Modulation and TPI control

[DaveSteveBob]

So what is the general consensus with the following?

How does TPI control from a cm 927 interface with a Baxi Duo-Tec that has internal modulation.

Is there a potential for conflict or do the two work together all be it independent to achieve max efficiency?

 

[DaveSteveBob]

I think I understand this better now after speaking to Baxi but please speak up if I am mistaken.

The burner modulation is determined by the boiler internal temperature, in other words if the system is calling for heat and the boiler is already hot the burner will be modulated down. On the other hand when the house is close to the target temperature as set on the room thermostat, the TPI control reduces the firing time accordingly within the 6 periods of every hour. The reduced firing times will therefore allow the boiler to operate at a lower temperature and consequently the burner will modulate up. In other words there will be no conflict.

 

[ianniann]

Read the CM927 thread currently near the top of the page. It discusses TPI without using that particular fancy acronym (TPI stands for "for switching the boiler on and off"!). Plenty of people think it is great. I think it is crap

Baxi's explanation is largely correct. However the whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature, inevitably causing the boiler to fire at high power whenever it is allowed to be on by the controller. Call it conflict or call it something else, I don't think it is very good.

 

[D_Hailsham]

The whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature, inevitably causing the boiler to fire at high power whenever it is allowed to be on by the controller.

You are assuming that the boiler goes to max power each time it lights and then modulates down. I know that many believe this is the case, but I haven't seen any evidence and the manufacturers don't give any information.

 

[DaveSteveBob]

Read the CM927 thread currently near the top of the page. It discusses TPI without using that particular fancy acronym (TPI stands for "for switching the boiler on and off"!). Plenty of people think it is great. I think it is crap

Baxi's explanation is largely correct. However the whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature, inevitably causing the boiler to fire at high power whenever it is allowed to be on by the controller. Call it conflict or call it something else, I don't think it is very good.

Why do you suggest that high power it is not very good?

 

[MJN]

However the whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature [...]

Completely wrong. The whole point of TPI is that it has to be used when such direct proportional control is unavailable. As you so eloquently put it: 'it switches the boiler on and off'. It us unable to lower the flow temperature directly and so can only vary the on/off duty cycle to lower the overall, average, heat output. Flow temperature control remains solely in the realm of the boiler.

The OP's question is very valid - in the described scenario we have a controller attempting to effectively reduce the heat output using a variation of pulse-width modulation i.e. lowering the average output through discrete on/off duty cycles however we also have a boiler attempting also to reduce the output based on its own assessment of demand (indirectly through measurement of flow and/or return temperatures). In the absence of any direct linkage between the two I think the OP has a very valid question and one which I am unsure of the answer to, particularly when you add into the mix the behaviour-learning properties of most of these TPI stats as whilst it sounds like a grandiose capability they seem to be doing little more than recording the long term rate of rise/fall of the system as a whole which will of course vary in the short term if the boiler starts lowering its output.

The solution of course is simple, albeit no good for existing systems - choose a boiler that allows the external stat to directly control the heat output and modulation of the boiler hence it is in control of every aspect.

Mathew

 

[DaveSteveBob]

However the whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature [...]

Completely wrong. The whole point of TPI is that it has to be used when such direct proportional control is unavailable. As you so eloquently put it: 'it switches the boiler on and off'. It us unable to lower the flow temperature directly and so can only vary the on/off duty cycle to lower the overall, average, heat output. Flow temperature control remains solely in the realm of the boiler.

The OP's question is very valid - in the described scenario we have a controller attempting to effectively reduce the heat output using a variation of pulse-width modulation i.e. lowering the average output through discrete on/off duty cycles however we also have a boiler attempting also to reduce the output based on its own assessment of demand (indirectly through measurement of flow and/or return temperatures). In the absence of any direct linkage between the two I think the OP has a very valid question and one which I am unsure of the answer to, particularly when you add into the mix the behavior-learning properties of most of these TPI stats as whilst it sounds like a grandiose capability they seem to be doing little more than recording the long term rate of rise/fall of the system as a whole which will of course vary in the short term if the boiler starts lowering its output.

The solution of course is simple, albeit no good for existing systems - choose a boiler that allows the external stat to directly control the heat output and modulation of the boiler hence it is in control of every aspect.

Mathew

Makes sense, is that not the whole point of Open Therm control?

Back on track,what about when you add into the equation weather compensation. We now have 3 sperate control methods all trying to achieve the same goal.

Is there a flow chart to enable the control process to be visually explored?

 

[D_Hailsham]

However the whole point of TPI is to reduce the flow temperature of the water when you are near the desired room temperature [...]

Completely wrong. The whole point of TPI is that it has to be used when such direct proportional control is unavailable. As you so eloquently put it: 'it switches the boiler on and off'. It us unable to do this directly (i.e. lowering the flow temperature) and so must do so by varying the on/off duty cycle to lower the overall, average, heat output.

If a modulating boiler is only controlled by a basic on/off thermostat, all the modulation does is to control the water temperature to the value set on the boiler thermostat. The basic on/off stat is providing a form of pulse width modulation to provide a mean water temperature approximating to what you would get if you had weather compensation.

The purpose of TPI is to reduce under/overshoot. The traditional mechanical stat keeps the boiler running flat out until the stat opens. Because the rads have been giving out full heat, the room temperature continues to rise, may be a degree or more. The room then cools down and, as mechanical stats have a fairly wide hysteresis, the room temperature can drop 3 or 4 degrees before the stat closes and turns the boiler back on.

TPI reduces the under/overshoot to about 0.5 degree by reducing the percentage of time the boiler is on as the actual temperature approaches the required temperature.

 

[DaveSteveBob]

My original question was based on my misunderstanding of what influences the boiler to modulate. I originally thought the modulation was determined by the return flow temp hence my question about a potential conflict with TPI.

On reflection, with the help of the comments I now believe that all the associated systems work in harmony. The following is my understand so far :-

TPI controls how often the boiler fires to achieve precise regulation of the desired temperature at the room stat.

Modulation regulates the flame depending on the boiler temperature and boiler stat setting.

Weather compensation, automatically adjusts the boiler stat setting depending on the outside temperature.

Open therm has the ability to control all of the above based on input from all of the above.

The Baxi Duo Tec does not support open therm however, everything seems to work just fine as is. Shame Baxi do not promote the weather compensator by default.

Many thanks for the help.

 

[MJN]

Makes sense, is that not the whole point of Open Therm control?

Yes, or at least it is one aspect of it. The primary goal was open interoperability between vendors however the protocol does include native support for direct control of the boiler.

Back on track,what about when you add into the equation weather compensation. We now have 3 sperate control methods all trying to achieve the same goal.

It's not as complex as you might think as the external temperature measurement is used as an input to one of the existing control loops (boiler or stat) hence it is not at odds with whichever it is, but might be to the other (as per your question).

Is there a flow chart to enable the control process to be visually explored?

Not that I'm aware of it, not least given that there is very little detailed information about operational functionality of each component part. I dare say part of the reasoning for this is that they're probably not quite as capable and 'intelligent' as one might otherwise be led to believe from the marketing blurb!

Mathew