Large boiler, small .......

[bernardgreen]

The need to know what Opentherm does and its advantages. They do not need to know the electronics behind it.

I might be wrong but I believe "OpenTherm" specifies only the physical link and the data format used between the modules of a heating system.

The data sent ( functions, values and commands ) are not specified in the OpenTherm. Various manufacturers can create their own functions and commands which are transmitted between modules using the data formats specified by OpenTherm.

Installers should be aware of what to expect ( voltages currents etc ) on the OpenTherm bus if they have to fault find on a malfunctioning system.

 

[hard-work]

Ericmark, as you have found out, a TRV and wall stat can coexist. The setpoint of the wall stat needs to be higher than the TRV in most cases.

The best is TRVs on all rads. This needs a buffer cylinder/thermal store/low loss header and the likes.

 

[hard-work]

Opentherm can use any makers of stat, timer, etc. Plug and play. It is to give a wide range of advanced controls with competition making it all cheaper.

But many makers stick to their own proprietry controls and rip off.

 

[Dan Robinson]

I might be wrong but I believe "OpenTherm" specifies only the physical link and the data format used between the modules of a heating system.

The data sent ( functions, values and commands ) are not specified in the OpenTherm.

You are wrong, partially. Suprise suprise. To carry the opentherm mark, a control and boiler have to use a minimum set of control parameters. There are two levels of opentherm.

Ericmark, as you have found out, a TRV and wall stat can coexist. The setpoint of the wall stat needs to be higher than the TRV in most cases.

The best is TRVs on all rads. This needs a buffer cylinder/thermal store/low loss header and the likes.

In all cases, rendering that trv pointless.

 

[dilalio]

Is the earth flat?

 

[hard-work]

In all cases, rendering that trv pointless.

See Eric's post. It is a suck it and see thing.

 

[Dan Robinson]

See Eric's post. It is a suck it and see thing.

No, he just doesn't know what he's doing.

 

[John D v2.0]

Everyone is right - apologies for the long post but it may help understanding of complex systems.
A TRV and a thermostat in the same room is indeed not correct design for a system.
Having said that, in particular cases the instability and inertia between different control systems that affect each other, can actually help the system behave overall. That doesn't mean it's the right solution, it just works OK and shouldn't get worse. That has appeared elsewhere in this thread where someone mentioned turning down the lockshields to stop the boiler going on full blast and then cutting out the burner before the room warms up. That is the correct solution as the system must be matched to the required heat output hence the even temperature difference to each zone. That is because there is no "feedback" or "cycling" going on there.

As another common example many people have experienced a shower that goes hot/cold/hot on a combi, that is usually on a basic level caused by the heat exchanger not transferring heat effectively enough. However in terms of control system interference, it's caused by the burner increasing to get enough heat into the DHW, then the primary water getting too hot and the burner being cut off or reduced that way. A thermostatic shower might improve the situation, but it'll it'll start taking more or less hot water, thereby potentially exacerbating the situation. That isn't the right solution, and it will probably get worse, the solution is to improve the plate exchanger by cleaning or replacing.

Here is a rather theoretical description of the feedback cycles of two relativly quick cycling pulses cause a much longer pulse. Ovbiously it relates to music, and to two independent waves. In a heating system those two sources may be dependent on each other making the problem worse.
http://www.animations.physics.unsw.edu.au/jw/beats.htm

 

[hard-work]

No, he just doesn't know what he's doing.

He got a result to a problem. I would not design such a control system from the outset. I would try and get TRVs in sll rooms, but that can be a little involved.

In Eric's case it may have been better to have the room stat in the living room and a TRV in the hall. A wall stat in the hall is a hit and miss compromise. All depends on insulation value, set out of house etc.

 

[ericmark]

The evaporator is underneath the steam turbine, it causes a part vacuum, the problem is cooling the water, hence huge cooling towers, the hotter the water going into the boiler the less energy is required to turn it into steam. In theory I will admit one could extract energy from the flue gases, but what is more important is to remove particular emissions, so yes the flue gases are often cooled but in the main to remove nasty stuff rather than gain the latent heat, and it is not done in the boiler, the scrubbers are well after the boiler.

Low temperature water heaters may use the energy form the latent heat, but super heated steam boilers would not have this built into the boiler, if it exists it would be a separate unit.

I know the one I was involved in, the waste heat was used to make the steam, as the gas burns the expansion drives the gas turbine, then it makes super heated steam which is fed to the steam turbine, and then into the scrubber to remove nasties.

The domestic water heater 1) does not boil the water. 2) it does not use the expansion of the gas as it burns, and 3) it does not catch the nasties and store them for safe disposal. I am sure there were domestic central heating boilers, but I have never seen one, except when there was a fault.

