Y Plan / Mid Position Valve - Odd Boiler Behaviour

[Steve3948]

Is the Hot Water Off wired correctly

 

[Wayne Thomas]

Phantom voltage?

The wiring is completely correct according to 3 port diagrams. We've gone over it 3 times, hes checked each wire for damage, re terminated all connections, taken components (cylinder stat, programmer, roomstat) out of the equation. it always comes back to 89v staying there once heating is satisfied, which the manufacturers say is what it should be doing. The valve and everything else is working fine according to MI's 

We cant work out why the boiler is using this 89v  on the switch live to fire the boiler when it didnt on the old valve which is damaged. 

 

[Sidewinder]

What are you measuring the voltage with and, which terminals is it causing issues on?

 

[Wayne Thomas]

What are you measuring the voltage with and, which terminals is it causing issues on?

Forgive me as im just the heating engineer so Im a little out of my depth when talking complex electrics.

voltage is being measured with my tester which is a Megger Fork Multimeter DCM330 and my electrician has a megger multifunction tester for doing 17th edition and part p tests.

From what i remember the voltage is on the orange wire which is connected to the switch live and also a terminal on the cylinder stat.  

 

[steptoe]

using modern digital type meters they can sometimes show an 'induced' voltage,

this is not a real voltage measurement,

if you really are getting 89v on the boiler fire when satisfied then you have a problem

 

[Sidewinder]

A phantom voltage is a voltage capacitively or inductively coupled from a loaded source onto an unloaded source.

This is seen on high impedance de-energised wiring.

One of the major issues with digital meters of the type that you describe above is that they have a high impedance input on voltage ranges.

Typically megohms per volt.

This is by design as it reduces the current they draw from the supply, this is more often than not desirable as it minimises the affect of the measurement being taken upon the system being measured.

They would typically draw microamps to measure the voltage in a circuit.

Now, if this is a phantom voltage then the high impedance of the source will not traw enough current to drop the voltage to a realistic level.

A phantom voltage has negligible current so as soon as it is put under load it drops to virtually zero.

You need to ascertain if it is real or phantom, first.

A moving coil analogue multi-meter such as an AVO8 would give you a more realistic measurement, or, a digital meter with a low impedance input characteristic such as a Fluke 289, or the use of a specific device to eliminate the stray voltage such as the Fluke stray voltage eliminator, SVE225.

It is difficult from here to say whether your voltage is real or phantom.

A bit more information on the wiring system might help.

 

[ProDave]

What might be happening here, is the boilers call for heat input is an electronic input rather than an electrical input.  In the old days this would drive something like a relay coil with a fairly low impedance. but if the manufacturers have now re designed it with a new electronic control board, then it could well have a very high input impedance and worse it could trigger from a stray voltage.

Sadly this sort of electronics in boilers is much more common now.

As an experiment try loading the call for heat input with a resistor to neutral or even a low power lamp, just to see if it behaves with a load on the input.

An example of how some manufacturers can ****** things up, I have been helping my plumber friend fault find a new Grant combi oil boiler.  For years Grant used the same basic controls on their boilers, just a couple of relays. they were reliable and simple. Hardly ever went wrong, but if they did they were simple to fix.

But now the new one has replaced all that with a board of electronics. It doesn't work and there's no information to fault find it, so Grant's own engineers are coming out to sort it.

If you have a proven reliable design, why mess it up and replace it with something new and risk the reputation of the company?

 

[Wayne Thomas]

Thanks for all you help.

You guys on here give alot more clear useable information than the plumbers forum im on. 

In short all i got there is "stick a relay on it" or "rewire" 

I have solved the problem now. It turns out that honeywell and similar 3 port valves have a residual voltage left when the heating is satisfied. Talking to Biasi the boiler manufacturers, they told me this is becoming a common problem which can be solved in one of two ways. Replace the honeywell valve for a Drayton as the residual voltage is less than 20v on that one or put a 0.47 microfarad capacitor across the switch live and neutral to take the power away.

Something interesting did happen which i forgot to mention which goes along with what you guys are saying. When my sparky put his test on, it would switch the boiler off and read 60v which clearly now shows what you are saying about testers no giving a true reading as they take some away.  

Anyway i took the easy route and swapped the valve as i can imagine the blank expressions i would get from my local merchants when i ask for a capacitor like that. "a micro what??? Try screwfix, they might have one" 

All working fine now. this is definitely one to store away in my memory. 

Thanks for your constructive help.

 

[ProDave]

Looks like the boiler design is deficient, and if fitting a 0.47 microfarad capacitor solves the problem, then they had better start making that a standard fitment inside the boiler.