Direct Outside venting draft question

[rayttt]

I have a Harman p38 (and love it)
that I was using a straight out pipe...but because of
smoke in the house from a power failure...I removed the straight out pipe
and replaced it with the home depot out and up kit..
given me a 1 foot out, a tee cleanout ..up 3 feet and an 90 to a cap...
and since Ive done this the draft inside the stove actually appears to be higher than
just the straight out pipe...

I would think that the out and up which has to have more resistance
than just a straight out pipe (3 ft). Is it possible to have a higher draft amount with
more pipe? I would have to assume the draft should be less due to higher resistance of the t and length,
is that even possible.

 

[SmokeyTheBear]

I have a Harman p38 (and love it)
that I was using a straight out pipe...but because of
smoke in the house from a power failure...I removed the straight out pipe
and replaced it with the home depot out and up kit..
given me a 1 foot out, a tee cleanout ..up 3 feet and an 90 to a cap...
and since Ive done this the draft inside the stove actually appears to be higher than
just the straight out pipe...

I would think that the out and up which has to have more resistance
than just a straight out pipe (3 ft). Is it possible to have a higher draft amount with
more pipe? I would have to assume the draft should be less due to higher resistance of the t and length,
is that even possible.

The resistance talked about is that caused to the blower, the blower has to push the air through the pipe as well as pull the exhaust out of the firebox. That is not the same as draft although there is a relationship.

A draft is caused by a pressure (or temperature [which induces an additional pressure difference]) difference, the higher the chimney the lower the pressure at the top and the greater the pressure difference the higher the draft.

In addition air flow by the cap also lowers the pressure with the same effect.

What happens when the power goes out is that the blower blades form an air flow restriction in the flue and the only thing that will cause smoke to exit the system is a natural draft if you have a straight out flue then the pressure difference is too low to overcome the restriction caused by the blower blades. The difference doesn't need to be much and in fact with a properly sealed system and an OAK even a straight out setup shouldn't dump any smoke into house.

 

[Delta-T]

could just be perception. only by taking a W.C. reading could we really know. but, very intersting

 

[SmokeyTheBear]

Because resistance is not draft.

What the OP is seeing isn't likely perception at all.

 

[Dr_Drum]

Heat flows from hot to cold based on temperature difference. The greater the delta T (temp difference) the faster the air velocity. One thing to also consider is that air blowing across the end of the vent can also create vacuum. Which can be confused as increased draft.
Mike -

 

[SmokeyTheBear]

Air flows from high pressure to low pressure the greater the pressure difference the faster the flow.

 

[SmokeyTheBear]

OK, now that you all know which way the air will flow, whenever the resistance has been overcome you have a draft.

When a pellet stove runs it has a draft induced by the combustion blower, provided the blower can overcome the resistance caused by the exhaust system.

When the exhaust blower stops it adds resistance to the exhaust system.

In this case if both the air flow caused by the temperature and pressure can overcome the resistance of the blower fins plus the exhaust system a draft will be setup resulting in the smoke exiting the exhaust system.

The smoke will always follow the path of least resistance this is usually the air intake if there is no OAK or the hopper if it isn't sealed.

Edited to add: That is if there is no draft to cause it to exit via the exhaust system.