Boiler backplate - Hot water needed for heating

[matburton]

Hi,

I have a pretty old gas boiler system, it's a two channel setup.
Oddly for the central heating to work the hot water must also be on

Recently I've swapped out the mechanical timer with a Drayton Wiser Hub.
So for the first time I now have a thermostat

But that leads to a problem, the Wiser Heat app has no way to specify that heating requires hot water as far as I can see, making the room thermostat quite a bit less useful.

I was wondering if I could solve this problem with a modification to the backplate wiring?

Maybe I could do something like link CH ON to HW ON with a diode?
(Sorry my electronics knowledge is pathetic, I'm a n00b)

 

[oldbutnotdead]

Depends how your system is plumbed. You may have an old-school system- gravity hot water and pumped heating- in which case you can't have heating on without hot water. Clue will be any motorised valves on the system...none= gravity/pumped.
Diodes won't switch ac as your plan, the Wiser doesn't have volt free contacts (as i've just discovered, costing me £5 for some relays).
Your original room stat-2 wires? Bridge it out. Then at the backplane, CH on should connect to the pump & boiler power. HW should connect to boiler direct (assuming gravity). Pics of the original wiring would help...

EDIT
Ah yes. Your original controller will have been mechanically interlocked (so heating wouldn't/couldn't be commanded on unless hot water was on. So i'm wrong above- switched live to boiler comes from HW on, switched live to pump comes from CH on.
2 options.
1 Tell Wiser you want hot water 06.00-23.00. Heating you can then have between those times (controlled by app).
2 (maybe beyond your current skill set) connect HW on to boiler as normal.
Acquire a 240v coil DPDT relay. Coil live goes to CH on. COM1 and COM2 connect to permanent live. NO1 connect to boiler switched live. NO2 connect to pump live.

 

[ianmcd]

As already said sounds like you have Gravity HW, post a pic of your HW Cylinder

 

[matburton]

Thanks for the responses!

Diodes won't switch ac as your plan

Blast! Any other simple options if I can't solve this via wall plate wiring alone?

Your original room stat-2 wires? Bridge it out.

Not a problem as it looks like this system has never had a room thermostat.

at the backplane, CH on should connect to the pump & boiler power. HW should connect to boiler direct

If the heating is turned on without the hot water I can hear 'pump-y' sounds but the boiler doesn't fire-up.

Pics of the original wiring would help...

Here's the wall plate wiring, which I haven't made any changes to.

Your original controller will have been mechanically interlocked

The original controller was an SM2.

Clue will be any motorised valves on the system

There does seem to be a motorised valve, sorry this pic is a bit dark at the bottom.

Tell Wiser you want hot water 06.00-23.00

I was hoping to avoid that, I think that could be fairly expensive in terms of wasted gas.

(maybe beyond your current skill set) connect HW on to boiler as normal.
Acquire a 240v coil DPDT relay. Coil live goes to CH on. COM1 and COM2 connect to permanent live. NO1 connect to boiler switched live. NO2 connect to pump live.

Ah that sounds like wiring that would indeed be beyond my current capability.

As already said sounds like you have Gravity HW, post a pic of your HW Cylinder

Sorry about the mess. There are no valves etc around the cylinder.



Thanks for your help! I realise the system simply needs replacing, but that's something I'd need to save-up for over a few years and I was hoping maybe there was a simple way to tide us over until then.

 

[ianmcd]

When you switched the CH on with the SM2 did the HW slider move accross at the same time ?

 

[matburton]

When you switched the CH on with the SM2 did the HW slider move accross at the same time ?

I admire your optimism!

No, the LED would light, there would be pump-y sounds but the boiler wouldn't do anything.

This was never a problem for the previous owners (or us up to now) due to the lack any thermostat.
(I found a date, the boiler is 36 years old)

Either you moved the pins around together for timing or for manual control you'd just have to know that if you want heating you need to slide both.

This annoyance has only been highlighted now because it's the first time the system has had a thermostat attached I guess.

