Fan control on the furnace

[Mushroom Man]

I am getting close to initial firing of the boiler but have no separate thermostat to control the oil furnace fan, in a system with a water to air HX. Do most users have a separate thermostat? OR, Is there a way to disable the oil burner and retain the fan’s ON/OFF function.

I’m not looking for the cheapest solution; I’m looking for the easiest.

 

[stee6043]

I think most folks add a second thermostat. The easiest way to do this is to wire the new thermostat to the old thermostat's AC circuit (assuming you have air conditioning). Throw the breaker on your AC during the winter and enjoy the super easy way to control your existing fan....

 

[rwh442]

I used the AC circuit as stee stated. Placed a toggle switch at the furnace for AC or BOILER operation. My furnace has a variable speed blower and this was the easiest way to get the higher fan speeds. The "fan on" setting was too low.

 

[Mushroom Man]

I do not have A/C . I'm not sure what to look for to see if the unit is wired for A/C. Can anyone indicate what to look for? I know this is a generic question so likely difficult to pinpoint. The separate thermostat part is no problem and seems like an easy way to keep the systems separate.

 

[stee6043]

If it's wired you'll have four or five wires going into your existing thermostat. If not you'll only have two or three (I think).

If you don't have the extra wires you'll need to run a new wire from the thermostat location. That's exactly what I wanted to avoid. Zero fun...

 

[leaddog]

just got done doing this with my neighbors. ran new ther. with 2wires. connected to R and G. Set oil furnace at 65* and wood at 72*. If wood temp goes down the oil furnace comes on.
leaddog

 

[Donl]

Hi Tim, Glad to hear you are almost ready to fire your boiler for the first time!

I use the same thermostat for both oil and wood operation. Last year I just used a DPDT switch to toggle the thermostat between controlling the oil or wood. When toggled to wood the thermostat activated a 24 volt contactor (relay) which turned on the furnace fan when there was a call for heat. This worked well except when there was no heat coming from the boiler, at which time the furnace fan would blow cold air.

This year I have rewired everything in anticipation of installing a NFCS which I will be building this winter. Right now I have replaced the DPDT switch with DPDT relay and I control the furnace fan with the contactor which in turn is activated by a heat sensor located on the water to air HX in the furnace plenum. This way the furnace fan will only come on when there is heat available. The relay makes it easy to automatically control switching between oil and wood when and if required. Works really well.

Good luck with your first firing!

Don

 

[Mushroom Man]

Hi Don:

"Almost ready" refers to only the plenum loop. I need to test for leaks, and wire three pumps; then can launch.

The other zones are partially completed and should come on quickly after the initiation.

The main lines to the barn are buried, insulated and await linking to the manifold in the barn. The 3 barn rooms that will be heated this winter have staple up pex installed.

The storage is all built and has held water for 13 months. The lines are run to storage but the plumbing of that loop is still rough.

Controlling this monster will be challenging but considering the learning curve over the last year and the challenges surmounted, it seems like a reality.

Your help with the design and understanding the principles has been invaluable. This forum is invaluable too. Thanks to all contributors.

As far as the thermostat goes, I have opted for ultimate simplicity. A 110v baseboard heater thermostat wired directly to the furnace fan wires should fit the bill. I wanted to leave the oil furnace alone. Never know....I might need it.

 

[Mid Michigan]

Mushroom Man
Is your furnace fan a multi-speed or single speed fan. I believe if you have a multi-speed fan and both thermostats are calling for heat at the same time there is a possibility of sending power to the fan on two different speeds at the same time. This could be detrimental to your furnace fan.
In my humble opinion.

 

[Mushroom Man]

I have a single speed fan on the furnace. I was planning to use two wire nut splices near the motor of the furnace fan on each of the black and white wires. The baseboard heater thermostat would be connected between the fan connections and the 110V power source.

I have paused to consider whether the 110V power to the fan will impact or damage the relay that reacts to the firing of the oil furnace under normal oil furnace firing conditions.

I realize that I could switch off the oil furnace but that is not my preference since I'd like to use it as a backup.

Is there anyone aboard who knows what to do in this instance?

 

[Mid Michigan]

I believe with the single speed fan you are OK to go the route you are thinking. Use the same circuit for the 120V Stat as your furnace is on and be sure the hot is spliced to the hot and the neutral to the neutral. It might not be good to make the assumption the white factory wire is the neutral. It probably is but take the time to follow it back. It should tie into the neutral on the feed to furnace sooner or later. Better to be sure that to smoke something.

 

[jpowell1979]

What I did for a friend is super simple. The existing thermostat was connected to the oil burner TT leads. When it called for heat the oil burner fired off and the fan kicked on when the low limit switch in the plenum reached temperature.

What we did was relocate the temp sensor for the limit switch close to the water to air HX and lowered the low limit. We then wired the switched fan lead from the limit switch to a normally open relay that was wired to the second thermostat. All you have to do is keep the second thermostat a few degrees warmer than the oil burner thermostat and it all works perfectly.