Smart Replacement for Central Heating Thermostat

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#3228708 10-May-2024 12:46

I'm leaning towards a smart relay and control it with automation and a temperature sensor.. and replace the wall thermostat with an override switch. But assuming this thing is wired like a no-neutral light switch I'm limited to no-neural options like a shelly 1L (no longer available) or perhaps a Shelly Dimmer.

I really need to confirm how the existing unit is wired but to do that I have to get the thermostat off it's wall mounting plate to see... and it's not clear how to get it off. It might pop off, but is resisting and I don't want to break it. There's a screw under the dial but it looks like a set screw. I wish I could find some documentation for this thing...

johno1234

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#3228812 10-May-2024 17:35

FYI - fernwood.nz have Shelly 1L in stock. These are no longer available from Shelly and hard to find!

Handle9

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#3228814 10-May-2024 17:38

I’m not sure why you can’t get a neutral? Won’t you mount the control relay at the furnace where there would be a phase and neutral.

You’d switch phase onto the furnace contact.

johno1234

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#3228927 11-May-2024 08:29

Hmm. Maybe I was too focused on the thermostat itself. There is power and a manual override switch at the furnace end.

davidcole

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#3228933 11-May-2024 08:55

johno1234: Hmm. Maybe I was too focused on the thermostat itself. There is power and a manual override switch at the furnace end.

That where my relay is. I have a control board for my radiator boiler. It had a dumb timer (on at x off at y - regardless of temp) and a manual override. The relay was out on the automatic circuit to replace the timer.

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johno1234

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#3229314 12-May-2024 16:33

davidcole:
johno1234: Hmm. Maybe I was too focused on the thermostat itself. There is power and a manual override switch at the furnace end.

That where my relay is. I have a control board for my radiator boiler. It had a dumb timer (on at x off at y - regardless of temp) and a manual override. The relay was out on the automatic circuit to replace the timer.

OK fought my way though the cobwebs in the basement and checked the furnace... it is plugged into a 3 pin power socket... so could just use a smart plug and set the old thermometer to something warm so it stays on. Then use an automation and smart thermometer to turn the power plug on when the time of day and temperature says so.

Does that sound like a sensible idea?

The furnace doesn't appear to use much electrical power for the igniter and fan.

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#3229329 12-May-2024 17:20

Yeah exactly. We didn’t have an existing thermostat. So I didn’t have that problem. But I’ve done the same. Control it at the boiled/furnace level. And use a temperature sensor somewhere else. Mine is actually the average of 4.

If Using a power plug. I guess check the rating

Handle9

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#3229330 12-May-2024 17:25

I wouldn’t use a power plug, it likely isn’t rated for the number of switches you will use for temperature control. Smart plugs are designed to be switched a few times a day, not a several times an hour.

Regardless of how you switch it make sure you put in minimum run and off timers and have at least 2 degrees of hysteresis. You really need to avoid rapid on and off switching.

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#3229332 12-May-2024 17:28

Why would it be switching many times an hour?

johno1234

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#3229343 12-May-2024 18:18

I guess it all depends on how fast the space heats up on heating and how fast it cools down when the heater is off. It’s all logged so will soon see. By building in hysteresis the switching interval will hopefully be extended.

As to rating it appears that the power consumption is only a few hundred watts. That’s measured too so will confirm. The smart plug claims to be rated 10A but should only be passing through a fraction of that in this application. It’s only driving the fan most of the time

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#3229346 12-May-2024 18:33

davidcole: Why would it be switching many times an hour?

If the furnace is properly sized it won’t run continuously.

If the space starts at 16 degrees with a set point of 22 degrees what happens is the unit runs continuously until the space hits 23 degrees (1K above set point). Then the unit switches off until the space hits 21 degrees (1K below Setpoint). That will happen 2-3 times and hour in a typical space to maintain comfort.

If the house is really efficient or the unit is under or oversized it could be more or less frequent which is why you need anticycle timers. Furnaces and compressors (for AC) like to run for a decent period of time and then be off for a bit. Short cycling causes all sorts of problems.

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#3229349 12-May-2024 18:44

I guess I’m thinking of radiators which stay on longer. As each radiator has its own thermostat

Handle9

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#3229351 12-May-2024 18:46

davidcole: I guess I’m thinking of radiators which stay on longer. As each radiator has its own thermostat

Radiators usually have individual thermostatic valves. They are a bit different.

johno1234

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#3229701 13-May-2024 11:44

Ugh, the smart plug idea probably no good when I think about it. When the thermostat switches off the heat, the furnace stops the heater but keeps the fan going for a while to cool it down. If I just cut the power it won't do the cool-down phase.

Back to the smart relay on the thermostat circuit...

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#3229799 13-May-2024 14:29

I have a Shelly 1L v2 here not required if you're after one. Is used, but worked fine, just replaced with a Shelly 2.5 to get an additional circuit controlled.

Let me know if interested

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