NEST Y PLAN

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Y plan only works when nest is in manual mode and valve is manually opened.

Done some electrical tests.

HW ONLY - Orange 130v, Grey 240v, White 130v

CH ONLY - 240v on all

BOTH ON - 240 on all

BOTH OFF - Orange 130v, Grey 240v, White 130v

Could this be a faulty nest heatlink?
 
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You should not have 240 on the grey when calling for hot water only, you have either miswired it (most likely scenario) or a faulty nest.
 
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You should not have 240 on the grey when calling for hot water only, you have either miswired it (most likely scenario) or a faulty nest.

Grey wire on valve is going too 4 (HW OFF) on nest heatlink and NC on cylinder stat.
 
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You should not have 240 on the grey when calling for hot water only, you have either miswired it (most likely scenario) or a faulty nest.
Unless the hot water had reached the temperature the cylinder stat is set to.
 
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What are you testing between for voltages? Eg, is there a potential difference?
 
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When calling for DHW only the motorised valve has no part in the control, it is off, no power to it or from it, however you will get a back feed to the orange.

In other words you can remove all wires to motorised valve, and DHW will still work.

So Orange is out, but back feed when only DHW called for will result in 230 volt on Orange, but this is a back feed, it does not come from White or Gray.

White powers the motorised valve half way, and connects normally to room thermostat.

Gray is powered when DHW demand is OFF. And if powered will cause the valve when also powered by White to go all the way and stop DHW from heating.

Due to resistors, and diodes in the motorised valve, once connected you get some odd voltages, but for DHW both Gray and White are NOT powered, there will be voltage at the Orange but this is from the boiler and pump not to the boiler and pump.

So for DHW power goes to the programmer, and from programmer to tank thermostat, and from tank thermostat to boiler and pump, it does not have anything to do with the three port valve.

Gray only has power when DHW is off, or temperature satisfied.
 

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