Central heating always on!

[Leelums]

Hi there,
My radiators on my central heating system have started to remain on all the time regardless of what the thermostat is set to.

The system we have requires that the hot water is on for the central heating to work.

I have checked the output from the thermostat with my multimeter and everything functions correctly here i.e. sends live to the valve when room is below set temp.

So venturing up into the airing cupboard and tracing out what is going on. It seems that the 2 way valve which is supposed to isolate the central hating flow pipe is not closing off the pipe although the motor seems to be working since the contact arm which triggers the microswitch to run the pump and boiler is working according to visual inspection and my trusty multimeter. It's not until the cylinder stat finally reaches temp, which it is finding hard since the radiators are demanding constantly that the pump and boiler cut out. The 2 way zone valve is normally closed so with the evidence above is unlikely to be the motor. The only way I can shut off the cental heating is to turn the manual red tap below the zone valve.

Any suggestions on what is at fault here and what needs replacing and how difficult is it?

Also is it possible to upgrade my system by adding another zone valve and upgrading the timer switch so that I don't have to always heat hot water when i want central heating as this seems very wasteful?

Thanks in advance
Lee

 

[felix]

What do you mean by a two way valve. Is it the type with one inlet (from the boiler) and two outlets, one each for HW and CH? These are more commonly known as three port valves. Or do you mean it has two positions: open and closed? If it's the latter then how does it close?

Just because a motor drives a switch arm that doesn't mean it's moving anything inside the valve body. A shaft might have sheared.

As for the other question, if you have a three port valve then all you need is the right controller. These valves are designed to give HW, CH or both. What you need from the controller is a 'No-HW' output. This is not just an absence of any HW output; it's a wire that goes live to indicate that you don't want HW. If your valve is an Open-Closed type then you can add another one to the HW circuit. It's motor will close a switch when it reaches the fully open state and you wire this in parallel with its mate on the CH valve.

 

[Leelums]

Thanks for reply felix.
Its just a 2 port valve, i.e. 1 input 1 output A > B.
Its actually my parents house and they recon that its the same valve that was installed in 1981, and it certainly looks it.

like I say its a normally closed spring return valve which opens when energised by the room thermostat, which is working ok.
I like your suggestion about the broken shaft. If this is so can it be replaced easily without draining the system and what exactly do i need to buy, a whole new valve or just the head?

as for the independant central heating question. A pipe comes into the airing cupboard which then goes into the pump comes out and splits, one pipe to the CH valve and another one to the cylinder with no valve.

Thanks
Lee

 

[Leelums]

So anybody have any ideas on rapairing the zone valve. i.e. do I need to replace the whole lot? Will I have to drain the system?

And could there be anything in the current system design that would be stopping me from adding the extra zone valve to the HW, as I can't understand why this was not added in the first place.

Thanks
Lee

 

[felix]

I think the only way you're going to be able to do anything with that valve is to get it out and have a look. This will be a lot easier if you can isolate the water on both sides of it. From what you've said earlier, you have one stop valve in there already and pumps usually have one on each side for maintenance. If no combination of these will do the job then you'll have to partially drain the system.

I can see just one reason why you might not be able to put a zone valve in the HW feed pipe. Is the vent taken from the top of the cylinder's heating coil? It's a popular place because it's usually the highest point in the circuit. You MUST maintain a free flow from the boiler up to the vent pipe. This will not stop you from putting a zone valve in the HW return pipe but once again there is a proviso.

Where does the feed (aka expansion) pipe come in. In a fully pumped system the feed and vent should be close together but not everybody appreciates this. A pressure differential between these pipes can send water up the vent. If you insert a valve between them you'll get a very big differential!

 

[Leelums]

Yeah I was afraid there may be such issues, i'm not near the system right now but I will have a look when I go there tonight. I will maybe include a picture if im not 100% sure on the setup.

Thanks again, really appreciated
Lee

 

[Leelums]

Ok here are some pics of the system. What do you think is it ok to add the valve?

please click a thumbnail for a larger image!

 

[felix]

First things first, I can see now why that zone valve might appear to operate while not actually doing much. The switch arm you talked about must be in the valve head. The motor drives this to the open position but what happens when you cut the motor power? All you can see is the switch arm moving back. You can't be sure that the valve is closing fully. If the valve has become stiff, the spring might not be strong enough to do the job. Perhaps you can get the lid off and try moving it by hand. This would also be a good time to look for a manual over-ride lever that has been left in the open position!

I think your second problem has a solution. I can't be 100% sure from the pictures so you'll have to double check the run of the pipes yourself.

I can see that the pipe from the top of the pump branches left to the zone valve and right to what looks like the top of your cylinder heating coil. There are gate valves in both branches. The small pipe and gate valve to the left of the pump and zone valve look like a bypass. It's valve should be almost closed.

The two most important pipes in the pictures are the ones to the right of the pump running vertically. Is the big one coming from the top of the boiler and does it carry straight on up to become the vent? Is the small one the feed? There is another small pipe coming off the top of the pump. Does this just go to a bleed valve? If the answer to all these questions is yes then you're in luck. You can put another zone valve in the big pipe leading from the pump to the cylinder heating coil. Just below the gate valve would be fine but if space is a problem you can replace that entire section of pipe.

I cannot over-emphasize the importance of maintaining a permanently open pipe from the boiler to the vent so don't accept my interpretation of your pictures. CHECK THAT PIPE.

PS: If that small pipe from the pump top is a dead end you can isolate the zone valve by closing both big gate valves, the bypass gate valve and the upper pump isolating valve. Follow the pipes and you'll see what I mean. Count the turns carefully when you close the bypass so that you can put it back where it came from later.

 

[Leelums]

Yes your diagnosis of the zone valve problem is tatally correct! Today I partially drained the sysyem and removed the zone valve, after I finally managed to remove it, I noticed that the arm was indeed stiff. I also noticed that the valve was fitted with the input to the valve (A) at the bottom and the output of the valve (B) at the top! On talking to some guy at B&Q that seemed to know quite a bit about plumbing, he said that it is possibly like this for a reason and that if the existing one as been working this way for 25 years then I should replace it in the same way. Is this correct? The only differnece in the system I can think we may have is that it is a drop system because of the concrete floor downstairs!
Anyway the valve is now replaced and seems to be operating correctly.

Also your tracing of the pipes seem to be correct, and the smaller pipe does indeed go to a bleed valve. The large pipe from the boiler carries on into the loft and vents over the heating tank.

 

[doitall]

where does that small pipe just above the pump go, the one thats hiding behind the valve in your picture.

Another picture shows it going into the floor.

And did the new valve stop the heating getting hot

 

[Leelums]

The small pipe that goes down into the floor is the bypass.
The new valve stops the heating getting hot perfectly!

Lee

 

[doitall]

give that man a pat on the back

and one for felix

 

[kevplumb]

well p*ssed john

 

[felix]

I don't see any good reason for putting that valve in backwards. It has been working for a long time but maybe it got blocked precisely because it was in backwards - and maybe not. The simple answer is I don't know. If it was mine I'd fit it the right way round.

Either way round (no pun intended) you are free to fit a second zone valve in that pipe with the big gate valve leading from the top of the pump to the cylinder heating coil. You'll need to do a bit of wiring too. Without knowing what you've got already it's hard to be specific. A reasonable scheme would be to have the controller and stats open either or both valves and use the switches on the valves, in parallel, to control the boiler and pump.

 

[doitall]

A good reason for not putting a valve to the cylinder is the by-pass would need to be changed to an auto by-pass, possibly 22mm depending on the system and pump