Blower cycles on and off regardless of thermostat setting.

[DerrickH]

Hello everyone! We’re new to our house and it’s wood burning furnace and trying to figure out why the blower is turning on and off again so quickly. I have attached a link to a video below of the fan limit switch in action, as well as a few photos of our furnace.

Mostly, I’m trying to first understand whether this is normal behavior, or if there is cause for concern.

It seems to behave this way all the time regardless of the thermostat setting in our main room upstairs. It seems like the only thing that the thermostat actually directly controls is the forced air blower, and doesn’t seem to have a direct affect on the main blower, pushing substantial air into the house.

I have determined that whenever there is a full hot fire in the furnace the blower does seem to act more like I would expect with as much as 10 minutes worth of the blower, pushing our warm air through the house.

So to recap, I’m not sure where this is perfectly normal or if there is something wrong with the fan limit switch or if there’s something wrong with the fan blower, or that it’s purely operator error and ignorance on my part.

Thanks in advance for all your guidance.

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[sloeffle]

Perfectly normal. If it doesn't cycle the blower, heat will build up and you will have bigger issues on your hands. The thermostat on a wood furnace is pretty worthless IMHO. Wood doesn't stop burning when the thermostat turns off like a gas or electric furnace. On a warm day, do smaller loads more often. On a colder day, you can probably get away with a few bigger loads.

 

[DerrickH]

Thank you. That makes a lot of sense. I was beginning to think of the thermostat is really more of a switch to turn on the forced air blower to aid in starting a fire or getting a small one going strong again.

So if the blower motor is cycling back-and-forth to protect the internals of the furnace, I totally get that. My follow-up question would be if we want to pump more warm air into the house that is happening through these short protection cycles is the best thing to do is to simply switch the fan limit switch to manual and let it run like that for a while and before switching it back to automatic?

 

[DerrickH]

Perfectly normal. If it doesn't cycle the blower, heat will build up and you will have bigger issues on your hands. The thermostat on a wood furnace is pretty worthless IMHO. Wood doesn't stop burning when the thermostat turns off like a gas or electric furnace. On a warm day, do smaller loads more often. On a colder day, you can probably get away with a few bigger loads.

Thank you sir. That is helpful.

I’m learning a little bit more about the system all the time. I now understand this is actually a forced air system. Knowing that, would your opinion of the thermostat change? If I understand correctly, the forced air blower that is driven by the thermostat “calling for heat“ is the only way that the fire can really get substantial air at all. I realize the thermostat can’t shut off the fire but I do believe that because it’s a sealed system the forced air blower turning off effectively dampers down the fire. Does that make sense?

 

[brenndatomu]

. I was beginning to think of the thermostat is really more of a switch to turn on the forced air blower to aid in starting a fire or getting a small one going strong again.

It kinda is...that and building the fire big when there is a call for heat from the Tstat. The switch you have in the video does not control the fire though...only the forced air duct blower (house blower)...unless things get too hot, then the fan/limit switch will cut power to the fan or servo, whatever is controlling air flow to the fire.

My follow-up question would be if we want to pump more warm air into the house that is happening through these short protection cycles is the best thing to do is to simply switch the fan limit switch to manual and let it run like that for a while and before switching it back to automatic?

No, that will cool the furnace/HX too much and cause creosote.

 

[brenndatomu]

What controls the fire? A small combustion blower, or a flap/servo setup somewhere there? Basically, what does the Tstat (it has one?) control?

 

[DerrickH]

What controls the fire? A small combustion blower, or a flap/servo setup somewhere there? Basically, what does the Tstat (it has one?) control?

It controls a small combustion blower on the front of the furnace.

I’m learning more about the system all the time. I now understand this is a forced air system. If I understand correctly, the forced air blower is driven by the thermostat “calling for heat“ and that is the only way that the fire can really get substantial air.

So I think this means the thermostat being set properly and the combustion blower driving the fire intensity is very much required for this system. Based on some other comments I've read about forced air (combustion blowers) greatly increasing the amount of wood a furnace will chew through, I was previously wondering if its use was optional, but now I'm thinking that is not the case. Is that right?

 

[brenndatomu]

So I think this means the thermostat being set properly and the combustion blower driving the fire intensity is very much required for this system. Based on some other comments I've read about forced air (combustion blowers) greatly increasing the amount of wood a furnace will chew through, I was previously wondering if its use was optional, but now I'm thinking that is not the case. Is that right?

Well, not necessarily...the last forced air furnace I had I couldn't stand the way it worked when using the comustion blower so I just set it (the air opening on the blower) so that the furnace burnt at kind of a fixed rate all the time...then its just like a big whole house wood stove...you control the temp by loading the amount of wood that is needed for the weather you are having at the time...it has a learning curve, but works pretty well once you figure it out. Some people still prefer to use the blower and the Tstat though too...

 

[DerrickH]

Well, not necessarily...the last forced air furnace I had I couldn't stand the way it worked when using the comustion blower so I just set it (the air opening on the blower) so that the furnace burnt at kind of a fixed rate all the time...then its just like a big whole house wood furnace...you control the temp by loading the amount of wood that is needed for the weather you are having at the time...it has a learning curve, but works pretty well once you figure it out. Some people still prefer to use the blower and the Tstat though too...

