I currently have two line voltage ceiling radiant heating systems in my condo each with their own DPST thermostat. I'd like to use low voltage thermostats such as the nest. I found this transformer/relay, but it only has 3 wires. Will this work with a system designed to use DPST thermostats? If not, is there something else out there that will allow me to use low voltage thermostats with line voltage heaters?
Electrical – Convert line voltage heating system to use low voltage thermostats
electricalheatinghvacthermostat
Related Solutions
Yes, it is easy to control a swamp cooler with any thermostat. The problem is that in order to use a swamp cooler effectively, you need strategic openings (typically open windows) to let the air out. This is the part which is hard to automate. The best way to use a swamp cooler is to open windows in the rooms that need the cooling the most. You want as much windowspace opened as possible without any outdoor air coming back in through the windows. There are some automated devices I've seen (such as UP-DUX that vent into the attic but they cause additional resistance so they are much less effective.
Your drawing has a couple of problems...
- You need to use the Rc terminal
- The coil on your R8222B1062 needs to connect to the Y2 and the C (not the R)
- I don't think you want to connect to the C terminal on the nest because your heating system is already providing the C to power the thermostat.
- I think you're right about this one but I wouldn't rely on the thermostat powering Y1 and Y2 simultaneously when it called for high. I would rather to design it so that if the Y1 is not powered but the Y2 was, it would still run on high. This gives you a little more redundancy in case of a relay or wire failure.
Below is a drawing that would fix these problems but it is much more generic, the only specific part is that Packard contact delay because I am unfamiliar with a generic part name for that. If you fix 1 - 3, your design should work flawlessly. Again, the exhaust ventilation is your biggest problem.
I would put a nice 24v DPDT relay and a nice 24v 3PDT relay on the appropriate mounting sockets and mount them to a DIN rail. This will make it easy and clean. Put it all in a box with the transformer and the Packard and voila.
That particular device, the RC840T-120, is a combo relay and transformer. The problem is the transformer is just big enough to operate the relay itself, and not also a smart thermostat. It is intended for a passive/dumb thermostat.
Someone else may make a combo relay-transformer which has an appropriately sized transformer. Or, you can just go with separates; it's not that big a deal. Transformers and relays can both be found in that same form factor.
The relay can be down at the service panel. So can the transformer.
Related Topic
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- Installing Nest From HoneyWell T8575D Condo 8-Wire thermostats
Best Answer
The device you linked to will work. You are essentially converting to a SPST "thermostat" from your current DPST thermostat. Some people feel a SPST thermostat is dangerous because the heating device is always connected to a hot leg of the power mains. Both legs are disconnected with a DPST device. Yet this SPST practice is common and acceptable in many jurisdictions. It's quite safe, there is no way to contact energized components in a proper installation. And when the heat is on, all components are energized under either system, and yet it's still safe.
The only conceivable danger is if someone assumes it is a DPST system and foolishly attempts to work on the heater without turning off the power at the breakers. If you're OK with this marginal increase in risk, check with your local building authority, ask them if you can use a single pole line thermostat on a 240 volt (assuming US/Canada voltage system) resistive heater. The remaining details aren't pertinent.
If all is OK, the device's installation instructions are fairly clear on how to hook up both 2 and 3 wire low voltage thermostats. Just ensure you order the correct device for your heater's voltage and power or current rating. To be safe, the device rating should be 125% more than your heater's actual power or current draw.
If you or your building authority do not like SPST type installations, you should be able to find a similar relay that is DPST, even if you have to supply the transformer separately. Any possible solution will certainly involve some combination of transformer and relay, it's just a question of packaging and proper ratings.