Nest Thermostat and Biasi M96.24 Combi Boiler Install issue

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Gavin_T

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Hi All,

I have recently purchased a Nest to install alongside my combi boiler.

My boiler currently does not have a thermostat, it has a timer / controller on the boiler that I use to turn on / off the CH.

The thermostat connections on the boiler are 'volt free' and currently there is simply a loop across pins 1 and 3.

I've had a read through a number of posts and from what I have read installation seemed simple. I needed to use the low voltage nest install taking two wires from pin 1 and 3 on the boiler and running them up the the nest heat link unit on pins 2 and 3.

When I press the override on button on the nest heal link the unit just appears to attempt to fire over the relay but cant. It sits in a loop trying to fire up the boiler with the green lights going on / off on the nest unit.

The nearest thread I came across was http://www.diynot.com/diy/threads/nest-thermostat-and-baxi-combi-105e.416527/

My boiler manual states the following:

M96.24SM.png


and the nest instructions state the following:

full


Any help would be greatly appreciated.

Many Thanks, Gavin
 
Gavin_T said:
I have recently purchased a Nest to install alongside my combi boiler.My boiler currently does not have a thermostat, it has a timer / controller on the boiler that I use to turn on / off the CH.
Click to expand...
OK, so this will now be set to be 'on' 24/7


Gavin_T said:
The thermostat connections on the boiler are 'volt free' and currently there is simply a loop across pins 1 and 3.
Click to expand...
The loop should be removed when the terminals are wired to the Heatlink.


Gavin_T said:
I needed to use the low voltage nest install taking two wires from pin 1 and 3 on the boiler and running them up the the nest heat link unit on pins 2 and 3.
Click to expand...
Yes, that's fine.


Gavin_T said:
When I press the override on button on the nest heal link the unit just appears to attempt to fire over the relay but cant. It sits in a loop trying to fire up the boiler with the green lights going on / off on the nest unit.
Click to expand...
Has the loop across boiler terminals 1 and 3 been removed?

How have you powered the Heatlink, Where is the N and L connected to?

How have you powered the thermostat? is it connected to T1 & T2 or Plug in power supply?

What happens if you disconnect the wiring from the the boiler to the Heatlink and operate it, does the Heatlink green light stay on then?
 
Hi Stem, thanks for the prompt response.

The boiler timer will still be utilised but using the nest as the thermostat when active.

Yes I have removed the original loop that was installed between 1 and 3 on the boiler.

The heatlink has a separate independent L and N feed run through a fused spare.

I haven't tried disconnecting the connections from the boiler and seeing if the heatlink light stays on. I'll give it a try when I get home today.
 
It all looks correct so will be interesting to see what the problem is.
 
I thought as much......spent an hour looking at it and scratching my head last night!
 
Gavin_T said:
The boiler timer will still be utilised but using the nest as the thermostat when active.
Click to expand...
Bit of an odd way of operating it. The Nest has both time and temperature control functions. It is also a 'learning thermostat' and is designed to be clever enough to work out when to put the heating on as it learns your lifestyle. Using the boilers time control will prevent this functioning to its full potential.

It will also work out how long your home takes to heat up based on the current temperature, but if it thinks the boiler is on and it isn't because the boiler timeswitch doesn't let it be, it will learn the wrong warm up time for your home and may do some odd things.

I recommend that you leave the boiler timer 'on' permanently and use the Nest as it was designed to operate.

Gavin_T said:
The heatlink has a separate independent L and N feed run through a fused spare
Click to expand...
That's not really good practice, the boiler and its controls should be fed from the same source, so that one isolator turns off the whole system, and as shown in the diagrams from the Nest manual that you posted.
 
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It is a three minute job with a meter to see if the Nest is switching!

DIY installs take days and usually just involve replacing parts rather than any diagnosis!
 
stem said:
Bit of an odd way of installing it. The Nest has both time and temperature control functions. It is also a 'learning thermostat' and is designed to be clever enough to work out when you want the heating on as it learns your lifestyle. Using the boilers time control will prevent this functioning to its full potential.

It will also work out how long your home takes to heat up based on the current temperature, but if it thinks the boiler is on and it isn't because the boiler timeswitch doesn't let it be, it may do some odd things.
Click to expand...

No problem, happy to take your advice. I can simply set the boiler status to be on all the time and utilise the nest once I get it working.
 
Agile said:
It is a three minute job with a meter to see if the Nest is switching!

DIY installs take days and usually just involve replacing parts rather than any diagnosis!
Click to expand...

I did put a meter across it last night, the meter demonstrated the same as the lights on the heatlink.
 
Agile said:
The meter flashes green?
Click to expand...

Ha what meters are you using that flashes green :-) , no the meter shows that power is attempted to be passed from pin 1 to pin 3 on the boiler via the relay in the heat link but then returns to 0. This continues as the lights flash on the heatlink and you can hear the relay clicking over inside the heatlink unit.
 
I'm about to give this a go with this boiler. Did you get it working? Would appreciate any pointers as it's a rental property and don't want to break anything!
Cheers!
 
I have a M90F32S. Exactly the same interface for the thermostat and excerpt from instruction manual. I have a Honeywell CM927 with a relay box connected across connections 1 and 3 on the boiler block. Whilst it is not a nest, it is the same principle of operation. This has worked fine for the last 3 years or so.

Follow the good advice by Stem in this thread. The boiler timer needs to be set to continously on. Little red time switch all way down on my boiler AFAIR. The fused spur should feed the relay box (heatlink) and boiler.

You'll need suitable cable available for the supply to the nest thermostat (from the Heatlink T1 and T2) or use the supplied power pack.

All as outlined by Stem in this thread.

I suspect a wiring problem or a boiler fault may have been an issue with the OP in this thread (or maybe even faulty nest). The use of the boiler timer also with the nest would also cause problems....

Edit: on my control timer is little red switch all way down AND you need to press the on / off button to toggle boiler to always on!
 
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