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PostPosted: Sat Mar 24, 2018 10:33 am 
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New thermostat fitted and I note it has N/O and N/C contacts, and the heating or cooling is controlled by moving a jumper so be it used for heating or cooling one would still use the N/O contact. So why have a N/C contact? Putting the jumper in the wrong setting would allow one to use the N/C so if battery fails it would fail on rather than fail off.

Only a single relay so can't do heating and cooling, it is either one or the other not both. So why is the N/C contact provided? Is it so if the batteries fail the heating stays on? Since my boiler is not modulating and I don't have TRV on all radiators for me not an option, but just wonder if on modern boilers the N/C contact is used?


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PostPosted: Sat Mar 24, 2018 6:02 pm 
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Simple, the addition of a relay with no/nc close contacts gives the device additional functionality at probably zero cost to the manufacturer as getting a relay with 'just' a simple set of contacts isn't actually that easy.

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PostPosted: Sat Mar 24, 2018 6:58 pm 
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I can't remember why, but I have a frost stat that actually works the other way round. (It uses C and NC) I tried using a normal stat and it just would not work. (I will no doubt remember why at some obscure hour in the morning) It's for my shed, it controls a tubular heater to just "take the chill off" the shed inside.

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PostPosted: Sun Mar 25, 2018 2:51 am 
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I also asked on another forum, it seems some motorised valves need to be powered both open and closed, also if heating and cooling supplying the cooling thermostat from the heating N/C contact means both can't run together, so now added reason clearly with frost thermostat failing safe is failing on not off, if batteries go flat don't want pipes to freeze.

However for years I have remembered which way around to connect a thermostat as N/C means cool and N/O means heat that is now clearly wrong, all the N/C and N/O means is what will happen if battery fails.


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PostPosted: Sun Mar 25, 2018 9:04 am 
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..............another reason not to use stats that have batteries.

I have said before (in a post on this forum) I was cold more than once because of battery failure in a stat, and as I said then, who looks at a stat that is working to see if it needs new batteries, no one does because its working.

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PostPosted: Sun Mar 25, 2018 2:29 pm 
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someone-else wrote:
..............another reason not to use stats that have batteries.

I have said before (in a post on this forum) I was cold more than once because of battery failure in a stat, and as I said then, who looks at a stat that is working to see if it needs new batteries, no one does because its working.

I have to agree, so with my late mothers house there are two thermostats wired in parallel, so both need to fail before central heating fails, however at home with a Horstmann DRT2 I renewed batteries at start of each Winter, they should last 2 years, but I never tried to span 2 years after the first time they failed.

The Horstmann DRT2 Image image shows both DRT1 and DRT2 it seems has been superseded by the Flowmasta 22199SX (2659G) and the Celect plus Horstmann new DRT2 Imageseem from pictures to be identical other than the name on the front. The iQE battery powered, 7 day programmable room thermostat was first I tried to buy but that also seems to have been discontinued, so in essence new thermostat is same as old one, the old one has been in some 20 years so no real complaint.

The problem with the old unit was the batteries became too weak to work relay before the battery warning light comes on, so no amount of checking would stop the unit failing due to discharged batteries.

However today it seems we have to rely on battery power, my son looked for hard wired TRV heads and failed, they all seem to be wifi and battery powered, however mine did show the battery state on the PC when accessing the system.

But the main reason for the post is to alert people N/C and N/O means simply normally closed and normally open, it does not mean cooling and heating. Before buying this new thermostat I would have told any one asking N/C is for cooling and I remember which way around as C is for cooling. However this is not the case.

The main problem as I see it is if you loose the instructions there seems to be no on line version except the one I have now put on my own website. So as an electrician we get called to a fault and we have to guess. The default setting is no delay on heat, 12 hour clock, and N/O for heating, but other than the very small label on the circuit board which says heat cool Image there is very little to show us how to set it, and there is another circuit board on top of the one with jumpers so very easy to miss, sorry not a good photo used my phone not proper camera.


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