Room thermostat wiring diagram

Hi, I am fitting a simple room thermostat to my combi boiler.

https://www.vaillant.co.uk/downloads/vrt-15-2018/vrt-15-supplement-261436.pdf

The wiring diagram is simple enough but I have questions nonetheless.

1, If the top two terminals are both neutral why have two terminals when they could be bridged internally?

2, What are the meanings of the symbols on the neutral terminals? I recognize the snow flake symbol but what does it signify, and what is the other symbol?

Thanks for your advice

.

snowflake, I would guess at frost setting.

The two terminals aren't neutrals; they are switched Lines(Lives).

The conductors are only Neutrals after the appliances - pump symbol being the boiler if it has 230V control.

However, presuming the snowflake is a frost stat, then its outgoing wire should also go to the boiler or if you don't have one, ignore it and just connect NO (Normally Open) to the boiler control if 230V.

If the boiler does not have 230V controls then L will be the RT 24V terminal at the boiler and any Neutrals will not be involved.

Telnet,
The 'N' is not really neutral, it is actually "Switched Live", ie teh output is the Live being switched on or off.
Do not connect a neutral to this themostat for normal applications.

For me the two Symbols, are Heater and AirConditioning (AC).

Heater connection is a 'Normally Open' switch.
Airconditioning connection is a 'Normally Closed' switch.

For the Heater connection with the Normally Open switch:
- When temp is above set point (say 20oC) then Switch is open showing room is warm.
- No voltage is transmitted from L to Heater Symbol.
- The Connected heater does not turn on.
- When temp falls below set point (say 20oC) then Switch is closed showing room is cold.
- Voltage is transmitted from L to Heater Symbol.
- The connected heater receives voltage and turns on to increase room temp.

For the AC connection with the Normally Closed switch:
- When temp is above set point (say 20oC) then Switch is closed suggesting room is warm.
- Voltage is transmitted from L to Snowflake Symbol.
- The connected AC receives voltage and turns on to reduce room temp.
- When temp falls below set point (say 20oC) then Switch is open showing room is cold.
- No voltage is transmitted from L to Snowflake Symbol.
- The connected AC does not turn on.

SFK

EFLImpudence said:

However, presuming the snowflake is a frost stat,

Click to expand...

Snowflake is the switched live to an air conditioner or othe cooling equipment. Becomes Live when room temperature is higher than thermostat setting.

@SFK has given you the correct answer

SFK said:

Telnet,

For me the two Symbols, are Heater and AirConditioning (AC).

Heater connection is a 'Normally Open' switch.
Airconditioning connection is a 'Normally Closed' switch.

SFK

Click to expand...

Got it ! That makes sense. One terminal is the reciprocal of the other so you can use it for heating or cooling applications.

bernardgreen said:

Snowflake is the switched live to an air conditioner or othe cooling equipment. Becomes Live when room temperature is higher than thermostat setting.

Click to expand...

Exactly.

EFLImpudence said:

The two terminals aren't neutrals; they are switched Lines(Lives).

The conductors are only Neutrals after the appliances - pump symbol being the boiler if it has 230V control.

View attachment 165901

Click to expand...

I see, the strange symbol is for a pump. OK, it all makes sense.

***************

The other end of the wire from the thermostat goes to a pair of 24V terminals in the boiler that are currently bridged, so I understand about the neutral no really being neutral but a switched live.

My multimeter agrees with the consensus opinion. And now I understand the meaning of the symbols.

Thanks for your help.

telnet said:

I see, the strange symbol is for a pump

Click to expand...

Not a pump but boiler, to be precise the air blower ( fan ) that blows air and gas into the combustion chamber,