Range rating, modulation and manually setting the CH temperature question.

[vulcancontinental]

At what point does it try to modulate down? I thought a boiler would modulate down as it approached the set point, rather than increasing further?

EDIT: I think I read your comment a bit too literally.

On my tests I wanted to make sure the boiler did not modulate or turn off which I can do by ensuring the setpoint wasn't even approached as that would not allow me to complete the trial.

I also only allowed the Vision Plus two 10% increases but have little doubt that the incremental increases would continue at 10%.

 

[JonathanM]

On my tests I wanted to make sure the boiler did not modulate or turn off which I can do by ensuring the setpoint wasn't even approached as that would not allow me to complete the trial.

I also only allowed the Vision Plus two 10% increases but have little doubt that the incremental increases would continue at 10%.

Thank you. Makes sense.

 

[Johntheo5]

The challenge IMO is when the boiler is in cycling mode due to low heat demand, is in being able to modulate down fast enough after ignition settings are released after firing, these settings can be anything from maybe 55% to 70%, some then. like vaillant hold these settings for up to 60 secs, in which time the the flow temp is at or aready > the SP.
You have to allow a very long anti cycle time to undercool the boiler temp to allow for this, Vokera (at least the vision 20S) seem to have a very responsive control system and will actually start modulating down well before the SP temp is reached and I have seen overshoots as low as 2 deg with only a 3 or 4 kw heating load, eventually of course the flow will exceed the SP by 5C and the boiler will cycle but makes no big deal about it.

 

[JonathanM]

ScottishGasMan, thank you, that is all so useful, and has helped me get a few things straight in my head.

 

[ScottishGasMan]

maybe 55% to 70%, some then. like vaillant hold these settings for up to 60 secs

Just to clarify, Only the "Regular/Heat Only" Vaillants do that, as a test to see if there is enough circulation in the system as they can't see pressure or pump movement. This "shouldnt" be a problem as it's fairly easy to size a heat only boiler up to be pretty close to actual demand unlike a combi, however its incredibly common to see grossly oversized heat only boilers as very few people perform a heat loss calculation.

Vaillant System and Combi boilers, fire around the 60% mark and immediately drop to minimum once a safe flame patter has been established, the can stay there for upto 5 minutes before deciding how high to fire.

 

[JonathanM]

The challenge IMO is when the boiler is in cycling mode due to low heat demand, is in being able to modulate down fast enough after ignition settings are released after firing, these settings can be anything from maybe 55% to 70%, some then. like vaillant hold these settings for up to 60 secs, in which time the the flow temp is at or aready > the SP.
You have to allow a very long anti cycle time to undercool the boiler temp to allow for this, Vokera (at least the vision 20S) seem to have a very responsive control system and will actually start modulating down well before the SP temp is reached and I have seen overshoots as low as 2 deg with only a 3 or 4 kw heating load, eventually of course the flow will exceed the SP by 5C and the boiler will cycle but makes no big deal about it.

For people serious about running at low temperatures and maximising condensing and modulating, I think it would be really useful to get a list of all the boilers and their firing/ramping patterns, and how configurable they are. I was reading a manual for an Alpha boiler, and it seems you can change both the ramp up period and the anti-cycle period. Whereas I read on here that Ideal Logic boilers are very hard to run at low temperature because the pump is too fast. So many factors!

 

[JonathanM]

Just to clarify, Only the "Regular/Heat Only" Vaillants do that, as a test to see if there is enough circulation in the system as they can't see pressure or pump movement. This "shouldnt" be a problem as it's fairly easy to size a heat only boiler up to be pretty close to actual demand unlike a combi, however its incredibly common to see grossly oversized heat only boilers as very few people perform a heat loss calculation.

Vaillant System and Combi boilers, fire around the 60% mark and immediately drop to minimum once a safe flame patter has been established, the can stay there for upto 5 minutes before deciding how high to fire.

I've recently been getting quotes with a few installers for a heat only boiler. Only one seemed to really take sizing it seriously and suggested 19KW. He was a top guy. Others all suggested 30KW, saying it was always better to have plenty in reserve. Even after I told them current boiler was 24KW and was never close to its limit.

 

[ScottishGasMan]

I think it would be really useful to get a list of all the boilers and their firing/ramping patterns

I run full Weather comp, but I set my 18kW system boiler to 6kW max output for CH, and think I left it at 7kW for the hot water reheating.

I have a graph on my phone of recent heating/cycling I'll post up later, as the heating has only just come back on the last couple of weeks, it targets around 35-37°C at the moment, and cycles about twice an hour. When it gets a little colder outside the cycling will just stop once the heat loss is greater than the minimum output, then the burner can stay lit all day.

 

[ScottishGasMan]

I've recently been getting quotes with a few installers for a heat only boiler. Only one seemed to really take sizing it seriously and suggested 19KW. He was a top guy. Others all suggested 30KW, saying it was always better to have plenty in reserve. Even after I told them current boiler was 24KW and was never close to its limit.

Even 19kW would be a pretty big heat loss, however the old way of doing it was to add XkW on for DHW, where more modern systems will run priority hot water, so they will heat the cylinder at high temperature and full output, then switch back to heating at low temperature based on either load compensating or weather compensating controls.

Although this is not the "norm" yet unfortunately, it's gaining a little popularity but still a bit of a niche installer base.

