I have a Vaillant ETS 600 system. It has 240 to the actual unit (to heat the ceramic bricks) and a separate circuit to the thermostat which operates the units fan. The thermostat is an 859M. The unit works fine with that thermostat but heat regulation is difficult due to the thermostat's sensitivity. I want to get better heat regulation. I'm fairly certain there is some sort of control panel or relay behind the wall, as that room was recently renovated and my contractor had a diagram of the wire configurations.

I tried to replace the thermostat with a Honeywell programmable RLV430A, but I'm getting a low power indicator when the fan kicks on and the fan will not run for more than a few seconds. The display in the RLV430A also dims, which according to the directions, means that the heating system is less than the required minimum load.

So either it's connected wrong, a wire is loose or I need a different thermostat.

Any ideas?

Any suggestions on a programmable thermostat that can be used in this application?

Is this a Viessmann system?

The manufacturer is Vaillant, they are no longer sold in the United States. If Viessmann is a manufacturer then the answer is no, if it's a type of system then I have no idea. I know that it operates off two breakers and since I'm on time of day and my water heater was already connected to a fuse relay or bypass (I can pull the fuse to bypass time of day), there is an additional relay by the fuse box for the thermal storage unit. I also think there is some sort of control panel behind the wall near the thermostat as I only have a black and white wire coming to the thermostat.

I still have the book on the Thermal Storage Unit, if that would help. I've already taken out the new stat and replaced it with the old one and it's working again... So at this point, I just need a thermostat that is a little more sensitive, hopefully programmable which will work with my setup.

Sorry but I have no information on your type of unit.

I know nothing about your system, but have figured out how to make many things work. Does your system have transformer that supplies 24 volts AC to the thermostat, and is that what the RVL430A requires? One place that may have an answer to the fan relay. What does it give as the coil voltage?

I know there is some sort of control box behind the wall, because it was disconnected during a recent remodel. There were several wires there, but I have no other information on it.

The line voltage thermostat only controls the fan, and the RLV430A was expecting more power (I guess to provide heat).

Where would I find information on the fan relay coil voltage?

Contact these folks for all your answers.

17 West Street
West Hatfield, MA 01088
[email protected]
413.247.3380
800.582.8423

If the thermostat is a line voltage one, it won't have a transformer and relay. Try connecting the black and white wire together. If the fan doesn't come on, the problem is with the fan or in the control box.

Following the link from HVAC1000 I've learned that the fans run on 240V but only draw .5 Amps. There doesn't appear to be any programmable thermostat for this setup.