# two independent zones from central heating unit

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


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)

UPDATE: Here's a tentative ladder diagram, as I said it's not my field so it probably sucks :-D

• Just a safety question: are you 100% certain that this is fail-safe? You need to be very confident that there is pressure relief on the system should any of the relays stick / weld / fail to energise / fail to release / power fail / fuse fail / electro-valve coil failure / electro-valve sticks open / etc. Why must both valves open if the stats are off and circulator is off? Commented Oct 17, 2020 at 15:25
• about fail-safe, most contractors I talked to undermined the issue and wanted to just put two valves with two thermostats and no fail-safe, so in my idea this was already a lot better, but I'm definitely open to suggestions. About your question: they shouldn't, at least according to my truth tables Commented Oct 17, 2020 at 15:30
• @Transistor fixed thank you, there was a typo in the question! Commented Oct 17, 2020 at 15:31
• Guys, this question is electrical and logic. The fact that the application is home heating does not automatically make it a DIY question. I think it's on-topic here and the OP has done an excellent job in framing the question (migrate to photography?), drawing the truth table (a table! - migrate to carpentry), working out the boolean logic (SuperUser?) and creating the ladder diagram (OK - ladder may be DIY). +1. Commented Oct 17, 2020 at 16:17
• No problem. Wait a day or two to encourage other answers which may give you some other insights and then accept the best to mark it answered. Commented Oct 17, 2020 at 16:42