 

[JonathanM]

I run full Weather comp, but I set my 18kW system boiler to 6kW max output for CH, and think I left it at 7kW for the hot water reheating.

I have a graph on my phone of recent heating/cycling I'll post up later, as the heating has only just come back on the last couple of weeks, it targets around 35-37°C at the moment, and cycles about twice an hour. When it gets a little colder outside the cycling will just stop once the heat loss is greater than the minimum output, then the burner can stay lit all day.

At that sort of flow temperature, does it produce masses of condensate?

EDIT: Mainly asking because BG suggested soakaway, and the ground isn't very good here, and I wondered if I ran it very cool, might I get a puddle of condensate.

 

[vulcancontinental]

The challenge IMO is when the boiler is in cycling mode due to low heat demand, is in being able to modulate down fast enough after ignition settings are released after firing, these settings can be anything from maybe 55% to 70%, some then. like vaillant hold these settings for up to 60 secs, in which time the the flow temp is at or aready > the SP.
You have to allow a very long anti cycle time to undercool the boiler temp to allow for this, Vokera (at least the vision 20S) seem to have a very responsive control system and will actually start modulating down well before the SP temp is reached and I have seen overshoots as low as 2 deg with only a 3 or 4 kw heating load, eventually of course the flow will exceed the SP by 5C and the boiler will cycle but makes no big deal about it.

It is also possible to reduce the ignition fan speed on certain Vokera boilers, something I am wary of because the ignition fan speed serves two purposes one to ensures good ignition and the other the pre-ignition purge to ensure a chamber clear of previously created products of combustion even though there is a post firing purge.

I have of course tried this I couldn’t resist it, but not incrementally.

I selected minimum fan speed as the new ‘ignition’ fan speed. The idea was to see what would happen if this was tried in order to avoid precisely what you describe, a too high heat input at minimum system restart. There was a small noise at start up but at the time I wasn’t particularly interested in pursuing things, if there was a problem I can lengthen the anti-cycle delay up to 20 minutes from the factory default of 3 minutes to ensure a longer burn time.

It’s permitted with a combi as an anti-oscillation special hot water function in poor flow situations or higher than normal cold water inlet temperatures. The fan is kept running at minimum fan speed following a shut down after DHW set point is reached but demand is continuing requiring a refiring sequence so I knew I was within a safe fan speed region.

 

[vulcancontinental]

The challenge IMO is when the boiler is in cycling mode due to low heat demand, is in being able to modulate down fast enough after ignition settings are released after firing, these settings can be anything from maybe 55% to 70%, some then. like vaillant hold these settings for up to 60 secs, in which time the the flow temp is at or aready > the SP.
You have to allow a very long anti cycle time to undercool the boiler temp to allow for this, Vokera (at least the vision 20S) seem to have a very responsive control system and will actually start modulating down well before the SP temp is reached and I have seen overshoots as low as 2 deg with only a 3 or 4 kw heating load, eventually of course the flow will exceed the SP by 5C and the boiler will cycle but makes no big deal about it.

It is possible to adjust the hysteresis both over and under setpoint on Vokera boilers if that is your desire.

 

[amcluesent]

On my Worcester Greenstar CDi classic (30kWh) I've been experimenting with range rating to 50% of capacity for CH and also set the flow temp to about 55°C to (hopefully) encourage condensing. Also set the pump so it steps along with the fan using Map 4 rather than runs at constant pressure.

The Nest has a 'time-to-temperature' feature so it's learning that to reach the setting of 19°C by 07:30 with that boiler configuration it needs to start heating sooner.

 

[ScottishGasMan]

At that sort of flow temperature, does it produce masses of condensate?

I've not measured it accurately but yes, its much more than when run at high temperature, soakaways are the least favoured way of terminating condensate due to these sort of issues, although if their done right shouldn't pose much problem (always depends on what the ground is like though for waterlogging and drainage though)

 

[mpgw]

Yes and no.

First the Vision Plus, this boiler starts at 60% of the range rated heating output for the first 15 minutes then increases output in 10% increments until a timer or thermostat stop it or it reaches temperature setpoint. The modulation is 1:7. I had to go check this myself as it's not in the manual (I range rated the output on a 35kW combi - 30kW to heating to 10kW and timed it watching fan speeds).

The Unica Max is also a 35kW combi model that I range rated to 12kW. It starts at 75% of the range rated heating output and ramps up to 100% after 15 minutes. The fan is also subject to being part of the active combustion control so is dynamically involved in fuel/air mix. Modulation is 1:8.

I don't have system boilers on the test rigs but the software is the same.

This was an interesting part of my day as there is no training today

Thank you for the response.
I'm currently thinking of replacing an old Ideal Classic FF250 boiler (in S-plan) with either the Vibe Max System or Vision Plus System boiler given both have a great soft start & internal modulation capability. The Unica MAX but it doesn't seem to be as good as the other two in this regard however it's ability to control pump modulation for a specific delta T is making it hard for me to choose between the three.
How does weather compensation and Opentherm controls e.g. BeSmart/Nest/EPH impact soft start & internal modulation for these boilers please?
Also, what happens if these boilers are connected to an Opentherm controller via OT1/OT2 AND they receive a 240v switched live connection from 2 port zone valve - do the boilers get confused or do they take the 240v switched live connection to run at max rated output and then revert back to Opentherm once the 240v connection drops to zero?