Boiler Power Off Sensing Circuit

[WL7JA]

I am trying to build a Sensing circuit for a Bosch on demand Boiler for a Heated floor. Right now I have switched to boiler via relay to the set temp thermostat. So when the
Boiler calls for heat switches the AC Load on and from there I just have an array of 12volt LEDS lights up on the AC Leg. What I want to do is build a simple circuit or not. That when
the boiler is not longer calling for heat it switches power off state (Already Does this) will switch over to say a 9 volt or 12 volt battery and lights say a Red LED and when the boiler
calls for a Heat Green Led Comes on. I haven't looked at the Circuit board on the Bosch 2400ES yet. Although I have been looking a  UPS Circuit board.....But still looking....Any Ideas
would be really helpful....


73's de WL7JA  

[fcb]

Your boiler is likely to have a fan in it, you could tap that (even with a mains rated relay or opto-coupler) to sense the boiler running, else there will be a gas control valve - sense the feed for that. If you want something a little less invasive then you could sense the mains current draw to the boiler, or even the exhaust gas temperature with a therocouple.

[SirNick]

I didn't quite follow your description, but... (disclaimer: I'm not an HVAC expert)

Usually there will be a 24vAC PSU somewhere with one leg of the transformer going through a thermostat switch, the other side of the switch going into the boiler, and the boiler returning to the transformer.

Depending on how integrated your system is, the switch may actually connect to a zone valve instead of the boiler itself.  The zone valve may then have an end-switch that connects to the boiler to tell it when to fire up, or the boiler may sense this based on water temp and/or flow.

Either way, if this is what your system looks like, you may be able to tap into the 24vAC loop with an opto-coupler.  When the thermostat calls for heat, it will close its switch and complete the loop.  Therefore, you could connect an opto-coupler directly on the boiler side of the thermostat switch, with the other side directly to the 24v return.  This will light up the opto when the switch closes.  Keep the load light -- those transformers are usually on rated for an amp or so... give or take half an amp.