Another question - to bypass or not bypass?

Thomas mentioned installing a bypass in another thread.  I saw a video with Matt Walker, and he said he doesn't like to use a bypass.  When two people I respect have different opinions on something like this, it generally means the "thing" whatever it is, is dependent on your circumstances, knowledge, or abilities.  That said, can someone give me a list of the pro's and con's of the bypass?  One pro Thomas mentioned is ease of starting, but I'm assuming there are others I am unaware of?

Hey Trace;
The real purpose behind a Bypass is to have a happy spouse, whose home does not get smoked out when you are lighting your brand-new build for the first time!

Once a batch is up to temperature for the season, the bypass usually remains closed.

A bypass is nice during shoulder season, allowing you to have a fire for cool damp days without heating the entire bell.

If a stove has a bypass the operator must be made aware of its function and how to use it.

Unknowingly leaving a bypass open during winter, would send most of your heat out of the chimney.

Every pipe connection is a potential leak.

I wonder if a bypass might be useful in a locale where there is sometimes a downdraft coming off the trees or hills (risking an annoying backdraft). A taller chimney helps a lot, but pushing some direct heat gives a little extra oomph during startup.

thomas rubino wrote:Hey Trace;
The real purpose behind a Bypass is to have a happy spouse, whose home does not get smoked out when you are lighting your brand-new build for the first time!

Once a batch is up to temperature for the season, the bypass usually remains closed.

A bypass is nice during shoulder season, allowing you to have a fire for cool damp days without heating the entire bell.

If a stove has a bypass the operator must be made aware of its function and how to use it.

Unknowingly leaving a bypass open during winter, would send most of your heat out of the chimney.

Every pipe connection is a potential leak.

Now that you mentioned it, I think I do remember Matt saying something about people using the bypass incorrectly and leaving it open as you said. That makes perfect sense. Thank you.

I built a bypass into our Matt Walker tiny cookstove, and I wouldn't want to be without it.  However, I can see Paul's issue that if you have different people with no experience lighting the stove, it offers another way for them to get it wrong.

Here we live in a warm climate (compared to, say, Montana) and it can be chilly and damp so you want the fire alight but it's not ideal conditions for starting a rocket and then the bypass is a big help.  The bypass opening is approximately 6" x 3½" and is at the back of the core directly into the top of the brick flue just under where the chimney connects.  You can see it in my build thread

My normal lighting procedure is as follows:
1) make a small kindling fire (with cardboard or a pine cone or other tinder and thin, 6"-long or so sticks) right at the back of the box just in front of the secondary air tube.  With the bypass fully open it normally takes less than a minute to start that roaring nicely.
2) add full length fatter sticks and some larger pieces on top of those until the box is mostly full.  Those catch light at the end from the small starting fire, and in turn start roaring nicely.  As soon as that wood has caught I mostly-shut the bypass.
3) monitor the stove for a few minutes, especially the lower oven door, which is a plain 3mm steel.  When that door gets to about 26-27°C (78-80°F) the bypass can almost always be shut fully.

If, on a marginal day (e.g. bad wind direction or too warm outside), I shut the bypass completely too soon it can stall and belch smoke into the house which nobody wants.  This stall is unmistakeable: it will be burning fine and then quite suddenly will stop and clouds of smoke blow back from everywhere.  From what I see, this is due to the colder air in the lower spaces of the stove getting into the flue and reversing the temperature gradient.  

The other thing I would have built into my stove had I thought about it is flue priming access.  This would be a removeable plug of some kind at the bottom of the brick flue space near the back of the stove, and would allow for placing a small amount of cardboard or other tinder at the bottom of the flue.  I have a means to do that which is a small shovel on a long handle, which I can insert through the lower oven door into the bottom of the flue but that's more hassle.  Priming the flue like that makes a difference on marginal days when the outside temperature makes it hard to get the flue drawing.  Likely in a colder climate you'd find less use for that feature though.  In the same way, we don't generally burn the stove enough that it stays all that warm 24/7, so we do more cold starts than you would if you have freezing weather outside.

Burra says I should mention the idea of using the Walker stove as a pure cookstove, and she's right, I should have .  On an otherwise warm or hot day we might want to use the stove for cooking, if for example we were out of gas or the electricity was off. (we normally cook in a mix of gas stove and combi microwave/convection oven)

By having the bypass available, it would be possible to light a fast fire with small sticks, and leave the bypass fully open, so minimal heat would stay in the house.  Under those conditions, half the hotplate on the stove would heat up quickly.  We also have lids we can open in the hotplate, one of which is right above the outlet from the core and would be great for boiling water or for setting a wok on to stir fry stuff.

Having experienced cold plugs, shoulder season burning, wind gusts, new builds etc which can all be potential back drafting material, I would not build another RMH without a bypass of some kind.
A priming port that Austin suggested would be my second choice... Lower tech (meaning slightly less complex than a bypass to install and bullet proof for anything to go wrong with) but not quite as functional as a bypass (can only be used for priming and as a cleanout/ inspection port).
After deciding on a bypass, the next question would then be what style to choose?
There are simple to complex based on your skill level.
A topic already discussed on Permies but could easily be revisited to help keep that resource updated.

Just chiming in with support for the pro-bypass crowd.

When your RMH has sat idle for a period of time and the system is now at the ambient temperature of your room it would suggest that the outdoor temperature is not real cold. In this case the bypass will be invaluable in overcoming the pressure required to get a draft through the system and up the chimney. If you visualize the system with the combustion core leading the heat up the riser, the heat hitting the top of the bell (steel barrel or masonry) then reversing direction in a downward path to heat the bell and finally dropping down to the lower area to again reverse direction and exit up the chimney... it is easy to visualize why a cold system will have a very difficult time overcoming the resistance to start the draft out the chimney. The bypass solves this problem of overcoming two 180 degree direction changes and the associated frustration of starting your system. It may only be used for 10 minutes to start up a cold system but you'll probably be thankful that you have it.

Think of it like a choke on a gas engine, try starting your gas chainsaw without a choke. Then think about how much time you spend running your chainsaw with the choke on vs the choke off.