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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)

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

tentative ladder diagram

filippo
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    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? – Transistor Oct 17 '20 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 – filippo Oct 17 '20 at 15:30
  • @Transistor fixed thank you, there was a typo in the question! – filippo Oct 17 '20 at 15:31
  • I’m voting to close this question because it should be migrated to DIY,SE – Elliot Alderson Oct 17 '20 at 15:46
  • @ElliotAlderson can this be done automatically? (didn't even know about that community) – filippo Oct 17 '20 at 15:48
  • Someone more powerful than I am might be able to migrate your question. The other option is for you to copy the question to DIY.SE and then delete the question here. Having the same question in two SE sites is considered bad form. – Elliot Alderson Oct 17 '20 at 15:56
  • Draw a truth table to check your logic. Headings `A B T C V1 V2`. Then the logic will be easy. I think we can answer it here. It's electrical design. – Transistor Oct 17 '20 at 15:58
  • @Transistor already did that, that's how I came up with the logic expression, will add it to the question – filippo Oct 17 '20 at 16:01
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    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. – Transistor Oct 17 '20 at 16:17
  • @Transistor thank you for the answer, and for the good laugh – filippo Oct 17 '20 at 16:28
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    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. – Transistor Oct 17 '20 at 16:42

1 Answers1

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Your logic looks fine.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. OP's logic redrawn using changeover contacts. 2-pole relays may be easier to source.

This does exactly the same as your logic but is more difficult to read.

Transistor
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