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Interfacing a Jeenode to a thermostat

Added by JohnO almost 5 years ago

I would like to influence my heating thermostat using a Jeenode. While exercising this influence is it also important to retain a manual control such that my wife may override it of thing go pear in my absense. I enclose a couple of photographs of the internals of the thermostat as one point to exercise influence.

I think it has one of those difficult dangerous power supplies and looking at the relay, an NEC MR301-24H 8K4 I think things are at 24 volts. There is a thermistor out on the edge and a LM324N quad op amp presumably deciding when to flip the relay. The relay appears to be switching 240VAC.

I would like my Jeenode to know the status of the relay, be able to trip the relay under its control and in an ideal world safely derive the Jeenode power from the 24V.

Should I put it all back together and forget it or is this doable?


Replies (26)

RE: Interfacing a Jeenode to a thermostat - Added by martynj almost 5 years ago

@JohnO,

Not a project for the faint hearted.
What was in the PCB designer’s head when that meander was done for the thermistor??

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

My faint heart hears you @Martyn.

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

@JohnO,

Why bother with the old thermostat, they are so localized anyhow.

I had this problem (few years ago), but my thermostat was already a wireless one, still the base was connected to the boiler like any other install.

I found out my boiler was being driven by a volt free, so I tapped a jeenode v5 between the base and boiler. From this I have various relays that handle ops such as taking the old/existing thermostat on/offline. Once it is offline, my Jeenode (or rather my carambola) has full control of the boiler and the various Zone valves (by way of relays) - it also talks to a master controller (aka HouseMon).

On the back of my old wireless thermostat is a JNu (CR2032) in a custom printed backpack that has only two functions - bypass thermostat (for automated control) or back to manual (aka std wireless control).

TBH - its never been on manual unless I am updating systems.

All functions handled by a ‘derivative’ of HouseMon in combination with various HouseMon PID functions. Still if you take Housemon out of equasion, you can still control heating/HW via this basic Jeenode setup.

:)

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

lightbulb wrote:
> @JohnO,
>
> Why bother with the old thermostat, they are so localized anyhow.
My wife likes it and is clinging on to it.

> I found out my boiler was being driven by a volt free,
I wonder how that works.

>so I tapped a jeenode v5 between the base and boiler. From this I have various relays that handle ops such as taking the old/existing thermostat on/offline. Once it is offline, my Jeenode (or rather my carambola) has full control of the boiler and the various Zone valves (by way of relays) - it also talks to a master controller (aka HouseMon).

Very nice.

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

@Johno,

I did not mean “get rid of it”, as I understand why swmbo needs it. What I mean’t was why bother interfacing with it - leave it in its plastic case and do your interfacing between it and the boiler if possible - like i did with the VF inteface on mine.

Its a case of reviewing your boiler and determining how your thermostat was/is interfacing with it. Then finding a way of taking it out of the loop when you want to automate, and getting it back in the loop as simply as possible when swmbo needs direct control.

I must say, my wife never needs/wants to touch ours now, as she has finer control via various interfaces about the house. She can turn on just the rads in the kitchen, or top up the water until it reaches a specific setpoint etc etc - she can even do this on her phone if needed, but prefers physical buttons.

—lightbulb

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

lightbulb,

Thanks, the wiring to the boiler is straight forward, a switched 240v. I didn’t want anything on the boiler to confuse British Gas and the space is limited and it would need protecting from heat. Then I need to power it or using a latching relay etc. I am being lazy or over thinking it.

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

@JohnO,

If your worried about BG et al, your not going to get much done! - but I do understand you.

Interfacing between your old thermostat and boiler need not be done in the boiler room, after all your thermostat is perhaps wired from somewhere in living space back to the boiler. Essentially your after “mimicking” your thermostat once its OOB. In other word you are electrically isolating your old thermostat feed and replacing it with your own MCU? derived one. Depending upon the communications process, this could be as simple as a relay to take it offline.

—lightbulb

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

My thoughts are to parallel with the existing thermostat and manually turn it to a low value to allow the Jeenode to take over. If it went pear and the Jeenode stuck in the off position then the thermostat could be adjusted to bring back the heating. If the Jeenode stuck in the on position I would have an isolation switch to disable my relay output contacts.

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

@JohnO,

If you have them both running (in control - controllable) then when the setpoint (via jeenode) exceeds the old stat, then the old stat will send its OFF command, this will happen rapidly and probably damage your boiler or at least wear out components - you need take one/or other out of the control loop - this generally involves what I described earlier - a relay or some such to “disconnect” one of the control feeds. If you have an OpenTherm system then maybe you have additional mechanisms, but for a dumb stat, you must really have only one controller in charge at a time.

