Hello HVAC guys .. I would like to re-route the main return plenum for the furnace from the basement ceiling (low) into an existing bulkhead on the main floor. If I do this I can only get it there by panning an existing 3 1/2 X 12 interior stud bay. Once in the bulkhead space is not an issue. The existing plenum is 6 X 20. Would the reduced cross section be a problem, or is the rate of air flow through this section just increased? Thanks in advance ..
ps .. I am a daytime BT guy, so if you post in the evenings I’ll get back to you later ..
Replies
This reduction in plenum x-sectional area will never work. Your furnace will definitely shut itself down from the "High limit" temperature control which protects from overheating.
Your definition of a "plenum " is misleading.
The Plenum is the box type houseing that is directly attached to the unit. Some units have a supply AND a return sitting on the top of the unit. Some units have only a supply plenum on the top and have a return 'drop leg' into the bottom fan/filter compartment.
What you are describing (I think) is a return air trunk duct,(main)
a 6x20=120 sq inches, so if it was sized right when installed, you still need the equivalent area. Starvin the unit for return air is not good. If starved, it not only lessens the amount of supply output, it also puts adds to the INEFFICIENCY of the system, meaning less cooling or heated air.
Bottom line, make sure you obtain AT LEAST the 120 sq inches along the route. Larger than 120 in some spots will not hurt. Panning is ok, return air only needs a contained path back to the unit to be effective.
thanks, yes you are correct, I am talking about the main return air trunk, I understand that you have to balance air intake and output .. here's what I don't understand, if you strip all the duct work off the furnace and turn it on, the fan will suck a certain volume of air through in a given time ... now if you add a duct, at the same fan speed you will still get the same volume of air in the same time ... right? .. reduce the duct by half, you will get the same volume in the same time but the flow through the duct will be twice as fast .. so at what point does the fan stall, or not be able to pull enough air through .. I kind of knew this wouldn't work, but I still like to understand the limiting factors ... thanks everyone ..
Fans, duct sizing, airflows is a fairly scientific undertaking. Each fan has its own "fan curve" chart that tells designers how the fan performs under different duct resistances (external static pressure or esp). You have to know all the resistances you're designing into the system to make sure you get the performance you desire (the right volume of clean air at the right velocity, at the right temperature, and at the right humidity....see where the complexity comes in) In my area the residential HVAC trade is in general, one of the poorest trained.
When you decrease the duct size by 1/2, volume will suffer a bit and the air velocity will increase possibly with an increase in air noise. As you decrease the return air duct size, the fan will not have the blade configuration and horsepower to draw the same volume at the higher velocities necessary to maintain constant volume. The fan will start to cavitate (spin in a vacuum) without an increase in volume. If you could draw the volume needed with another fan/blower, the air movement would eventually be very noisy and undesirable.
Experienced; when a ducts volume is decreased by a HALF ,that ducts volume WILL suffer by a HALF, not just a BIT,(as you previously mentioned)
I don't know what area of N America you are in, but in the most parts of Canada (especially in Ontario) the HVAC trades have exceptionally high methods and standards of training.
Since our summers are sometimes very warm and our winters are very cold, we need the best tech's to maintain our htg/cooling equipment in proper working order, and efficiency.
Wane; In order for one to fully understand what has to be done in sizing of a ducted system,fan curves, static pressures,etc, one would have to engage in a MINIMUM 4-6 month training course just to learn a very small part of the Hvac trade. In some cases, it can take as many as 4-5 years to accomplish a fuller knowledge of htg/cooling, etc.
65064.5 in reply to 65064.4
Hube:
"In my area the residential HVAC trade is in general, one of the poorest trained."
Yes, that's very true with respect to duct design (especially duct alterations) and following codes with regard to clearances to combustibles. Since I now inspect homes, I see fires waiting to happen- how about the flue pipe from a forced air oil furnace actually touching the drywall when the required unshielded clearance is 9". I find about 40-45% of the fluepipes not meeting code. Many HRV installations are atrociuous with many upscale homes getting stripped down entry level "contractor" units with poor ducting. (let's put those $50 tires on my Mercedes/BMW)
In 1990, I set up an Indoor Air Quality and Air Balancing subsidiary for an engineering firm. I thought "Good, this gets me up out of the residential craziness to the professional ranks (you know, degreed engineers, certified techs, plans for everything including "as builts") where every thing is neat and tidy!! One of the biggest surprises of my life.
I got to commission a new bank with the first Atlantic Canada installations (2) of a new system (one of the bigger brand names in HVAC) of a V.V.V.T. for roof top HP/AC's up to 25 tons. The sales engineer drove 6 hours one way to meet on site to explain the system to me and how it was controlled/programmed by a circuit board in the main t'stat. He checked them out, explained basics to me and said they're ready to go!! By 11PM that evening, I figured out that the 2 separate systems were cross wired!! The HVAC installer and the expert both missed this.
On many jobs, I found the first take-offs from supply trunks were too close to the fan discharge (at least 8 duct diameters or duct widths downstream from the discharge are recommended) to be able to balance the system normally-some of this was from engineers and others from design-build HVAC companies with technicians only on staff. These people had never seen a list of recommended practices or taken courses on air balancing.
I just saw one last night at my daughter's high school gym. TThree years ago, the school had been identifed as a sick school, the students moved out a year and new ventilation systems installed, new boiler and higher chimney, etc. I looked up to the exhaust air duct grille in the gym and saw dust/dirt hanging from the metal cross members! The grille must be well over 50-60% closed off. Asked the custodian about servicing and he said that they were here over the summer. There's no way that that grille could get that dirty 1+1/2 months of school. That much dirt would have gathered from the time it was installed 3 years ago when my son was there!! (The gym ventilation is on a part time cycle (wasn't on at all the other night-just a practice for 12 students- and there are no trade shops in the school for dust production.) Where are the service people???
"When you decrease the duct size by 1/2, volume will suffer a bit and the air velocity will increase possibly with an increase in air noise."
This statement was a bit loose! For example: in a residential system with a drive belt blower, if 1000 cfm at a velocity of 750 fpm is area restricted by 1/2 , the flow may drop to 6-650 or so cfm but the air velocity has risen and may become noisy. Actually this is a situation I've see fairly often with returns blocked by furniture or renovations (a new return is not ire-installed) and the units are not kicking out on high limit as one would expect. Ocassionally, some people complain of poor heating characteristics for the system due to lower air flows or no return in an area of the house.