My problem is my 1hp unit I has a 30 micron bag that spews dust all over the place and doesn't pull hard enough to effectively evacuate the machines it is hooked up to.
Ouch, 30micron is not only dirty, but it also leaves all the very worst dust airborn! You are wise to investigate replacing such with a newer 1-micron dust collector.
When looking at dust collectors is the HP rating the running rating or rating under heavy load?
The horsepower rating is the horsepower rating. Nearly all dust collectors (except, perhaps, some ultra cheap models) use [much quieter] induction motors, not the screaming universal motors that you are accustomed to seeing with all sorts of wild hyped horsepower claims.
How much force can be added to a fixed fan? What happens when you close all of the blast gates?
A dust collector is ulitimately rated by its maximum cubic feet/minute of air volume and the maximum amount of static pressure (suction) that it can develop before the air volume -- and thus air speed -- drops unacceptably at its intake. Efficient transport of the suctioned dust and debris depends on certain minimum air speed -- the exact speed really depends on the mass and cross-section of the debris, but usually you are aiming for something in the vicinity of 4000ft/min (around 40-45MPH) -- necessary to keep the debris airborne along the entire journey back to the dust collector.
If you shut off all the blast gates, then the airspeed and volume will drop to nearly zero (in truth there will be some leakage, so not truly zero). That is acceptable in this case since no debris/dust is entering the system in this state. You will likely find (especially with a 2HP collector) that one or two 4-inch blast gates open is the practical minimum and maximum for practical and efficient dust collection. If you have a 2-1/2" gate open, then you may also need to open a single 4" port further down the line to ensure adequate air speed if the 2-1/2" line/hose opens up to a much larger trunk line.
You will also want to use the largest hose practical to your tool. For instance, if you have a choice, use a 4" hose to connect to your 2-1/2" router table port and adapt that 4" down to 2-1/2" with a conical reducer just before it enters the table. If you have to use 2-1/2" hose then use the shortest length possible. If you can, use a 5" or 6" hose to connect to your 4" tablesaw inlet as well. That said, when starting out you can get buy with short (no more than 20ft max, preferably no more than 10ft) lengths of 4" hose and you will observe a substantial improvement in dust collection and a much cleaner shop -- but not as good as if you had run a 6" line to the tablesaw and reduced it in those final few inches to 4".
Any negative effects such as overheating?
With a few rare exceptions (it comes down to design of the impeller), nearly all dust collectors ship with a single common warning: DO NOT RUN WITH NO HOSE/DUCT CONNECTED. With all of the most common impeller designs the ONLY way to overload your dust collector is to run it wide-open with no hoses or reducers connected. This occurs because the impeller depends upon a certain amount of resistance (either from moving air or from developing a vacuum) to keep it from overspeeding. This resistance assumes that atleast a few feet of hose or ductwork is connected to the inlet.
Other than overloading, the only other common way of overheating the dust collector is to block the motor's internal fan OR place the dust collector in an enclosed space with inadequate cooling airflow (e.g. a poorly designed closet).
Keep in mind that the air that exhausts from the 1-micron pleated filter tends to be quite warm (it can easily be 80F+ in a 68F degree room) which needs to be taken into account when tucked away in a closet. In the case of a closet, you also need to provide a suitable path for makeup/exhaust air to return to the shop. If you are exhausting outdoors than you need to ensure that your shop allows adequate makeup air to flow in from outdoors (especially if you have any gas burning equipment in your shop -- water heater, central heater, etc) so as to not draw in combustion fumes.
In the event I soon buy a cyclone. Should I decide to keep it running how important do you feel a seperate closet is needed for noise reduction?
This really comes down to personal preference and tolerance. My 2HP cyclone has a noise level of 74-78dB, which if you consult OSHA is considered a safe level of exposure for 8+ hours stretches. Nonetheless, it is loud enough that I do shut it off when it is not needed for awhile, or I will wear [foam] earplugs if I will be running it for many hours straight (it is just enough sustained noise to eventually give me a headache, but then I am prone to such).
If you go the route of placing it in a closet, just beware that you will have to provide a return path for the exhaust air so that it can return to the shop with *minimum* resistance. Also beware that this return path, if not carefully designed, will become the prime path for noise return to the shop.
Venting outdoors remedies the noise issue, but then you have to ensure an adequate safe supply of makeup air is provided for (see above). You also wind up venting all your heated and/or A/C conditioned air and risk drawing in excess humidity from outdoors with such a setup.
I hope this answers all your questions. I lost track of this thread for a few days while I was not feeling well or I would have gotten back to you earlier. I think I answered all your questions, but if you have any others feel free to ask them as well.
PS - If I may, while you are investigating purchasing a new cyclone dust collector, it is also a great time to purchase a good ambient air cleaner for your shop as well (if you have not already done so). These units take up little to no shop space (they are typically ceiling mounted) and they excel at scrubbing from the air the fine dust that inevetibally escapes (even with a dust collector). Ambient air cleaners are the perfect companion to a dust collection system and help to keep the air cleaner, the occupant healthier, and the shop much cleaner than a dust collector alone.
Good luck with your decision... whatever you decide your health will definitely be the better for it.