Hearthstone Mansfield 8013 - Auto temperature control

[5U4]

A neighbor has an old Vermont Castings with a control system, very simple, a bi-metallic spring controlling a flapper that regulates the amount of air that enters the stove. The spring monitors the temperature of the casting at the back of the stove.

A system to automatically control the burn rate and limit the maximum stovetop temperature would be a nice addition to the Mansfield. The question is how to implement such a system as far as where physically to sense the temperature and how to actually control it?

Where to start, any thoughts?

 

[Todd]

I would rather have a Blaze King thermostat on my stove.

 

[stoveliker]

But I think that is more or less the same?

I'd put it near the flue exit.

 

[5U4]

Sensing on the casting at the flue exit seems right, the question is how to control burn rate with a low force bi-metallic spring? The blaze king nicely integrated the control at the flue exit and the spring attached to the control plate; this is ideal requiring very little force to rotate the plate. The Vermont casting mounted the spring on the back casting with a chain going down to operate a small plate covering the air inlet; this system also requires very little force to operate. On the Mansfield there is no convenient location to put a plate to control the inlet air flow.

Looking at the air inlet location on the Mansfield it may be possible to add an inlet air control plate however would require the installation of the outside air kit, the kit would have to be modified to incorporate the control plate.

One concern is how the air control system works on this stove. There are two air paths, one to the primary air and one to the secondary that may or may not both be controlled by the control lever. When observing the operation it appears the control lever only adjusts the air flow to the primary air, and the secondary air flow rate is fixed. I can't prove this but suspect it. If this is true, and the fact that both air paths have a common inlet at the bottom of the stove, adding the outside air kit with a plate to control the air flow would interfere with the intended operation and causing the burn process to degrade.

Any thoughts or information on exactly how the air circuits are controlled in the Mansfield?

 

[stoveliker]

Secondary air often is not controlled. It is regulated by the strength of the draft in the flue, which increases when it gets warmer (hotter fire).

I am not sure a thermostatic controlled primary air makes a lot of sense when there is uncontrollable secondary air.

I think decreasing primary air can actually increase heat output (at least in some range). A thermostat wouldn't work well then.

 

[John Lehet]

I think decreasing primary air can actually increase heat output (at least in some range). A thermostat wouldn't work well then.

That was definitely the case with my old Jotul 3 CB. If it was cold and the draft was strong, the hottest fire was just off of fully shut down. Definitely non linear.

 

[stoveliker]

Yes. Even worse, the sign of the result (more or less heat) changes with the cutting of the air in some range.

 

[Highbeam]

I’ve read the Amish like to use wood furnaces but without electricity so they attach an off the shelf thermostat contraption to open and close the air intake that uses a bimetallic coil. The device exists.

Controlling the primary air only with a thermostat is just like adjusting the intake control manually so you have the same level of control as normal. A valve over the OAK is a very reasonable place to begin experimenting with this concept.

 

[stoveliker]

Except that it will act opposite to what you may want; the stove getting hotter, it decreasing the primary air, leading (due to secondary air system) to an even hotter stove .. and this at the range where the primary air is almost closed.

It will act the other way when the air is almost open, I think.

Due to this, I think a coil thermostat is unsuitable for a stove that (also) has secondary air.

 

[begreen]

A neighbor has an old Vermont Castings with a control system, very simple, a bi-metallic spring controlling a flapper that regulates the amount of air that enters the stove. The spring monitors the temperature of the casting at the back of the stove.

A system to automatically control the burn rate and limit the maximum stovetop temperature would be a nice addition to the Mansfield. The question is how to implement such a system as far as where physically to sense the temperature and how to actually control it?

Where to start, any thoughts?

VC stoves have always had thermostatic air control. Some of the older Hearthstone's had thermostatic regulation too. It probably would be possible to rig up something on the outside air control. For some stoves, a servo driven thermostatic control has been employed. There was a cool thread on this several years ago.

Thread 'Wood stove automation using the Raspberry Pi'

Jan 2, 2018

Happy New Year!

It has been a couple of years since I started working on my wood stove automation project. Its based on Python and currently runs on a Raspberry Pi3 Model B. I got quite far but until now I never documented any part of what I did. Took me about a week to take pictures and write up just the first phase. I don't think Ill ever be done so I'll just post this before I get lost in trying to make the doc perfect. Yes, it needs more pictures that I should be able to add soon. Feel free to use any part of this project for your own builds. I have learned a lot about...

• MisterFixIT
• automation raspberry pi
• Replies: 55
• Forum: The Hearth Room - Wood Stoves and Fireplaces

• 

 

[5U4]

I'm confident controlling the primary air knob will produce good results however a bi-metallic spring can't produce the force required to move the poorly designed air control, the one thing that bugs me on the Mansfield! The primary control is rough to move and much too sensitive in controlling the air flow rate, it's more like on and off than linear.