PS: I know this because I started this whole thing from your initial point of view; and quickly discovered that I was trying to create a bodged/false safety net with attempting to keep existing circuits unchanged.

You know what happens when two trains are on the same length of track with no signalling components in between

—lightbulb

RE: Interfacing a Jeenode to a thermostat - Added by ozzy almost 5 years ago

@johnO by all means go for it. But start out by drawing a circuit diagram, then take it from there.

martynj that meander was done for heat decoupling the thermistor from the rest of the circuit. How good it work’s that may be questionable.

Interfacing a Jeenode to a thermostat - Added by pbadger almost 5 years ago

John0,

You measured 240volts at the relay? I’m in the US so standards may vary considerably, but here, thermostats are almost always low voltage wiring. Switching higher voltages at the thermostat increases the cost (code requirements) of the wiring.

In theory, you could just wire another relay in parallel of the existing relay. (This would be a wired OR).
The relay should have the same rating as the original.

In practice, you would need to think out the following.
How will the logic work with the Tstats logic - It’s easy to call for heat but not to prevent the thermostat from calling for heat.
You could just turn down the Tstat when you wanted to use the Jee-stat.
Then you have all the issues of designing a Tstat.
Safety limits etc, rebooting when power goes out. If you live in a climate where it freezes, any glitch in your system has the possibility of freezing pipes and doing a lot of damage.
Also you may be voiding your home insurance.

Paul Badger
Modern Device

On Apr 20, 2015, at 5:14 AM, redmine@jeelabs.net wrote:

RE: Interfacing a Jeenode to a thermostat - Added by martynj almost 5 years ago

ozzy, Thanks - I missed that; odd if a minimal heat path is required that the GND trace is still so fat !

@JohnO, Hmm, the temptation to tinker is too strong? How many individual wires do you have from heating panel to thermostat? Is it a common grey PVC sheath over three core + bare copper ECC?

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

I am still minded to continue researching alternatives but in the spirit of tinkering here is the backplate. I would happily put in a commercial wireless unit if they would let me know the wireless protocol. But I also dislike the modern units that talk back to an internet server to function.

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

@JohnO,

I’m still a fan of my original proposal(s) - using a relay (mains or LV) to take your old Stat “Out Of Band” whilst your MCU takes over.
If you design this properly, after a powercut your old stat will continue to be master (and your MCU can send an alert of the revert of control - if it was in charge at time).

I see no need to touch your existing stat at all, and only a relay tap in your existing wiring between stat and boiler is required + one other relay for the MCU->boiler control.

—lightbulb

RE: Interfacing a Jeenode to a thermostat - Added by lightbulb almost 5 years ago

JohnO wrote:
> I am still minded to continue researching alternatives but in the spirit of tinkering here is the backplate. I would happily put in a commercial wireless unit if they would let me know the wireless protocol. But I also dislike the modern units that talk back to an internet server to function.

You’ll be needing an OpenTherm compliant device in that case (or a Stat->Bolier protocol thats been hacked/opensourced).

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

pbadger wrote:
> John0,
>
> You measured 240volts at the relay?
Not measured but I think the backplate wiring looks strongly that way. The backplate appears to support a volt free switching but that would mean the wiring and boiler interface would need to change.

> In theory, you could just wire another relay in parallel of the existing relay. (This would be a wired OR).
> The relay should have the same rating as the original.
Yes, you also made me think about sensing the magnetic effect in the existing relay coil.

> In practice, you would need to think out the following.
> How will the logic work with the Tstats logic - It’s easy to call for heat but not to prevent the thermostat from calling for heat.
> You could just turn down the Tstat when you wanted to use the Jee-stat.
That is the approach I was considering. In fact I only want to control between 18 and 22 degrees C. I am happy to allow the old system to deal with frost etc. I don’t feel the need for the Jeenode to prevent the Tstat demanding heat. This would also make it fail soft for my wife - just in case I don’t outlive her.

> Also you may be voiding your home insurance.
Another very good point, this may also apply if I install any home-brew equipment I guess perhaps more tension to a wireless commercial unit.

> Paul Badger
> Modern Device

Thank you for your insight.

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

I also don’t like the Tstats that attempt to learn the heating & cooling profile of the house.

