I think most (if not all) low voltage (i.e. 24 volt) thermostats have a dry contact, but not for 220Vac. Also most are limited to about 1 amp.
I doubt you will find what you described as a 220VAC contact carrying any significant current will require a significant current of its operating coil.
I would find a thermostat I liked and connect its output to a 24 volt relay whose contacts are capable of controlling your water heater.
OR if you are not comfortable with the above,
Get a Hubitat capable thermostat and a Hubitat capable 220Vac control and use RM to control your water heater.
If this is in the US (and maybe Canada?) the voltage of each line (w/ respect to neutral/ground) is 120V. The two 120V lines are out of phase with each other and therefor the voltage (line-line) is 240V. If you're not in the US, then please disregard.
A If you pair a Zooz ZEN16 or 17 with your desired thermostat, you should be able to meet these requirements. The thermostat can be wired to the switch terminals of the ZEN16, and the device can be configured to directly control the relay output based on the switch state. The relay contacts should be able to handle the control voltage necessary, but please confirm before installing. I could not see the voltage for the control circuit from the diagram, only the current (3A). Also, this will continue to operate without Hubitat. The thermostat will be able to turn the heat on and off as needed. This essentially turns the ZEN16 into a "dumb" relay device. Perhaps this is an acceptable compromise?
Here in Europe we have effective 230/400V in the households depending on whether you have 1 or 3 phases. I have 3, so I have 230V between one phase and the neutral and I have 400V between any two phases.
I have a Qubino Flush 1D relay, that is a Z-wave dry-contact relay, officially supported by hubitat. I could buy a Heatit Z-Temp2 thermostat (quite affordable and neat). In this way I have a battery operated z-wave thermostat and a relay and I can configure them in HE, but I want to have a system that is operable without HE ( to replace the thermostat I use now). So I'm looking for a single device. Without luck
I'm thinking the same after failing to find a device that match.
Here in Hungary we traditionally used thermostats that switched the gas furnace directly. 20 years ago, these thermostats (mostly mechanical, later electronic) switched the 230V power supply of the furnace. That was 2-3 amps most. Later, 15 years ago, they still switched 230V AC, but that was a control signal, not the real power supply. They choose to use the mains, because this way the voltage drop was not significant, you had 20-40 meter long cables, using low DC voltage on those long lines were problematic. So I think they went the easy way. So you don't really need heavy duty relays. My programmable thermostat has a max 8 Amp relay (2A inductive), that switches the furnace, and it keeps operating on a pair of quality AA batteries for 1,5 years.
Today's modern Gas furnaces has both low (24V) and high voltage (mains) control inputs. Mine has one low voltage control input that goes to the master bedroom, it is a thermostat and the main control interface of the furnace controlling the high temp circuit (55C water, radiator heating of the 4 bedrooms) and two high voltage inputs of which one is used for the low temp circuit (40C water, underfloor heating of the rest of the house. I wanted to replace the latter.
I could leave the thermostat on the wall and use the second mains input for a z-wave relay and use Heatit Z-Temp2 thermostat somewhere else. I can program the furnace to use the 2 mains input with an OR operator for switching ON the low temp circuit, so I can switch on the heating with Z-wave thermostat or remotely from HE, but I can not switch it off if the "dumb" thermostat is in heating mode. The furnace will heat if any of the two mains input is HIGH.
This is not how thermostats function - at least every thermostat I’ve come across. The only way I know to achieve your goal is to use a battery powered thermostat to power the coil of 220V NO relay.
This is how all the currently available wired weekly programmable "dumb" thermostats work in my country. There are hundreds of different kinds and they all work by energizing the coil of an electromagnetic relay by the use of 2 or 3 AA battery as power source.
They are so common, I thought that there must be a Zigbee or Z-wave capable version, but nada.
thermostats here have not operated that way for at least 50+ years. I have old mercury switch thermostats dating back to the 1970s (mercury switch controlled by bimetallic strip expansion), and even those function like a modern thermostat ...
However, from a practical perspective, combining a battery-powered zigbee thermostat with a NO relay would be the simplest way to achieve your goal.
