Room thermostat: connect to CH pump?

[mike004]

Wiring a room thermostat into a Danfoss 102E5 controller.
Traditional gravity fed system, 1 x room thermostat.
I thought it would be just a matter of connecting the thermostat wires to the controller outputs. So the thermostat turns the controller on and off.

But the Danfoss manual shows a connection to the CH pump. Is this needed for basic system?

If so, it is outside my DIY comfort zone.

 

[flameport]

The thermostat is used to switch the pump on and off - which will circulate the hot water through the radiators.
The pump N & E are permanently connected. Live is output from controller - thermostat - pump.

If you wire it so the thermostat turns the controller on/off, your hot water won't work unless the heating is on.

Is this replacing an existing thermostat? Or are you adding a thermostat where there was none before?

 

[mike004]

Adding a new thermostat where there was none.

 

[mike004]

I'll have to check this out. But I might be able to make all connections in the loft, near to the pump.

 

[flameport]

The connections can be made in a junction box near the pump.
If the thermostat you have doesn't require a neutral, then you just need the two live wires.

Untitled

• flameport
• 28 Jul 2011
• 1

 

[mike004]

Yes, that's the same diagram from the Danfoss manual I downloaded.
It doesn't look too complicated if I get a 2-wire stat.

 

[DP]

You need three wires and earth cable i.e. four conductors.

Get a programmable room thermostat or a battery operated thermostat and wire that between CH terminal on Timer (having removed the wire from that terminal) and second wire from new thermostat connects to wire that use to terminate at CH terminal on timer. Hope you can understand what I mean.

Better option is to rewire the controls so that CH is not dependant on HW internal link to fire the boiler.

 

[mike004]

You need three wires and earth cable i.e. four conductors.

Get a programmable room thermostat or a battery operated thermostat

What about mechanical thermostats? Are they unreliable? A programmable stat is too complicated for my needs.
Don't know about battery operated. How long do the batteries last?

 

[DP]

Mechanical stat invariably needs three cores plus earth (live, switched live, neutral and earth conductors) whereas programmable roomstat or battery operated stats need live and switched live (two conductors only).

Batteries last a long time so that is not an issue

Programmable roomstat is the way to go for enchanced ambient temperature control. When I installed one in my house (in fact I have two), my wife threatened to call a heating engineer to come fix the system. Now that she is acclimatised to unit operation (i.e. she does not need to set maximum temperature on it) as it fires the boiler when temperature drops below set points. There are time during winter the stat will not fire the boiler, will take care of frost protection overnight and during times when house is not occupied. Same stat has fired the boiler couple of times for very short periods over the last couple of weeks

 

[flameport]

Mechanical thermostats are reliable. They don't need batteries and are much cheaper than electronic ones.

If you really can't deal with installing 3 wires, then get this one:
http://www.heatingcontrolsonline.co.uk/siemens-raa20-room-thermostat-p-311.html
which only needs 2 wires - live in from the controller & live out to the pump.

Better option is to rewire the controls so that CH is not dependant on HW internal link to fire the boiler.

On a newer system yes - but this is an old system with gravity hot water, so there would be a lot more involved than just a bit of rewiring.

 

[DP]

On a newer system yes - but this is an old system with gravity hot water, so there would be a lot more involved than just a bit of rewiring.

Advice given is based on what I have done many times. As a service/ repair guy I have fitted plenty progstats to old gravity hot water systems. End result is HW works as normal for gravity circulation. CH demand from progstat runs the pump and fires the boiler till ambient temperature reached.

 

[mike004]

Mechanical thermostats are reliable. They don't need batteries and are much cheaper than electronic ones.
...

Better option is to rewire the controls so that CH is not dependant on HW internal link to fire the boiler.

On a newer system yes - but this is an old system with gravity hot water, so there would be a lot more involved than just a bit of rewiring.

Yes. I'm looking for a simple solution. A mechanical stat sounded like it would be less bother.

With my current controller I can have HW or HW + CH. I don't need CH only. Was DP suggesting this with his rewiring suggestion?

 

[DP]

You cannot have CH without hot water on account of way your system is plumbed.

Fitting a standard roomstat will now allow you to control the pump, thus the heating side of the system, BUT the boiler will continue to run for the duration of timer being on.

My suggestion switches the boiler off as soon as thermostat is satisfied UNLESS you already have a HW demand running.

 

[mike004]

You cannot have CH without hot water on account of way your system is plumbed.

Fitting a standard roomstat will now allow you to control the pump, thus the heating side of the system, BUT the boiler will continue to run for the duration of timer being on.

My suggestion switches the boiler off as soon as thermostat is satisfied UNLESS you already have a HW demand running.

Yes, I see.
So a room stat that does not switch the boiler off is useful for controlling temperature?
But if the boiler is still running it won't be as efficient and save me on gas bills.