Jensen Wood Furnace fast cycling

[Mrgreen73]

I have an old jensen model 24 ( I think ) furnace inside a 30x30 shop. It has been inside this shop for 30yrs probably unused for the last 15yrs. The furnace has two motors, a big blower on the bottom and a draft blower above it. I have a few questions. I get fire going all the blower kicks on for 8-10s then shuts off for 30s. It basically repeats this over and over on for 8-15s then off for 25-30s and yes this does vary depending on how hot the furnace is. I have never seen the smaller draft motor turn on ever, its possibly dead. Unit is probably working as intended but wow is it annoying fast cycling like this. I rewired motor to bypass thermostat and just stay ON and it heats shop much faster. Anybody know if its ok to just let motor rip. Im not sure if its making stove hotter i cant really see much difference. And what exactly does the smaller draft motor do ? Do I even need it? Would it help stop the fast cycling if I bought a new draft motor? Everything surrounding furnace is steel cnd brick so Im not in any fire hazzard situation yet anyways. Thanks for info.

 

[brenndatomu]

Wood furnaces cycle, some models more than others, and if you had the draft blower (forced combustion air) working, it might cycle the duct blower a lil less, but mostly just during the time the draft blower is actually on...which by the way, while that draft blower is on, your load of wood is disappearing, and fast! Many people don't use them because of that. But if you are looking for a fast warm up, wood usage be danged, then yeah, use the draft blower.
What type of switch runs the duct blower? If it's a Honeywell fan/limit switch, then you could widen the gap between the on and the off setting...that would help some.
You could try running the blower non stop, but that may produce more creosote in the chimney too (flue temp too low) but if you are just gonna run it hard then shut it off, it may work good enough for a shop...if it was in the house I'd say it'll probably give you a cold draft feeling that the family may not appreciate.
Another heads up is having a wood heater in the shop...some insurance companies will not allow it (if they know about it) and some will allow them in a "workshop", as long as there is no gasoline (paint thinner, etc) or gasoline powered items in there...if you call it a "garage" most just plain will not allow a wood heater in there at all.
Some places make you put the heater 18-24" off the floor too...fumes tend to be heavier than air.
Just some FYI for ya...

 

[Mrgreen73]

Wood furnaces cycle, some models more than others, and if you had the draft blower (forced combustion air) working, it might cycle the duct blower a lil less, but mostly just during the time the draft blower is actually on...which by the way, while that draft blower is on, your load of wood is disappearing, and fast! Many people don't use them because of that. But if you are looking for a fast warm up, wood usage be danged, then yeah, use the draft blower.
What type of switch runs the duct blower? If it's a Honeywell fan/limit switch, then you could widen the gap between the on and the off setting...that would help some.
You could try running the blower non stop, but that may produce more creosote in the chimney too (flue temp too low) but if you are just gonna run it hard then shut it off, it may work good enough for a shop...if it was in the house I'd say it'll probably give you a cold draft feeling that the family may not appreciate.
Another heads up is having a wood heater in the shop...some insurance companies will not allow it (if they know about it) and some will allow them in a "workshop", as long as there is no gasoline (paint thinner, etc) or gasoline powered items in there...if you call it a "garage" most just plain will not allow a wood heater in there at all.
Some places make you put the heater 18-24" off the floor too...fumes tend to be heavier than air.
Just some FYI for ya...

It does have that adjustable limit switch, it looks like it was set at 100-200 roughly. I think it can only be changed to aprox 80-200. I will adust that and see what happens. I believe Ive had the stove setup pretty poorly might need to make some adjustments. I notice the ash tray vent was open, and the draft blower vent was open so im thinking the fire was pretty volatile probably massive temperature swings making blower turn on off so fast, just guessing.

 

[Mrgreen73]

There is a 12" 90* duct coming out of top running to a 4' straight pipe that just ends right there.
I would assume that causes the fan to start and stop pretty fast, with it being able to withdraw stove heat so rapidly.
Im thinking if I extended that into a long duct pipe overhead across entire shop maybe that would extend run time?

 

[all night moe]

The blower is going to exhaust that heat with or without more ductwork. You want to set the differential of the fan limit switch longer.
My furnace is operating with no thermostat, to call for heat also. My limit switch is set for cutoff at 100* and on at 200 or 250*. I don't recall which. Your draft door controls how hot the fire gets. Just like a woodstove. Now that it isn't controlled via thermostat, you have to do it manually. With it open wide, and good dry wood, furnace should heat right up.

 

[brenndatomu]

There is a 12" 90* duct coming out of top running to a 4' straight pipe that just ends right there.
I would assume that causes the fan to start and stop pretty fast, with it being able to withdraw stove heat so rapidly.
Im thinking if I extended that into a long duct pipe overhead across entire shop maybe that would extend run time?