Interesting. How did you go about setting it? Again our furnace has a small blower on the front of the furnace, just above the ash box, but there are no other openings or adjustable dials that I have seen on other furnaces.

 

[brenndatomu]

Interesting. How did you go about setting it? Again our furnace has a small blower on the front of the furnace, just above the ash box, but there are no other openings or adjustable dials that I have seen on other furnaces.

It has a teardrop shaped cover over the intake? Most do...you just adjust that to where the furnace doesn't get too hot, but also doesn't smolder the wood and make creosote. Sometimes running the fan for a few minutes to get things rolling after a reload (or especially a cold start) is a good idea.
If yours is not adjustable then you could cut a piece of that magnetic sheet you can get to block registers to put over part of the blower intake

Edit, here is a pic of one with the teardrop cover on the intake

 

[DerrickH]

It has a teardrop shaped cover over the intake? Most do...you just adjust that to where the furnace doesn't get too hot, but also doesn't smolder the wood and make creosote. Sometimes running the fan for a few minutes to get things rolling after a reload (or especially a cold start) is a good idea.
If yours is not adjustable then you could cut a piece of that magnetic sheet you can get to block registers to put over part of the blower intake

Edit, here is a pic of one with the teardrop cover on the intake
View attachment 302937

Interesting. I don't believe mind has a cover like that. But I was under the impression that the circulating blower cycling on and off was the feature that kept the furnace from getting too hot. Is that not sufficient?

 

[brenndatomu]

Interesting. I don't believe mind has a cover like that. But I was under the impression that the circulating blower cycling on and off was the feature that kept the furnace from getting too hot. Is that not sufficient?

Yes, that cools the furnace (and heats the house) but the combustion blower (or how far its air intake is open) controls how big of a fire you have inside...you can have a total rip roaring fire in the furnace and the house blower can hardly take the heat away fast enough...plus you can actually overheat the internals of the firebox...but those are not normal situations.

 

[brenndatomu]

Sorry, hope I am not confusing you more...was just checking in here n there throughout my workday.
If someone else feels like they can explain things better, please feel free to jump in!

 

[Tonty]

We will discuss two blowers: the draft fan (blowing into the firebox) and the forced air fan (blowing into the plenum/ductwork)
Most likely your T-stat is connected to the draft fan. When it calls for heat, the draft fan kicks on and the plenum heats up, which causes the forced air fan to kick on. The forced air fan will cycle on and off by itself, trying to keep the plenum the optimum temperature so that: 1. The plenum doesn’t get to hot, and 2. The plenum doesn’t get to cold and cause creosote build up in the plenum. No T-stat control for the forced air fan. When the T-stat closes the heating cycle, the draft fan shuts off.
It’s imperfect heat because unless you build just the right size of fire, you will either not have enough wood/fire for your heat load, (which isn’t a big deal, you just get cold), or your will have too much wood/fire for your heat load. This is a bigger deal, that causes creosote because if the T-stat isn’t calling for heat, the draft fan is off, and the fire banks way down, smoldering all day and creosoting everything up.
You could also have an over fire situation if you have high btu wood, put a lot of it in, and have a high heat load. Then the T-stat will call for heat for a long length of time, and perhaps the forced air blower won’t be able to cool the plenum off fast enough. But this probably isn’t a normal situation, just something to be aware of.
I think what @brenndatomu was saying was that he bypassed (or regulated the opening of) the draft fan, and just built his fire according to the needs for the day as he saw them and let his furnace burn as a stove with no draft fan/T-stat control.

 

[DerrickH]

Thank you both so much. Your insights are helping connect the dots in my imperfect understanding.

@Tonty your description perfect matches how I believe our system is setup, and what I've been thinking/worrying about relative to having the "just the right size fire". If I was to replicate what @brenndatomu described, the only thing I'm still confused about is if we essentially stopping using the draft fan actively in conjunction with the thermostat, what is to keep the firing banking way down, smoldering all day and creosoting everything up, just as you described it doing when it is off? Does this type of system not necessarily need the occasional or regular induction of air that the draft blower was designed to provide?

 

[brenndatomu]

the only thing I'm still confused about is if we essentially stopping using the draft fan actively in conjunction with the thermostat, what is to keep the firing banking way down, smoldering all day and creosoting everything up, just as you described it doing when it is off? Does this type of system not necessarily need the occasional or regular induction of air that the draft blower was designed to provide?

The way most of these forced draft furnaces work is when the Tstat is satisfied the fire is kinda on the low and smoldery side...and because the blower suddenly shuts off when the Tstat is satisfied the fire is kinda starved for air then too...fire has a momentum to it, so it doesn't want to just suddenly stop, or suddenly get big (at least not without extraordinary means)
Now, when the combustion blower is on then the fire is kickin pretty good (depending on how the air is set, if it can be) and if it stays on very long, then the wood disappears pretty fast. (They'll make some serious heat though!)
The problem is that depending on the weather, and the heat load of your house, you never know how regularly the fire will get a "boost" from the fan (Tstat calling for heat) so you may occasionally have a scenario such as you describe, where the fire idles for a little bit, then get a nice lil boost for a lil bit...but that would much more commonly be the exception, rather than the rule. After owning one like this I quickly learned to hate it...ended up running it like I described earlier, just found an air setting that worked and left it there...other than maybe starting the blower for a few minutes to build a fire after loading.
I've never heard of your model furnace, and some are better than others, but I guess in the long run you will just have to experiment to see what works the best for you...using the combustion blower, or not.

