Meaning of COM, CALL and STAT

[Alastair]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

[Chris Pink]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

 

The thermostat is "common", "normally closed" and "normally open" but that's not what you asked!

[MoominPapa]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

COM is common, equivalent to CO

CALL is "call for heat" ie the terminal which is connected to COM when the temperature is below the setpoint.

STAT is err dunno, the terminal connected to common when the temperature is above the setpoint.

 

How the last two map to NC "normally closed" and NO "normally open" rather depends on what the normal is defined as, but a bit a experimentation with a multimeter should reveal all.

 

MP.

[johnjo]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

 

Without the diagram I'm not too sure, but at a guess I would say Com = common; CALL = return from the stat; and STAT = feed to the stat.

I'm sure someone will confirm or ridicule this.

John

[Big COL]

COM is common, equivalent to CO

CALL is "call for heat" ie the terminal which is connected to COM when the temperature is below the setpoint.

STAT is err dunno, the terminal connected to common when the temperature is above the setpoint.

 

How the last two map to NC "normally closed" and NO "normally open" rather depends on what the normal is defined as, but a bit a experimentation with a multimeter should reveal all.

 

MP.

 

MP

 

Is stat a typo?

As Co= Common, Call= Call for heat, Sat= Satisfied,

Stat is an abbreviation for thermostat

And depending on the manaufacturer they could also be numbered connections

Edited January 8, 2009 by Big COL

[Smelly]

Ah, where's a plumber when you need one...

[dor]

I seem to recall that the typical thermostat is not a simple NO/NC relay as they have a ballasting/heating resistor in them to introduce hysteresis to stop them switching in and out too quickly.

[Alastair]

Thanks, that helps.

 

I do have another diagram - on the thermostat.

 

CO-NC close on fall

 

CO-NO close on rise.

 

So I guess CO is common/COM. That would connect to the CH programmer/controller HW ON terminal.

 

NC should then be CALL. So the programmer/controller says HW should be on. COM is energised. Cyl temp falls, and NC closes on CO. That energises the boiler and pump.

When the programmer/controller calls for heating, it energises COM for the room therm. On fall, that energises the Y valve, which opens. The Y valve has a switched live, which energises the boiler and pump.

 

So that all seems to make sense. I think.

[twocvbloke]

Ah, where's a plumber when you need one...

 

They've trained for a job in IT and b*ggered off...

[NBMike]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

I would guess you've mis read STAT. Should it be SAT ie (temp) satisfied

Mike

[MoominPapa]

I seem to recall that the typical thermostat is not a simple NO/NC relay as they have a ballasting/heating resistor in them to introduce hysteresis to stop them switching in and out too quickly.

Room thermostats do (or did - most of the are electronic these days) but not pipe and cylinder stats. If the OPs stat is a room thermostat of the old bi-metallic type with a built-in compensating heater then it will probably be rated for 240v and he's likely to be using it on 12V in a boat. If so the compensator will not function effectively but shouldn't cause any problems.

 

MP.

[Alastair]

NBMike is right - I'm misreading SAT as STAT. Blame scruffy paper and eyesight.

 

I'm running all of this on 240V ac. Why on earth would I try doing an installation running 12V? To ensure it was 12V, I'd have to install some electric gizmos and huge cables. No point. On a large boat, 12V is so inefficient, I'm better off using the inverter (that's a guess, btw, I haven't bother calculating it).

[MoominPapa]

I'm running all of this on 240V ac. Why on earth would I try doing an installation running 12V?

Depends what you're doing. The only thermostat in the heating system on my boat controls a Bolin circulation pump which uses 100mA@12V. Doing that with a 240V circulating pump would use a whole lot more power.

 

MP.

[johnjo]

I'm running all of this on 240V ac. Why on earth would I try doing an installation running 12V?

 

Unlike me, a newby, most people on here would most likely assume 12v because this is a BOAT forum and would not be expecting anyone to be installing a 240v central heating system on a boat [i could be wrong]. Perhaps when asking a technical question it would help if ALL relevant detail was given to avoid misunderstandings.

[Alastair]

I'm running all of this on 240V ac. Why on earth would I try doing an installation running 12V?

 

Unlike me, a newby, most people on here would most likely assume 12v because this is a BOAT forum and would not be expecting anyone to be installing a 240v central heating system on a boat [i could be wrong]. Perhaps when asking a technical question it would help if ALL relevant detail was given to avoid misunderstandings.

OK, I can accept that new-ish boat owners would assume 12V. Except that I was talking about Y valves. Can you even get a dc-operated Y valve?

 

12V dc causes lots of problems on a long boats. The voltage drop is one issue, the voltage variation is another. If you look at Gary Peacock's posts about the mk2 Hurricane heater, you'll see that even supposedly designed-for-marine-use products can't cope with dc voltage variations.

 

Even stepping voltages up to 24V doesn't overcome all problems. There is some dc lighting on my boat, but I intend to replace it with 240v ac. 240V lighting is cheaper, safer (fewer concerns with excess currents) and more efficient. Fittings are readily available and in a greater variety.

 

Once you step away from a small narrowboat, you are talking about a house-sized space. Why not use the products designed to cope with such a space? A 12V Bolin is not going to cope with pushing water through 8 radiators.

