of -6C.

Also to my question above, if I set the pump to be relative to heat output. If it does still cut the burner out, wouldnt that then mean its not constantly pumping for that period, and therefore better? Or is that just a silly statement.

The pump must/should keep running all the time that there is a demand for heat, it should only stop when the heating is programmed off or all roomstats, if installed, are satisfied, the burner will then trip, followed by pump over run for 2 min (if set to its default time of 2mins, 3-d6), the pump then stops.

Also suggest increasing the Stand By time which I think is the anticycle time from its default of 5mins to 7 mins.

 

[GeddyMortgage]

I see, so even if the burner goes out, setting 1 on the pump settings wouldn’t then stop the pump as the boiler knows there’s still demand for heat? Despite the burner being out?

Will setting the max flow temp as above prevent it exceeding the target flow temp?

 

[MNW67]

I’ve noticed this boiler gets to target flow of 70c in about 25 mins. But when it gets there, the burner goes out for about 1minute during which time the flow temp rapidly goes down to maybe 50 or so. Then the burner comes back on at a much reduced output then gradually creeps back to target temp again over the next half hour, then stays there.

I’m assuming that’s due to the constant pump still running when the burners gone out for that minute. So I’m guessing option 1 would change that?

Should it be even doing that though? It’s always struck me as quite inefficient that it does that. Why cant it just stay at temp and modulate down?

You seem to be saying that the boiler only cuts out the first time when starting from cold. And only for one minute. After that it modulates and keeps running constantly. If that is the case, then the inefficiency that causes is vanishingly small.

A relative has the regular version of this boiler. It does something similar. It is often very confusing and difficult to understand what is actually happening. Especially why the temperature creeps up so slowly sometimes. I have occasionally been into the first level information menu and it can show the burner at 20% or less when the boiler is nowhere near the target.

If you go into the information menu you can monitor the burner modulation throughout the cycle. This may give you a better understanding of what is happening. An explanation given to me on here was that the pump might actually be too slow, so sometimes the boiler can't run at a higher output because it would exceed the maximum dT through the heat exchanger.

For the moment, it might be better to concentrate on getting all the radiators hot using the higher pump settings. This might sort out any gremlins. If not then deal with the modulation issues later. There are so many settings on this boiler that it would be easy to get lost and confused if you make changes.

 

[GeddyMortgage]

You seem to be saying that the boiler only cuts out the first time when starting from cold. And only for one minute. After that it modulates and keeps running constantly. If that is the case, then the inefficiency that causes is vanishingly small.

A relative has the regular version of this boiler. It does something similar. It is often very confusing and difficult to understand what is actually happening. Especially why the temperature creeps up so slowly sometimes. I have occasionally been into the first level information menu and it can show the burner at 20% or less when the boiler is nowhere near the target.

If you go into the information menu you can monitor the burner modulation throughout the cycle. This may give you a better understanding of what is happening. An explanation given to me on here was that the pump might actually be too slow, so sometimes the boiler can't run at a higher output because it would exceed the maximum dT through the heat exchanger.

For the moment, it might be better to concentrate on getting all the radiators hot using the higher pump settings. This might sort out any gremlins. If not then deal with the modulation issues later. There are so many settings on this boiler that it would be easy to get lost and confused if you make changes.

hear hear, my old Bosch boiler just has one dial that controls the flow temp and output … whack it on 5 out of 7 and bobs your uncle.

Glad im not the only one experiencing this. And your assessment is right,

turn on from cold…

takes 25 mins to get to target (incrementally upping its % output during this period )….

Exceeds target by a few degrees….

Burner goes out flow drops by 20c or so

Burner comes back on at 16% output or so and gradually creeps back up to target and generally doesn’t exceed 45% output once target re-achieved.

 

[MNW67]

takes 25 mins to get to target (incrementally upping its % output during this period )….

That's interesting. I've never been at the house at the right time, or had sufficient time, to do proper monitoring. My relatives main complaint was about how long it took for the house to get hot compared with the very old boiler of the same output. But they decided in the end just to put the heating on an hour earlier and now they have got used to it. If it was my boiler, though, I would be intrigued to monitor and find out what is happening.

So from a cold start, how does the output progress? How low does it start and how quickly does it get up to 100%?

The 16% to 45% is also what I have noticed.

