bypass alternative?

[iau461]

I'm getting close to starting my DSR3 build. Have not been able to find/design a bypass valve design that pleases me. Having a bypass valve inside the bell seems to mean:

    1) requiring human intervention to remember to open/close the bypass as needed, before/after start of burn
    2) using a leaky bypass, partway up the bell, which will allow some heat loss up the flue even when "closed"
    3) burying the bypass inside the bell, essentially inaccessible

Instead, I am considering putting an inline fan in the flue, above the bell. Probably this model:
www.amazon.com/gp/product/B07BQS1GB9/ref=ox_sc_act_title_1?smid=A2AW0W4FKP16S5&psc=1

FWIW, I already have a 6" version of that fan working for another purpose, and I like it. It is rated up to 230F, and the goal of course is to keep the flue gases well below that.

Using the included temperature probe, plus a separate Arduino mini-computer/temperature sensor, I would have the fan turn on whenever

    a) temperature inside the bell reads hot, maybe >200F (i.e., there's a fire)
    b) temperature inside the flue reads cold, maybe <120

When the fan turns on it would get the draft going, then shut off, while avoiding the human element, and eliminating heat leakage through the bypass.

Obvious downside is the electronics etc. but I am comfortable with that, as I have built several automated/Arduino systems that have been bulletproof.

Thoughts?

[gadget]

The temperature limit is a bit low for comfort. You are going to have a huge restriction when the fan isn't running.

[masonryrocketstove]

Somebody correct me if I’m wrong, but I thought one of the major upsides to a true “bell” was the hot gas stratification effect which allows the design to retain heat without a bypass.. so long as the exit flue is low at the bottom of the bell below the firebox door.

I guess if you really felt the need for a bypass, you could build the first few feet of the chimney out of masonry (maybe four or five feet up from floor level, higher than your firebox) and install the bypass at the top of that masonry chimney, just before the transition to double-wall / class A chimney pipe.

Doing it that way would keep the bypass out of the bell and you could do maintenance on it by pulling the first section of chimney pipe and only having 1 or 2 courses of brick above that bypass flap. Just keep the bell exit low near the floor and you should be golden, I’d think.

With a bypass In that location, the negative pressure in the chimney stack would prevent flue gasses from leaking into the room. If there were any leakage at all, it would only suck a tiny amount of room air *in*, rather than letting flue gas *out*.

Gadget makes good points also about the temp limit of the fan ..and the constriction when the fan is off, or goes out, or slows down when the bearings or shaft gunk up with fly ash and slow the rpm down.

[Dan (Upstate NY, USA)]

I think it will be destroyed by flue gases

[iau461]

Thanks all. I had the same thought this morning about the temperature limit. Somehow I had it in my head that "target" flue gas temp was around 150-200F... unfortunately 150-200C is what appears desirable. Back to the drawing board.

[josephcrawley]

Are you talking about a cold start bypass or a gate to close the flue when the burn is complete?

Either way there are several simple and reliable ways to do it. They will not be perfectly tight but a little loss is just not a big deal.

In regards to a bell holding hot gasses after the fire is out. Go up on the roof and you can see heat continues to cause a stack effect after the fire is out. If everything is air tight in the system it will be reduced but we aren't dealing with the theoretical here so it isn't.

[fiedia]

There are some mass heater designs without bypass.

When the mass is cold, the draft is started by burning a paper sheet at the bottom of the chimney vertical pipe. They have a door at this place which also helps cleaning the flue.