Fireplace Insert Modification - Boosting Airflow

[Murphy2000]

Hi All,
I have an Enviro Venice 1700 fireplace insert and I have a question about making a modification to the air circulation fan.

I am not really happy about the amount of air volume coming out the front of this thing and I am considering boosting this flow by installing a remote mounted auxiliary fan.

Let me explain, My fireplace insert has a lot of room behind the shroud and there used to be this box (cavity) within the cabinet space about 3 feet off to the side where the original fireplace insert had a remote mounted blower fan.. I think they called it a "Heat-o-later" or something funky like that.

Anyhow, I am considering cutting a round 4 inch hole in the side of the outer sheet metal of my stove and installing a flange to accept a 4 inch pipe. (probably the clothes dryer ducting stuff)
I would then install a squirrel cage blower fan and furnace type air-filter and duct that air into the 4 inch hole in the outer skin of the wood stove. The fan and filter would be in a permanently cool spot about 3 feet beyond the brick were it was originally installed.

Has anyone ever done this? My only real concern is that I may blow too much air through the thing and cool down the stove too much.. Is that even a legit concern?

If anyone else has any issues, I'm all ears.. Just to be clear, I am an industrial engineer and have designed large ovens and burn chambers over the years so I'm not just some idiot who's going to burn down his home doing something stupid.
I'm just wondering if I'm missing something obvious I should be thinking of.

The fan this insert comes with is a dual-wheel type.. each wheel is about 3 inches wide and 2 inches diameter and it does not blow much air even on high... And the stove manufacture had the idea they'd install a speed control that is totally useless.. I want to speed it up, not slow it down.

Another issue is the noise.. By installing a remote fan, I should be able to eliminate 80% of the noise coming from the stove.. its annoying trying to watch TV with that fan blower going.

Replies appreciated..!!

 

[Slow1]

The mechanics of what you are suggesting sounds reasonable. Clearly as soon as you go cut into the side of the stove you will be voiding and/all certifications etc and thus if there was a fire that in any way were traced back to the stove you likely would be on your own (i.e. insurance could argue to deny coverage). However, as you said you are confident you can do this safely...

The question that comes to my mind though is the air that is currently being blown off your stove - how hot is it? If you increase the volume of air I imagine it will be cooler (hard to guess how much) due to the higher velocity giving it less time to pick up heat as well as the fact that it will be cooling the stove surface more. If the air being blown out by the slower current fans isn't particularly hot then you may not get much more heat out and rather may just get more air that is cooler for your efforts. Less noise may make it worthwhile though....

 

[Murphy2000]

I thought about the insurance issue but they'd have to prove it was something to do with my modification, not just the stove.. IE: if I had a chimney fire they wouldn't be able to say it was my blower modification.
As per the warranty, its almost up anyhow.

As for the air temperature, it is very hot when the stove is burning on high.. You can hold your hand about 8 inches in front of the discharge vent for about 3 seconds before it starts to hurt... Its freaking hot! In design engineering we call it the Delta-T.. (the difference between inlet and discharge temperature).. Generally, the lower that temperature difference, the greater the efficiency of the heat exchanger. A higher volume fan should drop that delta-t to something reasonable. Higher volume of cooler air should mean more heat pulled from the stove.. which would also mean less heat up the stack... which is what I am concerned with. If I pull too much energy away, I'm worried I might start condensing smoke on my chimney flue. I have a stainless flex pipe insert in my chimney and I clean it myself every year. I burn only 2 year old dry wood and the amount of soot I clean out wouldn't fill a coffee cup.

 

[KaptJaq]

IE: if I had a chimney fire they wouldn't be able to say it was my blower modification.

As you state in your message, your fan may cool the stove enough that it no longer burns cleanly. The insurance company can argue that because the stove was running cooler than designed due to your modification it created more creosote causing the flue fire. They don't have to be right, you will have to prove them wrong. Since your modification is one off, there are no other examples to test on. With a factory spec stove the company will back you.

KaptJaq

 

[Slow1]

I thought about the insurance issue but they'd have to prove it was something to do with my modification, not just the stove.. IE: if I had a chimney fire they wouldn't be able to say it was my blower modification.
As per the warranty, its almost up anyhow.

As for the air temperature, it is very hot when the stove is burning on high.. You can hold your hand about 8 inches in front of the discharge vent for about 3 seconds before it starts to hurt... Its freaking hot! In design engineering we call it the Delta-T.. (the difference between inlet and discharge temperature).. Generally, the lower that temperature difference, the greater the efficiency of the heat exchanger. A higher volume fan should drop that delta-t to something reasonable. Higher volume of cooler air should mean more heat pulled from the stove.. which would also mean less heat up the stack... which is what I am concerned with. If I pull too much energy away, I'm worried I might start condensing smoke on my chimney flue. I have a stainless flex pipe insert in my chimney and I clean it myself every year. I burn only 2 year old dry wood and the amount of soot I clean out wouldn't fill a coffee cup.