Perhaps a piezoelectric heating and cooling of the thermistor. :-)

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

lightbulb wrote:
> I’m still a fan of my original proposal(s) - using a relay (mains or LV) to take your old Stat “Out Of Band” whilst your MCU takes over.
> If you design this properly, after a powercut your old stat will continue to be master (and your MCU can send an alert of the revert of control - if it was in charge at time).
Thanks lightbulb, I haven’t dismissed the idea, I am stirring the ideas around in my mind.

> I see no need to touch your existing stat at all, and only a relay tap in your existing wiring between stat and boiler is required + one other relay for the MCU->boiler control.

Any thoughts on Opentherm units or hacked wireless devices?

RE: Interfacing a Jeenode to a thermostat - Added by martynj almost 5 years ago

@JohnO,

The existing wiring is typical of pre-Eurosilly colour codes. There is a single, multi-core PVC run back to the control panel on the boiler.

  • RED: Permanent mains Live (as soon as the panel has power, independent of time clocks etc)
  • BLUE: Neutral tie back, permanent, unlikely to be switched anywhere unless the boiler has a double pole isolator (came in around 1995)
  • YELLOW: Demand circuit - energise this to demand heat. Is in series with timeclocks, programmers etc, but eventually turns on the boiler.
  • BARE/GREEN: ECC in the cable - linked to the metal of the backing box, not connected to the thermostat since nothing in there is at earth or can be put at earth potential

This is just enough connectivity to change the ground rules and restrict mains potential to the relay contacts only.

  • RED: Permanent mains Live (as soon as the panel has power, independent of time clocks etc) - used on relay contact ONLY
  • BLUE: DC + (pick some existing nice wall wart supply that is greater than the DC expected by the existing op amp/relay circuit)
  • YELLOW: Demand circuit - energise this to demand heat. Is in series with timeclocks, programmers etc, but eventually turns on the boiler.
  • BARE/GREEN: ECC in the cable - linked to the metal of the backing box, connect to the thermostat as DC -ve, putting all logic traces at ground or close to ground.

  • The existing relay acts as before, connecting Live to the Demand path when energised

  • At the control panel end, lift the blue from neutral and insert the DC supply +ve

  • The treacherous floating cap supply components can be removed (just take the big cap out)

  • The DC feed can be regulated down for whatever JeeNode flavour you are putting inside the thermostat housing.

Since the existing thermostat is permanently powered and is the default controller if the wireless commands are not functioning, the “wireless overide” can be done by injecting a little imbalance into the thermistor/op amp circuit. The WDT can kill this action if wireless comms cease.

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

Thank you for the analysis Martyn, I think the red AC feed to the thermostat is actually switched by the time clock but could be tweaked to be permanently on.

RE: Interfacing a Jeenode to a thermostat - Added by martynj almost 5 years ago

@JohnO,

No need to disturb the existing circuits too much - since the DC feed will be permanent, it doesn’t matter if the RED is switched or not.
This makes the minimal control panel changes as re-assigning the BLUE as DC +ve and connecting the DC ve to GND.
The link between P2/P4 is removed, P4 gets the BLUE and P5 the GND
the rest is then changes on the PCB.

As a first step, these changes can be made without adding the JeeNode - ending up with identical functionality as before.

Looking at the PCB underside, the creep distances between the hot contacts on the relay and the rest of the traces look adequate.

If you wanted to eliminate ALL mains potential in the thermostat box, then RED is no longer used and YELLOW could be switched low voltage back to the control panel, with an isolating DC/AC SPST relay placed there.

You may be wondering why the existing relay is SPDT, but not clearly marked as such on the backplate - this is so that the same thermostat can be used for controlling aircon (traditionally a fairly useless option for the UK market !)

RE: Interfacing a Jeenode to a thermostat - Added by jcw almost 5 years ago

I’m not (yet) convinced it’s 240 VAC. You could try to carefully measure the voltage across that switch symbol in AC mode. Wouldn’t be surprised if it turns out to be 12 or 24 V.

-jcw

RE: Interfacing a Jeenode to a thermostat - Added by JohnO almost 5 years ago

I have measured between 2 & 3 and it is 239VAC with the boiler on and surprisingly to me it reads 74VAC with the boiler off.

RE: Interfacing a Jeenode to a thermostat - Added by jcw almost 5 years ago

Ok, sounds convincing enough :) - the 74V could be anything from leakage to something in series, or a snubber circuit. Measuring an open circuit with a high-impedance multimeter says little about the voltage - that voltage could collapse under the slightest load (but not your fingers!).

RE: Interfacing a Jeenode to a thermostat - Added by martynj almost 5 years ago

Hmm - 2 & 3 ? 2 & 5 surely.

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