I have a central heating unit (a pellet stove) for a two floor house. I’d like to control it independently from two thermostats.
The stove at the moment is controlled by a single thermostat input and it turns on and off a circulator pump with an internal logic board. Circulator can be on both depending on thermostat input and on internal conditions, in particular if the heater overheats to dissipate some heat by sending water into the radiators.
Now I’d like to add two electro-valves. In the simplest scenario they would be controlled by two independent thermostats that go in OR into the thermostat input of the heater logic board. This way though I lose the safety mechanism. With valves closed the heater cannot dissipate.
Logic would be roughly like this:
- each valve opens if their zone thermostat is on
- one or both valves must open if both the thermostats are off and the circulator is on
- both thermostats in parallel go to the heater input
I believe I need some relay logic but I’m not an engineer and I’m a bit out of my comfort zone. Would you please validate my reasoning?
Let’s give some names:
A zone A thermostat B zone B thermostat C circulator output from the heater V electro-valve T thermostat input to the heater
I’d say that I should have a truth table like:
A B C V1 V2 T 0 0 0 0 0 0 0 0 1 1 1 0 0 1 0 0 1 1 0 1 1 0 1 1 1 0 0 1 0 1 1 0 1 1 0 1 1 1 0 1 1 1 1 1 1 1 1 1
That should lead to:
T = A or B V1 = A or ((A nor B) and C) V2 = B or ((B nor A) and C)
Which simplified should be:
T = A or B V1 = A or (not B and C) V2 = B or (not A and C)
Now my problem, assuming my reasoning is correct. How do I turn this into a ladder diagram and more importantly connection schematic? what kind of relays do I need? (everything is on 230V AC, if it matters)
Here’s a tentative ladder diagram, as I said it’s not my field so it probably sucks 😀