Wiring two boilers to one circulator

Can someone give me a heads up on the proper way to do this. I have installed my EFM and a oil boiler as a backup. I have plumbed them in such that I can isolate either from the system. There is one circulator in my system. I have a DPST switch to switch power from one to the other, and two heater cutoff switches (one near the boilers, one at the top of the basement stairway). I am nearly ready to bring the EFM online, but I do not know the proper way to wire the circulator to both boilers. Is it as simple as using a DPDT switch? Or am I looking at a relay of some kind?

Thanks,
Pete

I would recommend the relay. It would be automatic and require no human intervention to operate.

It may be best to use 2 relays w/ the loads paralleled to the one circ. pump, because, it is likely 2 different circuits from 2 different units.
If you knew for sure it was the 1 same circuit at both, then 1 relay would work and then you would parallel both at the relay coil. Hope this help.s

McGiever

Normally a circulator is controlled by a thermostat calling for heat rather than either the coal boiler or the oil boiler. If the circulator control circuit is separate from the oil boiler and the coal boiler then no relay is required.

I also have both an oil boler and coal boiler and the thermostats don't know/care which heat source is active. I use a DP switch to remove power from the oil boiler burner circuit and apply it to a separate circulator between the oil boiler and the coal boiler and that switch is thermostatically controlled (not a home thermostat but a sensor on the coal boiler) to do the switch when the coal boiler is at operating temperature. That in effect creates an automatic switchover from oil to coal when the coal boiler is operating. In my system the coal boiler controls are entirely separate--they simply keep the coal boiler at set temperature. While the coal boiler controls turn the forced draft on and off in response to changes in the boiler temperature they are electrically completely separate from the thermostat and the circulator for radiators controlled by a room thermostat.

But if your circulator is controlled by both the oil boiler and the coal boiler then relays to electrically isolate the two systems would be required.

Can you post a diagram of the your wiring--both line voltage and 24 volt controls?

I'll have to write a wiring diagram. Both boilers have Honeywell aquastats. I have a single thermostat wired to both boilers in series. The aquastats take care of hi and low boiler temps, as well as controlling the circulator. My line of think says replacing the SPDT switch with a DPDT switch would take care of isolating the boilers eletrically. One poles would be for 120v supply to the boilers, the other pole would be supply from the circulator circuit of the aquastats (two inputs in other words), and a single output to the circulator. The thermostat would still be connected in series to both boilers, but being 24v, I can't see any harm in that.

We ran two boilers with a single circulator using a DPDT switch. In this thread, scroll down a few pictures: Installation of a stoker boiler at efm

One box was for oil/coal selection and the other was for oil boiler or coal boiler. I'd have to look at the drawing to see how it was wired, if you aren't able to figure it out.

We used the standard Honeywell aquastats from each boiler and it was very easy to do.

Here is the wiring drawing. The W/O is an oil boiler and the other one is the coal boiler. The switch is on the left and labeled A B C D E F.

Thanks guys, I now have my solution!