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

[ericmark]

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.

 

[JohnW2]

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

As you say, given that the logical functionality Of the "N/O" contact can be changed (to N/C) by a jumper, the existence of the N/C terminal would seem redundant.

I imagine that one could probably use the "N/C" terminal and set the jumper to "Cooling" if one wanted to control heating, and vice versa, but that would be extremely perverse.

I can but speculate that the N/O terminal exists because (a) the relay has SPST contacts, so that thought they might as well connect it to a terminal!! or (b) there are variants of the device which don't have the jumper, but they have used essentially the same board, or (c) they are contemplating some possible controlled load which requires both N/O and N/C (although I'm not aware of one). Has anyone got any other ideas?

Kind Regards, John

 

[EFLImpudence]

Don't most thermostats have similar (some may just not have a contact)?

Is it not so that a 'heating off' signal can be given - if needed?

Honeywell -

 

[JohnW2]

Don't most thermostats have similar (some may just not have a contact)?

Indeed they do.

Is it not so that a 'heating off' signal can be given - if needed?

Presumably so - although, as has been said, I'm not aware of any heating system which 'needs' it. It could well be so that they can be used for cooling, in which case one would (normally) have to use the other terminal.

However, eric's point is that his one has a jumper which determines whether the "N/O" terminal is actually N/O or N/C - so that (whether one is using it for heating or cooling) one only needs the second terminal if the controlled load requires both N/O and N/C 'signals' - and I personally haven't come across one of those (which proves little!).

Kind Regards, John

 

[JohnW2]

Honeywell - View attachment 138939

Only just noticed that bit of your post. Do you know what (if anything) the connection to terminal 6 (of the 'zone valve') does? As I said, I haven't personally come across anything which requires both the N/O and N/C signals.

Kind Regards, John

 

[EFLImpudence]

one only needs the second terminal if the controlled load requires both N/O and N/C 'signals' - and I personally haven't come across one of those .

I have not seen it either, but most programmers have that facility to control the zone valve.

As in the diagram, it could be done with the thermostat if required.

 

[JohnW2]

I have not seen it either, but most programmers have that facility to control the zone valve. As in the diagram, it could be done with the thermostat if required.

As per my second post, I wonder what that connection does. All the 'zoning' valves in my CH system are standard 2-port motorised valves - one applies voltage to open them and, when the voltage is removed, a spring closes them - i.e. one doesn't have to apply voltage to something else in order to close the valve (and that wouldn't sound particularly 'fail safe'). Are 'zone valves' different from that?

Kind Regards, John

 

[EFLImpudence]

The mid-position valves are but - I may have been confused as they usually need DHW off signal.

However, as in the diagram, there are obviously applications which utilise CH off.

 

[JohnW2]

The mid-position valves are but - I may have been confused as they usually need DHW off signal.

As you will understand, I was talking about 2-port valves (which is what I imagine 'zone valves' are, as in my installation) - and, as I said, I have personally not come across one that involves 'active' closing (i.e. has to have a voltage applied to something to close it).

However, as in the diagram, there are obviously applications which utilise CH off.

Indeed - IF that connection to terminal 6 actually 'does something' (which is why I asked). Does the documentation shed any light on that question?

Kind Regards, John

 

[EFLImpudence]

Are mid-position valves not zone valves?

Here is the valve in question -
http://ecc.emea.honeywell.com/downloads/EN2R9011.PDF

Anyway, that is a separate issue. Have I not answered Eric's question?

 

[JohnW2]

Are mid-position valves not zone valves?

Well, I would have said that they were diverter valves (usually diverting between CH and DHW), whereas I regard 'zone valves' as being those which simply switch on/off the flow to a zone of the CH installation. However, that's just words.

Here is the valve in question - http://ecc.emea.honeywell.com/downloads/EN2R9011.PDF

Thanks. I'm currently trying to work out what is achieved by the SPDT switching arrangement. Watch this space.

Anyway, that is a separate issue. Have I not answered Eric's question?

Yes, I think you have. You have essentially expanded what I said in my initial response to eric ...

... or (c) they are contemplating some possible controlled load which requires both N/O and N/C (although I'm not aware of one).

... by apparently identifying something (of which I wasn't aware) that does (or may) require both N/O and N/C control signals.

Kind Regards, John

 

[JohnW2]

Here is the valve in question -
http://ecc.emea.honeywell.com/downloads/EN2R9011.PDF

Right - I've now pondered. It seems that all the 2-port motorised valves I've ever come across were as per Fig 4 in those instructions - and that I've never really bothered to think about exactly what goes on inside them (although I suppose that, had I thought, I would have realised that there had to be some mechanism which prevented the motor running continuously whilst power was applied!).

At first sight, I do not see any the advantage of the "Fig 3" arrangement other than it being slightly simpler (hence probably a little cheaper and possibly a little more reliable/long-lasting) inside - and it does, of course, require an additional core in the cables.

Kind Regards, John

 

[JohnW2]

.... At first sight, I do not see any the advantage of the "Fig 3" arrangement other than it being slightly simpler (hence probably a little cheaper and possibly a little more reliable/long-lasting) inside - and it does, of course, require an additional core in the cables.

Another thought/question ... one of the things which makes the Figure 4 system (the one I know, for SPST switching) 'less simple' is that they use a bridge rectifier to feed a DC coil of the relay which is used with that method - and that rectifier is probably the electrical component most likely to fail, particularly given taht it exists in a thermally-cycling environment.

Why, I wonder, did they not just use a relay with an AC coil, hence removing the need for that rectifier?

Kind Regards, John

 

[SUNRAY]

Sometimes a system may need to have some sort of priority, ie the orangery overrides any other heating requirement, this is easily achieved using the NC contact.

 

[JohnW2]

Sometimes a system may need to have some sort of priority, ie the orangery overrides any other heating requirement, this is easily achieved using the NC contact.

True - but that's a very specific/unusual requirement, and would normally be achieved within a 'control centre' (rather than from a thermostat) - and I doubt that such, in itself, would justify have N/C contacts/terminals on thermostates.

However, as EFLI has demonstrated, we now know the answer. Some (at least some Honeywell) motorised valves apparently do require both N/O and N/C feeds. However, I would imagine that is rare - I've dealt with a good few makes/models of motorised valves over the years, and haven't personally encountered that. It would usually require 5-core+E cable (N/O and N/C from stat, N, 2 cores for auxiliary switch and E) - and the stat would, if mains powered and requiring a neutral, need 4C+E.

Kind Regards, John