CH Boiler Controls

[Jeff]

Can anyone explain how boiler modulation works, its benefits etc and the same for weather compensation

Regards Jeff

[Tony Bryer]

In brief (as I'm just going home) modulation means that the boiler includes a facility for varying the fuel being burnt (and thus the output) - for example my Glow-worm 24Cxi combi will modulate between

5 and 18kW. When the temperature of the water coming back to the boiler starts to rise (because the radiators cannot get rid of all the heat being produced) the burn rate is reduced accordingly.

Think of it as driving a car: if you only had an on/off switch for the engine you'd have to fire up, then switch off and coast for a bit, start up again .... instead you ease off the throttle until the engine is producing just enough power to maintain the desired speed. Likewise a modulating boiler.

Weather compensation adjusts the boiler's output temperature to suit the current outside temperature, so the rads will be hotter on cold days, less hot on milder ones. This makes for greater comfort and also enhances the boiler efficiency.

[Ian Stirling]

Boiler modulation means that instead of the boiler running flat out for short periods, it runs for longer, at lower power. This can improve efficiency, especially with condensing boilers.

[Jeff]

So in the above analogy... weather comp varies the 'desired speed' in effect turning up or down the boiler temp knob ?

The control for the modulation must work on a fixed differential from flow to return ?

Tony you said 'in brief' - i would love the long and detailed version as i am trying to get my head around this for a future project.

Regards Jeff

[Michael Chare]

Modulation is like a light dimmer, the level of output can be varied, so the boiler can be on at a low level continously.

The alternative is like a normal light switch which can be off or on. You just keep the boiler on for long enough to generate the amount of heat that you need.

[Ed Sirett]

I seriously suggest you read all the FAQs available on heating, plumbing and boilers. (Sorry if you have done that already).

The mechanism for modulation is essentially one of two forms:

On conventional burners the gas pressure is varied typically over a range of about 5:1. Giving a power range of about 2.5:1

On a premix burner the fan speed is adjusted thus changing the flow rate through the burner. Typically the fan speed would alter over a range of

2:1 or even as much as 3:1 giving a power range of 4:1 or even as much as 9:1.

The gas valves for conventional modulation usually are fed with 230V for one/off and a signal of 0-9V DC which alters between the lowest and highest rates.

[Roger Mills]

Indeed - but I think the OP is looking for a slightly more in-depth explanation - like what algorithm the controller uses when deciding what power setting to select.

[Roger Mills]

OK - that tells us *how* it does it - but what's actually driving it? In other words what feedback parameter(s) - e.g. water outlet temperature, return temperature, temperature differential, etc. - is it setting out to control by modulating the burner?

[Ed Sirett]

Most boilers seem to govern the flow temperature. This is usually where the NTC is placed.

A few seem to monitor both flow and return temps and some alter the pump settings as well. I strongly suspect that on my Keston C25 the front knob controls the flow temp and that affects the burner power but the boiler senses the return temp to adjust the pump in an attempt to get the differential higher.

Quite a few models seem to have a "cruise-before-decision" approach, in which they operate at middling lowish power after ignition for around a minute or so then ramp up (usually) to the required power.

In almost every boiler I've fiddle with, the algorithm/electronics is usually sensitive to the rate of change of temperature as well as the temperature. For instance if you move the control quickly down the burner may cut out but if you move it slowly down the flames just reduce.

[Andy Hall]

I researched this quite a bit when I was selecting a boiler. You can deduce quite a bit from looking through the installation and maintenance manuals.

Factors I found were:

- Outside temperature sensor? yes/no

- Boiler water temperature sensor(s). Flow, return, both?

- Method of modulation i.e. gas pressure or fan speed

- Pump control. Fixed speed, 2-3 speeds controlled by boiler, variable speed controlled by boiler.

- Room temperature thermostat. On/off or sensing temperature.

- Settings of boiler behaviour for outside temperature changes.

- Settings of boiler for different types of heating load - e.g. radiators sized for conventional boiler, radiators sized for better efficiency from condensing boiler, underfloor heating.

