Replies

1. 
   NormanWB | Oct 22, 2018 11:44am | #1

   Have your contractor run a Manual D calculation on your ductwork. This will tell you if your current arrangement is up to the task.

2. 
   Yupster | Oct 22, 2018 12:19pm | #2

   What you want is to know if your current ductwork can handle the airflow of your new furnace and if it is well balanced. An hvac contractor can come and take site static pressure measurements to determine the performance of your existing system. Then you can determine whether your existing ductwork has any problems that should be remedied before new equipment is installed. Using the btu/h output of the existing system and a temperature rise calculation, the contractor should be able to roughly determine the existing system airflow. A flow hood over registers will help track down the root of comfort problems in rooms that tend to overheat or overcool. The contractor should be able to take this information and tell you whether you need to modify anything to accommodate the new furnace. A Manual D isn't that useful for existing systems, site measurements of real world performance are what you need.

   Installing new equipment is a great opportunity to repair an existing duct system to provide better comfort. Sometimes a couple simple modifications (an extra return duct, a bigger filter, a new drop plenum with a better transition, etc.) is all that's needed and as a bonus, your new equipment will last longer with well performing ductwork!

   You might also consider sealing your ductwork or adding insulation if needed at the same time.

3. Expert Member
   Dana Dorsett | Oct 22, 2018 03:19pm | #3

   Most of the time the existing furnace is 3x oversized for the load, and dialing back the oversize factor to a more appropriate 1.4x (or a bit less) , as prescribed by ASHRAE means lower cfm, lower static pressure on the ducts, which is a good thing. It gives more tweaking range for balancing vanes, reduces air handler driven pressure imbalances between rooms, etc.

   If there is a heating history on the place, run a fuel use based load calculation using the old furnace as the measuring instrument to size the furnace using this methodology:

   https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

4. 
   walta100 | Oct 22, 2018 07:51pm | #4

   In general older homes had ductwork sizes for heating only as at some point AC gets added every 15 or 20 years the system gets replaced often with the next larger unit because bigger is better. (NOT) At some point the ductwork way undersized for the equipment.

   Did your furnace run continually on the coldest nights?

   You need to decide what size equipment your house needs. With a manual J calculation or a fuel use to weather data calculation.

   Only after you know what equipment is need can you do the manual D calculation to decide if the current ducts will work.

   Walta

1. 
   Jon_R | Oct 22, 2018 09:30pm | #5

   To do it right, you need Manual J, S, T, & D.

5. GBA Editor
   Martin Holladay | Oct 23, 2018 05:27am | #6

   Rick,
   Older supply duct systems may be oversized, especially for heating -- and oversized is better than undersized. As Walter pointed out, if older ductwork was designed for heating, and air conditioning was added later, the ductwork may not be oversized any more, because in many cases AC airflow needs are higher than heating airflow needs.

   Even if the older supply ductwork is undersized, many older houses had poorly designed (or inadequate) return air systems. And most older duct systems are leaky. So there are a lot of issues to look at if you really care about duct design and duct installation.