Fan speed and heating efficiency with inserts

[Theo]

Standard instructions for wood inserts seem consistent in recommending higher fan settings with higher burn rates. This makes no sense to me, and I'd be interested in others' opinions or concrete knowledge from theory or practice. What follows is a semi-technical expression of what I've seen and inferred; happy to expand if useful.

Seems to me that the only reason to use a fan is to extract more heat from the insert. In my experience (with only a few different models), inserts deliver heat poorly with anything but a very hot fire: flames licking the glass, or a broad exposed area of glowing coals. This is consistent with the mid-infrared cutoff of relevant glasses - which only pass much energy in the visible and near-IR of a hot fire. They only radiate effectively if the window is itself quite hot, since emission goes like the fourth power of frequency. With my insert, the only time I can get much heat out without a fan is with a very fast fire; with anything less, most of the heat output is from blown air.

Another observation: the heat pathway between firebox and blower manifold makes it a high-impedance source. That is to say, increasing the load - by increasing fan speed - does not cause a temperature drop proportionate to flow rate. That, in turn, means that more heat is extracted with higher flow rate under any fire conditions.

I have also heard and read that the fan speed should be kept low until the firebox is hot, to avoid slowing the buildup of a hot fire. I'm dubious about this as well. In my current unit, there is no close connection between the active fire and the extraction manifold. Blower air is probably warmed only moderately by conduction through firebrick, and mainly (I'm guessing) by the relatively small area of bare iron above the firebox and separated from it by a solid insulating baffle. (I would have made this contact area much larger, but presumably the designers knew what they were doing.) So the only way extracting more heat would affect the fire is by lowering exhaust temperature slightly, which shouldn't matter with a proper draft. In any event, opening the secondary air gate allows a huge updraft that would overcome any such effect.

My insert, despite salesmen's promises, can only delver about three hours of strong heat on a full charge of dry wood. The way I usually use it during waking hours is with a steady feed of ca. 3-4 splits per hour. This maintains good flames and coals with the air gate completely closed. In this condition, heat output is directly related to fan speed, so I keep the fan near max to get the most heat with tolerable noise.

So - I don't get why the maker's advice is to match fan speed to air-gate setting. Can someone explain?

Thanks-

Theo

 

[iceman]

what kind of insert is it?
something is def wrong (sounds like it) but i think its operator error or possibly install error .. so shows us pictures and tell us more info and we will get you going

 

[Theo]

what kind of insert is it?
something is def wrong (sounds like it) but i think its operator error or possibly install error .. so shows us pictures and tell us more info and we will get you going

Wrong? Seems to me it's working correctly. Salesman made some promises that turned out to be bogus, and delivered unit was defective and had to be replaced, but the insert (VC Montpelier) seems to be working about like most other new designs I've seen, and like a different model I had installed elsewhere. I'm just questioning what seems to be common wisdom about fan speed settings for different air-inlet settings.

If any part of what I'm reporting is unusual, it would be helpful to know that.

Thanks-

Theo

 

[tfdchief]

I may not be much help as my stove is a 27 year old Little Buck. It has heat exchange space on 5 sides....both sides, top, bottom, and back. It has a 3 speed fan controlled by a 3 thermal disc thermostat. First is 110 F, Second, is 140 F, and high is 175 F (each has a 10 f differential). The thermostat discs are in the heat exchange space, against the fire box steel. As the air is increased and the fire burns hotter, the thermostat increases the fan speed and vise versa as it cools down or is burned lower. When I am burning low, not much draft air, it never gets off low speed. It seems very effective to me. In the morning when I have only coals left, the fan is cycling on and off on the low speed. Hope this helps.

 

[cycloxer]

When the stove is running at higher temps you can run the fan higher to extract more heat. At lower temps, if you run the fan too hard you will just cool down your stove. You want to keep the stove at least above 400 degrees to get a good burn and I think the 500-600 range is optimal.

 

[stockdoct]

Here's an experiment I did with my Lopi Freedom insert. I put a cooking thermometer in the outflow tract of my insert blower, to see what the temperature of the air is that's coming out of the fan. Of course, an important factor is "how much" air is coming out, since that is a big advantage of a fan on "high" rather than "low", but of course I couldn't measure that. But here is the temperature data:

With stove-top temp at and Blower on "high" ( 80% engaged), outflow air temp is:
500 135 degrees
450 131
400 126
350 114
300 102


With stovetop temp at and Blower is on "low" (30% engaged)

500 degrees air is 148
450 140
400 136
350 122
300 110

So, at low blower speeds, outflow air is roughly 10 degrees warmer than the same stove temp at a high blower fan speed. This makes sense. But to determine the optimal blower speed to distribute the most heat in the room, you need to know exactly how much of that heated air is being kicked out.

My experience so far is my insert works best and delivers heat most efficiently, when at a low temp the blower is on low, and at high temps, the blower is on high

 

[bokehman]

something is def wrong (sounds like it) but i think its operator error

Sound's like it to me too. 3-4 splits an hour would be impossible for me. Either the insert is enormous (I doubt it), the splits are too small (probably), or you are burning with the air wide open and all the fuel is going up the chimney. If I burned 3 splits an hour I'd have to live in the garden. With my insert (1.8' c.f. firebox), right now, I have 2" of ash in the bottom (for easy restart), decent coal bed (1"). Two hours ago I loaded 3 decent sized splits which filled it almost to the baffle. Closed the air completely after good secondary burn was established. It's burned secondaries a good 90 minutes and the load is still within 1" of the baffle. It will carry on burning primary for at least another 90 minutes. After that it will continue to make heat and after about 8 hours the glass will still be at 300 degrees.