 

[Tonty]

Thank you both so much. Your insights are helping connect the dots in my imperfect understanding.

@Tonty your description perfect matches how I believe our system is setup, and what I've been thinking/worrying about relative to having the "just the right size fire". If I was to replicate what @brenndatomu described, the only thing I'm still confused about is if we essentially stopping using the draft fan actively in conjunction with the thermostat, what is to keep the firing banking way down, smoldering all day and creosoting everything up, just as you described it doing when it is off? Does this type of system not necessarily need the occasional or regular induction of air that the draft blower was designed to provide?

It would probably depend on how good your draft is on how well it would work without the blower running. I’d say you will have to just start using it, experimenting with different things to see what works, starting small and building up from there. Make sure your flue pipe/chimney is good to go and the fire box and plenum are rust free, but maybe you are sure of that already.
Would the previous owner be of any help?

 

[DerrickH]

It would probably depend on how good your draft is on how well it would work without the blower running. I’d say you will have to just start using it, experimenting with different things to see what works, starting small and building up from there. Make sure your flue pipe/chimney is good to go and the fire box and plenum are rust free, but maybe you are sure of that already.
Would the previous owner be of any help?

 

[DerrickH]

Once again, thank you both so much for all of your help. I do indeed plan to continue experimenting and making careful adjustments along the way.

Thankfully, I recently had a chimney sweep come out and not only had a recently serviced it before we purchased the house, but he verified that everything is good to go.

 

[DerrickH]

Hey fellas, one more question. I’m now realizing that the return air side of the furnace is not hooked into any duct work.
what I’ve recently read I’m thinking that that blower and the combustion fan or likely competing for air in my basement. Now the basement is rather large and the furnace is not in a super confined space but I’m wondering if this is really a correct set up.

Now from what I’ve recently read I’m wondering if the circulation blower and the combustion fan are competing for air in my basement. Now the basement is rather large and the furnace is not in a super confined space but I’m wondering if this is really a correct set up.

I didn’t initially realize that’s what it was designed for from the manufacture so far as I understand now. Is this thing safe to run like this? I know for a fact the previous owner did but I don’t know if that gives me any comfort.

 

[brenndatomu]

Do you have a door between the basement and the 1st floor? If so, leave it open and see if it makes any difference.
As long as there is a way for the air you just pumped up through the warm air ducts to get back to the inlet of the (duct) blower, you should be fine.
Now, if you have lots of air leakage out of the top level of your home (especially a 2 story home) that can actually compete for combustion air for the furnace (especially when the combustion blower is off)...a good way to test this is to crack a nearby window or door in the basement, see if it helps. This phenomenon is referred to as the "Florida bungalow syndrome"

Florida Bungalow Syndrome

 

[RockyMtnGriz]

I too removed my combustion blower on a similar unit. I put an electric duct damper in place of it to damp the fire during a power failure, or when I wanted to preserve coals. Normally the combustion inlet was unrestricted (left the room thermostat set to max temp). The biggest help was a key damper in the stove pipe that I set to the same "just short of making the stove smell" setting anytime the door wasn't open. Similar to a fireplace, I tried to construct a fire to burn in a clean manner and controlled the heat by controlling the size of the fire I built.

Run on my strong chimney, configured as intended by the mfg. (Clayton), I found it to be an almost frightening and wood hogging beast. It just bounced from one inefficient state (incomplete combustion) to the other (psycho blast furnace that put a lot of the heat up the chimney). Adding the key damper and removing the incompatible combustion blower made a huge difference in fuel consumption and noise. The price of the clean burn and wood savings is more frequent tending.

Once you're making and keeping the heat, it becomes a question of distribution. If it's not going up the chimney, it's going somewhere in the house. Depending on the house layout, if you have free flowing ductwork, convection may do most of the work, which is wonderful if it's getting the heat to where you want it. If set up as an add-on to a conventional forced air setup, this is even more likely because the ducting will have been designed for a conventional furnace with maybe 4x the distribution blower. If you have rooms distant from the furnace, it becomes a disappointment when convection is delivering most of the heat to the rooms close to the furnace. In that case, restricting the outlets to closer rooms will force the distribution blower to run more and with longer cycles which will put more heat to the distant rooms.

Restricting outlets near the furnace is a balancing act. You won't be able to restrict as much as you might like for the small wood furnace blower, while still keeping a conventional furnace happy if you use both. Also, the wood furnace still needs to convect effectively when the power goes out - if it can't, you're getting into fire hazard territory.

You'll find what works best for your setup. It took me a good, long winter to figure it out, as well as cautiously breaking from the manufacturer's directions to suit my situation.