The number and amount of detail with these has an influence on what it is possible for the boiler's controller to do to optimise its operation. Unsurprisingly, the more expensive boilers tend to have more sensing and more ability to control.

Examples of these are MHG and Viessmann.

one has a document explaining condensing operation and controls.

[Jeff]

Thanks for the input, that last link was very interesting. There is very little on the 'net about weather compensation which is what prompted the question here.

So here is my list of current assumptions :-

1. I need to keep the return temp as low as possible below 57 degrees.

1. A low temp, long burn is more efficient than a high temp short burn

and here's the long shot

1. Weather compensation does the same as turning the temp knob up or down relative to the outside temp.

If point 3 is correct then why have an outside temp sensor, surely you could just monitor the return temp to keep it constant thus compensating for different losses due to different outside temps.

I'm still struggling to fit modulation into this

Regards Jeff

[Doctor Drivel]

Yep.

Yep.

The boiler temp knob, yes.

If you want to stand there turning knob up and down...well, yes.

A weather compensator mated to the boilers controls (usually an integral part of the boiler), will drop the flow temp and hence the return temp to the dictates of the outside temperature. If say outside is 10C and the compensator controller demands 50C flow (about right), then the burner will modulate down to maintain 50C flow. But if the control is more sophisticated, when the return temp rises close to 50C, it know the building is up to temp, so modulates down even further, increasing efficiency. When the temp differential between the flow and return widens the burner modulates up, as the control system sees the building cooling, but never gives more than 50C at the flow, modulating up or down to maintain 50C. So, even when 50C is called for it could be sending out less than 50C temps on the flow. Also, if there is room temp influence, say set to 22C, then when approaching 22C the burner will modulate down too. Once over 22C room temp the burner is off. Once over 50C flow temp, the burner is off.

As you can see, there is a number of different parameters to take into account in programming such a control system, and one level overrides another.

[Andy Hall]

That's somewhat true, but don't get the impression that there is some magic step change in efficiency when condensing commences. What actually happens is that the rate of change of improvement in efficiency increases below the dew point.

In essence, the lower the better. This can most effectively be achieved for more of the time by having the radiators adequately sized such that they can produce enough output to meet worst case outside temperatures at 70 degree flow (or lower if you like) rather than the conventional 82 degree design.

Yes and no. External monitoring provides for increased comfort because both the temperature as well as the rate of change of temperature outside become known to the controller more accurately than by the other means. think of it this way. Changes in outside temperature (eventually) affect the heat loss through the building surfaces and air changes. However, the boiler, if it has return temperature sensing only learns about this indirectly as a result of TRVs adjusting flow rates through radiators. In most systems the only additional monitor is an on/off room control.

There are room thermostats which will do things such as optimised starting to start the boiler based on rate of temperature rise during previous heating periods. However, these can't be as accurate as being able to measure the actual conditions.

If the boiler has that information, then being able to adjust the heat output finely and continuously will produce far better control tracking leading to greater comfort and saving of fuel.

[Jeff]

"Andy Hall" wrote :-

Bingo !

So, you can do it by monitoring return temp, but with outside temp monitoring the system is more accurate and responsive

Regards Jeff

[Andy Hall]

This is a simplification, because of course there are all kinds of complexities if one really wanted to produce an accurate model and control system.

A temperature sensor is an electronic component costing a few tens of pence. The electronics on the boiler controller to interface it are not a lot more. The complexity comes in the algorithms in the firmware. In one way it is surprising that more manufacturers don't give their boilers the ability to sense more information and to do so more accurately - the component cost isn't high and the firmware is initial development cost (which can be substantial of course.

Nevertheless, I suspect that part of the story is that this strategy allows manufacturers to have optional features for which much more money can be charged than the unit cost. I suspect that another part is that setting up of some of the mopre sophisticated capabilities is deemed to be beyond what the manufacturer believes the capabilities of the average installer to be.

If you want to look at more general information, look for references to industrial control and the techniques used for that. Some of these are borrowed in simplified form.

e.g.

links from it is a place to start.