nest thermostat not activating central heating

[Chris-H1711]

Good Morning.

I've been having an issue for a couple of days, that my nest thermostat is demanding heat, but the boiler does not activate. However, when using the heat link manual button, the heating works fine for around 2 hours.

I have recently replaced the pump from a wilks gold rs50 to a Grundfos UPS3. After replacing this, the heating ran fine for 2 days, then the thermostat started playing up. When I changed the pump, only a small amount of water was lost from the system, however, i can't find any pressure gauges anywhere to check if it is pressure related?

I have contacted Nest, and completed a continuity test, and the heat link is working as it should. When the demand for heat comes through, there is a click, then the actuator opens as normal, however, the pump does not kick in and neither does the boiler. As mentioned above, the heating comes on when the manual button is pressed, but only seems to stay on for around 2 hours before it stops heating the radiators. (the boiler and pump continue to run, presumably to continue producing hot water)

The central heating actuator is a Drayton ZA5 and the hot water actuator is a Range (cant find model number) Its an open vented system (i think) with a water tank in the loft.

 

[Deleted member 120897]

If it’s open vented, then you won’t find a pressure gauge as these apply to sealed systems. Sounds more likely that the ZA5 isn‘t making the microswitch each time, or it’s sticking. Next time it fails, try measuring the voltage at the orange wire from the 2 port. The ZA5 brings on the pump and boiler, not the thermostat, the thermostat sends the signal to the ZA5.

 

[Chris-H1711]

If it’s open vented, then you won’t find a pressure gauge as these apply to sealed systems. Sounds more likely that the ZA5 isn‘t making the microswitch each time, or it’s sticking. Next time it fails, try measuring the voltage at the orange wire from the 2 port. The ZA5 brings on the pump and boiler, not the thermostat, the thermostat sends the signal to the ZA5.

Thanks for the response. will test this and report back, if this is the case, I'm assuming its a new ZA5?

 

[Deleted member 120897]

Thanks for the response. will test this and report back, if this is the case, I'm assuming its a new ZA5?

Usually yes, but you can just get the actuator (head).

 

[Chris-H1711]

Usually yes, but you can just get the actuator (head).

Ok, thanks. What am i looking for when i test the orange wire? Apologies for my ignorance, I've only really used multimeters for continuity on PCBs.

 

[Deleted member 120897]

Ok, thanks. What am i looking for when i test the orange wire? Apologies for my ignorance, I've only really used multimeters for continuity on PCBs.

Select correct voltage (AC) if manual ranging meter select closest to 240vac, but make sure it’s higher not lower, so 300vac, 750vac et al, and test between neutral or Earth and the orange and you should have approx 230 - 240 Volts AC.

 

[Chris-H1711]

Select correct voltage (AC) if manual ranging meter select closest to 240vac, but make sure it’s higher not lower, so 300vac, 750vac et al, and test between neutral or Earth and the orange and you should have approx 230 - 240 Volts A

I've tested the orange cable against the earth & neutral, and nothing comes through, when tested with live its around 230-240v. i've tested it both against the terminals coming from the boiler, and also from the Nest heat link (im assuming both are the same). Both neutral and earth have no voltage when paired with the orange cable

 

[Deleted member 120897]

I've tested the orange cable against the earth & neutral, and nothing comes through, when tested with live its around 230-240v. i've tested it both against the terminals coming from the boiler, and also from the Nest heat link (im assuming both are the same). Both neutral and earth have no voltage when paired with the orange cable

Sorry I might not have explained it properly, did you test this with a demand?

 

[Chris-H1711]

Sorry I might not have explained it properly, did you test this with a demand?

Yes, was tested with demand

 

[Deleted member 120897]

Ok, so if you’ve nowt on the orange with demand, and it’s definitely receiving voltage from the heatlink, then your ZA5 is kerput.

 

[Chris-H1711]

Ok, so if you’ve nowt on the orange with demand, and it’s definitely receiving voltage from the heatlink, then your ZA5 is kerput.

Ok, just so i understand what is going on, the valve is opening when it receives demand, but i am assuming it is not returning the signal to the boiler or pump to turn on?

Would you say this is related to the change of pump? I assume its just a coincidence that they have both gone around the same time?

 

[Deleted member 120897]

Yep that’s what’s happening, the valve can open, but if it doesn’t make the microswitch or the microswitch has failed the the voltage won’t be delivered to the pump and boiler. Coincidence they’ve gone at the same time.

 

[Chris-H1711]

Yep that’s what’s happening, the valve can open, but if it doesn’t make the microswitch or the microswitch has failed the the voltage won’t be delivered to the pump and boiler. Coincidence they’ve gone at the same time.

Spot on, thanks so much for the assistance

 

[Chris-H1711]

Ok, so if you’ve nowt on the orange with demand, and it’s definitely receiving voltage from the heatlink, then your ZA5 is kerput.

This was indeed the case, picked up a new zone valve from screwfix, installed and the heating is now working as it should. You have saved me a fortune in a callout, so an enormous thank you to you