Nest Heat Link Wiring Oil Boiler Not Firing Up

[Samwa]

Hey guys

I have moved to a house with a oil Combi boiler and a thermostat Danfoss which is on the wall in the kitchen. The system is configured where the Heating can't be on without the hot water being on too.

Can I replace this with my Nest Learning Thermostat?

From the attachments you can see the current wiring in the Danfoss.

I attempted to wire as seen in Google Nest (1) diagram but the boiler was not firing up.

I asked the previous owners who said he had a Hive and changed the settings to gravity fed, but I can't do this with Nest.

If I wire up as seen in diagram Google Nest (2), will this fire up the boiler?

I had a look around and found a diagram here https://www.diynot.com/diy/threads/...gravity-only-hot-water-pumped-heating.493177/

Any tips or help would be greatly appreciated.

Thanks guys.

 

[stem]

I've done a couple of Nests with pumped heating and gravity circulation of hot water. And, you are right, it doesn't have a gravity setting, but with a bit of a rejig of the terminals it can be done. The drawing below I produced for future reference when I did the first one as it took me quite a while to work out how to do it.

The bit that catches most people out is that the boiler live connects to the HW Common, and not the HW Call for Heat. There are also quite a few links between the various Heat link terminals, be careful to get them in the right ones.

If your boiler needs a permanent live (some boilers do, some don't) then that would simply come from the Heat link live terminal.

If you still have the Danfoss room thermostat connected, that will need removing and the wiring modified so that it matches the drawing.

 

[Samwa]

Hey, thanks so much for this.

Not sure if this is a stupid question, but, the flexes currently in the Danfoss at the wall as you can see in diagram 2, there are only 2 flexes, the one on the right is the 3A fused spur? And the left flex only has 2 wires, black which was linked to HW common on Danfoss and red wire, connected to call for CH on Danfoss. There are no neutral wires as seen in your diagram, are these necessary?

Do you think my diagram 2 would work?

Thanks again for your help.

 

[stem]

I have just noticed something rather odd in your description....

a house with a oil Combi boiler and a thermostat Danfoss which is on the wall in the kitchen. The system is configured where the Heating can't be on without the hot water being on too.

You are here describing two different systems.

1. A "Combi boiler" heats water instantaneously when a tap is turned on without the need for a hot water storage cylinder. It will have an internal pump, that is controlled by the boiler and not connected to the external controls. If this is what you have, my diagram doesn't apply to your system.

2. A "System configured where the Heating can't be on without the hot water being on too" describes a pumped heating system with gravity circulation of hot water to a hot water storage cylinder. This appears to be what you have based on the connections at the existing Danfoss programmer and therefore is what the diagram I supplied is for. I will continue on this basis.

Unfortunately there are many ways to wire heating systems up. Yes your boiler and pump will need a neutral connection to work so if they are working now they must have it. Whilst the diagram I have drawn shows the 230V supply coming to the Heat link. They could be connected to N elsewhere in the heating circuit. It's down to the personal preferences of the installer and the layout of the various components in relation to each other inside your home.

The red wire in Danfoss 1 is the 'heating on' connection and so should be the live to the pump and would go to Heat link 3 Heating call for heat

The black wire in Danfoss 4 is the 'hot water on' connection and so should be the live to the boiler and would go to Heat link 5 Hot water common

Based on that, your diagram 2 would appear correct.