vaillant vrc400 questions

[kylieo]

Hi everyone,

We recently moved into a house with a Vaillant ecotec plus 831 and a vrc400 mounted on the boiler. There is also a Drayton Digistat rf1 connected up.

The external sensor for the vrc400 is clearly broken as it has been registering temperatures of about -45 to -60C. It's cold but it's not that cold! However, I have set the time programme on the vrc400 for the on periods to be a couple of hours morning and evening. So when I set to auto I would expect for the heating to be on in those times and off at other times (I have the setback temperature at 13 and it's never that cold in the house). However the heating never comes on at all. The heating curve is currently set to 2.1 although I have tried it up to 3 and never had the heating come on. The only way I can make the heating come on is to turn up the digistat rf1 and then it does not pay any attention to the times set on the vrc400.

Obviously the fact that it thinks it is -60 outside will stop it from functioning as expected but I assumed that all that would mean was that the heating would be on a lot more than we wanted it if it thinks it's trying to make up a 80C temperature difference between outside and inside.

Can anyone explain what is happening here?

Thanks.

 

[Agile]

I would have expected the stat to have been connected so that it works in conjunction with the VCR400 whereas yours seems to have been connected independently.

You could measure the resistance of the outside sensor at the boiler connection end. Probably should be about 10 k.

It does rather sound as if the VCR400 might be at fault but there are other things to be checked before replacing an expensive item. Of course many engineers would have a spare one to check by substitution which is quicker.

Tony

 

[kylieo]

Thanks for your quick reply.

I would expect the stat to work in conjunction too, the way it is now it seems unnecessary. Until the vrc400 stopped working at least!

I don't think I have anything to measure resistance but I'm sure I can borrow from a friend.

Is there anything else you can suggest that I can check myself? I'd rather avoid the expense of calling an engineer if possible.

 

[Agile]

Not really apart from seeing where the stat is connected to.

Engineers are usually very useful for getting something repaired quickly!

Tony

 

[Tipper]

Not really apart from seeing where the stat is connected to.

Engineers are usually very useful for getting something repaired quickly!

Tony

You'll be lucky to find one that understands Vaillant controls though!

 

[mikely]

The VRC400 is not designed to be used with a separate thermostat, so it is not very clear how the system is supposed to work. Maybe the thermostat was added when the VRC stopped working properly?

Have you looked in the external sensor unit to see if there is any evidence of corrosion or poor connection? Failing that then, as suggested, you need a multimeter to check the resistance of the external thermistor. It is unlikely that the sensor is actually broken; more likely to be a broken connection somewhere. Disconnect the sensor wires at the boiler and then measure the resistance of the sensor between the wires. If this is open circuit, check the sensor resistance at the external unit.

 

[kylieo]

No I haven't had a look in the external sensor unit yet, it was cold and dark by the time I was doing this last night so it is on my list of things to look at tonight. I've also sent my partner on a multimeter hunt so hopefully I'll have one of those by tonight too and will be able to check the resistance.

The thing I don't understand is that the vrc400 is registering -60 so it is clearly getting some reading (even though it's wrong). I don't see why this wrong reading would cause the heating to never come on. If it was reading a high outdoor temperature I could understand it but surely if it thinks it's -60 outside the heating should be on full whack constantly for it to think it's even maintaining the setback temperature? Or maybe it assumes at -60 we're probably all dead so there's no need to heat the house!

 

[mikely]

Or quite likely it’s seeing an open circuit, which results in an indicated temperature of -60.

Try disconnecting the external sensor at the boiler to see if it still indicates -60.

 

[kylieo]

Thanks for the explanation Mikely, it makes a little more sense now. I shall try disconnecting it tonight and report back tomorrow on my findings.

 

[Agile]

Dont bother fiddling outside when all you need to know is what resistance you find across the wires at the boiler.

It certainly sounds as if your unit is faulty. But as suggested the room stat might have been fitted because it was never correctly installed or set up or even the user just could not get to use it.

One engineer here says that in his experience over 30% of newly installed WC units have to be removed because of user incompetence!

Tony

 

[Onetap]

Read the manual? It's available on the Vaillant site. I'm fairly sure there's a facility on there to make a correction to an out-of-calibration sensor; I don't know if you can put in a 60 deg offset, though. Probably the cable is broken.

 

[kylieo]

Well I got home too late last night to do anything but now have a multimeter and shall be having a look tonight.

Onetap - I hvae read and re-read the manual a few times now, it's a feat of awful translation I think. You're right though, there is an offset but only by a maximum of 5 deg so not very helpful in this case.

 

[kylieo]

Finally got time last night to check out the external sensor. Turns out it was full of water and the connection to the resistor had corroded through in one part. That explains why it wasn't working then! It was badly positioned and the rubber grommit where the cable went in wasn't in right so it had filled up with water.

My first solution attempt will be to try and solder it back together but if that fails does anyone know where I can get a replacement sensor without having to buy a complete vrc400 unit? Google hasn't come up with anything for me. The only other option I can think of is to scrape the plastic coating off the thermistor, find out what it is and see if I can replace that.

 

[BarryCambs]

Try this

//www.diynot.com/forums/plumbing/vaillant-weather-sensor-help.288642/#2100272

Barry

 

[kylieo]

Thanks Barry, that looks really helpful. Not sure how I missed it in my searching!