So if the secondary air inlet is not adjusted with the primary control and the primary air inlet is not accessible to fit a flapper things become more difficult, but depending how the air channels are routed in the casting it may be possible to cut a slot in the casting into the primary air circuit and put the flapper there.

Option II, any thoughts on installing a flue pipe damper and controlling the damper with the bi-metallic strip? A mechanically balanced damper would take little force to control and simple to fabricate, also be in a good location for lining up with the spring mounting location.

 

[stoveliker]

I like the flue damper idea. Gotta figure out if the time it takes for the coil to move is enough to regulate draft.
And you'd need to build a contraption to keep some heat on the coil. May not work for a double wall stove pipe?

 

[John Lehet]

When I was quite young for a couple of years I burned with a VC smoke dragon with a thermostat. It was in a group household rental with some other young men. We were busy with our lives, and not particularly thinking hard about the wood stove. In that case the thermostat just meant that we could continue to be stupid about the stove for the whole time and never really learn to burn wood properly in that period. I think it might be that way for a lot of people: there is some tendency to just treat it like an appliance. Maybe lots of people would rather treat the wood stove like a don't-think-about-it-appliance. But in fact a wood stove is complicated, and also dangerous. Everyone who uses one should understand and think about it as much as necessary.

That said, no experience with Blaze King, but it seems possible with serious design consideration the thermostat could work. From what I hear I'm pretty sure the thermostat on the BK is well done and overall helpful. VC, who knows.

The one case where I wish I had something like a thermostat, or a timed control on the intake is night burns. With my Progress Hybrid, I'll put 18 - 30 pounds of wood in it at 9PM or so and then spend 30 to 40 minutes fiddling with it to get as minimal as possible secondaries running, but not cycling between black box and woosh flame. This is a little fiddly, depending on draft, how hot the coal bed is, how much wood. It'll change during that period. No way to automate that, I don' think, I have to watch it and fiddle with the intake a bit before bed.

But then in maybe 4 or 5 hours or something, in the middle of the night, it's very low flaming coals, and then just a lot of coals. At that point ideally the thermostat would be shut as far as it will go. I'm not going to set an alarm to get out of bed to shut it that last bit. Some kind of auto thermostat would be great in that case to get less air moving through those coals and up the chimney. Some kind of "it's coaling, shut it down" automation would be a breakthrough.

 

[stoveliker]

The BK thermostat is indeed set it and forget it - after dialing it in initially (which is not that different from a tube stove: run high for a bit and dial down...).

But the thermostat opens when the stove cools down during the coaling phase.
And I think that's more desirable than one that closes then - the goal is consistent (even) heat output. Coaling gives less heat, so one needs to speed up the process then to get similar heat as earlier in the burn. So the thermostat opens.

At least for 3 cu ft stoves one does not need to close things off to keep some glowing coals for a relight in the morning; enough fuel to reach the morning without such middle of the night shenanigans.

 

[John Lehet]

With the Progress hybrid, that heat is already in the stone. The sides of the stove if not the top are about peaking in output at coaling time when doing long burn cycles. So far it hasn't been cold enough for me to run this stove with flaming fires back to back very often in this house. With this stove, in this house, I usually like to let the coaling last about 6 hours, let it coast down until just enough coals for the next fire. In the evening I will run a couple of active fires back to back to build up some coals and momentum for the night -- makes it easier to put the big load in at bedtime, it seems. Still figuring it out.

Chilly stretch coming up though, so everything might be different.

 

[John Lehet]

Was also thinking the OP has a Mansfield. A different stove from my Progress I guess, but for sure whatever is going on on the outside of the stove temp is lagging what is going on in the firebox by a long time interval. Any change to the damper needs to be a response to the firebox interior and not the stove exterior with a massive stone stove.

 

[begreen]

Qwee pointed out this regulation device in another thread. You may need to use Google Translate, but this device may be worth checking out.
https://www.ofen-schraps.de/kamin-und-kachelofenbau/elektronische-abbrandsteuerung/
Explanation of operation.

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

Accept third party cookies

 

[John Lehet]

That is very cool. I'm sure it's worth 600 euros, but maybe not to me, right now. I'm surely not going to live long enough that it would save me that much on wood non-optimally burned.

 

[5U4]

Determining the relationship between the three zones, metal casting, flue, stone, is the challenge along with getting a bi-metal spring with the right temp profile characteristics.

As far as the ABS control, that's cheating, it's a fairly simple task when you through a micro-controller at it! I'm surprised there isn't an o2 sensor in the mix.

