fire bricks for bell

[Forsythe]

Oct 4, 2022 7:04:36 GMT -8 hammer said:

can you give me a link or brief explanation of a cold start bypass, thanks

A bypass damper allows a short-circuit pathway directly to the chimney stack (externally-venting exhaust exit) which bypasses the longer, more circuitous flue pathway through the thermal storage mass. This bypass flue pathway is used when the chimney and the thermal mass are cold — allowing the hot gasses directly into the chimney during the early part of the burn cycle — which warms the chimney stack and establishes the strong draft needed to pull the gasses through the cold mass.



Once the chimney stack is warmed and drafting well, the bypass damper is closed — when the chimney's draw then has enough suction force to pull the flue gasses through the longer thermal storage pathway.




These pics are borrowed from Trev's Vortex thread (starting approximately here: donkey32.proboards.com/post/23834/thread ) — and give a really great example of how bypass flue pathways can work in a confined, space-efficient heater. (Hope Trev doesn't mind me borrowing these pics to label them)



There are 3 main types of bypass damper flap used: Pivoting-, Swivel-, and "Guillotine" AKA "Slot-" or "Sliding-" plate.

Trev used a Pivoting-flap style bypass damper plate, which is the easiest to fabricate and tends to give a good seal without having to do precision machining of the damper plate to the exact internal dimensions of the flue (as in the swivel-plate style,) and it avoids the problem of sticking / binding in place (or a slowly, incrementally decreasing width of opening) caused by any build-up of fly ash or brick sediment over the longer term (as can happen with the Guillotine/Slot-/Sliding-plate style, as the build-up of grit impedes the sliding motion of the plate within its channel slots and is said to be more of an issue with vertically-oriented slots than those positioned horizontally over a flue pathway, as the bottom channel of a vertical plate's slot will become the lowest point in the flue pathway for gravity to accumulate ash and/or grit.)



Swivel-plate type
(requires more-precise machining to block the flue pathway — and is a little more difficult to align with the exterior handle, to prevent overtravel of rotation, which translates to a bypass flap not fully closing or fully opening when the handle appears to be in the "fully open" or "fully closed" positions.)




Guillotine or Sliding-/Slot-plate type
(more prone to binding with long-term stove use, from ash / grit / sediment caught in the slot channels. The one on the lower right is from a German-language "self-build" (DIY) stove builder's forum... and appears to compensate for the ash-buildup problem by using half-rounds of tubing for sealing the flue, rather than sliding within a full "slot" (allowing for ash to be pushed out of the way) —and instead uses two sections of stainless tubing on the backside of the plate assembly to act as guide rails, keeping the plate rigidly oriented through its length of travel. Kinda neat, I think.)

[hammer]

Amazing, thanks once more Forsythe, truly appreciated. This seems doable but I think I will see if it comes good if it was moisture issue, all the bricks I used were salvaged and so most of them have been sitting outside in the weather for a few years.
I lit it again today ( half full) I just started it as Peter suggested, small fire at the back and build up slow, that worked great. I took the outside layer of insulation from the outside of the outer flue(inside the bell) leaving the well insulated flue inside the lager flue. I dont know weather that helped or not. The uninsulated flue that is outside of the bell does not get hot until there are only embers (hours later). When it is going full bore you can put your hand on it indefinatly not even uncomfortable. So in short, it seems to work well but am wondering what is going on with the flue.

[Forsythe]

Oct 5, 2022 6:24:42 GMT -8 hammer said:

Amazing, thanks once more Forsythe, truly appreciated. This seems doable but I think I will see if it comes good if it was moisture issue, all the bricks I used were salvaged and so most of them have been sitting outside in the weather for a few years. [...] The uninsulated flue that is outside of the bell does not get hot until there are only embers (hours later). When it is going full bore you can put your hand on it indefinatly not even uncomfortable. So in short, it seems to work well but am wondering what is going on with the flue.

It sounds like a whole lot of residual moisture, which just takes time and continued use to dry out.

I lit it again today ( half full) I just started it as Peter suggested, small fire at the back and build up slow, that worked great. I took the outside layer of insulation from the outside of the outer flue(inside the bell) leaving the well insulated flue inside the lager flue. I dont know weather that helped or not.

It all sounds like it's on the right track, and it's actually kind-of a good thing for it to dry out slowly, as that allows for all the wet brick, mortar, coatings, ceramic fiber rigidizer, etc. to dry evenly and avoid cracking. A month (or more) drying period isn't unheard of. I'd keep with that small-fire, slow build-up method Peter advised for at least a full month before hacking into it to add a bypass... especially given that your insulating bricks had been exposed to the elements prior to being installed. Over that period, you should have a very slow, gradual improvement to the rate of draw through the system with each successive firing. (Liquid water's evaporative phase shift to steam takes a LOT of energy... that's why evaporators are so effective as air conditioners.)

[hammer]

Its getting easier every time now, the bricks i used for the riser were very light weight and the refractory glue/mortar would dry as soon as it saw them so i dunked them in water for about 5 seconds and it was amazing it would just suck up amazing amount of water, but made it so much easier to lay and felt much more stable, but the drying process must really take a long time for reasons that you mentioned.
Overall I am really pleased with the way it is beginning to preform, much easier to light uses a lot less wood and stays warmer than my old slow comb log burner

[hammer]

I have finally finished (nearly), I have a bit of repointing on a couple of minor mortar joints on the outer bricks. I have used slate tiles that I used to build my house 25 years ago! I also want to tidy up the air intake. Here are a few more pics  www.flickr.com/photos/196601896@N05/shares/Ck5nxdf4ji

[Forsythe]

That's great work, hammer !

[martyn]

Yeah that looks great!