 

[ericmark]

He got a result to a problem. I would not design such a control system from the outset. I would try and get TRVs in sll rooms, but that can be a little involved.

In Eric's case it may have been better to have the room stat in the living room and a TRV in the hall. A wall stat in the hall is a hit and miss compromise. All depends on insulation value, set out of house etc.

Hind sight and hind site is easy, I should have fitted EvoHome. Even though the boiler is not OpenTherm EvoHome would have been a fit and forget even if the boiler did not modulate but used a mark/space ratio to regulate output.

However once I had paid out for eTRV heads I was not going to bin the Energenie ones and start again, the Energenie ones will link to Nest, and that was the original idea, however the whole idea of TRV is each room can be a different temperature, and the eTRV allows different rooms to heat at different times. However the follow command using IFTTT has every TRV set to same temperature at same time, so rather defeats the whole idea.

I will agree fitting the wall thermostat in the coldest ground floor room with no independent forms of heating and no outside doors will work, however in mother's house that room does not exist.

To my mind whole idea of wall thermostat is to turn off the water heater once whole house is up to temperature to stop the boiler cycling, in theory you could fit one in every room all wired in parallel, however with eTRV heads which report the target and current temperature in each room, really there should be no need for wall thermostats, it is really simple, if any room has a target lower than current boiler runs, other wise boiler is switched off. The question is why other than EvoHome is there not any hubs which do this?

 

[D_Hailsham]

I might be wrong but I believe "OpenTherm" specifies only the physical link and the data format used between the modules of a heating system.

The data sent ( functions, values and commands ) are not specified in the OpenTherm. Various manufacturers can create their own functions and commands which are transmitted between modules using the data formats specified by OpenTherm.

You may change your mind when you read the attached document. It's not the latest version (V3), but that doesn't matter as backward compatibility applies.

 

[hard-work]

1.2 Key OpenTherm Characteristics

• Compatibility with so-called “dumb” or non-intelligent boiler systems.
• Compatibility with low-cost entry-level room thermostats.
• Compatible with electrical supplies typically normally available within boilers.
• Two-wire, polarity-free connection for concurrent power supply and data transmission.
• Provides a suitable power supply for a room controller so that it can operate without the need for an additional power source such as batteries.
• Implementable in low-cost microcontrollers with small ROM / RAM / CPU-speed requirements.
• Installer friendly feature for boiler testing. Shorting the wires at the boiler provides a simulated maximum heat demand (similar to current on/off systems).
• Allows for the transfer of sensor, fault and configuration data between the devices.
• Provides a mandatory minimum set of data objects, which allows for transmission of a modulating control signal from the room controller to the boiler.

One of the key characteristics of the OpenTherm standard is the two-level approach which allows analogue-
type solutions at the low-end.

OT/+ The OpenTherm/plus protocol provides a digital communications system for data-exchange between two microprocessor-based devices.

OT/- The OpenTherm/Lite protocol uses a PWM signal and simple signalling capabilities to allow implementation on analogue-only products.

Both protocols use the same physical layer for data transmission and power-feeding ensuring that the two levels of communications are physically compatible.

 

[bernardgreen]

You may change your mind when you read the attached document.

That has created some confusion.

The connection between the heater and the thermostat consists of two wires and is set up in such a way that it does not matter how they are connected (i.e. the polarity is protected). The heater provides power to the thermostat over these wires.
To communicate to the slave, the master device changes the voltage on the wire. To communication back to the master, the slave changes the current through the wire. The voltage/current levels are as follows:

Which is master and which is slave on the OpenTherm protocol. ?

The master of the physical layer is the source of power to the bus. The boiler provides this function.

The master of environmental control data is the thermostat. ( the thermostat commands the boiler )

In other ( non OpenTherm ) applications of the physical layer (*) the master providing power to the bus sends data to the slave(s) by altering the voltage it provides to the bus.

(*) power and data bit transfer )

The master sends data by altering the voltage it provides to the bus.

9 volts = digital zero This voltage must be high enough to supply the power to the electronics in the slave(s)
15 volts = digital one

The slave sends data by altering the current it takes from the bus.

low current = digital zero ( typically the current required to power the slave's electronics )
high current = digital one ( current created by the slave connecting a resistor across the bus )

 

[Dan Robinson]

Which is master and which is slave on the OpenTherm protocol

For three purpose of control, it is the thermostat, but within the limits of the boiler. This is one of the reasons that Intergas (for example) and Vaillant are better than Viessmann under opentherm.

BTW, I have version 3 of the specification