Haven't the slightest idea about what is involved in cleaning a residential range hood output line. The important thing is the avoiding of damage, the climate it must function in, and amount of grease being produced. Grease will function as a water vapor repellent so getting ducting shiny clean is probably not a good idea in a moderate climate. In addition, grease will likely degrade over time and find some sort of equilibrium with the amount being produced by your stove use. All in all, probably the most cost effective idea would be to plan on a replacement of the ducting and complete exhaust fan assembly at a given target date within the next few years. Finding out the condition of the old system will be a free bonus. This post was edited by laat2 on Sat, Sep 28, 13 at 20:14

OK, since my name was called, I'll try not to make it in vain. :) Yes, more than one fan in series (but with enough separation to ensure blade to blade transient effects aren't present) will improve flow because the overall pressure drop is shared between fans (but not equally unless they are identical fans given identical power). If for example, the duct (and hood and hood filter and MUA) will cause a pressure loss of one-inch at 300 CFM, then two identical fans capable of 300 CFM at one-half inch will be required to flow 300 CFM at one inch. To predict the performance of two different fans, the overall pressure loss vs. flow rate is needed as well as the fan curves of both fans. Without knowing what the MUA pressure loss is, and the hood filter pressure loss is, estimating from just duct losses will not be very close to reality. I would add, however, that adding a fan may be a good idea when the goal is to not change anything else. Note that the roof cap in this case is not very amenable to adding a fan to one duct.

In my experience with my 1400 cfm Tradewind liner with baffles there would be no need to ever "clean" any parts except the baffles. My rationale for this is the following. Once a week ( more often if it warrants it ) I remove the baffles and clean them. depending on how much stir fry and deep fat frying we have done they are quite greasy, sometimes very very oily and one or two times , before I realized that they capture everything they were dripping droplets of golden oil. I also wipe down the surrounding area and there is never anything on this area at all. I have pushed the damper up and tried to wipe in that area and felt up there with my hand and there is nothing up there either. The process of using your hood correctly should remove any need for cleaning other than the baffles. The hood should be turned on at least 5 min. before you begin cooking and remain on at least that long at the end. This insures that all grease products are trapped on the baffles and then the remaining products, steam/heat/odors are removed to the outdoors. I hope this helps you with your decision. The baffles or your mesh should be very dirty on a regular basis, depending on your cooking style. Also there should not be any grease on any part of your surrounding area in the kitchen. If there is then the vent is not working correctly or you can adjust your use of it. Good Luck c

A few very quick thoughts. Wide sweep smooth 45's & 90's will be much better than tighter radius or segmented. Two 30's would be better than two 45's and two 15's better than two 30's. An oversized end cap (eg, 8" to 10" reducer + 10" cap) will be better than a typical 8" cap. IOW, almost anything that can reduce the static pressure will help. 10" duct would help as well but might also result in too low of air velocity. MUA is Make Up Air. If you're sucking 600 CFM (with your setup about 100 CFM is more likely) out of your flat then you need to bring that same amount back in. In your case it is doubly important as too little MUA, as @kaseki mentioned, will cause your hood blower to have to work much harder against the vacuum that will be created in your house and so it's performance will go down even more. You need to do everything possible to help your hood do it's thing. You need to talk to a competent HVAC person.