Boiler Reported Return Temperature

[webbm000]

I've got ebusd reading the flow and return temperatures from the boiler (red and Green) (Vaillant EcoTec Plus 637) and some thermocouples on the pipes about 1.5m away from the boiler (yellow and blue). The flow as reported by the boiler is matching the thermocouple both when the boiler is off and on (accepting that the boiler EBUS readings are 2.5 mins apart so misses some of the peaks).

However the return tracks when the boiler has been off for a while but then is ~10degrees hotter than the return 2m from the boiler when the boiler switches on. Our system has both the internal boiler bypass and an external one about 0.5m away from the boiler and I assume this is to caused by them but I can't understand why it is happening when the boiler has started from cold as all TRVs should be open.

Any ideas?

 

[Johntheo5]

The internal by pass is probably the problem, its default setting is only 250mb, 2.5M and the adjuxtable max is still only 350mb, 3.5M, depending on the pump speed setting it will probably be by passing constantly, even more so at low system flowrates, have a look at d.014 setting and the actual pump speed, d.015.
What is the external ABV setting?.

 

[webbm000]

External bypass has been screwed down to max pressure once I found out there was an internal bypass and I'll check tomorrow but I dont have d.14 just d.19 which is set to 1 slow speed pump

I guess the question is "Is it an issue that the internal bypass is constantly letting water through?"

 

[Johntheo5]

If the actual return temperature is below say 45/48c then you are losing a few % in condensing efficiency, if above say 50,C then a further 10C in temperature only reduces the efficiency by ~ 0.5%

 

[webbm000]

The boiler cycles horribly most of the time as you can see on the right hand side of the chart due to the boiler being oversized so not exactly efficient and condensing as much as it should. Turned boiler down to 12kW (lowest it will go) but still cycling so I'm trying to understand it a bit more before justifying the cost of converting to a modulating controller as these apparently modulate down to 6kW

 

[MNW67]

Turned boiler down to 12kW (lowest it will go) but still cycling so I'm trying to understand it a bit more before justifying the cost of converting to a modulating controller as these apparently modulate down to 6kW

It sounds like you've turned the boiler maximum output down to 12KW. But doesn't the boiler automatically modulate down to 6KW by itself? I can't see why a modulating controller would make the boiler modulate to a lower output. The minimum output is a function of the boiler, not the controller.

A modulating controller usually means that it modulates the flow temperature.

How old is the boiler?

 

[Johntheo5]

All/most boilers will cycle if the heat demand is less than the boiler's minimum output (6kw, above?) when the flow temp reaches taget temp+5C, this shouldn't be a big deal but Vaillant's hold the ignition settings of ~ 65% of max output (or used to) for 60 secs so requires a long anticycle time to get the flow/return temp as low as possible, the actual anticycle time depends on the set time and the target flow temp. A SP at its default setting of 20 with a target temp of 65c will give a anticycle time of 4.5mins which may not be long enough to get the flow/return temps down by say 20/25C to 40C/45C to enable the boiler to get away on its first refiring, if this is a problem with this boiler.

 

[webbm000]

It sounds like you've turned the boiler maximum output down to 12KW. But doesn't the boiler automatically modulate down to 6KW by itself? I can't see why a modulating controller would make the boiler modulate to a lower output. The minimum output is a function of the boiler, not the controller.

A modulating controller usually means that it modulates the flow temperature.

How old is the boiler?

The lowest you can set it on d.00 is 12kW for the 637 but the Vaillant technical help line said it would modulate down to 6kW which I wasn't sure was correct.

I did wonder if the VRC720 weather compensating controller would be able to set the boiler to a lower output than you can do manually

 

[MNW67]

The lowest you can set it on d.00 is 12kW for the 637 but the Vaillant technical help line said it would modulate down to 6kW which I wasn't sure was correct.

I did wonder if the VRC720 weather compensating controller would be able to set the boiler to a lower output than you can do manually

But to be clear, we're talking about a lower maximum output? The minimum output will always be 6KW, whether you use weather compensation or not?

As far as I know, d.00 sets the upper limit for the output. So if you set it to 12kW, the boiler will modulate between 6kW and 12kW. If you set it to 37KW, the boiler will modulate between 6kW and 37kW.

Where is that table from? Do you have a link please and page number?

