Honeywell R845A transformer output?

chall2627

I have a two-zone hydronic system that is at least 25 years old, maybe older, house was built in 1969. I tried to replace the old mercury switch Honeywell round thermostat with a new CT-87K Honeywell round thermostat but it will not kick on the circulator or furnace. I measured the red and wire wires at the thermostat and am only getting 15 vac, the user manual from the CT 87K say it needs 20-30 vac. I checked both Honeywell R845A transformers at the "T" terminals and both read about 15 vac. Did the R845A's ever use 15 vac transformers? I called the 800 number in the user guide, but they could not help with the R845A. I put the old mercury switch thermostat back in and the system works fine. I am replacing the thermostats because the system starts, runs for about 5 minutes then shuts off, only to restart 10 to 15 minutes later, I am thinking the anticipator may be the culprit with the old thermostats…. Any help would be greatly appreciated!

Big Ed_4

Measure the amp draw of the circuit , it will be in milliamps and set the heat anticipator to that number as a start , or check the relay label it will be posted for close enough if circuit is tight…

EBEBRATT-Ed

Remember that the thermostat is just a switch. You can't measure voltage between T & T or at the thermostat you won't read 24 volts. You would have to measure it at the transformer secondary terminals or at the relay coil and those terminals are probably not accessible unless you pull the guts out of the enclosure…..use caution because the 120 volt will be exposed.

The transformer puts out 24 volts provided the primary is 120. They are small transformers and only sized to pull in the relay. Too much load will pull the voltage down.

HVACNUT

That thermostat has trouble switching if the voltage isn't there. The little internal battery isn't charging. Get a digital with AA batteries for switching like the Honeywell T4. If you don't want programmable, you can programmable it to be non programmable.

chall2627

Big Ed_4, do I measure the current draw with the circulator running or just turn the thermostat all the way down so it does not run the pump?

EDEBRATT-Ed, I took the measurements with the thermostat disconnected, so no current was flowing in the transformer-relay coil loop, there shouldn't be any voltage drop across the relay coil so why would I not see the transformer full output (in RMS)? I did measure the other thermostat and R845A with the thermostat connected and got the same reading,15.5 vac. Yes, the terminals 1 and 2 do measure the proper 120 vac supply voltage and the only thing connected to the T terminals are the relay coil and the thermostat.

HVACNUT, this is a brand new thermostat, shouldn't the battery in it be charged? Do these button batteries charge when the thermostat is connected to the 24 vac transformer?

Big Ed_4

Take off the thermostat and measure the amp draw across R and W ..

Jamie Hall

The button batteries might have enough charge to run when new — probably not. The batteries should charge when connected with three wires to a 24 VAC transformer. Sometimes they will when using two wires… sometimes not.

There is something amiss, however, if your transformer is only putting out 15 VAC. If that is really the case, disconnect the transformer secondary completely and check the voltage. If it's less than 24 VAC, it's toast…

Since the old reliable T87 worked, I'd put it back on and adjust the anticipator to get the desired cycle time. But that's me.

chall2627

Big Ed_4, ok thanks, I will do that.

Jamie, thanks, I will try that. For now, the old reliable T87 is working..

hot_rod

measure the voltage right on the transformer