HW call won’t light boiler

Gary Lapidus · 24 Jun 2021

I have a Vaillant ECO Plus, remote hot water cylinder, wired Honeywell S Plan with a Lifestyle LP722.
This time of year I have CH set to OFF and HW set to twice per day.

For the past day or two, the HW signal calling for heat doesn’t light the boiler. E.g., the boiler water was 19 C this morning despite it set at 60. The boiler status was S.30, which I understand is “no call for heat”. And yet the cylinder thermostat and Lifestyle were definitely calling for heat.

As a test, I turned CH ON and set house thermostat to call for heat; the boiler fired-up from the CH call signal. I then turned CH OFF, and the boiler stayed on, i.e. stayed on only from the HW call signal.

So, in summary, the HW call signal won’t wake up and ignite the boiler, but it will maintain the boiler if the CH call signal is used to wake it up. So strange.

any thoughts?

ianmcd · 24 Jun 2021

faulty HW zone valve actuator, if you want as its Honeywell you can just change the synchron motor, about £15 ish depending where you buy, or change the actuator, if the correct onew , post a pic , if the casing has a little dimple on the top it is removeable without draining anything, as a short term fix, if you want HW slide the manual lever to the Man position and lock in the man position, then put the CH on this will heat up the HW cylinder

Gary Lapidus · 25 Jun 2021

Thanks Ian!
Here are photos; I believe they are the "dimpled" with accessible/removable synchronous?
The valves are as I found them. The sliders are both on "Open / Manual". At time of photo the CH was Off, which suggests the CH diverter valve incorrectly left in manual position, yes?

Since you clearly know a lot about this, can you please explain:

1. how it is that the diverter valves are what control the signals to fire the boiler? Is it that that the thermostats send their signal to the diverter valves, which in-turn send a signal to the boiler? The electric signals would seem independent of the mechanical action of the valve. For example, if a HW synchronous motor was dead, I'd think that if the cylinder thermostat called for heat, the HW diverter valve would remain closed (normally-closed), but that the HW call signal would still ignite the boiler, in which case the boiler would heat up but no hot water would flow to the cylinder heat exchanger.

2. what steps do you recommend I take now?

All very exciting to tackle this on my own, with your help of course.

kidgreen61 · 25 Jun 2021

The programmer sends power to the zone valve via cylinder thermostat to open the valve, when the valve is fully open, a switch in the valve closes, which powers pump & boiler.

Isolate heating system electrically, remove silver cover from valve (often a Torx screw), then undo 2 screws holding actuator to brass valve body.
Replace with new actuator, & wire in new actuator taking care to wire exactly as before.

Gary Lapidus · 25 Jun 2021

kidgreen61 said:
The programmer sends power to the zone valve via cylinder thermostat to open the valve, when the valve is fully open, a switch in the valve closes, which powers pump & boiler.

Isolate heating system electrically, remove silver cover from valve (often a Torx screw), then undo 2 screws holding actuator to brass valve body.
Replace with new actuator, & wire in new actuator taking care to wire exactly as before.

Thanks for the reply. Is it just replacing the synchronous motor, e.g. one of these?

SYNCHRONOUS MOTOR

Gary Lapidus
25 Jun 2021

It looks like one can also buy complete replacement motorized valve heads, which I assume include the electric switch in the valve that signals the boiler.

Is there a way to know if it is a bad motor, as opposed to a bad electrical switch in the valve?

Does it make more sense to replace the complete motorized valve head for an extra £20?

ianmcd · 25 Jun 2021

Neither valve is in the manual open position, once you manually open the valve , there is a small notch at the end , you lift the lever up till the spring pulls it baxk, and that little latch holds the valve in the manual position, this however will not fire the boiler, as the motor has not moved enough to make the micr switch that tells the boiler to fire, to test the synchron motor, turn oth CH &HW off and slide each lever over manually, you should feel a slight resistance, then turn both CH & HW on and do the same with the levers, this time there should be no resistace as the motors have opened, if one has a resistace then that is the faulty one , completely up to you if you change the whole actuator or just the motor, and yes you do indeed have the removeable actuators so no need to drain anything

