Honeywell ST699 to Hive

[frqnck]

Evening Echo the husky!

That's not a good start.

That's how I inherited it, looks messy indeed

Both the HW and CH are currently working -- it is a gravity feed. The system does not have any thermostat. I am not sure which pump and valve are linked (hard to reach).

Looking at the ST699 manual, it would seem it was setup as follow:

ST699-Basic-System-without-thermostat

• frqnck
• 16 Apr 2017

I understand that the COM would no longer be required -- not sure about terminal 8/6.

Can these be rewired to fit the Hive?

 

[Echo the husky]

If it is as the drawing there, it is possible to alter the wiring to fit the hive, then set the hive to gravity mode.

However, I'm concerned that the existing wiring is unsafe and not suitable for continued use...

 

[frqnck]

It looks messy but it is safe. Cables will be replaced by a 5 core flex + new junction.

But as it stands, would rewiring as follow work:
L -> L (W.Brown)
N -> N (W.Blue & G.Blue)
3 -> 4 (G.Brown) CH on
4 -> null
5 -> null
6 -> null
7 -> 1 (G.Brown) HW off
8 -> 3 (R.Brown) HW on

 

[Echo the husky]

Sorry I was mixing this with another thread There are so many poorly wired heating systems on here it is hard to keep up...

That is correct except for the connection to HW off, that terminal should be left empty. When you set it up you need to set the hive to gravity mode or the boiler wont come on when heating is selected.

Also make sure there is no inner insulation visible like there is now, and that the bare earth wires are sleeved with earth sleeving and all the earth wires are properly connected together.

 

[frqnck]

Thank you Echo! Still 5 core so that should be good hopefully. Wil try today.

So I will be rewiring as follow:
L -> L (W.Brown)
N -> N (W.Blue & G.Blue)
3 -> 4 (G.Brown) CH on
4 -> null
5 -> null
6 -> null
7 -> null
8 -> 3 (R.Brown) HW on

The ST699 two modes were set as CH+HW (i.e. CH is also HW) and HW-only. With the Hive and the wiring above, will CH also heat the HW?

I am new to gravity system generally -- very impressed by how efficient and strong it feels. Does it make sense to replace such system by an unvented?

 

[frqnck]

For clarity here is the final drawing plan:

Rewiring-Plan-Final

• frqnck
• 17 Apr 2017

 

[Echo the husky]

Yes that's right (Make no connection to HW off though), with the setup you currently have it will not be possible to have heating without hot water. You mention a valve, where is this connected? If it is plumbed as a C plan with a 28mm 2 port valve in the pipework to the cylinder, it will be possible to have separate heating and hw control. If there is nothing to stop the boiler circulating hot water to the cylinder when it is up to temperature it will not be very efficient, new systems are required to be fully pumped to meet Part L of the building regulations.

 

[frqnck]

I have plugged the Hive in and set as gravity (blue light).

The CH+HW is definitely working.
But I don't think the HW by itself does (does seem to kickstart the boiler).

 

[Echo the husky]

If you have a gravity system the boiler firing is the HW working, the water will circulate up to the cylinder. What with the HW doesn't seem to be working?

 

[frqnck]

In HW-only mode (for some reason) it is not starting the boiler... in CH+HW it is working as expected.

Could the terminals 7 and 8 of the ST699 were doing something extra?

 

[Echo the husky]

The wire in 3 should go straight to the boiler to switch it on.

 

[Echo the husky]

where does it go?

 

[frqnck]

I am not sure at this stage where terminal 3 (on the Hive that is) really goes to... the junction box where all these are connected is hard to reach.

I will try to get to the bottom of this over the next WE. At least we have CH+HW for now.

Maybe 3 needs to be on 1 (hot water off) and hard link 3 to 4 instead? That how the ST699 seems to have been wired.

 

[Echo the husky]

Maybe 3 needs to be on 1 (hot water off) and hard link 3 to 4 instead? That how the ST699 seems to have been wired

No, the old programmer had lots of extra connections because it did not have a gravity mode option. All the extra connections allow the boiler to switch on when hot water is selected, without starting the heating pump. and for the heating pump and boiler run together when heating & hot water is selected.

Unlike Hive, Nest thermostats were not designed with a gravity option, but have separate common terminals that are not linked to live so they can be made to work by wiring in the same way as your old one.

 

[frqnck]

Are you suggesting that the Hive is not suitable with this installation? And that the Nest would be a better fit?
Shame as the Hive was very good otherwise.

BTW, I don't thing there is a valve in the system, just a Grundfos pump. Which as it stands is connected to terminal 3 HW on (instead of 1 - HW off?) but still the issue is to be able to turn the boiler on when in HW-only.