Constant call for heat from cylinder thermostat?

[bernardgreen]

Could the thermstat on the cylinder be set to a temperature that is higher than the hottest water that the boiler can produce ?

 

[oldbutnotdead]

Has the system ever worked properly? As above posters have mentioned, a multimeter will be useful- first job is draw up a rough schematic of what is actually connected to what. You can get problems with latching if the switched live from the thermostat is used on the microswitch on the motorised valve.

 

[Madrab]

The chain in a Y plan for HW only is - programmer sends power to the cylinder stat - the cylinder stat throughputs that power to switch live on the boiler, if the HIVE isn't actually calling for hot water (green ring on the receiver HW) then test the Common @ the stat - is it getting 240V? Given your symptoms this should be a yes, as you say turning down the cylinder stat shuts off the boiler. 3 port should be @ idle, @ port B so 3 port shouldn't be in the power chain.

If there is power @ the stat's Common then the HIVE receiver is sending out a SL when it shouldn't be.

 

[jamojamo]

I'm busy today, but have dug out a multimeter, (no battery, so will pick one up tomorrow).

Even with the boiler set to off (powered but on/off dial on off), the pump still fires and also only goes off if you turn the Danfoss to zero.

 

[Mister Banks]

It’s probably a faulty cylinder stat, get a new one they are cheap as chips.

 

[Madrab]

Even with the boiler set to off (powered but on/off dial on off), the pump still fires and also only goes off if you turn the Danfoss to zero.

It’s probably a faulty cylinder stat, get a new one they are cheap as chips.

All the cylinder stat does is switches the live in (C) to the SL out (1) - so the stat may be faulty but it isn't that which is switching the boiler on and off, the stat doesn't have a live of it's own. The only thing that feeds the stat with 240V is the HIVE receiver.

So its - FCU 240v out > 240v into Receiver 240v out switched live (SL) > 240V into Stat 240v out SL > 240v SL into Boiler

 

[DP]

The thermostat on the cylinder completes a circuit to the boiler ( motorised valve not powered during hot water demand only)

I am wondering if NO of the programmer is completing the circuit for HW to run. Turning the thermostat to 0 would disconnect circuit. Equally, Hot water demand on the programmer would not cause the boiler to fire and selecting CH as well MV would power up to motor to close the HW port and start heating the radiators

If the heat is not transferred to the switching element ( Bimetaljc strip) or the switch itself is faulty issue described in the post is possible

Good suggestions already made in above posts.

Removing the programmer removes the power to the thermostat this removes demand to run to the boiler.

Checking with multimeter essential as then can trace back to the point that is to the cause.

Cylinder thermostats are not expensive. If changing ensure correct wires go to the correct terminals. With call for hW to run. C would be live as would be NC of thermostat

 

[Deleted member 267285]

A few things that I haven’t asked and appears nobody else has: is this a new thing? How old is the Hive? Anything changed recently other than the 3 port?

 

[Madrab]

is this a new thing

The boiler was doing the same before the 3 ways was replaced

??

 

[Deleted member 267285]

??

Must have been half asleep last night reading it and didn’t re-read

 

[RandomGrinch]

A few things that I haven’t asked and appears nobody else has: is this a new thing? How old is the Hive? Anything changed recently other than the 3 port?

And relating to the 3 port...

The pin was stuck, the motor was gone and the gearing was knackered.

Which may, or may not be a contributing factor to a welded/sticky relay!

 

[Madrab]

Must have been half asleep last night reading it and didn’t re-read

I didn't want to say much in case you had seen it but were wondering if there were other things previous to the 3Port

 

[Madrab]

Ultimately @jamojamo you need to get in there with your multimeter and check back to the cylinder stat and see if it's getting power when it shouldn't be - if it isn't powered then the next thing to check is the orange from the 3 port. If that is energised then there's a 3 port micro issue but if it's new there wouldn't normally be.

Please take all safety precautions when in testing in and around 240v.

 

[Mister Banks]

All the cylinder stat does is switches the live in (C) to the SL out (1) - so the stat may be faulty but it isn't that which is switching the boiler on and off, the stat doesn't have a live of it's own. The only thing that feeds the stat with 240V is the HIVE receiver.

So its - FCU 240v out > 240v into Receiver 240v out switched live (SL) > 240V into Stat 240v out SL > 240v SL into Boiler

What makes me think there is a cylinder stat issue is the fact that it doesn’t turn off until it is turned to 0. But having thought about this a bit more was the OP talking about the stat? My Honeywell cylinder stat doesn’t go down to 0.

 

[Madrab]

My Honeywell cylinder stat doesn’t go down to 0.

Very few things apart from frost stats and boiler controls, in a CH system, actually go right down to 0 TBH.