# Combo wood/oil furnace question ....



## drewmo (Oct 13, 2012)

We recently bought a house and there is a combo wood/oil furnace made by Powrmatic. I can't seem to find any information online about operating this unit. I have used the oil side a few times to take the chill out of the house, but have yet to use the wood side for heating. There are two thermostats, one for the oil side, the other for the wood side. The oil thermostat is working as it should. When I turn up the wood thermostat, nothing happens, and I'm assuming this is because there is no fire/the unit is not up to temp. Am I assuming correctly that once a fire is going, I will be able to control the wood side with the thermostat? Or, am I missing something altogether different?

I should have asked this question to the local oil company when we had it checked over.


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## laynes69 (Oct 13, 2012)

I've heard of those furnaces, but never seen one in person. Usually your thermostat will control either a servo motor for a damper, or a forced draft. The idea is if heat is needed, the fan or damper will open allowing more air for the fire. Once the thermostat meets the set point on the home, that damper closes or fan shuts off. Make sure your chimney is clean before operating the wood side of the furnace. This time of year, if the heat demand is low burn smaller hotter fires to reduce the chance for creosote. Hope this helps.


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## drewmo (Oct 13, 2012)

Thanks for the advice. The chimney sweep will be here in a couple of weeks, then I'll start using the wood to heat the house.

It seems like very little is known about these systems. I'm still wondering how the furnace knows that there is a fire in the box because at this time when I turn the thermostat up on the wood side (with no fire), the blower does not start. Do wood furnaces need an active fire to allow thermostat control or is there an on/off switch I have missed?


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## laynes69 (Oct 13, 2012)

Yes, there is a fan/limit control on the furnace. When you add wood, the thermostat should open a damper or start a small fan (not blower). Once that fire starts to heat the furnace, the fan control will sense heat and once it hits a set temperature the blower will turn on and push heat through the air jacket into the house. Once the fan control temperature drops to the off position, the blower shuts off. So in other words, it takes an active fire to activate everything. A wood furnace is not like a normal furnace. As long as there is a fire, that blower will run, even after the thermostat is met on the wall. That's where learning how the unit operates will help you. I would recommend start with a smaller fire and don't completely load it right away until you get a understanding how things will work.


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## drewmo (Oct 13, 2012)

laynes69 said:


> Yes, there is a fan/limit control on the furnace. When you add wood, the thermostat should open a damper or start a small fan (not blower). Once that fire starts to heat the furnace, the fan control will sense heat and once it hits a set temperature the blower will turn on and push heat through the air jacket into the house. Once the fan control temperature drops to the off position, the blower shuts off. So in other words, it takes an active fire to activate everything. A wood furnace is not like a normal furnace. As long as there is a fire, that blower will run, even after the thermostat is met on the wall. That's where learning how the unit operates will help you. I would recommend start with a smaller fire and don't completely load it right away until you get a understanding how things will work.


 
Great, thanks for the info. Above the fire box door, there is a control with two wires coming out of it and some sort of adjustable knob labelled "Guage." It's tough to move and really gives no indication of what we're guaging. I'll assume this is for the fan control temperature?? One wire goes to the damper, and the other goes to the box that controls the blower. This might be starting to make sense. And if I understand correctly, the blower will run constantly, but if the thermostat is met, the damper will close forcing less hot air through the jacket until the fire dies down enough to shut everything off? Looks like I have some experimenting ahead.

I mastered a small wood stove the first season I had it, and I will do the same with this beast (hopefully without relying on oil too much).


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## laynes69 (Oct 13, 2012)

Yeah in a round about way, once the damper closes the fire will die down. The blower still might run, but not as often. Could you post a pic or 2 of the unit?


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## fabsroman (Oct 17, 2012)

Your furnace sounds pretty much like the Yukon Polar/Eagle II that I just installed. Did a test run a couple days ago with the natural gas, but have yet to try out the wood. Still too warm here to have it going very long.

The fan limit control is usually right above the heat exchanger and below the A-coil for the A/C.  Mine is set to turn the blower on once the fan limit control is reading 150 degrees. This is for both the natural gas and the wood fire. So, until the plenum area is around 150 degrees, the blower will not turn on. Then, if the plenum area drops below 150 degrees, the blower will shut off even if a fire is burning or the gas burner is on.  Also, if the fan limit control ever reaches 250 degrees, the blower will come on even if neither thermostat is calling for heat. This is to prevent the furnace from overheating, by distributing the heat throughout the house. So, with a strong wood fire going, the house might reach temps above what the thermostat is set at.

Now, the reason for the two thermostats is because one controls the damper for the wood fire and the other controls the natural gas burner. I have the wood fire one set at 73 degrees and the natrual gas one set at 65 degrees.  The damper for the wood fire will open once the temp drops below 73 degrees, thereby increasing the amount of oxygen to the wood fire and increasing its temperature. Once the house temp drops below 65 degrees, the gas burner will come on regardless of whether there is a wood fire still burning.  The way to start the wood fire in this unit is to load the unit with wood and then termporarily raise the thermostat setting for the gas burner, thereby using the gas burner to ignite the wood.  I am really looking forward to using this furnace this season. Just ordered a manometer to set the flue draft and will order another instrument to measure the gas burner efficiency. Last thing I need to figure out is how to do a smoke analysis for the gas burner.

Good luck with your furance and I hope you hardly have to use any oil this season.


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## SmokeEater (Oct 17, 2012)

drewmo said:


> We recently bought a house and there is a combo wood/oil furnace made by Powrmatic. I can't seem to find any information online about operating this unit. I have used the oil side a few times to take the chill out of the house, but have yet to use the wood side for heating. There are two thermostats, one for the oil side, the other for the wood side. The oil thermostat is working as it should. When I turn up the wood thermostat, nothing happens, and I'm assuming this is because there is no fire/the unit is not up to temp. Am I assuming correctly that once a fire is going, I will be able to control the wood side with the thermostat? Or, am I missing something altogether different?
> 
> I should have asked this question to the local oil company when we had it checked over.


I had a Powrmatic wood/coal/oil combo for over 30 years and every one of those winters the wood/coal portion basically kept the house warm.  The wood thermostat controls a selsin type motor that opens and closes the damper and thereby allowing the fire to pick up or die back according to the setting.  The trouble with that damper motor was to adjust the opening chain from the motor to the damper so that if the damper was opened too far, the wood side could overheat.  A barometric damper is absolutely necessary on your flue to be able to use the wood thermostat correctly.  Does your furnace have the heavy shaker grates for burning coal?  I feel that with today's technology, the Powrmatic is not nearly as efficient as most modern wood burners and it therefore will use more wood than a modern furnace.


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