Y plan system, CH only comes on when HW also on.

[chillyfly]

Hi All

Long time since I've been back to these forums and I have looked around for the answers but as yet with no luck.

I have just moved into a house with what I am assuming is a Y plan installation. It has a Honeywell 3-Port 40003916-003 valve, with Port B going to the Boiler, and Port A going to the Central Heating circuit. The Controller is a Horstmann Centaur Plus C27. It is programmed for separate times for the hot water to come on (just twice a day, morning and evening) and the central heating (morning, early afternoon - late evening).

During these times however, the central heating is not on (i.e. the radiators don't get hot and the pump doesn't run) unless the hot water is also on. When the CH is commanded on, either by the timed program or by pressing 'advance', the red light illuminates on the controller, and the motorised valve opens up with the lever being free all the way across the gate. it also moves in response to the thermostat being turned up or down. But there is no pump activity, no boiler light up, and thus no hot radiators. If I then turn the hot water on manually however (by pressing 'Boost' on the controller, or it just turns on by timed program), the pump and boiler both fire up and the rads get hot (as well as the hot water heating up.)

So from the outside it would appear that the valve is being commanded to the correct position by the Central heating controller, but it doesn't result in pump activation or boiler light up.

I had read somewhere that the controller could accidentally be commissioned to gravity type system, but if this was the case, the controller would not then show separate time programs for central heating and hot water.

I contacted the previous owners and they said they always had the hot water running 24 hours a day (!) and the central heating was always on, just controlled by the thermostat, so they have never known it operate any other way.

In a few weeks I am getting British Gas to install a Hive system which I am guessing will result in ripping out the old controller and wiring to the valves / motor anyway. But I would like to make sure the bones of the system are working first, so that the engineer isn't left chasing his tail with what might be a fault elsewhere. Any ideas what the fault could be?

Thanks a lot, Derek

 

[picasso]

Its either been miswired or the valve head is faulty, surprising how many are wired up wrong.

 

[chillyfly]

Thanks. The fact that the motor moves the valve to the correct position but the pump doesn't start and the boiler doesn't fire up - is that pointing more to a wiring fault?

Is the boiler and pump commanded on by the valve reaching the correct position and not by the signal from the controller? I am guessing so, because that's why you can start the CH by manually moving the lever...

So does that suggest a problem with the microswitch? If so, is it something easily tested or should I just replace the head unit?

Thanks again

 

[picasso]

On balance I would say the microswitch is knackered, just replace the head.

 

[chillyfly]

Ok thanks, I'll do that first. It's not too expensive. Fortunately it's the type that can be replaced dry...

 

[DP]

Check if the Orange wire has 240 volts on it during CH demand

 

[Johnmdc]

It sound to me like there is no HW off feed from the clock to the valve.
Turn the power off first then take the clock front off, see if there is a wire in terminal 1.
If there is this then the valve may be faulty.
If there isn't when you're having the Hive fitted get the installer to fit the receiver near the valve instead of where the clock is now then you won't need the extra wire running in.

 

[chillyfly]

OK thanks. I had to go ahead and order a valve head as most of the threads I searched also suggested a duff microswitch. So I'll try that first when it turns up tomorrow.

Regarding the Hive receiver - showing my ignorance, but wouldn't that need to be next to the valve anyway? I am assuming that's what commands the valve position anyway so it will replace the box that is currently wired in to the pump and valve?

For what it's worth, the online adviser at British Gas said the installer of Hive would probably not be qualified to troubleshoot any existing problem during installation of Hive, so if I can't get it to work first, I would need to call out another engineer first. Sounded like BS to me, as Hive involves ripping out the existing wiring/controller/thermostat anyway?

 

[chillyfly]

The other thing I should have mentioned before, the valve has a cryptic message on the label that says "valve used for hot water only" But why would you have a three way valve with A and B going to the CH circuit and the HW tank if it was only for HW!? Confused slightly by that. It's a fairly modern and big house, and a fairly recent installation with very small bore radiator pipes, so it must be pumped. I also can't imagine a modern system that only runs when the HW is on...

 

[Johnmdc]

The receiver would normally go in place of the existing clock, using the original wiring.
For the CH to work independent of the HW on a Y plan system you need a feed from the HW off terminal (1) on the clock. If there isn't one on your system then it would be easier to fit the receiver near the valve to save running an extra wire in.
As to your "cryptic message" I don't know.

 

[St0rmer66]

If there is no HW OFF behind the clock then tell him to commission it as a Gravity set-up (it's in the instructions). If he doesn't know how (they send electricians to install, not heating engineers when you order it standalone) then come back here and I can tell you how. That'll get around it and will work properly; the only minor downside being that having CH on will also get the HW up to temp at the same time.

 

[chillyfly]

Thanks very much all. The clock is a long way from the valve and pump, on a different floor. The present installation is not set up as a gravity system (which is an option), so I guess either there is a fault like the microswitch, or it was not installed correctly in the first place and hasn't worked as an independent HW/CH system since day 1.

The point of Hive was to save money as I live in the house alone and go away every week, so I don't really want to compromise the savings by having to heat the HW all the time the CH is on too. I'd rather bite the bullet and get it comissioned as it should be. Would it be installed with a three way valve if it was supposed to require HW on for the CH?

I'll try the head unit first (and hope it's a duff microswitch!) And if not I'll check for the HW off wire you mention. I tried to get the clock off the backplate the other day but almost bust it due to some dodgy excess plaster around the unit. I take it it's just one screw on the bottom to get the clock off the backplate?

Thanks again, much appreciated!

 

[St0rmer66]

Depends what clock it is to tell you about the screw. Sounds about right though.

Remember it will only heat the HW to the temperature set on the cylinder thermostat, I'm not saying it will heat indefinitely in that mode .

It could well be the microswitch, had that happen a fair few times. But going by the information that it's always been that way for the previous owner... It could have been converted to a Y-plan without the required number of wires at the timer.

 

[Johnmdc]

There are 2 screws one on the left side and one near the middle. Just undo them a few turns.

 

[chillyfly]

I checked the PDF manual I found online and it is indeed a single screw and tilt up to remove.

I also now understand the function of pin 1. It seems to mimic the function of the cylinder Stat, sending a signal to the valve to close as if a SAT signal was sent from the cylinder Stat. I suppose having the HW cycling on and off at the same time won't make that much difference if it's a well insulated cylinder, which it is.

All will be revealed tomorrow, when I get home and can try the head unit / search for the wire!

Thanks again.