1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Installing and Wiring a Potterton PTT2 Cylinder Thermostat

Discussion in 'Plumbing and Central Heating' started by Philip134, 18 Feb 2013.

  1. Philip134

    Philip134

    Joined:
    4 Jul 2012
    Messages:
    3
    Thanks Received:
    0
    Location:
    Southampton
    Country:
    United Kingdom
    Faced with this problem I found lots of confusing and incomplete information on various sites.

    So this note gives the benefit of my experience in getting this cylinder stat working.

    Most commonly people will be replacing a bi metal mechanical thermostat which sits on the cylinder with this electronic, remotely controlled stat.

    The PTT2 has a much more sensitive response than most bi metal stats. Typically the PTT2 will cut out at the temperature you set and cut back in when the temperature falls by 5 or 6 degrees centigrade. A bi metal stat will need a much bigger temperature drop before it cuts back in.

    So if you set a low temperature (say 45 to 50 degrees) to save fuel then with a bi metal stat the water will run cold before it has time to be reheated. To avoid this problem with a bi metal stat you will need to set a higher temperature (60 degrees plus) and you will use/waste more energy.

    The PTT2 avoids this problem.

    The PTT2 has five connections. This means you will need to run a 2 core and earth wire (Brown, Blue, Earth) plus a three core and earth wire (Red, Blue, Yellow, Earth) to the stat. To distinguish the two blue wires, strip one wire back at each end and sleeve it in red (say).

    The lead from the sensor to the stat can be fed into the stat on the surface or from the rear if the stat is mounted on a 25mm box.

    The sensor can be strapped to the HW cylinder or if the cylinder has the foam moulded to it, cut a hole slightly bigger than the sensor, push the sensor against the cylinder, fill round with foam insulation cut from pipe insulation, push down and seal over with brown parcel tape!

    The PTT2 fitting instructions are at the end of this note.

    Now for the wiring.

    The PTT2 terminals are labelled 1 to 5 (left to right), connections are

    Terminal Symbol Function Wire Connect to
    1 H HW On/Call Blue with sleeve Terminal 8
    2 C HW Off/Satisfied Yellow Terminal 7
    3 L Permanent Live Red Terminal 1
    4 TL HW on Call from Programmer/Timer Brown Terminal 6
    5 N Permanent Neutral Blue Terminal 2

    At the Stat both earth wires should be sleeved and connected to the earth terminal in the 25mm box. In the Wiring Centre both earth wires should be sleeved and connected to Terminal 3.

    The Connect to Terminal Numbers are those in the standard Y plan Wiring Centre with a mid position valve. These are shown in the diagram below.

    If you have a different Wiring Centre set up, then you can deduce the correct terminals by choosing

    Blue with sleeve (H) connects to the terminal which connects via the orange (usually) wire to the mid position valve.

    Yellow (C) connects to the terminal which connects via the grey (usually) wire to the mid position valve.

    Red (L) connects to the terminal which has a wire running to the live on the fused connection unit supplying power to the boiler.

    Brown (TL) connects to the terminal which has a wire connecting to the HW Call terminal on the programmer/timer.

    Blue (N) connects to the terminal which has a wire running to the neutral on the fused connection unit supplying power to the boiler.

    How it works

    If timer is switched on for water heating then if the temperature not satisfied the water will heat (green light on Stat) until it is hot enough (red light on Stat). If the water temperature falls (say through use) cycle will repeat.

    If timer is not switched on for water heating, no lights will show.

    If timer is switched on for water heating the boost button has no effect – as it should because either the water is hot enough or it is already heating!

    If timer is not switched on for water heating and you push the boost button, yellow and green lights will show and a tank of water will be heated at which point the red light will show.


     
  2. petertheplumber12

    petertheplumber12

    Joined:
    5 Dec 2012
    Messages:
    694
    Thanks Received:
    96
    Location:
    Surrey
    Country:
    United Kingdom
    Not many people set a bimetal stat at 45-50C it is usually 55-60C Bugs get bred if too low.
     
  3. Sponsored Links
  4. Philip134

    Philip134

    Joined:
    4 Jul 2012
    Messages:
    3
    Thanks Received:
    0
    Location:
    Southampton
    Country:
    United Kingdom
    Yes - UK recommendations particularly for non domestic buildings are for 60 degrees.

    However in North America (and perhaps elsewhere) the recommended temperature is 49 degrees. Regulations state

    WHILE THERE IS A VERY SLIGHT RISK OF PROMOTING LEGIONELLAE BACTERIA WHEN HOT WATER TANKS ARE MAINTAINED AT 49 (120 F) DEGREES, THIS LEVEL IS STILL CONSIDERED SAFE FOR THE MAJORITY OF THE POPULATION. IF YOU HAVE A SUPPRESSED IMMUNE SYSTEM OR CHRONIC RESPIRATORY DISEASE, YOU MAY CONSIDER KEEPING YOUR HOT WATER TANK AT 60 (140 F) DEGREES. HOWEVER, THIS HIGH TEMPERATURE SIGNIFICANTLY INCREASES THE RISK OF SCALDING. (Sorry regulations are printed in UC).

    Reducing the temperature will save about 8% on your water heating bills.
     
  5. Agile

    Agile

    Joined:
    26 Jun 2004
    Messages:
    63,583
    Thanks Received:
    4,520
    Location:
    London
    Country:
    United Kingdom
    How long has Southampton been in the US ?
     
