doctorclean wrote:Hello Gentlemen,
I hope you can give me some advice on my Harmon MKII setup. . I do have a manual damper and no manometer at this time.
did you ever get this sorted out ? a few thoughts...
nix the manual damper, Harman actually specifically states not to use one on their new stoves. Next time it backdrafts open the damper wide open, see if it stops. If you have a Mk. II it may be the top outlet stove ? I believe all the Harmon free standing stoves went to rear pipe outlets, but I'd have to check their product line to be sure.
one definite, change the chimney cap- get the type that has a separate band between the top and bottom, those are specifically designed to increase draft upwards no matter what direction the wind is from. See below, it's called a "windcap". I'm using one on my Harman Mk I and it never backdrafts, ever- even though the huge oak and maple trees around my home are quite close and tower over the chimney and house by at least 20 feet.
A chimney with no cap is the most vulnerable to the adverse effects of wind. A cap, particularly one that has baffles to prevent direct line of sight access to the opening (as opposed to a simple flat rain cap) provides significant protection from the adverse effects of wind. In fact, research has shown that caps with baffles (of the sort common on factory-built chimneys) can actually enhance draft regardless of wind direction.
This kind of cap can take adverse winds and convert them to upward flow in the chimney.The cap shown at right is the type of design common on factory-built
chimneys. Note that the baffle, in the form of a band between the cap and the skirt at the base of the cap, prevents direct access of the wind to the open top of the chimney. This simple design consistently produces a driving pressure at the top of the chimney, regardless of wind direction or speed.
Adverse pressure can also occur when the top of the chimney is in a positive pressure zone caused by the velocity pressure of the wind as it flows against a raised part of the building behind the chimney (below). This is one case in which adding to the height of the chimney may help to resolve a wind-related venting problem.
next issue, you have the classic pressure differential problem created by when a rooftop is over and near a lower chimney, see below, someone else pointed this out earlier with red arrows showing airflow downward into chimney
For example, wind can often flow down towards the top of a chimney after passing over an obstacle like a roof, adjacent building or trees. Wind may also approach the top of a chimney from below after flowing up a roofline to a chimney penetrating the peak. Wind tunnel testing has demonstrated that wind flowing from either above or below the chimney top can be adverse to upward flow by creating positive pressure at the top of the chimney.
Note that the thick black line in this and the other house drawings on this page is the building envelope, which contains the insulation and vapor barrier that encloses the warm spaces of the house.
and you also have a variation of this related problem, pressure zone created by the lower chimney next to higher rooftop
Adding height to this chimney could get its top above the positive pressure zone and also make it higher than the second floor ceiling.