How to install a Hive Thermostat to a Vaillant 938 Combi Boiler

14 Feb 2017
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United Kingdom
Hi, I am a new member on this forum. Not sure if this query should be in the Plumbing or Electrical section. I guess it’s relevant to both.

I had bought a Hive Thermostat from Ebay about a year ago. As I was still renovating my house, I was not yet ready for it to be installed, hence why I opted for the package without installation.

I have recently bought a Vaillant 938 Combi Boiler and wanted to install the Hive system. I don’t have a thermostat already installed, so this would need to be from scratch.

I would be grateful if someone could advise me on how to wire this up.

Below are pictures of my boiler's electrical compartment and the Hive installation instructions:

IMG_3683.JPG IMG_3684.JPG
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Live and neutral you would normally piggy back those from the boiler. Where your white main cable goes to that aqua coloured plug, L & N... easy..

The white 24v Rt plug on your boiler you remove that red link wire, get some 2 core or 3 core heat flex wire, and wire them into the hive 2 & 3, it's normally doesn't matter which wire you put in 2 & 3 from the 24v Rt plug. ( I use nest and it didn't matter with that ) that should be everything done. It's just down to you to set the hive up at the router and connect to the display.
Oh and I better say make sure all power to the boiler is off before you start wiring,
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Got it. Thanks. However, I have noticed that the Hive version that I have is for Hot Water and Central Heating, which is not really meant to be used for a combi. I have heard from others that have also made the same mistake but it should still work with a combi. My question now is, how would the wiring change?

Below is the difference between the two units.
With the 2 channel Hive the switch wires are 230 volts.
Connect live neutral and Earth as advised and connect terminal 4 to the lilac coloured plug marked 230 volt RT.
Remove the link in 24v RT.
If you have an integral clock you will need to set it to constant.
Job done.
Ok, great. Thanks for that. I'll give it a try. Hopefully I won't fry the motherboard in the boiler.
I would like to do the same install with a NEST. Would the L, N, E piggy back off the 230v, and the call for heat into the 24v?
I would like to do the same install with a NEST. Would the L, N, E piggy back off the 230v, and the call for heat into the 24v?

The Heatlink doesn't really need an Earth so at least Live & Neutral from the same mains feed as the boiler.
Then 2 wires from the Heatlink terminals 2 & 3 can go to the two 24V RT connectors (remove the red wire link across these two in the boiler if it's still connected).

(It can also be wired to trigger the 230V RT if you would prefer it but be careful and ask again if you want to go down that route. The most important thing is to avoid mains voltages getting onto the 24V boiler connectors which would be expensive)
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If I have wired give from same spur as boiler will this then put mains voltage on boiler?
If I have wired give from same spur as boiler will this then put mains voltage on boiler?

Sorry, I don't understand what you are asking, but the bottom line is:

The single channel Hive receiver is designed for use with combi boilers and is provided with voltage free contacts. This means that it provides a simple on / off switch (Terminals 1 & 3) that can be wired to switch either 24v or 230v as required by the boiler. This switch is totally separate and isolated from the 230v mains supply (Terminals N & L) that is required to operate the receiver's electronics.

However, the dual channel Hive receiver is designed for use with systems with separate stored hot water systems (ie a hot water cylinder) here, the Hive is not designed to be connected to the boiler, but instead to motorised valve/s that operate at 230v mains. So, the switch in this version of the Hive is wired internally to the mains live supply and can only provide a 230v live output. Whilst it is possible to use the dual channel with a combi that has 230v switching, this version must not be connected to anything that should only have 24v control.

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