Daikin Air Source and Nest

[Iain Carr]

Good afternoon!

I've bought the Nest Thermostat 3, I'm just looking for a little thermostat wiring assistance please to go with my Daikin Altherma Air Source Heat Pump LT Split.

I know the Nest cannot assist with hot water controls on this boiler.

So do I just need to wire the existing thermostat cable from the boiler to terminals 2 + 3 on the Nest heat link (these are labelled Common and Call-for-heat).

So boiler to heatlink. 2 and 3.

Then the existing thermostat cable will go to T1 and T2 on the heatlink and into the new Nest thermostat (in place of the existing thermostat)?

Any help appreciated!

 

[stem]

The Heat link terminals 2 and 3 do indeed provide a voltage free switch, designed to be used in the place of a standard room thermostat of the type that has a basic open/closed voltage free contact, that closes when heating is required and opens when it is not required.

The Nest is really designed for controlling heating systems with boilers, (BTW, a heat pump is not a boiler) so for anything else, you should consider:

1) The Heat link is designed for switching the low current associated with central heating systems with gas or oil boilers. Would your system draw more current through the Heat link relay contact than it is designed for. (3A resistive, 1A inductive, see below from the Nest manual)

2) Heat link terminals 2 and 3 are for heating control. Do you need cooling control also? If so, does the existing thermostat provide this, or is the cooling control selected elsewhere?

If in doubt about any of the above, it would probably be better to get in a professional.

 

[Iain Carr]

Thanks for your response.
I will double check but I'm sure my single Honeywell rotary thermostat is 230v.

I will have to check by putting my fluke in series with the thermostat exactly what current is drawn across it, I'm not sure! I would like to think the current would be very small because the thermostat is just a ON / OFF signal really?! That's an assumption.

My Daikin does heating and hot water, no cooling. I appreciate Nest cannot do anything with my hot water, I'll just leave that in auto (the tank stays around a set temperature) but I really want the ability to control my heating remotely and think the Nest looks best.

So as my heating is not controlled by a programmer, it is literally ON / OFF by thermostat demand. Am I right in thinking the thermostat wiring OUT from the indoor unit (that I was calling a boiler) which I think is 230v 2 core, will just go into Heat link 2 and 3?

I then will use the existing thermostat wiring to go from the Heat link low voltage output to power the Nest thermostat.

Thanks!

 

[stem]

The heat pump itself will draw more than the Heat Link can handle, but it's unlikely that it would be wired directly to the thermostat, it will probably have other controls that switch the heavier current, and the thermostat just operates operates them, especially if your heat pump is heating a water circulation system. This looks all the more likely if you already have a standard Honeywell central heating thermostat in place now.

And yes, standard thermostats (including the Heat Link) are wired as an off / off switch. Often you will hear thermostats 'click' as the switch opens and closes. The technical part comes in deciding when to tell the switch to operate.