 

[webbm000]

All/most boilers will cycle if the heat demand is less than the boiler's minimum output (6kw, above?) when the flow temp reaches taget temp+5C, this shouldn't be a big deal but Vaillant's hold the ignition settings of ~ 65% of max output (or used to) for 60 secs so requires a long anticycle time to get the flow/return temp as low as possible, the actual anticycle time depends on the set time and the target flow temp. A SP at its default setting of 20 with a target temp of 65c will give a anticycle time of 4.5mins which may not be long enough to get the flow/return temps down by say 20/25C to 40C/45C to enable the boiler to get away on its first refiring, if this is a problem with this boiler.

View attachment 337578

I had read about this and was on the list of things to change to see the effect.

Consistent with what you said this morning was 3degC outside. Rads, DHW and UFH all had demand at the same time so boiler ran for ~1.5hrs before reaching setpoint 65 + 5degC = 70degC and switched off. Looks like it came on after 3-8 mins and immediately was back up at 65degC and cycling. Will increase the anti cycle time to 60 to see what 11.5 mins does to the cycling

Also will changing the pump speed to high make it better or worse? Higher pump speed means higher return temperatures but as the return is 15degC below the return in the boiler it probably wont make much of a difference there but it could increase the amount of water coming through the internal bypass. Is this correct?

Also boiler is 2006 model but has had PCB and main heat exchanger replaced

 

[MNW67]

Also boiler is 2006 model but has had PCB and main heat exchanger replaced

I've just checked the manual for the 2006 version and it seems to have a minimum output of 12kW.

Later versions seem to have a minimum output of 7kW. Is it possible Vaillant got confused and gave you info about a later version by mistake?

 

[Johntheo5]

I had read about this and was on the list of things to change to see the effect.

Consistent with what you said this morning was 3degC outside. Rads, DHW and UFH all had demand at the same time so boiler ran for ~1.5hrs before reaching setpoint 65 + 5degC = 70degC and switched off. Looks like it came on after 3-8 mins and immediately was back up at 65degC and cycling. Will increase the anti cycle time to 60 to see what 11.5 mins does to the cycling

Also will changing the pump speed to high make it better or worse? Higher pump speed means higher return temperatures but as the return is 15degC below the return in the boiler it probably wont make much of a difference there but it could inchuge difference rease the amount of water coming through the internal bypass. Is this correct?

It will probably increase the internal bypass flowrate as it will be running with a higher head, but difficult to what effect this will have on the refiring affect flow temperature, the above trends above show a apparent huge bypass as it is, rad flow/return ~ 68C/43C & boiler flow/return, 68C/57C, are you sure there is no bypassing through the external ABV, feel the pipe and note the setting.
Theoretically of course, the combined higher flowrate with a smaller dT should result in the same flow temp with no by pass and with the lower flow at a greater dT.
Did you note the actual pump speed while firing normally?

Also boiler is 2006 model but has had PCB and main heat exchanger replaced

View attachment 337626

 

[Johntheo5]

I've just checked the manual for the 2006 version and it seems to have a minimum output of 12kW.

Later versions seem to have a minimum output of 7kW. Is it possible Vaillant got confused and gave you info about a later version by mistake?

I've just checked the manual for the 2006 version and it seems to have a minimum output of 12kW.

Later versions seem to have a minimum output of 7kW. Is it possible Vaillant got confused and gave you info about a later version by mistake?

I get the same, the model number may give a clue, VU GB 376/5_5 A for the 7.1/6.4kw one.

 

[webbm000]

I've just checked the manual for the 2006 version and it seems to have a minimum output of 12kW.

Later versions seem to have a minimum output of 7kW. Is it possible Vaillant got confused and gave you info about a later version by mistake?

That would explain it as they didn't ask how old it was. I didn't know they changed it in later models

 

[webbm000]

It will probably increase the internal bypass flowrate as it will be running with a higher head, but difficult to what effect this will have on the refiring affect flow temperature, the above trends above show a apparent huge bypass as it is, rad flow/return ~ 68C/43C & boiler flow/return, 68C/57C, are you sure there is no bypassing through the external ABV, feel the pipe and note the setting.
Theoretically of course, the combined higher flowrate with a smaller dT should result in the same flow temp with no by pass and with the lower flow at a greater dT.
Did you note the actual pump speed while firing normally?

Setting d.19 = 1 should mean that everything is on low speed but I'm not sure if there is any way to check pump speed?

I have a slight worry about the external ABV and was going to remove it to make sure it wasn't the cause as the pipe the other side of it is hot but that could just be conduction along the pipe? Something else to add to the list of things to try