Hearthstone Craftsbury in a tight home, strong draft.

[49thparallel]

This is my second winter in our new well insulated and tight SIP house. We brought the Hearthstone Craftsbury from our previous smaller but much less insulated house into the new one.

I immediately discovered that the draft of the new chimney is quite strong and apparently more than the stove can handle. It is a straight 6” flue with double wall stove pipe for the first 6’ to the ceiling and 20’ of stainless triple wall Security chimney from there.

I had run this stove for 5 years in the previous house and never had issues controlling it. The first fire in the new ran out of control quickly and I had to plug the air intake on the back of the stove.

I have since added a probe type flue gas thermometer and damper above the stove and have been very careful about how I build fires, tightly stacking the wood and burning top down.

While I can keep things under control, it requires vigilance and shutting down the air and damper very early to keep the flue gas below 800. Also, despite having clearances 30% over the minimums to combustibles for a corner installation, I’ve seen the drywall get upward of 160f on a regular basis. Additionally, given the fast, hot burns that are inevitable it is impractical to use the stove for much more than ambiance in the evening.

I am considering moving to a Blaze King Sirocco 30.2 in order to achieve a lower and slower burn. In the experience of fellow forum goers, would doing so address the issues I’m having or am I just throwing money at the problem? Can the Sirocco handle a strong draft (as near as I can measure 0.06” when all is shut down and 0.08 or more when opened up) and would the ability to burn lower fill the need I have?

It just so happens that in arbitrarily oversizing the clearances and hearth pad for the Craftsbury, I made it the exact size necessary for the Blaze kings. Perhaps it is providence…

 

[stoveliker]

I would first try adding a second key damper.

Too much draft is not good for a Bk either. Some folks see their cat getting clogged by fly ash sucked into it and cats don't live long with overdraft.

If a second damper works, you spent $20 rather than a few thousand, and are operating a stove you know and had life left.

That said, sometimes things may work better if the secondary air is restricted. Other folks (with tube stoves) know better.
@Todd maybe (though he doesn't have this model).

 

[Todd]

Looks like you already have a pipe damper but something else to try is finding your unregulated boost air which is usually located at the lower front of the firebox under the glass. Blocking that should give you a little better control. I also wouldn’t rule out partially blocking some secondary air wherever that comes in with magnets.

every stove pretty much is set up and tested to burn on a 15’ chimney so with taller chimneys you really need to adjust the draft with a damper or cut back so air somewhere. Especially for non cat stoves since they let in more air than a cat stove will.

 

[49thparallel]

Thanks for the input. So if I were to get a Blaze King for the reasons I mentioned, I would still need to address the strong draft, correct?

 

[Todd]

Hard to say for sure but yeah all these modern stove are tested on a 15’ chimney with optimal draft and there’s so many variables it makes it tough to figure the right way to go.

 

[stoveliker]

Can you buy a manometer (up to 0.2) and measure the draft during a burn?
That'll tell you.

 

[BKVP]

117+ambient is value for clearance. So if inside home is 65 + 117 = 185. So your sheet rock at 160 would still pass. These are done on high burn. Close, but still ok.

BKVP

 

[BKVP]

Hard to say for sure but yeah all these modern stove are tested on a 15’ chimney with optimal draft and there’s so many variables it makes it tough to figure the right way to go.

Stoves are tested on dilution tunnels. Where the dilution tunnel ends, the stack is centered in a draft hood, where velocity is controlled with a fan. Over length is much more than 15'.

I would say, based upon manufacturers experiences, most recommend 15'.

BKVP

 

[49thparallel]

I used a digital manometer I got to balance my ERV. Measuring the draft through the hole for the flue gas thermometer (which is above the damper) it reads 0.15"H2O with the stove shut down and the damper closed. If I open the damper it reads 0.20. If I close the air intake as much as possible with a magnet and stick the manometer pipe into the small opening it reads 0.12. It is -16F outside right now with a 5mph wind, so decently strong drafting conditions.

 

[stoveliker]

That is more than most numbers being recommended; 0.05" wc.
But it's above the damper so this being dynamic pressures, there will be a small difference.

 

[49thparallel]

An update on this- I got a magnet to restrict the air intake on the stove and it seems to be the answer for this stove. When I measure the draft on the intake side with the air intake 90+% closed off it is 0.06-0.08" water column and the stove functions much more controllably. I was able to burn it through the night at -30F outside and relight from embers in the morning.

Since I know that I can control the draft decently, could I reliably do the same with a Blaze King Sirocco? And in the forums opinion, would I gain enough with the long, slow burn to justify the $1,400 net to upgrade after selling the Craftsbury? I'm still debating here, especially as I don't know what the air intake on the blaze kings is like. Presumably a person could get the fresh air kit and restrict it with a gate valve, correct?

 

[stoveliker]

I would not advise messing with the air intake on a BK.

Moreover the problem is not on the intake side, it's on the "suction" side, i.e. the draft in the flue.

If you can control the current stove better now, why not keep it?

For a BK I would Add a second key damper instead.

Also the longer burn would be at a reduced BTU output. Would that be enough? Do you know your BTU needs?

 

[49thparallel]

I see. Is there an aesthetically pleasing way to get the restriction provided by two dampers?

According to the load calculator I used, my heating load is just shy of 40k BTU. This seems to match up with the energy use of my heat pump at around 5kW at full load assuming a COP of 2 or so when it’s below zero.

 

[stoveliker]

They can be . mounted from the back, though I would think it's good to have a visual check that they are open before you open the door.

I find two shiny metal handles to not be an aesthetic problem though - but (my) taste is a strange thing.

40 kBTU per hour? Max or average over the year?
That's is still quite a bit, and is in fact near the top of a model 30 firebox. I.e. it won't be enough in the coldest days if this is an average number.

On the other hand, the shoulder season could be handled all by the BK, if the heat spreads enough.

I still think that changing a stove to resolve draft problems is like changing the auto body to resolve engine issues.
You're still going to need to tame the draft. I believe all stoves are tested at the same (induced) draft anyway.
Folks with too much draft d see cats clogging with fly ash in BKs.

And when you have the draft resolved,. there's no need to change stoves...?

 

[49thparallel]

40k BTU is the manual J design load, so it’s the size of furnace you would install. I think it’s meant to be the load on the coldest 97% of days or something, so I think a 30 firebox would be ample?

I have enough solar gain that I don’t need any supplemental heat during the day. My thinking goes that a catalytic stove will have more modulation to get through the night, but based on what your saying maybe it’s not the panacea I think it is.