Boiler stays on heating water despite programmer being off

[thermostorebore]

Hi guys,

I am looking for a bit of advice on my system.

I have:

• an unvented 200l tank being heated from a system boiler via an indirect coil.
• The boiler is an ideal icos.
• A danfoss two channel programmer
• The setup is a standard S plan wired with two zone values (1 HW, 1 HTG)

The system is wired up inside a lifestyle wiring box. All pretty standard

The problem:

Mainly after the hot water has been switched on by the programmer and then switched off as per its normal cycle, the boiler and pump remain on. There is normally a circulating run on for about 5 mins but in the fault condition the boiler is constantly cycling between heating and just cycling for an hour. The programmer HW channel and CH channels are not on. The cycling of burner coming on and off will be due to the cylinder stat going off and on but it doesn't explain why it is on in the first place.

After reading the s plan schematic I did some probing about in the lifestyle wiring (I know what I am doing, don't worry). I can see there is 240v at the boiler (on and off) and pump (pin 12 and 13). So the boiler is being told to go. I can also see there is 240v at both pins 6 and 7 which are the outputs from the zone valves. So the zone values are being told to go as well. When I look at the HW control from the programmer (pin 10) it has 40V on it. I wonder if this is enough to switch and move the zone values.

If I switch off the boiler and leave it for a few hours then switch it back on again, the fault condition clears and it remains fine until the programmer goes through its normal cycle again and sometimes it jumps into the fault condition again. If I probe pin 10 after the boiler switch off, it is sitting at 0V and the boiler is off.

I was wondering if you think the problem is with the programmer itself as I suspect. I assume the programmer has some relays in it. I suspect the relay is not releasing fully. What do you think?

 

[PullerGas]

Perhaps the micro switch on the zone valve is jamming?

 

[AGAS]

I wonder if this is enough to switch and move the zone values.

100v iirc ,

 

[stem]

in the fault condition the boiler is constantly cycling between heating and just cycling for an hour.

The motorised valves have microswitches in that control the boiler and pump. When the valve is open, the microswitch is operated and the boiler runs.

As pullergas has stated sometimes the valves close but the switch sticks in the 'on' position. When this happens, the boiler and pump will operate, but hot water won't actually be circulating around the water cylinder or heating circuit because the valves are closed, so is the hot water produced by the boiler actually being passed through the motorised valves and circulating around the hot water cylinder or radiator circuit or not?

The cycling of burner coming on and off will be due to the cylinder stat going off and on

That would be unusual; unless you are using a lot of hot water. Once a cylinder is hot, if it is insulated, it should stay hot for several hours even if you are using small quantities of hot water for washing hands or a few dishes.

When I look at the HW control from the programmer (pin 10) it has 40V on it.

Taking voltage readings can be misleading when several items are connected together, voltages can be fed back via other components in the circuit.

I was wondering if you think the problem is with the programmer itself as I suspect. I assume the programmer has some relays in it. I suspect the relay is not releasing fully. What do you think?

I have known the contacts in programmers fuse together and not release, often this occurs after there has been a short circuit, or a fuse has blown. Has this happened?

Questions like yours often pop up in the winter, because a system with a frost thermostat fitted will turn on the heating when the programmer is off and the temperature drops to a point where a risk of freezing is possible. It is unlikely to be the cause now though, unless you have one that can be set to a high temperature.

To start with when the fault occurs, you need to check the motorised valves and see if they are actually open and allowing water to flow through them.

 

[thermostorebore]

Thanks to PullarGas, Agas and especially Stem(your reply really helped me). Sorry it has taken me a little bit of time to reply. I have been waiting on the fault condition again. Sods law that it seems to have been working ok. I think you guys are right that it is the limit switch inside the zone valve. The thing that really sticks out is what Stem said about how unlikely it is that the cylinder stat is switching the burner on and off. I have seen this cycling behaviour before when I used to have a thermal store. The heat couldn't get out from the coil in the thermal store because the coil was corroded. The heat would return from the thermal store and the boiler thermostat would eventually switch off the burner because it realised the return water was getting too hot. Exactly the same situation is occurring here. The micro switch is stuck on, calling the boiler and pump to heat but the valve is shut meaning the water is just circulating round the boiler loop tripping its stat. If it was the programmer then it wouldn't keep coming on and heating. I have ordered a replacement actuator to fix it.

As an aside I wonder why they don't wire the enable (the thing that asks the zone valve to operate) to the other side of the switch i.e if there is no 240V on the enable then there is no 240V to pass through the switch. At the moment the live is always there at the zone valve and just needs the micro switch to be closed for the boiler to be called.

Thanks again for your speedy replies and help

C

 

[stem]

As an aside I wonder why they don't wire the enable (the thing that asks the zone valve to operate) to the other side of the switch i.e if there is no 240V on the enable then there is no 240V to pass through the switch. At the moment the live is always there at the zone valve and just needs the micro switch to be closed for the boiler to be called.

If they were wired like this, when both valves were open, their 230v supplies would be linked together through the microswitches. Then, when one valve was turned off by its thermostat, and its live was removed, the valve would still be fed via the microswitch from the other valve and so would stay open.