I suppose then the way to verify the design would be to stick a probe in your flue now and monitor temps then once you have the modification done you can compare pre/post modification to see if the flue temps have indeed fallen. Secondary advantage is that with the flue probe in place you can monitor to see if the temps are "too low" - i.e. reaching the point where you expect significant condensation and perhaps adjust your fan accordingly. Maybe an added feature would be to put a heat triggered variable resistor on the inside edge of the stove to control the fan speed - i.e. hotter to increase speed of fan and cooler to reduce speed and thus help reduce risk of over-cooling the box. May not be necessary or helpful, but if you are in the experimental mode you never know what could bring additional benefits.

 

[Jon1270]

I haven't done this, but I've flirted with the idea for the same reasons. My thought was to use a quiet 2-speed bathroom exhaust fan (Panasonic makes one), and design a simple circuit that only kicks the fan to the higher speed when the stovetop temp reaches some threshold.

 

[Murphy2000]

As you state in your message, your fan may cool the stove enough that it no longer burns cleanly. The insurance company can argue that because the stove was running cooler than designed due to your modification it created more creosote causing the flue fire. They don't have to be right, you will have to prove them wrong. Since your modification is one off, there are no other examples to test on. With a factory spec stove the company will back you.
KaptJaq

The example I gave was to show you can't blame the car stereo for an engine failure... The clean burn argument doesn't apply in this example because there are times where all of us only put one or two pieces of wood in the stove and run it at a lower temperature... (and we all know it doesn't burn as clean in that mode) Second, the last thing I ever worry about is a chimney fire. I only burn 2 year old wood that is dry and I clean my flue myself every year so I monitor that closely....

Nope.. my example, however bad, was just a fishing expedition to see if I could knock loose some good ideas or legit concerns from you folks. I'm just one person and we all know that more brains thinking of the same subject will come up with more ideas....

 

[Murphy2000]

I suppose then the way to verify the design would be to stick a probe in your flue now and monitor temps then once you have the modification done you can compare pre/post modification to see if the flue temps have indeed fallen. Secondary advantage is that with the flue probe in place you can monitor to see if the temps are "too low" - i.e. reaching the point where you expect significant condensation and perhaps adjust your fan accordingly. Maybe an added feature would be to put a heat triggered variable resistor on the inside edge of the stove to control the fan speed - i.e. hotter to increase speed of fan and cooler to reduce speed and thus help reduce risk of over-cooling the box. May not be necessary or helpful, but if you are in the experimental mode you never know what could bring additional benefits.

Oh.. if I could only get a probe into that sucker I'd be really happy.... Its a fireplace insert and while I have some room on the sides, it was the absolute largest stove I could fit in there vertically. There is no free space on top and I would be lucky just to get a surface mounted thermocouple onto that flue pipe... And even that would require some tricky maneuvers that I'm not sure I am capable of.. Does anyone know someone who is 12 inches tall and thin?? LOL.

I do have a few PID Temperature controllers here with dual digital readouts, double throw output relays and alarm contacts.. I guess I could just do a surface mount right on the top plate of the stove itself.. It would still be a trick to get it in there but I'm sure I could do it. I would have to insulate and protect it from the blowing air causing a change in temps but that's doable.

So no big flashing red warning signs yet from anyone... Looking good so far...

 

[Jon1270]

I do have a few PID Temperature controllers here with dual digital readouts, double throw output relays and alarm contacts..

Well, since you have extras you obviously want to get rid of, my address is...

 

[Slow1]

Well, if you have a probe with a long enough lead then all you have to do is get it in there once... For what you are trying to do here I'd rather see data for the internal temps rather than surface temps so putting the sensor inside the exhaust flow stream would be preferable.. yeah, it would be a pain, but in the name of good science (and since I'm just sitting here at a keyboard not having to get it in there... ha!).

 

[Murphy2000]

Well, since you have extras you obviously want to get rid of, my address is...

LOL.. I sell them along with kits I make.. I do a lot of custom design and engineering and I always keep stock of a half dozen or so and some type J thermocouples.

 

[Murphy2000]

Well, if you have a probe with a long enough lead then all you have to do is get it in there once....

That's what she said! LOL

 

[velvetfoot]

My radon fan is way quiet and it pushes a fair amount of air. The insert fan is way noisy. Way.
The radon fan is some kind of axial fan. (Just remembered the name: fantech.)
You'd have to block the current inlet, which might not be that bad. A lot easier than attaching a duct, somehow, if you weren't gonna cut a hole, I imagine. But to get the maxium heating effect, wouldn't the air have to be introduced somewhat near the original inlet?

 

[Murphy2000]

My radon fan is way quiet and it pushes a fair amount of air. The insert fan is way noisy. Way.
The radon fan is some kind of axial fan. (Just remembered the name: fantech.)
You'd have to block the current inlet, which might not be that bad. A lot easier than attaching a duct, somehow, if you weren't gonna cut a hole, I imagine. But to get the maxium heating effect, wouldn't the air have to be introduced somewhat near the original inlet?

I doubt your radon fan will provide enough pressure to push the air around fast enough.. I could be wrong but something tells me its a low volume - low pressure fan unit. Axial fans are best for moving high volumes of air using minimum power.. That "math" can also be reversed to provide maximum power with minimum air.. IE: Wind Turbine.. But axial fans are not that great at providing pressure due to the large gaps between the blades and the housings. Squirrel cage fans have closely spaced blades and housing tolerances which allow them to build higher pressures.
I'm not saying your radon fan won't work, it just depends on the setup I think.. I doubt it would work for me.