3rd Gen Nest - Hot Water only works when Central Heating is on

[P_Hanley]

Hi

We've just had a new boiler put in and an unvented cylinder, while they were doing that we thought it would be a good time to have a Nest installed.
We had one in our previous house but had to leave it when we moved.

Anyway, it's in and appeared to be working okay but I've noticed the hot water doesn't come on, even when scheduled, unless the heating is actually on.

I've taken a picture of the wiring, and the guy who fitted it is coming back but thought I'd ask for advice on here in case it would help him.

 

[Deleted member 267285]

Your Nest installer should be able to figure out that the Commons have been left out and doubt it would have worked at all, other than the green light.

 

[P_Hanley]

Your Nest installer should be able to figure out that the Commons have been left out and doubt it would have worked at all, other than the green light.

So is it wired incorrectly?

What works at the moment:
Heating comes on if I rise the required temperature
Hot water comes on if I press the boost button (this worked earlier with the heating not on)
Hot water doesn't come on based on what the schedule defines (I added a test schedule earlier and watched it, with the heating not on, the water didn't come on)

 

[P_Hanley]

Ah, by commons do you mean T1 and T2? Just checking the install guide. The thermostat hasn't been fitted to the wall, we're using the stand.

 

[Deleted member 267285]

Yes wired incorrectly. The commons are the live linked to 2 and 5.

 

[P_Hanley]

Yes wired incorrectly. The commons are the live linked to 2 and 5.

Thank you for taking the time to reply, I appreciate it.
It doesn't mean anything to me though but I'll pass it on when I can get them back out.

 

[stem]

Unfortunately this is only half the story. To give a definitive answer we would need to know what the other ends of the wires are connected to.

You actually have wires in the correct terminals for an S-Plan system with two x 2 port motorised valves, (some installations also require a wire in terminal 4) However, It all depends upon what is connected to the other end the two wires in the common terminals 2 and 5. If they are both connected to a permanent live at their other end it would work.

However, as has been pointed out it is usual to link 2 and 5 to the L terminal as I've highlighted in the diagram from Nest below. But there is more than one way to skin a cat. (if I'm allowed to say that these days )

 

[P_Hanley]

Unfortunately this is only half the story. To give a definitive answer we would need to know what the other ends of the wires are connected to.

You actually have wires in the correct terminals for an S-Plan system with two x 2 port motorised valves, (some installations also require a wire in terminal 4) However, It all depends upon what is connected to the other end the two wires in the common terminals 2 and 5. If they are both connected to a permanent live at their other end it would work.

However, as has been pointed out it is usual to link 2 and 5 to the L terminal as I've highlighted in the diagram from Nest below. But there is more than one way to skin a cat. (if I'm allowed to say that these days )

View attachment 280747

Thank you, the installers are coming back out tomorrow to take a look at the wiring.
They're very good so I'm sure they'll figure it out.

 

[ericmark]

I have read and re-read can't see where you say using S Plan? Mine is wired for C plan, since my boiler has a 230 volt control circuit it can be linked, but not all boilers use 230 volt control a lot use 24 volt.

I found when fitting my own Nest Gen 3 there was not enough room to fit the links I wanted, so all linking done in a wiring centre, but for C plan yes if no tank stat there is a wire missing, but we don't know which plan has been used.

 

[stem]

I have read and re-read can't see where you say using S Plan?

He doesn't. I just said that there were wires in the right terminals for an S Plan, and not a Y Plan, or for that matter a C Plan.

It's very unlikely to be a C Plan, I haven't come across one for years. Apart from yours that you keep mentioning of course. Personally, my guess would be that less than 1 in 1000 of the heating systems currently in use are C Plan. But I suppose by the law of averages one day we will come across another one........

 

[ericmark]

It seems C plan is still common with oil fired boilers, although most seem to have moved to Y plan, they don't like using S plan with old oil boilers as they have no run on, and the thermal syphon allows the boiler to cool by heating DHW.

25 year old is common for an oil boiler, but no so with gas, but they are more expensive than gas to start with.