So I rewire. If I can pull out the cable (I've installed it), with the new cable attached to the other end, I can pull a new multi core cable. That way I can use a mains powered thermostat (2 wires needed L+N) and I use one returning core to deliver the L1 output from the thermostat and use it to operate a finder 220V relay to get a potential free NO contact. So I need at least 3 wires. This way I can use any of the wired HE compatible thermostats and don't even need a dry contact, because I can do that with a relay on the other side.
The problem is the IF in the first second line. The cable is at least 10 meters long, runs in the concrete under the floor (in a pipe) and has at least 2 90 degree turns.
With a pair of quality alkaline AA batteries.
On the Honeywell around 2 years, the computherm has on demand backlight, etc, so around 1,5 years at most.
This is how my old programmable honeywell worked as well. For a basic heating call, it energizes the relay to connect R to W and thats it. I can even hear the relay clicking back and forth. 2 AA batteries last long enough that I can't recall how often I needed to change them.
Any reason you don't want to use the Zooz ZEN16? All the "smarts" are contained in the unit. Once it's peroperly configured, you can probably even exclude it from the Z-wave mesh.
I can propose another solution later when I have more time...gotta get to work!
Yepp. But I have a Qubino Flush 1D relay, that already works. Plus I have 2 Aqara relays, one equipped with a Finder 220V relay to give a dry contact), leftover from earlier project.
So here's my second idea. I'll explain it in terms of common HVAC control voltage (24VAC) here in the US, but the same principle should apply anywhere. Forgive me if I seem to be going into too much detail, but I want to make sure we are both on the same page. This is also just a more long-winded version of @aaiyar's response above.
A thermostat is basically just a switch. Using a simple heating only application like this, when the switch (thermostat) contact are closed (ON state) the heating equipment runs, and when the contacts are open (OFF state) the equipment turns off. Here in the US, the "switch" only carries 24 VAC and minimal current. This is what most thermostats in the US are designed to handle. Anything intended to control the 120 volt power to the heating equipment is typically called a "line-voltage" thermostat. There are significantly fewer options for this type of thermostat in residential applications.
The 24VAC controlled by the thermostat is supplied from a small transformer at the heating equipment. These are commonly available in 120-to-24 volt and 240-to-24 volt versions.
In your case, you need to control a 220VAC, 3 A power circuit.
So, here is what I propose.
First, turn off power to your HVAC equipment and disconnect everything from both ends of your existing thermostat wire.
Second, install your desired thermostat. Any battery-powered smart thermostat should do fine. Connect your thermostat wires to the appropriate terminals for a heating application. Here in the US that would be the R and W terminals.
Next, install a 240-to-24 VAC transformer in or near your heating equipment (wherever the "business end" of your thermostat wiring goes). Connect one wire from your thermostat (preferably the same one connected to the R terminal) to the 24VAC side of the transformer. Connect the primary side of the transformer to your 220 power source. Ideally the power source would be the same as the main power to the equipment.
Install a basic relay somewhere convenient for both the 220VAC HVAC control circuit and the 24VAC thermostat signal. This relay's contact should be rated for the 220VAC, 3A load shown in your wiring diagram. The coil voltage should be 24VAC and should require less than 1 amp to activate the relay. One popular example is linked below, but there are literally hundreds (maybe more?) of different styles of these. Connect your 220VAC to the appropriate NO (normally open) contacts on the relay. Connect the coil side of the relay to both your thermostat and to the neutral or common side of your 24VAC transformer output.
Triple check your work and turn the power back on.
That should do it. When your thermostat calls for heat, it will close it's internal relay and send 24VAC to the power relay at your heating equipment. That will close the main contacts of the relay and send your 220VAC control signal to the heating system. The heating system should turn on and do it's thing. Just for safety's sake, someone else should read through this and confirm I haven't lead you astray or mis-typed some important detail.
I will replace all the TRVs with Eurotronic zwave TRVs - been using them for a few years.
To control the Combo gas boiler I need to hijack the no volt thermostat in the lounge.
Looking at it I am probably going to get to the thermostat wire going into the boiler and extending the 240v supply with a 5a fused box that goes into a Secure SSR303 zwave single channel controller.
The 240v is to power the SSR303 and it will expose an on/off switch to Habitat that can be wired to be no volt.
The trick is going to be adding in another switch - to allow switch between HomeAutomation Control and Thermostat control (in case I get hit by a bus and my wife still needs heat) so still figuring that out
so still figuring that out