  6. Sponsored Links
  7. Philip134

    Philip134

    Joined:
    4 Jul 2012
    Messages:
    3
    Thanks Received:
    0
    Location:
    Southampton
    Country:
    United Kingdom
    Here is an update which explains how to incorporate a changeover relay so as to ensure that the thermostat operates as intended with a 'standard' mid-position valve.

    Installing a Potterton PTT2 Cylinder Thermostat


    Faced with this problem I found lots of confusing and incomplete information on various sites.

    So this note gives the benefit of my experience in getting this cylinder stat working.

    Most commonly people will be replacing a bi metal mechanical thermostat which sits on the cylinder with this electronic, remotely controlled stat.

    The PTT2 has a much more sensitive response than most bi metal stats. Typically the PTT2 will cut out at the temperature you set and cut back in when the temperature falls by 5 or 6 degrees centigrade. A bi metal stat will need a much bigger temperature drop before it cuts back in.

    So if you set a low temperature (say 45 to 50 degrees) to save fuel then with a bi metal stat the water will run cold before it has time to be reheated. To avoid this problem with a bi metal stat you will need to set a higher temperature (60 degrees plus) and you will use/waste more energy.

    The PTT2 avoids this problem.

    The PTT2 has five connections. This means you will need to run a 2 core and earth wire (Brown, Blue, Earth) plus a three core and earth wire (Red, Blue, Yellow, Earth) to the stat. To distinguish the two blue wires, strip one wire back at each end and sleeve it in red (say).

    The lead from the sensor to the stat can be fed into the stat on the surface or from the rear if the stat is mounted on a 25mm box.

    The sensor can be strapped to the HW cylinder or if the cylinder has the foam moulded to it, cut a hole slightly bigger than the sensor, push the sensor against the cylinder, fill round with foam insulation cut from pipe insulation, push down and seal over with brown parcel tape!

    The PTT2 fitting instructions are at the end of this note.

    Now for the wiring.

    The PTT2 terminals are labelled 1 to 5 (left to right), connections are

    Terminal Symbol Function Wire Connect to
    1 H HW On/Call Blue with sleeve Terminal 8
    2 C HW Off/Satisfied Yellow Terminal 7
    3 L Permanent Live Red Terminal 1
    4 TL HW on Call from Programmer/Timer Brown Terminal 6
    5 N Permanent Neutral Blue Terminal 2

    At the Stat both earth wires should be sleeved and connected to the earth terminal in the 25mm box. In the Wiring Centre both earth wires should be sleeved and connected to Terminal 3.

    The Connect to Terminal Numbers are those in the standard Y plan Wiring Centre with a mid position valve. These are shown in the diagram below.

    If you have a different Wiring Centre set up, then you can deduce the correct terminals by choosing

    Blue with sleeve (H) connects to the terminal which connects via the orange (usually) wire to the mid position valve.

    Yellow (C) connects to the terminal which connects via the grey (usually) wire to the mid position valve.

    Red (L) connects to the terminal which has a wire running to the live on the fused connection unit supplying power to the boiler.

    Brown (TL) connects to the terminal which has a wire connecting to the HW Call terminal on the programmer/timer.

    Blue (N) connects to the terminal which has a wire running to the neutral on the fused connection unit supplying power to the boiler.

    Now for the Unfortunate Twist

    The thermostat will not create the correct switching through the mid-position valve when the boost button is pushed.

    This is hinted at in a review on heating controlsonline.co.uk which says ‘Not suitable for mid position valve without adding a changeover relay’.

    If the system has CHon and HWoff then the valve is ‘fully over’ and the grey valve wire is +ve.

    If the boost button is pushed then the boiler will fire but the +ve grey wire will ensure that the valve cannot return to its mid position. No HW will be heated, there will only be CH heat.

    This is not the result we require.

    Pushing the boost needs to switch off the +ve grey wire, allowing the valve to return to its mid position and provide both HW and CH.

    If the system has CHoff and HWoff then the valve is ‘fully rewound’ and the valve has no +ve connection.

    If the boost button is pushed then the boiler will fire and the valve will stay in its ‘fully rewound’ position because it is not receiving any power. HW will be heated, but (as expected) there will be no CH heat.

    This is the result we require.

    Using a Relay

    The relay needs to achieve the required result: Pushing the boost needs to switch off the +ve grey wire.

    An A4810 like this will do the trick:

    http://www.amazon.co.uk/MGR-1-A4810-Single-Phase-Solid/dp/B00WJD4B2E

    This is widely available for about £7.

    The required connections are:

    PTT2 Terminal C to A4810 control terminal, A4810 control terminal to Terminal 7 in the standard Y plan Wiring Centre. This is AC so the A4810 terminals can be used either way round.

    Programmer HWoff Terminal to A4810 output terminal Terminal 7 in the standard Y plan Wiring Centre to A4810 output terminal. This is AC so the A4810 terminals can be used either way round.

    How it works

    If the programmer is switched on for water heating then if the temperature not satisfied the water will heat (green light on Stat) until it is hot enough (red light on Stat). If the water temperature falls (say through use) cycle will repeat (while the programmer is calling for HW).

    If the programmer is not switched on for water heating, no lights will show.

    If the programmer is switched on for water heating the boost button has no effect – as it should because either the water is hot enough or it is already heating!

    If the programmer is not switched on for water heating and you push the boost button, yellow light (on the boost button) and green light will show and a tank of water will be heated at which point the red light only will show.
     
  8. DIYnot Local

    DIYnot Local

    Joined:
    3 Sep 2019
    Country:
    United Kingdom

    If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

    Select the supplier or trade you require, enter your location to begin your search.


    Are you a trade or supplier? You can create your listing free at DIYnot Local

     
Loading...

Share This Page