Nest Gen 3 to replace an old Horstmann Potterton controller to a Ideal Mexico 2 Boiler

25 Nov 2021
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United Kingdom
Hi, We have just moved in to an old property with a Ideal Mexico 2 boiler controlled from a Horstmann Potterton controller. The timer function has just stopped working and I thought I could upgrade the potterton controller to a Nest thermostat with heat link. I think the Ideal Mexico 2 is a Gravity fed Hotwater system with No cylinder stat and Pumped Central heating.

I can see the power goes in on the back row of the potterton, but don't understand the what the

Rating (BLK)
5A. (RED)
250v (BLUE)

would get mapped to on the Nest.

Thanks for you help in advance.

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WhatsApp Image 2021-11-25 at 21.36.36 (2).jpeg

WhatsApp Image 2021-11-25 at 21.36.36 (3).jpeg

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WhatsApp Image 2021-11-25 at 21.36.36 (1).jpeg
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The good news, apparently the timer is a vintage collectable! :)

It's a Horstmann Q MK3,
Just visible in your photo:

The not so good news,
Rating (BLK)
5A. (RED)
250v (BLUE)

Literally just means the controller switching has a 'Rating of 5A at 250V' and it's only to be used on AC.

It may help if we can see a photo of how the wiring is connected at the other end!

....or, it may be simpler to just buy the replacement from eBay! ;)
....or, a new boiler! :)

This is also an interesting thread:
Last edited:
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As @RandomGrinch, you are trying to connect modern controls to a very antiquated plumbing system (at least 40 years would be my guess possibly considerably more) so Nest isn't really designed for this old type of plumbing system that you have. i.e. one where the boiler is operated by the HW controls and the pump by CH controls. Obviously for the central heating to work, just switching on the pump won't do anything, so the HW has to be on at the same time to start the boiler.

Fortunately, electrically speaking it is very simple, just two parts to connect to the Nest, the boiler and the pump. When you have identified the wiring for each, there is a method of wiring Nest so that it works on such a system and ensures that the boiler will come on with the pump whenever the central heating is required. I produced a drawing a few years back, but follow it carefully, it's easy to get the 'links' between the terminals wrong.

Nest with Gravity Fed System.jpg

I see that with your boiler the live connections for both the 'pump' LP and the 'boiler' LB are connected here.


LB would go to Nest (5) Hot water common
LP would go to Nest (3) Heating call for heat

You don't have to use T1 & T2 to provide 12v power to the Nest thermostat you can use a separate plug in power supply for it if you wish.

Ideally conversion to a fully pumped system with motorised valve(s) and thermostatic control of the hot water cylinder would be the best way to go, it would save fuel, and allow total individual control of the heating and hot water. This would have to be done if in the future the boiler was replaced, as it is a building regulations requirement and modern boilers don't have the facility to provide natural / gravity circulation of hot water.
Summed up perfectly by @stem above, thank you.

....also as it is a new boiler to you, with a potentially unknown service history - it might be worth a full service, to check that it is still safe and functioning correctly.
The outcome of a service might inform any further choices :)
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Thanks for your help @stem @RandomGrinch I'll open it up this weekend and see if I can wire it up.

I am looking at a new boiler but taking my time as I want to go as green as possible, which is not easy living in a 1960s property that needs a lot of work and insulating... Present thoughts are to go for a Combi Boiler to reduce the need to heat a cylinder of hot water and upgrade to a flow / return setup on the radiators. The Nest has been purchased to go with the new system so thought I could make use of it before it gets installed next year.

+ Will make sure the antique controller gets a new home in a museum :)

Thanks again for your help!!
In the future, if as has been proposed, we are forced to get rid of boilers and install heat pumps, we will all need stored hot water. Heat pumps aren't capable of a high enough output to heat water instantaneously like a combi can.
Hi @stem, That is a really interesting point about the need to store hot water, I am being advised that are property will not work with an Air/water system and everything I read also confirms this, but I want whatever changes I make to be future-proof so my property can be as energy efficient as possible when the technology is available.


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