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I have posted this issue before at another forum, but I thought I could gain a little more insight. I have a 7 year old home with a Carrier 58MVP furnace. I have had a re-ocurring problem that happens once or twice a year. Once ina while during the winter, the furnace will shut off due to the preaaure switch to protect for a clogged flue. It happens only once or twice in the winter, but sometimes it takes forever for the furnace to come back on. One time we woke up and it was 52 F in the house! I tried to monitor conditions and I thought it only happened during a stiff wind that blew directly east (right at the flue vent). I thought maybe this was the cause, but last year for a couple days in gave me problems again (and then went away and never came back). I had the company that installed it come out, but they gave it a clean bill of health. Well, winter is approaching again and I know I am going to have the same problem. Previously, someone told me I have an incorrectly installed flue (other chat room). But, I read through the installing manual and it is the corect diameter pipe for the size run and number of pressure drops (pipe bends). I don't know what else to do to the flue, it looks installed properly based on the documentation.

On a separate (maybe related) note. I KNOW that there are not enough cold air returns in my house. This is definitely a big problem that has taken me years to finally figure out. The symton came after a year (carpet filtration soiling), but I didn't understand the root cause until lately. A nationally syndicated handyman column finally had someinformation related to this and highly recommended a patented product (equiliz-air) that attaches to the intake trunk of the cold air return and vents to the outside. The device also connects to standard household air. Anyway, could these two issues be related? I am going to install the product this fall so I can finally replace the rugs in my house without having them turn black around the endges after 6 months.
 

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Your pressure switch for your flue is totally unrelated to your return situation. A few causes of pressure switch problems can be (and not limited to) an object(s) caught in the vent/intake pipes (leaves,grass, wasp nests are notorious for "once in awhile" problems), improper slope on flue causing condensation to get into the pressure switch tube, debris (such as gasket material from assembly or from initial installation) in the inducer motor assembly causing a blockage at the pressure switch outlet, possibly ice build up on exhaust pipe outlet, an inducer motor losing speed/not turning full rpm's, and of course a possibly defective switch. When they checked it out before, did they use a magnahelic guage to read what the WC (water column) was on the inducer motor and verify the pressure it was making contact? Did they also check and verify it was maintaining steady WC during operation? Has the intake and exhaust pipes been checked to verify nothing hanging/lodge in the pipes? Has the condensate drain/pipe slope been checked to make sure no water was backing up into the inducer motor/pressure switch?
 

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Discussion Starter #3
Thanks HVAC DOC. You obviously have a lot more knowledge of the situation than myself. I don't think they used anything to check the flue (much less wc) and the condensate drain pipe probably was not checked either. But, at least with a couple more possibilities, I can check these things out, or get someone in here to check them before I have to pay a premium for weekend / 24 hour service.

As far as my other issue, why isn't there a code for # of cold air returns? This is a common problem that I am not sure many people are aware of. I am frustrated because I had an independent company that specialized in diagnosing carpet problems come in and they chracterized the problem right away. But, he only blamed it on car exhaust and the possibility that I cooked with too much oil (crazy). When the most common reason for carpet filtration (now that I know) is improperly balanced HVAC system. I would have known within the first year of owning my home and could have spoken to the builder (who I find out now, knew about the problem). Thanks again.
 

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Some areas (not all unfortunately) require a Manual J load calc and Manual D duct design to be submitted with permits. Proper sizing has only started really becoming "important" the past 10-12 yrs. Most often in new construction, it is get in, get out, and as long as it heats/cools great. Builders look to keep costs down and low bid guy gets the job. So subsequently, many things (HVAC included) get put in sub-par. Reality is, you aren't the only one who suffers from poor return problems and many cases, it is supply as well. And of course when you build a home, the last thing you want to hear is "We gotta rip this this and this out and redo this costing X dollars". As far as making a "universal code" to cover number of returns, it would be next to impossible since no home is exactly the same with same room sizes or even same size systems. A poorly insulated home could require (as an example) a 4 ton system. Build that same home with high efficiency in mind such as insulation, glass, etc. and it could take only 2 tons to cool it. That's why Load calculations are so important to have done and sizing by "rules of thumb" are just plain sloppy work. Now understand too that sometimes (not all cases) symptoms like you descibed can be homeowner caused (not saying this is your case). Cheap candles cause more issues (soot badly and burned constantly or large numbers, can cause major stains) than I have time/space to begin to dive into. And with todays tighter homes, it doesn't take much to cause "an ordinary action", to become one that causes major headaches. Infiltration of dirt, soot, ash, other chemicals can be bad when a home is so tight it operates under a negative pressure. The home will suck air from any opening it can and that commonly includes range vents (your cooking oils/grease) chimneys (wood burning or pellet type stoves), and even hot water tanks (flue pipe). not to mention when you open the garage door in your home (attached garages) and suck whatever dirt/particles are floating around.
 

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Thanks

Thank you for all of your insight. I am sure I will be back with more questions once I take a closer look. I don't remember any specifi slope to the flue. I know the builder moved the flue after he had installed it because the AC unit was not in the correct location. I have a feeling this could be part of it. As far as the cold air returns, I am going to install this lttle unit and that is supposed to equalize the air flow. That will hopefully take care of the filtration soiling.
 
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