 

[GeddyMortgage]

That's interesting. I've never been at the house at the right time, or had sufficient time, to do proper monitoring. My relatives main complaint was about how long it took for the house to get hot compared with the very old boiler of the same output. But they decided in the end just to put the heating on an hour earlier and now they have got used to it. If it was my boiler, though, I would be intrigued to monitor and find out what is happening.

So from a cold start, how does the output progress? How low does it start and how quickly does it get up to 100%?

About 20mins I’d say. Starts at 20%… It’s gradual and incremental over that period all the way up to 100%. I am running 30kw of rads mind you so in my inexperienced mind… 25 mins to get to target temp felt about right.

I’ve just never understood why it can’t just reach 100% much quicker…Bosch technical told me in passing it can do. But tbh, they also told me option 3 on the pump is plenty for 15 type22 radiators, so my faith in them is pretty darn low

 

[MNW67]

About 20mins I’d say. Starts at 20%… It’s gradual and incremental over that period all the way up to 100%. I am running 30kw of rads mind you so in my inexperienced mind… 25 mins to get to target temp felt about right.

I’ve just never understood why it can’t just reach 100% much quicker…Bosch technical told me in passing it can do. But tbh, they also told me option 3 on the pump is plenty for 15 type22 radiators, so my faith in them is pretty darn low

I agree, 25 minutes doesn't sound too bad.

I have found Bosch support hit and miss. And there are definitely small errors in some of the manuals. So you lose confidence. A classic example is about the meaning of those pump curves. Whether it is residual head or full head makes a big difference. If its residual head then 250 mbar should do your radiators. But if it's only 250 mbar full head then that looks very unlikely. I know it says residual head but I have had other small mistakes confirmed by tech support. Or it might be that some of the figures are residual head and others full head. The main reason I think John is suspicious is because that curve A is so high. At 30 litres per minute (1800 litres per hour) it would probably take 3m of head just to push the water through the heat exchanger. But it is showing 5.5m head. If that is residual head then that means 8.5m full head, which at 30l/min would be a massively powerful pump.

 

[GeddyMortgage]

Yeah makes sense.

I guess one easy test method would be to whack it in chimney sweep mode… that goes straight to 100% right? And assume it doesn’t override the pump speed settings… be interesting to see the difference regarding the heat of the rads and whether it still trips at target flow etc.

 

[MNW67]

Yeah makes sense.

I guess one easy test method would be to whack it in chimney sweep mode… that goes straight to 100% right? And assume it doesn’t override the pump speed settings… be interesting to see the difference regarding the heat of the rads and whether it still trips at target flow etc.

I think that has been suggested before. I've never done it. I'd be interested to see what happens.

 

[GeddyMortgage]

I agree, 25 minutes doesn't sound too bad.

I have found Bosch support hit and miss. And there are definitely small errors in some of the manuals. So you lose confidence. A classic example is about the meaning of those pump curves. Whether it is residual head or full head makes a big difference. If its residual head then 250 mbar should do your radiators. But if it's only 250 mbar full head then that looks very unlikely. I know it says residual head but I have had other small mistakes confirmed by tech support. Or it might be that some of the figures are residual head and others full head. The main reason I think John is suspicious is because that curve A is so high. At 30 litres per minute (1800 litres per hour) it would probably take 3m of head just to push the water through the heat exchanger. But it is showing 5.5m head. If that is residual head then that means 8.5m full head, which at 30l/min would be a massively powerful pump.

Again I guess it’s gonna be suck it and see…. Worst case I whack it up too high and the heat exchanger errors out because the waters coming back too hot?…

 

[MNW67]

Again I guess it’s gonna be suck it and see…. Worst case I whack it up too high and the heat exchanger errors out because the waters coming back too hot?…

At a higher pump speed the difference between flow and return will narrow. I don't know if that would ever cause an error. The errors are usually when the difference is too large.

I would go a step at a time with the pump but Johntheo5 and snb1 have more experience and seem bolder.

Instead of chimney sweep mode can you set minimum output to 100% in the menu. That might give you more control? Maybe you can't having checked the manual? That would be unusual for a fancy boiler.

 

[Johntheo5]

Time for action!, remember the cold rads problem and the thread title.
The best pump setting is stage 6, 4.5M head, even if the residual head is 4.5M it shouldn't cause the slightest problem when circulating through 15 rads or whatever, monitor 1-A5 flow temp, 1-b8 burner modulation & 1-C2, pump speed. I'm surprised that I didn't see somewhere to monitor the boiler return temp?, that would really help.