Flow Control Valve Operating Mechanism in Boiler

Think the easiest way would be to switch over to a constant 20mA source to fully open the FCV. Can you post a link to the burner controller manual? Google didn't help.

Honeywell's site doesn't show RM7879A, is this one of the Delphi's? Is this 'Flow control valve' the air damper (I don't know what else it would be, surely you're not opening the fuel valve during purge.)? The controller must include a start-up/purge process, with associated inputs/outputs - what does the manual say?

https://www.google.com.pk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDcQFjAA&url=http%3A%2F%2Fproducts.ecc.emea.honeywell.com%2Feurope%2Fpdf%2Fen-rm7897ac-primary-instructionsheet-66-1151-07-nl05r0812.pdf&ei=BhejUrTtMKqR5ASRmoDQBA&usg=AFQjCNEF540jckOwIq5dr__JIC68qKFqcA&bvm=bv.57752919,d.bGE

OK, I see you had a couple of digits transposed in your original post. This seems to be a fairly simple controller intended for use on a single-stage (on-off, non modulating) burner. The only output I see that is 'on' during the pre- and post-purge is the burner/fan contact, which is, of course, also 'on' during the firing. While it might be possible to invert and/or 'and' some of the other outputs to achieve your objective, you haven't provided a complete description of your system and what you are trying to accomplish. This may not be the best controller for your application.

If you are modulating the combustion air with a damper driven by an electric actuator, one technique is to close a contact during the pre or post purge cycle which drives the electric actuator to its full open position, as shown in the diagram for the Honeywell series 72xx Modutrol electric actuator with a 4-20mA input, (document 63-2640-08)

The issue, as bigg has pointed out, is that this particular flame safety controller has no separate 'purge' output and the burner motor output is on for more than just the purge cycles.

The box you are using is not normally suitable for this application.

Questions relevant to the OP.

1. Define small boiler particularly peak firing rate. The rules change depending upon the firing rate.
2. Where are you operating? The rules vary with the location. Why are you trying to re-invent the wheel (burner).
3. Why not buy a burner from a reputable burner manufacturer supplying your area? You can then be confident that it is going to comply with local regulations and keep your insurance company happy.