Danfoss CP715 programmer To Nest 3rd Gen Thermostat Issues

[diyhov]

Hello,

I rarely post questions, as answers to other posts have always answered my question.

I'm attempting to replace a Danfoss CP715 programmer with a Nest 3rd Gen.

From:
Danfoss CP715
Potterton Neatheat Profile Boiler
Y Plan config (Honeywell F-5 3 port valve) gravity fed




To:
Nest 3rd Gen
Potterton Neatheat Profile Boiler
Y Plan config (Honeywell F-5 3 port valve) gravity fed

After reading https://www.diynot.com/diy/threads/nest-danfoss-compatible-how-to-install.493478/ and http://lifeofman.co.uk/2017/06/install-nest-learning-thermostat-3rd-gen-y-plan-system/ I followed the same wiring:

CP715 Nest Heat Link
N-Blue N
L-Brown L
1-HW Off Earth colour 4
2-CH Off None
3-HW On Yellow 6
4-CH On Light Blue 3


I also read about having to connect LIVE to connections 2 and 5 (common), which I have done induced by the pic with the arrows from the CP715 to heat Link. The backplate photo shows 2 live and 2 neutral wires - I've connected each via its own Splicing Connector. (handy! https://www.clasohlson.com/uk/Wago-221-Compact-Splicing-Connectors/36-6521)


Problem:
The boiler fires up once water or heat are selected on the Nest Thermostat or app, but then the boiler stays on.

It will switch off (as usual) when the water is hot, but, it wont obey anything set on the thermostat or app. It wont switch off (water off/heating off). The pump keeps going. The only way to switch it off is to switch off the supply to the

I replaced the CP715 and everything is working as normal, water triggering as needed, and the heating.

Admittedly I didn't check the wiring of the Honeywell 3 port valve, but cant understand why Nest is not controlling the boiler.

Any help appreciated

 

[stem]

The wiring looks exactly as it should as to where it goes. (there are one or two installation issues that won't effect operation though) Well done with the post showing both items and the arrows showing the changes, very helpful . Have you changed any other wiring anywhere? Was there an existing room thermostat for example?

FYI The Nest doesn't actually directly control the boiler. In heating mode the motorised valve controls it and in hot water mode the cylinder thermostat. But the Nest does provide power to the aforesaid items.

 

[diyhov]

Hi,

I assume he installation issues you're referring to are the use of an earth colored wire for one of the controls?

I've not change anything else, I've not opened up the control box where the pump/valve/water tank thermostat connect into, that's probably messy, and I'll need more time to get my head around it.

Good point - yes, I realise the Nest controls the valve to give hot water/heating. I checked this last night, and it seems to control the value fine.

Once the boiler was triggered, it remained on (until it switched off triggered by its own internal thermostat), and the pump kept on whirring ... The only thermostat is on the tank...

 

[stem]

I assume he installation issues you're referring to are the use of an earth colored wire for one of the controls?

Sort of:

1. The green yellow wire should at least have a brown sleeve on it to identify it is being used as a live wire.
2. The outer sheath of the cables should extend below and be secured by the cable clamps, not the wires themselves.
3. You haven't made a connection to the Heat Link earth terminal. But to be fair according to Nest, this is only necessary if you are using the T1 & T2 terminals for the thermostat, and I can't see if you have or not.

So, why it stays on then is a bit of a puzzle, especially if it all works OK when you put the original programmer back, that suggests that the motorised valve and cylinder stat are doing their thing.

The only possibility I can think of is that there may be a frost thermostat somewhere? They are designed to override the normal controls and operate the boiler to prevent pipes etc, freezing. A bit of a long shot I know, and it would have to coincidentally come 'on' when you were trying the Nest and not when the old programmer was in place.

The pump will be wired such that it operates whenever the boiler does, and some boilers control the pump and actually keep it running for a few minutes after the boiler has gone off do disperse residual heat.

 

[diyhov]

Hi Stem,

Thanks for the replies.