[chris w]

240V lighting is cheaper, safer (fewer concerns with excess currents) and more efficient. Fittings are readily available and in a greater variety.

1. Cheaper? bulbs for 12v are just as cheap as bulbs for 240v

 

2. Safer? - it's not excess currents, it's excess voltage you need to worry about. It's the volts that give the jolts.

 

3. More efficient? - an incandescent or a halogen or a fluorescent has the same efficiency whether it's designed for 12v or 240v.

 

The big issue with relying solely on 240v is just that - the single point of failure. If cruising, and your inverter blows, you are stuffed.

 

Chris

[chris w]

This is in reference to wiring central heating controls. I *think* I know what they are, but need to be certain.

 

I have a wiring diagram that refers to COM, CALL and STAT, and a thermostat with CO, NC and NO on the terminals.

 

Does anyone know for certain what COM, CALL and STAT are (please no guesses)?

It's CALL, COM and SAT

 

COM is connected to the live output from the timer switch that controls the Domestic Hot Water (DHW)

 

CALL is connected to the ORANGE wire on the mid-position diverter valve (usually ORANGE but sometimes RED). That terminal also connects to the LIVE terminal on the boiler and pump.

 

SAT is connected to the GREY wire on the mid-position diverter valve

 

Chris

[chris w]

I seem to recall that the typical thermostat is not a simple NO/NC relay as they have a ballasting/heating resistor in them to introduce hysteresis to stop them switching in and out too quickly.

Actually, it's the opposite way round. The bimetallic strip inside the room thermostat has too great a hysteresis in normal use, ie: the difference between switch ON and switch OFF, resulting in the room temperature's fluctuating too much.

 

So, inside the thermostat is a small resistor, one end of which is connected to neutral permanently and the other end of which is connected to live when the thermostat is ON (closed). This causes the resistor to heat up very slightly which fools the thermostat into thinking that the room is actually warmer than it really is and so switches the thermostat OFF early. Since the thermostat has switched OFF a little earlier than it would have done without the resistor, the time before it switches ON again (due to room cooling) is reduced and therefore the room is held at a more constant temperature.

 

Chris

Edited January 11, 2009 by chris w

[Alastair]

2. Safer? - it's not excess currents, it's excess voltage you need to worry about. It's the volts that give the jolts.

It is excess current that melts wires and causes fires.

[chris w]

It is excess current that melts wires and causes fires.

Your logic (and knowledge) is severely flawed.

 

Excess currents don't just happen, they are driven by a voltage, like a water pump drives water current. So if a short happens, one will get a ton more current if driven by 240v as opposed to 12v. That's why, in both cases, fuses are absolutely essential.

 

12v systems are inherently much safer than 240v systems for this reason and, of course, the ever present electrocution issues with 240v which don't obtain with 12v systems.

 

Chris

[Dave Hannigan]

It's CALL, COM and SAT

 

COM is connected to the live output from the timer switch that controls the Domestic Hot Water (DHW)

 

CALL is connected to the ORANGE wire on the mid-position diverter valve (usually ORANGE but sometimes RED). That terminal also connects to the LIVE terminal on the boiler and pump.

 

SAT is connected to the GREY wire on the mid-position diverter valve

 

Chris

 

 

Not necessarily the pump if you are fitting a modern boiler with pump overrun. Check with the boiler manufacturers instructions- the pump may be wired directly to the boiler.

 

Dave

[Alastair]

Excess currents don't just happen, they are driven by a voltage, like a water pump drives water current. So if a short happens, one will get a ton more current if driven by 240v as opposed to 12v. That's why, in both cases, fuses are absolutely essential.

 

No

 

Current is not driven by voltage.

 

How much current do you think a car battery can supply?

[Semitrad]

No

 

Current is not driven by voltage.

 

How much current do you think a car battery can supply?

 

 

Not sure if you are just looking for an argument here or are serious Alastair, don't forget that resistance is a very important part of the equation. Would you rather touch a 240V (even dc) supply or 12V?

[Lee J]

Sorry, I realise that this is also off topic but I have installed 230V lighting on Helena, GX53 cfl's, 6 light the whole of the boat (though not the shower, I've used 12v in there for safety), and I have had to replace the 13w lamps for 9w ones because the originals were too bright.

 

Though I do concur, the single point of failure with the inverter is a concern so I have a spare inverter and made the reading lights all 12V

[chris w]

No

 

Current is not driven by voltage.

 

How much current do you think a car battery can supply?

 

Alistair - your local library will have a copy of "Janet and John Learn Electrics - (Part 1 - Basic Level)".

 

Read it... in it you will find an equation named after a certain Mr Ohm.

 

Current cannot flow unless driven by a voltage, anymore than water can flow along a pipe without a pump's pushing it along. The amount of current, driven by a particular voltage, is determined by the circuit resistance.

 

So, for example, if you have a circuit resistance of 10 ohms and a voltage of 12v dc, then 12/10 = 1.2 amps will flow. If you now apply 240v ac rms instead then the current will increase to 240/10 = 24 amps!!!!

 

The power dissipated in the resistance, in the 12v case, will be 14.4 watts. In the 240v case it will be 5.7 KILOwatts !!!!!!!!!!

 

Now do you understand?

 

Chris

Edited January 12, 2009 by chris w