DT90E working correctly?

[SpecialK]

I hope someone may bring some quiet to our home.
Our plumber recently exchanged our aged Honeywell analogue thermostat for a DT90E. Hurrah we thought, we know what the temperature actually is, and we can set the heating accurately.
All settings are factory.
Heating is on.
Set temperature is 21.5
Room temperature is 21.5
Every 10 minutes (presumably from cycle rate 6) the thermostat clicks. Flame appears, boiler fires up.
1 minute later (minimum on/off time 1 minute) it clicks again. Flame disappears, boiler shuts down.
Repeat until I change the Set temperature to 16.0 and the clicking finally stops (Proportional Band width >1.5)

Is this correct behaviour for the unit?
This is the second unit the plumber has fitted.

Maybe for many people the constant clicking is not noticeable.

Before I get the plumber back again, would someone tell me if it is working correctly?
Any advice to reduce the clicking to a simple 'it's cold start the boiler / it's warm shut it down' type clicks greatly appreciated.

 

[Chuck]

Before I get the plumber back again, would someone tell me if it is working correctly?
Any advice to reduce the clicking to a simple 'it's cold start the boiler / it's warm shut it down' type clicks greatly appreciated.

It's working okay. The clicks are the relay inside switching on and off. The cycling is normal for a so-called 'TPI' (time domain proportional integral) controller. At this time of year you apparently only require 10% of the boiler minimum output to keep the temperature at 21.5°C.

Most people have the thermostat in the hall so it's out of hearing range. You can reduce the number of cyles per hour but the room temperature won't remain as stable.

You can replace it with wireless version so the relay, which is in the receiver unit, will be out of earshot. The risk here is you will no longer hear the relay so you'll start hearing something something else, e.g. pipes creaking as they expand and contract.

My advice is that, now you know it's working correctly, just try to ignore it for a couple of weeks. You'll probably stop noticing it eventually. If not, get a wireless one installed.

 

[SpecialK]

It's working okay. The clicks are the relay inside switching on and off. The cycling is normal for a so-called 'TPI' (time domain proportional integral) controller. At this time of year you apparently only require 10% of the boiler minimum output to keep the temperature at 21.5°C.

Most people have the thermostat in the hall so it's out of hearing range. You can reduce the number of cyles per hour but the room temperature won't remain as stable.

You can replace it with wireless version so the relay, which is in the receiver unit, will be out of earshot. The risk here is you will no longer hear the relay so you'll start hearing something something else, e.g. pipes creaking as they expand and contract.

My advice is that, now you know it's working correctly, just try to ignore it for a couple of weeks. You'll probably stop noticing it eventually. If not, get a wireless one installed.

Thank you Chuck.

Knowing it is working correctly at least means I can have a different conversation with our plumber.

The thermostat is in the hall....but outside our bedroom door (we live in a bungalow) so getting used to it may take a while!
Again, thank you for taking the time to reply.
Graham

 

[Aquarius]

Every 10 minutes, why is this Chuck?

 

[Chuck]

Every 10 minutes, why is this Chuck?

While the room temperature is above than the upper limit of the proportional band the boiler is on 100% of the time. While the room temperature is below the lower limit of the proportional band the boiler is off, i.e. on 0% of the time.

Within the proportional band the controller wants to modulate the boiler so that its output is somewhere in the range 1%-99% calculated by the 'PI' algorithm to maintain the room temperature at the thermostat setpoint. It achieves this by an on-off duty cycle. The full cycle time is normally 10 minutes for a domestic gas boiler and 20 minutes for oil. There is usually a minimum on-time within this period, typically one minute.

If you make the cycle time too short the boiler won't like it. Too long and the room temperature fluctuates up and down and the occupants don't like it. 10 minutes is the usual compromise.

There are more sophisticated methods of of controlling boilers but this one works better than a traditional on-off thermonstat with 2°C+ hysteresis and is compatible with relatively unsophisticated boilers.

 

[SpecialK]

While the room temperature is above than the upper limit of the proportional band the boiler is on 100% of the time. While the room temperature is below the lower limit of the proportional band the boiler is off, i.e. on 0% of the time.

Within the proportional band the controller wants to modulate the boiler so that its output is somewhere in the range 1%-99% calculated by the 'PI' algorithm to maintain the room temperature at the thermostat setpoint. It achieves this by an on-off duty cycle. The full cycle time is normally 10 minutes for a domestic gas boiler and 20 minutes for oil. There is usually a minimum on-time within this period, typically one minute.

If you make the cycle time too short the boiler won't like it. Too long and the room temperature fluctuates up and down and the occupants don't like it. 10 minutes is the usual compromise.

There are more sophisticated methods of of controlling boilers but this one works better than a traditional on-off thermostat with 2°C+ hysteresis and is compatible with relatively unsophisticated boilers.

Thank you again Chuck for the detailed explanation.

I can understand the goal here, and can confirm the house does stay warm.

If the clicking wasn't so loud we would probably be saying how well the thermostat works!

Graham

 

[Aquarius]

Thanks @Chuck for that explanation.

 

[doitmyself]

While the room temperature is above than the upper limit of the proportional band the boiler is on 100% of the time. While the room temperature is below the lower limit of the proportional band the boiler is off, i.e. on 0% of the time.

I think you will find it's the other way round; i.e. boiler is in 100% when room temperature is below set temperature minus proportional band, and off when the room temperature is above set temperature.