Thermostats
Cylinder Thermostat (A)
In short
A cylinder thermostat (stat) is strapped to the side of the hot water cylinder. Its terminals: C, 1, 2. Units based on makers’ products sit under it in the parts list, such as the Danfoss ATC and the Drayton HTS3.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
A CYLINDER THERMOSTAT (stat) is strapped to the side of the hot water cylinder. It feels how hot the stored water is, and tells the system when the water needs heating.
Inside is one CHANGEOVER switch. Its common, C, is joined to 1 while the water is cool: it CALLS for heat. C is joined to 2 once the water is hot: it is SATISFIED.
Its dial is its SETTING, from 50 to 80 °C. It comes set at 60 °C, the temperature that kills LEGIONELLA bacteria. A new canvas's cylinder water is 54 °C, so a new thermostat calls at once.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example |
|---|---|---|---|
| C (Common) | The common (C): the hot water DEMAND in, from the programmer's hot water on (HW ON). | Wiring centre block 6 | Wiring centre block 6 |
| 1 (Calling) | The CALL, the normally open (NO) contact: live while the water is cool. On an S-Plan it goes to the hot water valve's motor. On a Y-Plan it goes to wiring centre block 8, which fires the boiler. The 3-port valve's orange is on that block too, so the orange reads live from the call. | Wiring centre block 8 | Wiring centre block 8 |
| 2 (Satisfied) | SATISFIED, the normally closed (NC) contact: live when the water is hot. On a Y-Plan it feeds the 3-port valve's grey. | — | Wiring centre block 7 |
Read from the wired example circuits: S-Plan, Y-Plan. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
The water it feels is the Conditions card's Cylinder water. Cool water: 1 is live. Hot water: 2 is live. Never both.
It calls once the water is more than 2 degrees below its setting, and is satisfied once the water reaches 2 degrees above it. In between, it stays as it was.
That gap of 4 degrees is its DIFFERENTIAL: it stops the thermostat (stat) switching on and off again and again at the setting. Set at 60 °C, it is satisfied at 62 °C, and calls again below 58 °C.
Its own dial is its setting. Moving the dial changes the setting, never the water.
In this simulator the cylinder warms only while the thermostat calls, so it stops just past the setting. So a thermostat linked out cannot overheat the water here. On site it would.
Units based on makers’ products
In the parts list these sit under the Cylinder Thermostat. Each is the simulator’s own unit, simplified for teaching, and may differ from the product it is based on: on site, read the unit’s own instructions. DrawWarm is not part of these makers and is not approved by them.
- Based on the Danfoss ATC
- Based on the Drayton HTS3
- Based on the Honeywell L641A
- Based on the Siemens RAM1T
DrawWarm’s own: Dual Cylinder Stat, Dual Cylinder Stat LCD.
Call-outs with this part on the path
- No hot water: heating fine, cylinder cold
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
- Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
- Heating stops once the cylinder is hot (Y-Plan)
“The heating's fine on its own. With the hot water on as well, the radiators go cold once the cylinder's hot.”
- Hot water only when the heating is on too (Y-Plan)
“We only get hot water when the heating is on as well.”
Where it connects
Lessons that use it
- The Manual, History and the examples
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- A thermostat is a switch
Foundation · Reading a circuit
- The cylinder thermostats, A and B
Foundation · The parts
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: S-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: S-Plan
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: Y-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: Y-Plan
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- Wireless S-Plan
S-Plan
- S-Plan with a frost thermostat
S-Plan
- S-Plan Plus
S-Plan
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
Read the explanation
- Heating on but no hot water: how an engineer approaches it
The radiators are warm, the cylinder is cold. What the working heating already proves, the hot water path to trace, and why you prove a part before changing it.
Practise it in the simulator.
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