Potterton PTT2 Equivalent Replacement

[JHS88]

Hi Guys

**Before I start, please feel free to reply as if I'm 5 years old, I'm a new homeowner and know next to nothing about plumbing and heating**

Hoping someone can help, after a quick google it seems a few questions in the past have been asked about this product.

To summarise what could otherwise be quite a long story:
- I have a unit in the kitchen near the boiler than controls hot water and central heating.
- Hot water was set to come on, on a timer, twice a day, central heating was off.
- Hot water stopped working all of a sudden.
- Was told it was the motorised valve near the hot water tank (upstairs) that was broken, so replaced it (Myson ACT 322 Mid Position Actuator)
- Fixed nothing, as I can now only get hot water by switching the central heating on, the actuator isn't being told what to do by whatever needs to tell it what to do.
- Took the Myson valve off, as it was getting ridiculously hot.
- I'm now being told it could be the hot water thermostat (Potterton PTT2) that is broken, and replacing it will help fix the actuator valve too.
- Potterton have told me product is obsolete and they never made a direct replacement. They also told me best course of action is to 'get a new boiler system' - the boiler itself was replaced just 3 years ago, and doesnt seem to have any issues.

I'm now having to manually switch the actuator valve myself when i want to flick between hot water and/or central heating, and always have to have the heating thermostat (located downstairs) switched on to get any hot water at all. Not ideal.

So two questions:
- Is replacing the PTT2 the right course of action?
- If so, do I replace it with a cyclinder stat with a bi metallic sensor (I believe that is the type the PTT2 was).

Any help would be appreciated, happy to give more info if what I've said doesn't cover it.

 

[stem]

Personally I think it unlikely to be the motorised valve, as they tend to fail in the hot water position, so what is happening here is probably that the cylinder thermostat is not switching the boiler on, but when the heating comes on, that starts up the boiler and allows both to heat up.

The best way to fault find is for someone that has a comprehensive knowledge of a Y Plan installation and how it all works to fault find with a multimeter by checking the correct voltages are present at the right places with the system in various states. So not really a DIY job.

However if you are electrically competent, then there is a basic test you can do. Unfortunately I wouldn't recommend a "5 year old" try it.

First the system needs to be in a functioning state, so back as it was. (Three port valves do get hot due to the hot water that flows through them, and even heat from their motor which under certain circumstances will be permanently energised. That's normal)

If you want to test the PTT2 and are competent to do this safely, remove the wires from terminals TL(4) and H(1) and connect them together (note where they come from first so that you can put them back) If you switch on only the hot water at the programmer, [make sure that the boost is not energised] and the hot water comes on now and it didn't before, the cylinder thermostat is faulty. [Caution. If the hot water does come on now, there will be no thermostatic control so it will get very hot if left on]

The PTT2 is slightly unusual in that the control dial and wiring terminals are mounted remotely from the cylinder and it has a separate remote temperature sensor.

If it is faulty, the easiest way to proceed is to replace it with like for like, but you may struggle to find one.

If you want to use an alternative replacement, such as strap on cylinder thermostat, (eg Honeywell L641A, or Drayton HTS3) you can do, but you would need to replace the present PTT2 with a terminal box having a flex outlet and then run a heat resistant cable to the new thermostat. Most cylinder thermostats including the examples above, would only need to use the following wires. The instructions will confirm if this is the case or not.

Timer Live (TL) from programmer - presently connected to TL(4)
Call for heat - presently connected to H(1)
Satisfied - presently connected to C(2)
Plus earth if required by the new stat

The permanent 230 VAC supply to the PTT2 (N & L to terminals 3 & 5) would not be required as this is used to power the device and provide the boost function when the programmer is off. So these wires should simply be terminated and made safe.