Time to log the different temperatures during the burn cycles and see if we can make sense of it. I'll post the results when available for further discussions.

 

[5U4]

cycling between black box and woosh flame.

In my limited experience with operating a wood stove, with the Mansfield, I believe the "black box" can be minimized by bringing the stove up to temperature at a slower rate. I figure when a fresh load is heated fast then the air is chocked down, the gas continues to emit for some period of time from the exterior hot wood layer then slowly diminishes as the exterior of the wood is cooled by the internal wood temperature causing the flame to slowly die off. Not saying this is fact, just speculation.

I now use a cycle based on flue gas temperature; load the fuel, door open a crack, wait for gas temp > 200f, close door, wait for >300f engage cat, wait for gas temp 500f, throttle down to 25%, wait for gas temp 300f, make final adjustment. This method is not perfect but 90 percent of the time the fuel load goes in at 8:30 and off the sleep at 10:00

Using temperature as a guide seems to compensate for all the variables associated with burning a piece of wood! The basic variables, stove temp, coal bed, draft, wood type, wood moisture, surface area....

Give it try, your temps will certainly be different but once established it may make for a more predictable burn.

 

[John Lehet]

An auto control for the main air intake I think might also be non-optimum for my Progress Hybrid, as I believe the primary and secondary air are both controlled by the damper but in a non-linear manner. They don’t open and close at the same rate with movement of the lever.

In my limited experience with operating a wood stove, with the Mansfield, I believe the "black box" can be minimized by bringing the stove up to temperature at a slower rate

Yes, this actually worked well for my overnight fire last night, as long as I stayed up to watch it.

 

[Leeks]

The older Hearthstone II had a bi-metal controlled primary air intake that worked very well. After 35+ years, we retired our aging Hearthstone II after the 2023-24 heating season so I can attest to how well it worked. The Hearthstone II air intake is likely very similar to the Vermont Casting. We've since purchased a Progress Hybrid and are on our 2nd year using it.

The Progress Hybrid air intake control seems to mostly control the secondary air/air wash with some (very little) air ending up at a single small hole in the bottom - middle of the andiron base plate directed at the coals. This very limited "primary air" configuration on the Progress Hybrid makes for really nice secondary burn displays and produces more smoke for the catalyst which is I guess how this stove is supposed to work. But trying to get a fire started or restarted from just a few glowing coals can be a real challenge.

I've since added a bi-metal controlled primary air intake to my Progress Hybrid. It works really well and IMO greatly improves the overall operation of the stove, especially start-ups and low temperature reloads.

Here's what the Hearthstone II bi-metal thermostat looks like. The added piece of wire on the upper handle was for a reference to set it.

 

[Todd]

The older Hearthstone II had a bi-metal controlled primary air intake that worked very well. After 35+ years, we retired our aging Hearthstone II after the 2023-24 heating season so I can attest to how well it worked. The Hearthstone II air intake is likely very similar to the Vermont Casting. We've since purchased a Progress Hybrid and are on our 2nd year using it.

The Progress Hybrid air intake control seems to mostly control the secondary air/air wash with some (very little) air ending up at a single small hole in the bottom - middle of the andiron base plate directed at the coals. This very limited "primary air" configuration on the Progress Hybrid makes for really nice secondary burn displays and produces more smoke for the catalyst which is I guess how this stove is supposed to work. But trying to get a fire started or restarted from just a few glowing coals can be a real challenge.

I've since added a bi-metal controlled primary air intake to my Progress Hybrid. It works really well and IMO greatly improves the overall operation of the stove, especially start-ups and low temperature reloads.

Here's what the Hearthstone II bi-metal thermostat looks like. The added piece of wire on the upper handle was for a reference to set it.
View attachment 345455

More info and pictures on your PH please!

 

[Leeks]

Not wanting to hijack the OP thread, but here are a couple pictures of the modification.

2 things are necessary:
1-You need the optional Progress Hybrid ash pan (for the new primary air intake)
2-You need the optional rear heat shield (to mount the bi-metal coil & control arm)

The design is basically a copy of the Hearthstone II air intake shown above.
Note: My Progress Hybrid has the left hand side loading door.
I have a 9 page .pdf file that I sent to Woodstock on my stove installation & details of the modification. I can send it to those interested or start a new thread.

Yes, it probably voids any warranty.
Yes, it probably invalidates any EPA certification.
But, IMO, it vastly improves the operation of the stove.

Control arm with bi-metal coil.

Ash pan modification. Note: the chain shown above attaches to the center of the flapper door.

 

[John Lehet]

Wild! So you are putting air into the ash pan and up through the grate instead of air washing down the front of the stove. What did Woodstock say about it?