Replacement Nest Link not working for my Gravity Fed Boiler

My Heat Link stopped working and has been replaced. The wiring looks the same but it doesn't trigger the boiler or the pump. I am wondering if it is a newer version of the Nest Link which needs a different wiring or if the new Nest Link is faulty. The wiring is the same as in an earlier post: https://www.diynot.com/diy/threads/...gravity-only-hot-water-pumped-heating.493177/

Can some one help with this problem

I would say the heat link is the same as at least 4 years ago, slight design change but functionalities are the same. I changed an old one recently and it was a straight replacement.

If you replaced yours, it seems to imply it is not heat link related.

You will need to start taking measurements with your multimeter or provide a more accurate description, ie. what does not work? Does it get power?

It is very hard to understand you current wiring from the thread you provided, are you able to include it in this thread? Are you sure you do not have any zone valve?

Other than the earth wires it looks like this which is what it was before the previous Nest Link stopped working

The diagram above looks as it should be for your system [with the exception of the extra E terminal on the left and the L & N terminals being transposed in your drawing, but I assume that's a drawing error and they are the other way around in reality].



So what happens when you press the manual operating button on the Heat link? Do the boiler and pump work then? If it so, it's likely to be a software problem, did you reinstall the app?

If it doesn't work when you operate the manual button, the next thing to check is that the wires are actually installed as per the diagram, it's easy to miss a link out, or transpose the wires because the Heat link terminals are close together, which also makes it fiddly to install the wires. Are they secure and are the terminal screws tightened onto the metal conductor not the insulation?

Is there a chance that the fuse has blown? Although if the Heat link light/s are working that would appear to be OK.

You say the original Heat link stopped working, if the conditions are the same with new one then maybe the Heat link isn't the problem?

I don't know if the older and newer thermostats are compatible either, so if you haven't changed the thermostat, maybe that's something to investigate.

stem said:

The diagram above looks as it should be for your system

Click to expand...

According to the diagram, is CH is OFF and HW is ON, only the boiler will run, not the pump. Is this normal on such install or am I missing something?

Since there is no zone valve and no cylinder stat on the diagram, is there a diagram of the actual pipework somewhere? I never came across such an installation and I am curious.

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The pipework seems to be as such:



Hence the wiring diagram seems correct as the pump is only for CH.

Can the HW really circulate through the cylinder coil only using gravity?

PS: still think the cylinder stat is missing from the wiring diagram though.

Nest Heat link isn't really designed for systems that only have pumped central heating and gravity circulation of hot water. So it requires some lateral thinking. The boiler has to be 'on' when heating the hot water, and the pump and the boiler both have to be 'on' for the heating. The difficult part is to isolate them electrically so that the pump doesn't come on when the hot water only is required. Normally this isolation is provided by the microswitches in motorised valves, but as there aren't any present, it can be achieved by using the hot water switch in the Heat link to provide isolation.

EDIT:
Here's a description of how it works:

1. When hot water only is set 'on' (5) and (6) are connected together by the Nest. (6) is connected to the live, therefore (5) becomes live and the boiler operates and water circulates around the hot water cylinder by natural circulation. The radiators don't heat up because the pump is off.

2. When heating only is 'on' (2) and (3) are connected together by the Nest. (2) is connected to the live, so (3) also becomes live and the pump starts. As hot water is not required, (4) and (5) are connected together by the Nest, As (4) is connected to (3) by a link, when the heating comes 'on' (5) also becomes live via the heating section of the Nest and the pump and boiler operate. This is the 'fly in the ointment' bit, because as the boiler needs to run for the radiators, the hot water will also heat up. But this is because of the plumbing restrictions.

3. When heating and hot water are 'on' (5) and (6) are connected together by the Nest. (6) is connected to the live, (5) becomes live and so the boiler operates. Also (2) and (3) are connected together by the Nest and the pump runs also. Water is thus pumped around the radiators, and the hot water is heated by gravity circulation.

Ant1 said:

Can the HW really circulate through the cylinder coil only using gravity?

PS: still think the cylinder stat is missing from the wiring diagram though.

Click to expand...

Yes it can circulate through the coil but it is slow so it takes ages to heat the cylinder. Larger diameter pipes are usually used to facilitate circulation. Generally they are a carry over from an earlier solid fuel boiler. There isn't a cylinder thermostat. These type of systems are old and not installed with new gas or oil boilers for over 45 years now. The plumbing restriction means that the hot water cylinder is heated whenever the boiler is 'on'. There isn't a motorised valve as per newer systems to control with a cylinder thermostat and you can't connect it to the boiler otherwise the heating would go off when the hot water cylinder had finished heating.

Very detailed explanation @stem , thanks for taking the time going through it, I was not aware and useful to know.