1. The earth cable - yes, i need to get some brown sheathing, the earth is courtesy of the last owners/installer, who didn't do a good job
2. Agree, I connected it all up temporarily, as, I want to use the 'holes', well, there seen to be 'outlined' parts of the back plate below the wire connections, suggesting they can be drilled out, so the wires can pass directly behind the Heat Link. The temp connections were also to test, and I'm glad I did!
3. I've got the earths which were terminated on the Danfoss back place tucked away, so I could connect easily. Not using any other thermostat; the hot water tank thermostat connects into the control box next to the tank, but to be honest I don't think it does anything.....

Frost thermostat, didn't know they existed! I'll try to investigate. The weird thing, the pipes seemed to make noises I've not heard before, like the clinks/expansion sounds when the hot water passes into a cold pipe/rad, but other pipes that don't usually make a noise.....

The pump - yes, it usually stays on a little while, after being triggered by the boiler; its as through the boiler was permanently live (when connected to the Nest) as the pump kept on whirring for 30 mins, the boiler firing on and off (as it can do). The pump didn't switch off as I would have expected.

I've got a multi meter, so could test if the boiler is permanently live - am I right in thinking the 3 way valve supplies the live to the boiler and tank? Wondering which ports on the Heat Sink I need to measure.

Never touched heating systems before so all a learning curve.

Appreciate your help

 

[stem]

With the three port valve, [I know a Honeywell V4073, not familiar with F5 though] The Heat Link terminal 3 (heating call for heat) should connect to the motorised valve's white wire, so that when the heating comes 'on' it winds the valve from rest, to one of the two heating positions. If only heating is required, the valve is fully wound across to the 'heating only' position and a microswitch inside the valve operates and makes the motorised valve's orange wire live, this is what controls the boiler.

When the hot water is running, the boiler is controlled by the hot water cylinder thermostat instead and not the motorised valve.

The cylinder thermostat gets its power from Heat Link 6 (hot water call for heat) and the cylinder thermostat switches the boiler on. If the central heating isn't calling for heat, a spring pulls the valve back to the hot water only position.

You would probably know if you had a frost thermostat, I thought it probably a long shot.

If the boiler runs the pump will run, they are inextricably linked. But as you have observed, when the Nest turns the heating off the boiler should stop firing, but the pump will run on for a while.

 

[diyhov]

Thanks Stem, I'll connect the Nest/Heat Link back up over the weekend and troubleshoot a bit more, and will check the valve/cylinder stat wiring.

One thought, when goig through the Nest thermostat setup, I didn't see a Y-Plan setting, so configured it as 'other', or what ever the choice was, selecting heating and hot water (I think). Im assuming this should not make a difference?

Cheers

 

[stem]

What were the other options you had to choose from? I can't remember without having it in front of me

Y-Plan is really a Honeywell brand that has come into common use to describe all systems with a 3-port valve. Similar to the way 'Hoover' is to describe all vacuum cleaners. So I'm not surprised it's not shown.

 

[diyhov]

Hi Stem,

I checked how i'd configure it, and its set as "Equipment: Heat source Gas, Heat type Radiator."

I was about to work through your wiring, and I worked out the 3 port valve is a V4073A (not F-5) so I also had that diagram ready, then contacted the Nest support via chat.

Very helpful ,got passed from the USA to the UK, and was advised to fist check if the Heat Link triggers the boiler correctly, as in the article https://nest.com/uk/support/article/How-do-I-turn-the-heat-on-if-the-Nest-Thermostat-isn-t-working. That way I'd know if it was the Heat Link or stat. They even gave me the support case number, thats impressive.

So today, I reconnected it within 10 minutes, and happily pressing the heat link button the boiler popped into life, and shut down with another press. Very happy as the wirings good!

The Heat Link would not play with the thermostat at all.

I reset the Heat Link (held the Heat Link button down for 10 second) ass in https://nest.com/uk/support/article...-reset-it-to-factory-default-settings#hl-nlt3 but no change, the heat link light remained yellow.

I then followed the "How to reset Heat Link to factory defaults", holding the Heat Link button down for 20 seconds (light flashes yellow - release, turns blue), paired it to the stat, and its working like a charm, water and heating triggering the boiler on and off as expected.

Many thanks for your help, and I can see you help a lot of people.

Thanks!