Single Zone Thermostat Circulation Relay Stuck ON

Have a customer that I installed a 5070THBRPG,WE

Single Zone Thermostat complete with 5 relays

Been working fine for a year.

For the last week the heater FAN will not stop running.

Checked the heater and the 5070 has the Circulation FAN relay closed.

The screen shows the thermostat is OFF and fan off.

Remove cbus power and then reapply and the Circulation fan relay turns off and all works fine again.

What would make the relay stay on ? these are not latching relays are they.

Charles

 

It is running 4.6 which I think is the current release

Tool kit also shows 5.4 as a firmware version but new thermostats are still shipping with 4.6 so I assume 5.4 was never released.


Charles

 

Hey Charlie,

It does looks like a firmware sort of problem, but there is a possibility that it is actually a 'sticky relay' and I would suggest you check the current requirements of the heater fan (unless it is running via a contactor etc). The relays in the thermostats are sort of micro relays and the contacts only rated at 24VAC 2A which is probably also resistive rather than inductive loading.

Some relays when close to or being overloaded start to get the contact surfaces pitting which slightly 'weld' themselves open when switching a big current on and sometimes this can look like the control circuit is keeping it open, when it is actually contact stickiness based on this switching 'welding'.


I would suggest you look at the current on your meter and also possibly measure the coil voltage of the relay in question when the fan is running to see if it is still getting an 'ON' signal from the thermostat. It also may be a relay driver transistor continuing to leak some current to the relay coil, keeping it open even after it's been switched off.

To reduce the loading of the thermostat relay, try using a contactor to switch the fan motor with a lower voltage eg. 12VDC on the contactor coil and see if this changes the symptoms.

Just a possibility to look at,

All the best,

Brad

 

thanks for the suggestion

It is connected to an ANALOGUE interface on a BRAVIS heater. 24v very low current.

Disconnecting the cbus made the relay drop so I don't think it was welded.


I had another person have the HEAT relay stick and again removing cbus power made the relay release. (this was the 4 zone thermostat)



what I did find weird about the one with the circulation relay stuck was that if you let the thermostat go through a heat and off cycle the circulation stayed shut after the heat cycle. I was hoping if the relay got out of sync with the firmware programming, it would resync after a heat and off cycle but no it did not. A heater and analogue interface power cycle did not fix it. it needed cbus power removed to reset it.

Charles


Charles