Need Help Controlling Blower Fan With 2 Different Thermostat

mcgills443 · Post by **mcgills443** » Fri. Sep. 25, 2009 3:28 pm

Hi everyone,

It's been a long wet summer here in the upstate NY. The heating season is just around the corner. I Purchased a Harman sf-260 and have all the plumbing and stove pipe hooked up. The wiring on my Johnson controls are all finished also. I'm having a problem wiring the oil thermostat, and the coal thermostat to control the same blower on my oil furnance and are wondering if it can be done? I have a oil hot air furnance that heats my house. I put a boiler in the garage with a water to heat exchanger in the furnance. I would like to have 2 thermostats in the house. 1 for coal,and the other for the oil. if the boiler goes out the oil takes over. the problem that I'm having is there is 2,24 volt currents always going to the furnance which is causing voltage bucking. I'm wondering is there a way to isolate one when the other is running? Any help would be great. Thanks Joe

DOUG · Post by **DOUG** » Fri. Sep. 25, 2009 6:16 pm

Yes there is a way to do what you want. I have a hand fired coal furnace, a stoker coal furnace, and a natural gas furnace. I use a different thermostat for each furnace and only control one three speed twinned circulation blower fan arrangement with any one or any combination of these furnaces running, to move the hot air through the duct work.

The key to making this happen is using some relays. I used 115 volt coils with 8 amp contacts. http://images.grainger.com/B292_57/images/products/4E658.JPG Grainger part number 4E941.

You take the 110 volt output of each circuit, hot and common, that is controlling the blower fan, from each furnace and you connect it to the 115 volt coil spades, on that particular furnaces own relay. Run a separate, independent, circuit for the blower fan that you want to activate, hot to the normally closed contact side of the relay and then to a barrier strip. The common runs right to the blower fan. The same separate, independent, fan circuit, will be used on all of the relays for the fan normally closed contact side.

By using a separate circuit to control the fan, all of the hot contacts, from the relays, can now be run to a barrier strip, which will now control the blower fan, when any of the appliances circuit activates the coil.

So what that means is that anytime one of the units wants to turn on the blower fan, it will, and they all can be doing it at the same time, without any problems.

It is really nice once it is all done. You'll have everything you want accomplished.

DOUG · Post by **DOUG** » Fri. Sep. 25, 2009 6:29 pm

I forgot to mention that you may have to use a fan center, http://www.grainger.com/Grainger/wwg/productIndex ... ontrols%2C or a different voltage relay coil to make your particular application function. The fan centers take the 24 volt thermostat line and convert it to 110 volt. You also have the possibility to lower the voltage coil for either or both relays, depending on how you plan to run your wiring.

I'll be happy to answer anymore questions if you have any.

mcgills443 · Post by **mcgills443** » Fri. Sep. 25, 2009 7:06 pm

Doug,

Thank you for the info. I have a friend that is a electrician that will help me wire this hookup. If I have anymore ??'s I'll give you a shout.Thanks again Joe.