RM 7850 Burner controllers

Does any body have experience with Honywell RM7850 burner controllers? We have been asked to replicate some existing circuits into new panels but I am having trouble tying up the schematics given to us with the circuits in the controller manual. The purged relay R11 normally closed contact is connected to terminal 6 on the RM7850, R11 is energised via purge prove pressure switch and a timer. My questions are, doesn't the purge prove pressure switch stay closed while the combustion fan is running? If this is the case the normally closed contact on terminal 6 will lockout the controller and stop the burner? Is this correct or do purge prove pressure switches work differently? Any input is gratefully received.

Pauly

I recognize the drawing style, it's from a company I used to work for - H*****k, I could even tell you the name of the guy who did the drawing but can't place the actual job, are you allowed to say which site/plant this is on? is it a ladle heater?

It looks like it originally had a Honeywell R4341B unit and someone has subsequently changed it to a RM7850.

The purge pressure switch is probably a DP switch piped across an orifice plate in the combustion air line, the switch should be set so that it only closes when the combustion air flow is at maximum.

The logic is arranged that on start up the air control valve drives fully open, the switch will then close energising the in-line timer. After the preset purge time the timer output will energise R11 a contact of which will seal it in irrespective of the purge pressure switch state.

The logic will then drive the air control motor to minimum. The low fire pressure swich will probably be another DP switch piped across the air control valve such that the switch closes when the valve is at minimum (highest DP). When this switch closes the burner programmer starts the ignition sequence. Once the burner is lit the programmer will release the air control valve to temperature control and will ignore the state of the low fire pressure switch.

Hope this helps, if you need any more help send me a pm - my rates are very reasonable!

Burnerman is correct

Yep you are quite correct!The purge prove is indeed derived from a d.p.switch, and I understand the sequence with respect to the purge cycle but what I can't quite understand is the n/c contact across terminal 6 will be permanently open until low fire condition! Is terminal 6 not an interlock?

I don't understand the N/C contact of R11 across terminal 20 & 6, I would check what's there in reality, I think you might find that it's a N/O contact, that would make more sense.

With the RM7850 module all the purge and low fire logic can be done from the unit itself, the purge time is governed by fitting different purge cards to the unit.