I wonder if there's any potential that I could smooth this over with software?

 

[ianmcd]

Nope sorry you have gravity HW and you cant use the wiser for that, it may be possible to alter pipework and install another zone valve, not really sure what that one is doing to be honest
The way your system is wired, HW on switches the boiler on
CH on switches the pump on
on HW only the water is gravity circulated
ON CH both are on, the HW switch brings the boiler on
and the CH brings the pump on

I admire your optimism!

Not really there is a little adjustment inside the old programmer that you can set so both sliders will move together if you select CH, the lazy installer has just been two lazy to set it properly

 

[matburton]

Nope sorry you have gravity HW and you cant use the wiser for that

That's fair. The Wiser wasn't expensive (it's used) and is still useful, I mainly bought it for the timer and remote control. The pins on the SM2 are pretty dodgy making it unreliable as a timer.

not really sure what that one is doing to be honest

Now I look at it closely I'm not sure it's wired to anything?

there is a little adjustment inside the old programmer that you can set so both sliders will move together

WHAT! I wish I'd known that! But then again I guess that wouldn't have avoided the problem of the dodgy pins.

I'll instigate whether I can 'supplement' the Wiser' behavior. Maybe there's some IoT approach I can take, or maybe the Wiser itself has an API etc.

 

[ianmcd]

I'll instigate whether I can 'supplement' the Wiser' behavior. Maybe there's some IoT approach I can take, or maybe the Wiser itself has an API etc.

Nothing to do with the wiser, it is a plumbing set up, you could install a Hive Dual Channel, it can be set to Gravity HW

 

[oldbutnotdead]

With the old controller the DHW will probably have been active all day so your gas bill won't be any worse....
Without a relay, you can't have real 2 channel operation. Your choices are;
DHW on constantly (doesn't mean boiler will be running all the time long as your cylinder is well lagged), heating whenever you want.
DHW only heating when CH is on (problem with yr new controller is CH ON will only go live when Wiser thermostat is calling for heat). This option will suck in the summer but work quite well in winter.

 

[matburton]

you could install a Hive Dual Channel, it can be set to Gravity HW

Darn! That would have been easier.

Serves me right for not educating myself better upfront.

Nothing to do with the wiser, it is a plumbing set up

After a bit of experimentation I can detect when the Hub has enabled the heating relay from http://hubhostname/data/domain/ since it contains:

{
  ...
  "HeatingChannel": [
    {
      "id": 1,
      "Name": "Channel-1",
      "RoomIds": [
        1
      ],
      "PercentageDemand": 0,
      "DemandOnOffOutput": "Off",
      "HeatingRelayState": "Off",
      "IsSmartValvePreventingDemand": true
    }
  ],
  ...
}

And I can enable the hot water relay by asking for a boost by doing a PATCH to http://hubhostname/data/domain/HotWater/2 with:

{
  "RequestOverride":{
    "Type":"Manual",
    "DurationMinutes":2,
    "SetPoint":1100
  }
}

So I guess maybe if I just check if HeatingRelayState is "On" every handful of seconds and if it is, but the HotWaterRelayState is "Off", then request an override boost for a few minutes. Something along those lines anyway.

Not nearly as elegant as a properly wired solution, but maybe good enough to make the thermostat useful until I can get the whole system overhauled.

 

[ianmcd]

Love your coding, but here it is in simple language
If you want CH , you need the HW on at the same time .....,,,,,,,////////

 

[matburton]

Love your coding, but here it is in simple language
If you want CH , you need the HW on at the same time .....,,,,,,,////////

He he yeh don't worry, I've known that from before I posted, and that's what the above can (probably) achieve.

 

[ianmcd]

OR be radical and set the HW on times to be on every time the CH is due to be on, it is called gravity HW settings

 

[matburton]

OR be radical and set the HW on times to be on every time the CH is due to be on

That certainly works, it's what we did with the SM2. I just want to make better use of the thermostat and avoid using gas when the temperature is already good