My furnace/AC needs to be replaced and my oil company is currently writing a quote to replace it. What I'm wondering is, does anyone have any experience with combination wood/coal & oil furnaces? I figured it would be good to have since I have plenty of firewood and coal is relatively cheap. It could help offset some of my oil costs.

Problem with a combo is they dont do any one fuel well.
Get a dedicated coal or wood burner and realize the best performance.

I had a wood/oil combo in the old farm house, really nice to be able to burn wood when we were home and let the oil work when not. Course there is the labor of cutting splitting, stacking, ash removal...….

We have a Yukon oil/wood furnace in the shop at the farm that I mechanic at. It works really well run wood when someone is there and oil comes on when wood is not burning. More maintenance due to wood ash but since we have an ample supply of apple wood it’s cheap for us. Ours is an old unit but parts are easily obtained. I heartily recommend Yukon but I do not think they are sold in the US anymore.

I have a HS Tarm, believe it was made in Scandinavia. Burns wood/coal/oil. Provides hot water for baseboard units and hot water for sinks, showers,etc. Pretty efficient for oil, very good if you burn wood, but it will suck it in as far as consumption goes. Coal is very good as well and it will really product a lot of heat or, damped down, burn for a long time.

As the others mentioned, let the coal or wood burn out and the thermostat kicks on the oil burner.

It has worked well for some 25 years. Still set up and with a few details can be switched on today. We used it here in PA for some 6 months/year, then shut it down and got domestic hot water from a water heater (oil fired, very efficient).

We stopped using it due to the installation of a geothermal system.

Just be aware you can screw up a chimney really fast with anything wood if you don't burn it right. I've never seen a wood furnace down here that didn't... but we have a milder climate. The problem is most of these have a damper on the air intake for the wood box and the thermostat will shut the air off when the house reaches the right tempt. The absolute worst way to burn wood... would you control the heat off of oil that way?

I'd most likely be using coal more often than wood. But I don't think I would kill the wood with the thermostat, more likely learn how much it takes to heat the home and adjust the load accordingly.