 

[LLigetfa]

I'm not familiar with the VC Montpelier and my stove is a ZC insert but there are probably some commonalities. My stove is a [box in a [box in a [box]]] with the combustion air being heated in the space between the two inner boxes. The temperature of said air is important for efficient secondary combustion.

Also, rather than a wimpy in stove blower, I have an external 650 CFM blower moving cold air up to the space between the two outer boxes. If I move too much air, I cool the combustion air and reduce the burn efficiency of the stove.

I've noticed the same sort of efficiency loss with providing too much combustion air. There is a non-linear point of diminishing gain increasing the combustion air supply and above that, there is actually a loss of efficiency. I'm not sure if it is just that more heat is going up the flue, that the secondary air temperature is too low for good secondary combustion, or both. I suspect both.

In the 16 years I've been burning in this model RSF stove, I've learned how much air to give to combustion and to heat exchange.

 

[velvetfoot]

When the weather gets cold you can see the limitations of a small insert more.
I do feel that higher fan speeds = more total heat delivered even when the stove is starting to get cooler.
The blowing air starts feeling cooler though too, just from moving.
The noise too is loud and you feel that slowing it down is the answer.

What would be cool if the fan was one of those quiet fantech units located in the basement so you wouldn't, hear it.
It'd be getting suction from where (for me) the heater is, so I don't know if that's be a good idea, though it's all in the same building envelope.

 

[cycloxer]

In the experiments I have performed with a blower in my hearth & a free standing stove, I have noticed the exact same phenomenon. You can cool down the stove too much with too much air. You just lose the efficiency of the combustion. It would be just like if you ran your car engine w/o a thermostat - yeah it would work, but the engine would never warm up to its comfortable operating temperature. I'm sure every stove is different, but mine seems to really throw the heat in the 500-650°F range. At 400°F it still works, but you can literally see, feel, & measure that it is starting to fall off the efficiency curve

Heat transfer is a funny thing. I hated those damn classes. You can only transfer so much heat from the hot stove surfaces to the air by blowing across them. At some point, you can keep increasing the fan speed w/ no gain. You would have to increase the surface area of the stove (via fins) to get more heat transfer.

 

[LLigetfa]

You would have to increase the surface area of the stove (via fins) to get more heat transfer.

That is my next project, to place finned heat sinks on the single-wall top section of the firebox (80% of the top is double-wall to heat combustion air). I also want to wrap some soft copper pipe around the single-wall section of flue leaving the firebox before it transitions to the class A pipe.

I have a vent at the top of the chimney breast to salvage heat coming off the class A pipe that would otherwise be trapped in the chase. The class A pipe is still too hot to hold where it meets the firestop at the ceiling. To meet 2 hour fire rating code, I built a metal chase within a chase on the second floor and vented between the two, so I salvage a little more heat there. From there, I heat the entire neighborhood.

Those with inserts into existing masonry don't have all these options for heat reclaimation.

 

[daveswoodhauler]

I found that leaving the fan on around the 35% to 65% range works well for me.
Basically, Once the stove is up around 500 or so, I kick the fan to appx 45-50% and it works great....plus, my wife doesn't like the noise.
Also, I am no engineer, but I don't think that increasing the fan speed from 50% to 80% will result in the same increase on air being flowed out....just think you need to find the sweet spot on the blower and stick to it.
Also, whats nice is, when the rooms are up to temp, I can reduce the blower to appx 25-30% and it keeps a nice even heat throught the first floor.
Actualy, gotta go check it now

 

[cycloxer]

I don't think that increasing the fan speed from 50% to 80% will result in the same increase on air being flowed out....

Exactly because the squirrel cage fan itself has an efficiency curve. They generally have one speed where they flow optimally. As you change the rpms, depending on where you are on the curve you may have a big gain or very little gain at all at the expense of more air noise. Every fan design is different. So you are doing the right thing by experimenting with what speed setting works best for the airflow/noise balance.

Fans are pretty cool and they can really help with the tranfer - imho they should be on every stove.

 

[gpcollen1]

It is going to vary markedly between inserts and the size of the stove. I get exceptional heat from my Olympic without the blower on - this is a large stove. I hardly even turn the blower on until it gets to real winter conditions, like this December.

As for the fan, I get good heat with the fan too obviously. The fan usually stays ON in that the stove never cools down enough for it to shut off automatically. I can get some pretty long burns around the 400 degree mark with the fan cranked up pretty good. If I have the stove at 600-700, that air coming out is damn hot and I can cook the room out in no time.

All in all. I think you may not be filling the firebox enough and/or cooking the stove hot enough.

I generally only use the fan in these positions:

Off
25%
50%

 

[Roxburyeric]

I use the fan on high all the time for maximum heat output. I will use the auto on function also so the fan won't come on until the stove is hot enough and it will shut off when it cools down to much. I can't be bothered or maybe not even around to turn it down through the burn cycle. They should make a fan that will adjust itself to the temp of the stove - that assumes it is most efficient to slow down the fan as the stove cools. Burning 3 splits an hour is really a question of the size of the splits and the size of the stove. On really cold days I can do 3 splits an hour in my large firebox insert with the fan on high.