We are about to begin a renovation on the lowest level of out house that will include an upgrade in systems. We currently have a 40 year old oil furnace and separate AC system. We want to replace with a gas furnace and AC unit and in rerouting the duct work in that level we’d love to reduce the size of it. Our contractor told us that smaller duct was outdated and didn’t work very well. I’m looking for a check here. Can anyone tell me if this is true? Am I asking for the right stuff? Our reasoning has to do with headroom in some of the rooms which have ceilings that will only be 7’8″.
Thanks in advance to any that can help!
Kell
Replies
There is a product called spacepak or something like that which is 2 inch high velocity duct work. I checked into it a few years ago for a.c. Maybe something like that can work for you.
Have a good day
Cliffy
We do mostly old houses and have had installed the high velocity ducting by "Unico?" in several recent projects.
Yes, smaller sizes are possible. Are they practical or desirable? Maybe not.
Given that you have to move a certain amount of air, smaller diameter means a higher velocity and a greater pressure drop.
The higher velocity will make more noise. That can be mitigated somewhat by PVC or another solid, sealed, high-mass piping (compared to sheetmetal duct). Also, if the transitions to registers and the registers themselves are smooth and larger diameter, there won't be as much noise from exit effects at high speed.
The greater pressure drop will require a higher horsepower blower. For a given technology (i.e. stamped sheet metal vanes) it will make more noise. Stepping up to more aerodynamically shapped impellor can help a lot. Consider the very few ceiling fans that look like airplane propellors. Versus the "Casablanca-look" slabs of pressboard or rattan being turned around. The Wright brother knew better in 1903! Similarly, in blowers, there's cheap. And then there's good. Look for decibel ratings. Air flow and pressure being equal, the more efficient, lower-hp blower will be quieter (because it has a better impellor shape).
Lastly, you know who the pioneers are? They are the ones with arrows in their backs. You don't want to be your contractor's first customer to use a high velocity system. ONLY use someone who has put in 6 or more that have been through a full year (heating and cooling) of use and that you can call as references. There's a learning curve to everything and you don't want to be on it.
After-the-last-thought: If it is only the clearance issue, sometimes you can just throw a bit of time and money at it. Without going high-velocity. Use two 6" ducts instead of a 8". Play with the geometry and consider a rectangular 4"x14", etc. Get a duct work shop involved. They can fab anything to fit. Your contractor may be limiting his imagination to what Home Depot stocks for ductwork.
David,
Thanks for a very well thought out, concise response. I appreciate the time and efforts!
Kell
David,
Do you think that the smaller ductwork systems can be installed by a reasonably skilled HO? I always thought that you had to buy their special heater to go with the small duct systems as standard heaters would not be able to handle the higher pressures?
Thanks,Daniel Neuman
Oakland CA
Crazy Home Owner
It is maybe a bit more DIY than, for instance, RFH. But only if you RTFB. And follow ALL their recommendations (heater type, blowers, etc). Things go downhill fast when someone, pro or DIY, gets creative on their first installation of anything.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Depends. One idea is to see how the ductwork is sized now. Short of custom equipment or at least non-residential equipment (one approach Dave covered, the other approach is described briefly below) a reveiw of the air flow needs (as determined by load calculations), the sizing and routing of the existing ducts may show some ways to reduce the size of the ducts. It may also reveal that they are actually too small now.
Two ways to get the same heating or cooling effect out of reduced sized ducting, one is to increase the pressure and or velocity of the air in the ducts, the other way is to increase the temperature difference between the supply air and the space temperature. For instance, most "standard" cooling systems are designed for supply air 55 degrees cooling the space to 75 degrees. The air flow is calculated based on the load divided by the 20 deg temperature diffenece (and a factor that adjusts for the physical peculiarities of heat transfer with low pressure air). If someone were to design the system based on 45 degree supply air the duct could be approximately 50% smaller than "standard". Typically, residential ductwork is designed for cooling air flow rates which are greater than heating air flow rates. There are, however, practical limitations to the low temperature systems. Freezing the coil is much more likely, the efficiency and capacity of the system is reduced are two of the immediate concerns. With a "supercool" system, every facet of the system requires greater attention to detail. Duct sealing and insulation becomes critical.
On the heating side, the changes are minimal, unless the system is already restricted and discharge temperatures are high. There are limits to the high side operation that cannot be exceded without scarificing the longevity fo the equipment or the safety of the occupants.
>>We are about to begin a renovation on the lowest level of out house
May I suggest NOT including a return air duct in that location!
(Sorry, I couldn't resist!)