Gary Lapidus · 25 Jun 2021

Ian, I appreciate your help!
After much trial-and-error, I find that both valve levers are giving resistance when OFF, and both have no resistance when ON.
I also found that the boiler would not fire with signal only from CH, rather it required signal from both CH and HW. This is the reverse of what lead to my initial post, where it would not fire just from HW but required CH as well.
The only consistent finding is that the boiler requires two heat-call signals, as neither alone are sufficient.
That finding makes no sense in the context of how these systems are wired. One could expect a problem with either of the motors and/or switches where either CH or HW would fail to ignite boiler, but I see no logical issue that would cause what I'm describing and seeing.

ianmcd · 25 Jun 2021

There is something else wrong, if both valves are opening with a demand, and working correctly then either opening should fire the boiler, it is common to find a faulty micro switch on either valve , so need both open to get the boiler to fire, where only one micro switch is making, but if this was the case at least 1 valve operating solo would fire the boiler, what programmer do you have ? did the system ever work correctly ?

Gary Lapidus · 25 Jun 2021

I confirmed the diagnosis, whereby with CH ON alone, or HW ON alone, the boiler sits idle at S.30, but when both are ON the boiler starts.
The controller is a Drayton Lifestyle LP722 with honeywell thermostats.
I've never thought the system worked properly, but for reasons different than what I'm describing here, and will only confuse the issue.
It's possible I never noticed issue in winter as both CH and HW come on at same time, but I'd not noticed this particular issue in previous summers, i.e. using only HW.
It does strike me as an electrical issue rather than anything mechanical.
Could it be the controller? I'd have thought that the circuitry for HW and CH on the controller are completely independent, and therefore unlikely to be the cause of this strange scenario.

ianmcd · 25 Jun 2021

if you only select the CH to come on, does the red led on the HW side also illuminate ? If you can see the wiring centre (with the power off) each zone valve has an orange wire and a grey wire, the grey wires should both be connected together and should be connected to a Permanent Live supply, the two oranges should be connected together and be connected to the boiler switched live, it sounds like the Grey wires may have been connected to a switched live somewhere, instead of a permanent live

ianmcd · 25 Jun 2021

Below the quote in the bottom right of your screen , if you hover your cursor over there, it will say like or thanks, if you click either I will get a notification that you have seen my message and I can check to see if you have replied, speeds things up

Gary Lapidus · 25 Jun 2021

CH LED only lights when CH is ON, and same for HW. What I find is that if I turn CH ON alone or HW ON alone, the respective red LEDs glow, but there’s nothing audible from the Drayton. But when I turn the second loop ON, which had been OFF, such that both are now ON, there is an audible click from within the Drayton, a click that sounds like a switch closing.
This makes me think that the problem is with the Drayton Lifestyle.

dilalio · 26 Jun 2021

@Gary Lapidus
Multimeter testing is what's really needed to resolve this.
However...
If you operate CH and DHW separately on the programmer AND room and cylinder stats are NOT satisfied, they will "make" the switch and send power on to the relevant 2-port (brown wire) which then motors open.

With everything calling, turn down the roomstat (or get someone else to do it) and listen for the CH 2-port to make a mechanical winding noise, as the spring pulls against the motor to close the valve.
Then do the same with the cylinder stat.
Then turn both stats back up.
Then get someone to turn the CH off at the programmer and again listen to CH 2-port which should again make closing noise.
Then do same for DHW at programmer.
This will prove if prog and stats are working as they should.
Then you'd really need to remove orange wires from wiring centre and see if 240v appears on each, separately when there's a CH and DHW demand, using a MM.

Gary Lapidus · 26 Jun 2021

I bypassed the Drayton 722, for both CH and HW, by connecting the respective wires to the Drayton live terminal. In both cases the respective motorised valves opened and the boiler turned on.
So, my conclusion is that the Drayton 722 has gone bad. I ordered a new 722 so it’s an easy fix.
Thanks to everyone above for helping me on this; very satisfying to work through this advanced DIY project. I learned a lot about the system.

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HW call won’t light boiler

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