Thermostat N/O and N/C is N/C ever used?

[ericmark]

Thank you good point, I have also thought about it, the thermostat can only heat or cool so if you want heating and cooling supplying a second cooling thermostat from heating N/C contact would ensure both don't run together.

Not got any cooling in my house, and only one of the radiators is suitable i.e. fan assisted, so tend to forget some people will have heating and cooling.

It has been pointed out on another forum that with a frost stat often the N/C is call for heat so if battery fails it fails safe i.e. nothing freezes.

It is however an eye opener, N/O and N/C are purely the non powered states and nothing to do with if heating or cooling.

 

[fixitflav]

Never considered it before replacing thermostat, fitted a Flomasta 6259G linked to instructions be it heating or cooling Com and N/O are used, it would seem to swap you move a link, this makes sense as then if batteries become discharged it fails safe. It has a single relay so not suitable for heating and cooling, it is only suitable to do one at a time.

I have a thermostat used for brewing with twin relays which controls heating and cooling and there is a gap between the two temperatures and without twin relays you can't have that gap.

So what is the N/C contact for? I just can't think of a scenario where the N/C contact is required.

If it's any help (or relevant) with most control schemes a DHW cylinder stat uses N/O and N/C contacts.

 

[EFLImpudence]

with most control schemes a DHW cylinder stat uses N/O and N/C contacts.

What are you calling a control scheme?

I cannot find any diagrams using NO and NC on DHW cylinder thermostats - C, 1 (& 2) is usual.

 

[ericmark]

There is one of the plans where the cylinder thermostat uses the N/C contact, it has caused many an argument where some one has used three core flex and used the green/yellow insulated wire for a line connection. However this is a room thermostat not a cylinder thermostat.

I think the main point is from now on, I realise the N/O and N/C contacts of a thermostat are not fixed to heating or cooling and can be swapped so for example with cooling the battery failing does not cause the heat pump to run.

 

[fixitflav]

What are you calling a control scheme?

I cannot find any diagrams using NO and NC on DHW cylinder thermostats - C, 1 (& 2) is usual.

The Honeywell W-plan and Y-plan use it. I can scan and post it if you need details.

 

[JohnW2]

There is one of the plans where the cylinder thermostat uses the N/C contact, it has caused many an argument where some one has used three core flex and used the green/yellow insulated wire for a line connection. However this is a room thermostat not a cylinder thermostat.

That use of of G/Y as a live conductor usually arises with room stats that have an 'anti-hysteresis heating resistor', and hence need an neutral, even though they only use one pair of contacts (whatever one calls them).

Kind Regards, John

 

[JohnW2]

The Honeywell W-plan and Y-plan use it. I can scan and post it if you need details.

Maybe they are using the Honeywell motorised valves [which, as we have been shown, quite unusually, require both signal inputs from the stat] controlled directly by the stat?

Kind Regards, John

 

[EFLImpudence]

The Honeywell W-plan and Y-plan use it. I can scan and post it if you need details.

Yes please - or a link

https://www.honeywelluk.com/documents/All/pdf/Wiring Guide Issue 16.pdf

 

[JohnW2]

https://www.honeywelluk.com/documents/All/pdf/Wiring Guide Issue 16.pdf

That does, indeed, seem to use both changeover contacts of the cylinder stat.

However, this is a rather special (and unusual) case. As I understand it, the 'W plan' (which I don't think is used much these days) is a 'water priority' system and the c/o contacts of the cylinder stat are being used so that the CH will only work when the cylinder is not calling for heat.

Most of these uses of stat c/o contacts (which is not something I've ever seen done, for what that is worth) we are finding seem to be specific to Honeywell.

Kind Regards, John

 

[EFLImpudence]

That does, indeed, seem to use both changeover contacts of the cylinder stat.
However, this is a rather special (and unusual) case. As I understand it, the 'W plan' (which I don't think is used much these days) is a 'water priority' system and the c/o contacts of the cylinder stat are being used so that the CH will only work when the cylinder is not calling for heat.

The contacts are also used in Y plan. See link and scroll down.

I only posted that with regard to the markings on the 'stat and fixitflav's post.
As it has C,1,2 on the stat itself I cannot see why NO,NC would be used in diagrams.

 

[JohnW2]

The contacts are also used in Y plan. See link and scroll down.

Indeed it does. In this case it seems that the c/o contacts are used so that the CH can work when the cylinder stat is not calling for heat.

I only posted that with regard to the markings on the 'stat and fixitflav's post. As it has C,1,2 on the stat itself I cannot see why NO,NC would be used in diagrams.

I realised that. I think we are agreed that it makes no real sense to describe a stat's contacts/terminals as N/O and N/C, not the least because of the confusion it can result in (per the OP in this thread). Eric is probably right that when an electronic stat bears such markings, they probably refer the the 'default' state (no power) - so that leaves one having to work out which of N/O and N/C corresponds to "1" and which to "2" (or "3") - indeed, as we know, Eric's one has a jumper which enables one to reverse that relationship.

Interestingly, many of the Honeywell diagrams clarify that a "closed" contact means "calling for heat" - although they do not use the "NO" or "NC" descriptors in their diagrams ...

Kind Regards, John

 

[fixitflav]

Yes please - or a link

View attachment 138980

https://www.honeywelluk.com/documents/All/pdf/Wiring Guide Issue 16.pdf

The stuff you posted covers the W-plan, so I've only posted Y-plan. Also my hand sketch of W-plan, which I think shows the relevant points more clearly.
Your comment "As it has C,1,2 on the stat itself I cannot see why NO,NC would be used in diagrams." - maybe it's because not all stat manufacturers use same markings for 1 and 2.
As JohnW2 said, W-plan isn't too common these days. I've had it since installing current system in 1998 (and in earlier houses) and find it OK. It takes about 20 mins to satisfy the cylinder, even when it only started because of heat leak, longer if just run a bath (but I only shower!). Doesn't help that the outside of the coil is probably well scaled up, but it's not a problem in practice.

 

[EFLImpudence]

...

 

[JohnW2]

Your comment "As it has C,1,2 on the stat itself I cannot see why NO,NC would be used in diagrams." - maybe it's because not all stat manufacturers use same markings for 1 and 2.

This discussion is, of course, all about the markings and (per the OP) how they can/could be confusing.

I don't think we have yet seen a case of any non-electronic stat, or any associated wiring diagram, using "NO"/"NC" terminology - they seem to invariably use "C"/"1"/"2" or "1"/"2"/"3". Are you aware of any (non-electronic) stats or diagram that use "NO"/"NC"?

As Eric has demonstrated in the other current thread, and as may be what the OP in this thread has discovered, at least some electronic stats seem to be different, in that they do use "NO"/"NC" descriptors - but, as Eric has suggested, this probably relates to the state of the contacts when the stat is not powered - leaving one to have to work out how "NO"/"NC" correspond to the usual "C"/"1"/"2"/"3". Indeed, as has been said, Eric's one has a jumper which enables one to change that relationship!

Kind Regards, John
Edit: too slow at typing again, probably due to greater verbosity!

 

[EFLImpudence]

I see that only the Danfoss stat uses NC,NO. I wonder if there is something different about them.