Put a damper on that duct and restrict it some...you need a little static pressure on that blower to calm it down

 

[Mrgreen73]

I did some experimenting this morning and had a bit of success. I spread temp range on tstat as far as possible and it did make the fan run longer than it ever it has, however that was only the case if the stove is really really rolling.
Seems to me when you first start up the stove the blower is pointless because its 5s on 10s off just completely all over the place.
Its somewhat finicky the stove is either too hot or too cold, its kind of a balancing act I gotta sort out.
I flipped fan off via breaker box and got a very hot fire going borderline too hot and then turned breaker back on and it ran for about 3 minutes solid, which was awesome, the shop temp just skyrocketed but Im a bit uneasy about that , I think the stove went up above
200* probably even hotter a time or two. I still have more questions the stove has a thermo disk that cant possibly be working correctly and there is still issue with the draft blower id like to get those two things fixed, and also do something with open exhaust just blowing into mid air unrestricted, maybe a plenum with a few ducts would spread out the heat real nice. Anyway thanks for replies, any help is greatly apprec.

 

[Gearhead660]

I have an old Daka furnace in my shop. No ducts, just a couple 90* elbows. I added a fan speed controller to slow the fan down. $15. Once it warms up and the fan kicks on, it stays on.

 

[Mrgreen73]

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I have an old Daka furnace in my shop. No ducts, just a couple 90* elbows. I added a fan speed controller to slow the fan down. $15. Once it warms up and the fan kicks on, it stays on.

I will deffinately find a speed controller then thanks for that. I have expeirmented more and just placed a piece of sheet metal in front
of exhaust and it made a massive improvement, thanks EPA. With no restriction on exhaust fan it varies 15-40s , then when fan shut down I simply held a piece of sheetmetal approx 50-60% over the pipe and the fan ran for a long long time.
A few times with the restriction it ran 3+ min and once it ran so long I just stopped my stopwatch at 6+ minutes.
That is all the heat i will ever need if I can keep it running like that.
It seems to me like the stove needs to be roaring hot tho because the tstat will go up and down so fast triggering fan. Once it hits that very hot zone the fan cant quite cool her down so quickly, making fan run a good bit longer.
It seems crazy inefficient just sitting there smelting with no fan running. Heck I could mount a squirrel cage behind it just blowing over it that would distribute a ton of heat.

 

[Mrgreen73]

Sorry to keep posting but yall have helped a ton so.... this stove has a Rheostat on side that says Fan speed control.
However I cant see it doing anything at all, no matter where its set max vs low makes no difference.
I am just guessing that it controls the draft blower thats not working at all.

 

[brenndatomu]

Sorry to keep posting but yall have helped a ton so.... this stove has a Rheostat on side that says Fan speed control.
However I cant see it doing anything at all, no matter where its set max vs low makes no difference.
I am just guessing that it controls the draft blower thats not working at all.

The draft blower normally does not have a rheostat...my guess is it was originally for the duct blower.

 

[brenndatomu]

Is this your unit? If so then I'm wrong, the wiring diagram says the rheostat is in case of low voltage...not sure what they are talking about, never heard of such a thing.
And it shows 2 small duct blowers...that was a common failure point on the hotblast furnaces too, many people replaced them with one larger blower, if that's what is going on with yours the blower may be oversized and that would make the cycling worse...but restricting that duct like you have will limit the cycling.
I personally wouldn't bother fixing that draft blower, I've had furnaces with them and eventually quit using the blower, just let it draft naturally. Just used way too much wood using the draft blower...stack temps high, very inefficient...but maybe not a big concern for you since it's for part time shop use, not full time home heating.

 

[Mrgreen73]

Is this your unit? If so then I'm wrong, the wiring diagram says the rheostat is in case of low voltage...not sure what they are talking about, never heard of such a thing.
And it shows 2 small duct blowers...that was a common failure point on the hotblast furnaces too, many people replaced them with one larger blower, if that's what is going on with yours the blower may be oversized and that would make the cycling worse...but restricting that duct like you have will limit the cycling.
I personally wouldn't bother fixing that draft blower, I've had furnaces with them and eventually quit using the blower, just let it draft naturally. Just used way too much wood using the draft blower...stack temps high, very inefficient...but maybe not a big concern for you since it's for part time shop use, not full time home heating.

The stove looks very similar to that with some differences. I have one blower and one draft blower. The wiring on mine (and the actual wiring of whoever wired it) is somewhat different , When I find the manual I will try upload a few pictures. Anyway this morning shop was 39-40, I fired it up to nuclear mode and was able to maintain around 65-70 inside shop so... It was simply amazing. I put a spare motor I had that can be wired for low-med-high configuration. I wired it for low speed and it just ran perfect. A speed control would make it even that much better. I never thought it would achieve 70 with drafty shop and no insulation but it will. Now I just need to keep making improvements for most heat with least firewood lol.

 

[brenndatomu]

Now I just need to keep making improvements for most heat with least firewood lol.

Well now that will be a lil more difficult...those old furnaces are hungry beasts...

 

[all night moe]

Insulation would save you wood, pun intended.
An interior vapor barrier would also be a huge gain in cutting down on draft invasion.