Boiler internal modulation vs OpenTherm

Valaki1234

Hello,

I would like to ask for your opinion, which is better:

1. Connect the Tado thermostat to the boiler's relay input and let the boiler doing the modulation using an external temperature sensor
2. Connect the Tado thermostat to the boiler via OpenTherm and let it modulate the boiler

In my particluar case I have a Baxi Luna Duo-Tec E boiler with an external DHW tank.

I am also not sure whether #1 is a valid possibility at all, does the boiler perform modulation when it is controlled by its relay input ?

GrilledCheese2

I think you're slightly confused with the different types of modulation.

In scenario 1 the boiler is modulating the gas burner to maintain a fixed flow temperature e.g. 65°C. In relay mode the smart thermostat will use PID control to ensure the room temperature does not overshoot. When the room is cold the relay is closed 100% of the time. As the room temperature nears the set point the relay close time is reduced. When the set point is reached the smart thermostat will calculate how long the relay needs to be closed to maintain a constant temperature e.g. the relay is closed for 5 minutes in every 20 minute (25%).

In scenario 2 the boiler adjusts both the flow temperature and the gas burner. So when a room is cold the flow temperature could start at 65°C and will be adjusted downwards as the room heats up. When the set point is reached the flow temperature to maintain a constant room temperature might be quite low e.g. 40°C.

Opentherm is compatible with most combi boilers but is not normally compatible with system boilers. The issue relates to controlling the motorised valves and the need to keep the DHW at 60°C or higher.

Valaki1234

Thanks for the explanation!

Do I understand it correctly that you suggest that the boiler internal modulation, using an external temperature sensor, only works when it is controlled by a dumb thermostat ?

GrilledCheese2

Boiler modulation, where the boiler turns down the heat output of the gas burner, will operate for any thermostat. The modulation is used to maintain a fixed flow temperature. E.g the heat output of an 18KW boiler might be turned down to 5KW.

Load compensation, where the boiler adjusts the flow temperature, works for OpenTherm thermostats. Modulating the flow temperature will increase/decrease the heat output of the radiators.

In addition to the above there's also Weather Compensation. This is where there's an outdoor OpenTherm thermostat that tells the boiler how cold it is outside. When it's cold outside the boiler is primed to user a higher than normal flow temperature.

Valaki1234

Thanks for the details!

Vitaminus

I also own a Baxi Luna Duotec E boiler with a water tank, and I find the modulation aspect confusing. This boiler has a 7:1 modulation capability. In the Tado interface, I only observe three curved lines (representing a heater icon or similar), indicating that the system operates at three levels: 1, 2, or 3. I'm wondering if this is linked to the boiler's modulation feature.

However, there's a peculiar observation: sometimes, there's one line in the living room and three lines in the bathroom. Considering we have only one circuit for heating water, it's puzzling how the boiler could operate at two different modulation stages simultaneously.

I find it satisfying to see the boiler running at a lower output, as I prefer not to have the small bathroom heater causing it to run at full capacity.

Unfortunately, I don't receive any feedback from Tado about the boiler's actual activities.

Thank you!

hugbilly

I think under the relay system the wiggly lines work as follows:

3 Lines, boiler being called for heat and TRV fully open.

2 Lines, boiler being called for heat and TRV partly closed.

1 Line, no call for heat, TRV nearly shut.

But I'm probably wrong !

Sguk

@Vitaminus
The three lines are the flow modulation. If you click “care & protect” it shows which room is requesting heat.
I’ve also seen two rooms with different percentages. I’m assuming the boiler is running at the higher of the two.

Eg if room one has nearly reached temperature and requests 60%, room two is still heating and requests 100% then full heat is going through both rooms.

Chances are room 1 will overshoot the target temperature until room 2 starts to modulate unless a trv in room 1 shuts off the rad.

It’s confusing when tado show two percentages, they should really show what the boiler is actually running at too, not just what the rooms are requesting.