# Why did Hearthstone stop using the Bi-Metallic Coil Primary Intake control?



## Motor7 (Mar 21, 2013)

I bought my first stove, a Heritage back in the mid 80's. It had the Bi-metallic coil on the back of the stove that controlled the amount of air that entered the primary. Basically it's a big heavy duty thermostat coil, with a wire hooked on one end. The other end of the wire is hooked to the intake flapper. As the coil gets heated up, it opens, closing the air intake, then again as it cools opens it back up. It was along time ago, but I don't remember having any issues with it at all. Fast forward to a few years ago when I bought my used and abused H1 off CL. It was missing a bunch of parts including the Bi-metallic coil. While talking back & forth with Jim Casavant at Hearthstone he told me that they stopped using the coil and replaced it it a all manual control. At the time I didn't think too much about it and had my work cut out re-building the beast.

Well, Once I fired it up and started using it I realized that the all manual control is kind of a pain in the arse requiring much more user adjustment. So after a almost 3 year search, Defiant here(THANKS AGAIN DEFIANT!) shipped me a proper coil he took off a parts stove. The fire does not get more air as it burns down with manual input unless you are monitoring it all the time, which in turn means more coals. With the coil, it opens up and gives those coals more air which has given me a noticeable more complete burn. Once adjusted correctly the coil controls the air intake perfectly. I can load it up and walk away for 8-10 hrs. Every time I look at the flue temp monitor it's running at optimum temps(usually between 425 and 500), and I have not touched the coil or intake flapper since I installed it over a month ago.

So the question is why would Hearthstone abandon a very simple air intake system that works better(in my opinion) then an all manual one? It can't be cost, the thing could not cost Hearthstone more then $20. Do y'all think it was because of user error....idiots unable to figure out how to set it up right? Or something else?

This is what one looks like for those that have never seen a Bi-Metallic coil:


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## begreen (Mar 21, 2013)

Not sure why they abandoned it. It was a great feature. VC kept it as well as Blaze King.


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## Motor7 (Mar 22, 2013)

I just sent Jim a update on the stove after the re-build 3 years ago. I asked him the same question, so maybe he has the answer.


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## Defiant (Mar 23, 2013)

Well Rick I am glad it worked out for you


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## Motor7 (Mar 25, 2013)

Well, I should have asked Jim the question first & here is _the_ answer:

"The bi-coil is one of those items that 50% of the users loved, and the other 50% requested a manual air control.  When we started  making the EPA certified stoves, the concern was that the bi-coil would start to shut the stove down before it got hot enough to start the secondary combustion (in the catalytic combustor originally – now via secondary air pipes).  Secondary burn requires fuel (smoke), temperature from the wood burn, and air supplied through the tubes.  If any of the three are missing, there will be no secondary combustion and therefore no certification."


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## Motor7 (Mar 25, 2013)

I had not thought about CAT operation but it looks like I was right about user error. I wonder how long it will be before we see digital & servo controlled air intakes on CAT stoves...or does that exist already?


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## Jags (Mar 27, 2013)

For the record, some of the old earth stoves also had a coil.  Different design, but same purpose.

There are some electronics coming to market as an after market add on, and I believe one stove MFG. is using it in the "wood stove design challenge":
https://www.hearth.com/talk/threads/wood-stove-design-challenge.106561/#post-1390448


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## blades (Apr 1, 2013)

Yep there were a couple threads on those subjects


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## MASohio (Nov 25, 2013)

I need the exact thing l believe, can you help me contact someone to get the right model number or representative to talk to and walk me through this.  The thermostat bi-metallic coil assembly kit,!!  thanks


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## Ed Gerken (Nov 25, 2013)

Hi, my first post. Nice forums here, hopefully I can contribute a little.

I've had a couple Ashley stoves over the years. Their damper flap is more accessible than the Hearthstone shown in the pics. For me, the bimetal was too slow in dealing with temperature changes. The fire would either roar or go out on me. I made a small wedge from scrapwood to set a fixed draft. I shut the damper down to low, inserted the wedge in the draft opening to hold damper open slightly. This worked for setting a manual temp. Just slide the wedge in further to increase the burn rate, withdraw it to shut the stove down. But, I still had to watch the fire till I got the wedge set to where I wanted the fire to burn, and as already noted, this needed changing often.

So, I converted my stove to electric thermostatic control, using a simple bitmetal switch stolen from the fan in a propane furnace. The switch is held to the stovepipe with some baling wire. An old 8-track tape player provided a 12-volt solenoid that I use to open and close the damper. The Ashley has a series of holes on a disc in the damper to choose a minimum burn rate. 

The whole thing runs on a wall-wart and you can hear the solenoid kick on or off for an audible signal of proper stove operation. I control the burn temp by raising or lowering the temp switch on the flue. A master switch provides instant off if need be. By just running a wire and adding the control, it could be converted to a wall-mounted thermostat, retaining the flue-mounted switch for a hi-temp safety override.

We've been using this setup for years now and it works great. I hold the doors open a bit to get the fire started, shut the doors and the system takes over from there. That's it!

Wiring couldn't be easier, simply daisy-chain the parts with the wall wart and the parts were scrounged, so zero cost!

-Ed


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## KDMANN (Nov 25, 2013)

The stove that came with this house I bought (Aunt Sarah) has a spring control for the air in. It was missing the knob ...so I made one. I adjusted the air- in when I first fired the stove...haven't touched it since....LOL . I add wood now and then ...and she's on auto pilot. Man , my first wood stove ever is spoiling me already. I think It's a nifty feature. I like the sounds of your setup too Ed...Very Cool !


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## Motor7 (Dec 2, 2013)

MASohio said:


> I need the exact thing l believe, can you help me contact someone to get the right model number or representative to talk to and walk me through this.  The thermostat bi-metallic coil assembly kit,!!  thanks



If you're looking for one for a Hearthstone, good luck....it's a white whale. They stopped making them decades ago. I even printed off a list of Hearthstone dealers and called them all....40-ish calls later....zip nadda nutten. The only place to find one is from a boneyard donor stove(I even considered buying a whole beater stove just for that part) & I was extremely lucky Defiant read my post and helped me out. 

Ed, glad you built a system that works, I was considering fabbing something up in that form before finding the old coil. I agree with you when starting a cold stove, the coil takes way too long to receive enough heat to close in a timely manner. But, if you burn 24/7 during winter like I do, the thing is amazing...I never touch it and the Heritage I bought back in '83 was the same way. Btw...what the hell is a wall wart?


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## BrotherBart (Dec 2, 2013)

Motor7 said:


> Btw...what the hell is a wall wart?



Those plug in power supplies that accumulate around a house like crazy.


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## Motor7 (Dec 3, 2013)

Weird.....56 years and never heard that term


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