Hi Don,
Hmmm, sorry for confusion... this is one of those things that would take about 5 seconds if we were sitting at the same table and I could
draw what I'm talking about, but is fairly difficult to convey with words alone. But I'll try.
DonJones said:
...I understand that not only must the vent area be roughly equal between hot and return vents, but the cross sectional area of the two ducting systems must be roughly equal, correct?...
Ok, it sounds like you got the gist of it: To put it very simply, if you have a great big trunk duct supplying hot air out to say, 6 registers, and you have a smaller return trunk being serviced by 6 return vents, you are going to have greater resistance on the return side even though you have the same number of heat registers and return vents due to the fact that the return trunk is smaller in cross-section than the output trunk. the whole system is a cycle, right? So the maximum rate of air flow in the system will be governed by the narrowest restriction; it's the "weakest link" principle.
DonJones said:
...Upstairs
228 square " heat 260 square " return
Downstairs
228 square " heat No returns
Doorway at bottom of stairs
2788 square " w/no doors...
Since we're dealing with this problem "after the fact" from a design standpoint, attempting to engineer an exact solution will be difficult at best, and I don't think we need to in order to address the bulk of the problem. To elucidate: If, in order to fix your spaceship 100%, it would take a year, but to fix it 85-90% (enough to get back to Earth) by estimating and making intelligent guesses would take a week, we choose the latter. There are sooooo many factors just to calculate max velocity pressures - any bends, how tight the bend (90*, 60*, etc., etc) angled hallways, ceiling shape and height, furnace location, any reducers, etc., etc. - that to try to second guess every feature in the sytem become overwhelming. What we need to do is to identify the problem in general terms, and then make an educated stab at resolving it.
With that in mind, let's assume that all trunk ducts are at least adequate in area and we'll leave them alone for now. Looking at your vent and register areas only then, we see that you have a total of 456 sq/in of register for both floors, but only 260 sq/in of ducted return, all of it upstairs. Remember, the "virtual duct" of the stairwell only serves to convey the downstairs return air to the top floor, not all the way back to the furnace. Once upstairs that (downstairs) air has to compete with the upstairs return air to get back to the furnace via the 260 sq/in of ducted return.
THE (POSSIBLE) PROBLEM: INADEQUATE OVERALL RETURN CAPACITY MAY BE SYSTEMICALLY REDUCING VELOCITY PRESSURE, THEREBY REDUCING RATE OF CIRCULATION. THIS MAY BE CAUSING "STALLING" OR STAGNATION AS A RESULT OF UNRESOLVED PRESSURE DIFFERENTIALS.
So, linear troubleshooting tells us that, though it may not be
the problem, it is
a problem, and it appears to be the "squeakiest wheel" so it's a logical and educated assumption that we start there. Our weak link, then, is that there's close to twice the cross-section of output to return. Providing greater return sounds like a plan. We already got to this point I think. So, where to put it?
I suggested locating any additional return capacity upstairs because, given no other info and not being there to see it with my own eyes, this is the solution for greater return that compromises the original design the
least. In your system, all the returns are upstairs... all we'd be doing is to increase the return capacity. We wouldn't be changing anything else. If we effect the return increase
downstairs, we'll
also be changing the overall flow path by reducing the volume of air coming up the stairwell from downstairs, savvy? And since neither of us know what effect this will have on system balance or circulation, prudence dictates that we take the path of least "disruption."
DonJones said:
..Something else that has me confused is all of the 2 story houses that I've lived in only had cold air returns on the ground floor and none on the 2nd floor, but they seemed to work just fine and to keep the temps more uniform than what this one does...
This is, IMO, a far better design. Warm air finds its way upstairs all by itself. Locating the returns low in the system ensures proper circulation in that part of a house that's most difficult to keep warm; the bottom floor. Let the top floor take care of itself! Unfortunately this is not the case in your home, but we can take a lesson there and try something. Have you tried restricting the warm air to the top floor by partially closing the upstairs registers? How about closing them completely? It might be worth the experiment just to see if using all the velocity pressure in the system to heat the downstairs only will get those temps up down there. This would also serve to further validate our assumption that your system suffers from inadequate circulation: We're assuming that increased circulation downstairs is the ticket, and closing all the upstairs registers simulates (at least downstairs) this change without tearing into wall and cutting holes in ducts by increasing velocity pressures and reducing the load on the return ducting. Not a fix in itself, but a decent indicator, savvy?
As far as where to physically locate an additional return, again, hard to say with being there, but the rule of thumb would be to find a place that lacks a return but has a warm air register and a door. Say a bedroom or bathroom. I specified "with a door" 'cuz a closeable space with a register and no return is an over-pressure candidate. As far as avoiding a short-circuit, that'll be pretty much just common sense which you seem to possess in sufficient measure. Avoid any placements that put a return vent in a direct line with a register unless it's a least 15'-20' away. Kitty-Korner placement is best; register along south wall, return vent along east or west wall. You can even get away with having the return on the same plane as the register as long as there is sufficient velocity at the register to propel the air column deep into the room.
The size (in area) of any additional return vents should be aimed at resolving the existing deficit: You're lookin' at what, nearly a 200 sq/in deficit in return cross-section? So maybe more than one new return? I would think, however, that
any significant increase would help, say a 9.5" x 4"? That'd be nearly 40 sq/in additional capacity. Maybe try one and see if the system responds; see if we're going in the right direction? Then, if we see any improvment, add another.
So, in broad strokes:
We're looking to increase circulation (rate of flow) by identifying the narrowest restriction and opening it up some while remaining conscious of the original design and taking steps to minimize the impact to other design considerations.
DonJones said:
...By the way what part of the country are you located in? I'm here in Spokane, WA...
Security considerations prohibit me from telling you the name of my city, but in code I
can tell you that it begins with "S" and ends with "eattle." Shhhh... If you successfully decipher this code please tell no one!
Try that "shut all the upstairs registers" thing and let me know what you observe. We'll go from there, huh?
~Snax
PS. I just had a thought, is your return ducting baffled anywhere? If you have central heat
and cooling you may have baffles in your return (or output for that matter) ducting to adjust for summer and winter conditions.