Replies

1. Expert Member
   Dana Dorsett | Jan 15, 2018 12:15pm | #1

   If you put the return at the ceiling level and the supplies are at the ceiling level it will just short-circuit- all the flow would be happening in the upper 2-3' of room air, which is the opposite of what you're looking to achieve.

   Unless there are fairly tight doors between the supply registers and the stairwell to the upstairs there is probably already adequate return flow paths. If not, converting partition wall stud bay into a jump duct, with a grille near the floor on the room side, and another near the ceiling on the side heading toward the return path/stairwell, using the stud bay as the duct between them removes the coldest air in the room (near the floor) rather than the warmest air (near the ceiling), which will improve the overall mixing.

   Installing narrower duct boots and register grilles designed for maximum throw would increase the exit velocity to achieve better mixing.

   If all else fails, low-temp panel radiators running off the hot water heater could heat those rooms.

2. GBA Editor
   Martin Holladay | Jan 15, 2018 01:50pm | #2

   J. Cryder,
   When a forced-air system has fundamental design issues, the fix isn't easy or cheap. In your case, your supply registers should have been installed in the floor instead of the ceiling, and each of your home's two floors should have had its own zone (and its own thermostat).

   Your problems resemble those of another GBA reader who posted similar questions recently: How to prevent/slow heat from rising up stairwell?

   Fixing the problems of your forced-air system would be expensive. It would probably be less expensive to install a new ductless minisplit unit to provide downstairs heat.

3. Expert Member
   Dana Dorsett | Jan 15, 2018 03:18pm | #3

   Also note:

   "The good news is that the air handler is powerful. In the summer it cools the whole duplex in 15 minutes with the AC."

   That's actually BAD news, because it means the AC (and probably the furnace) is ridiculously oversized for the actual loads, which leads to inefficiency, and more floor-to-floor temperature stratification when operated as a single zone (which is your current situation.)

   Is it in fact a single zone?(I'm guessing "yes")

   Where is the thermostat located?

   If the thermostat is upstairs and the heating cycles are short it hardly matters what the flow rates are- there isn't enough continuous air exchange to overcome the temperature stratification. Ideally you would have a right-sized furnace, that would take more than 15 minutes to satisfy the thermostat even with a narrow differential setting on the thermostat, that would run nearly continuously, at a 75% or higher duty cycle to keep up whenever the temperatures were below 20F.

   Using the nameplate efficiency of the furnace it's possible to use wintertime fuel use against heating degree-day data to measure the heat load, and compare it's D.O.E. output BTU to the calculated heat load. There will be some error due to the large difference in temp between floors, so use base 60F rather than base 65F degree-day data to compensate for that. Details on how to do that can be found here:

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

   A typical 1800' brownstone might have a heat load at +15F outdoors (NYC's 99% outside design temperature) and 70F indoors of about 35-40,000 BTU/hr, and a cooling load of ~20-25,000 BTU/hr. If the fuel use heat load calc comes up substantially higher than that it's probably due to high air leak rates and the higher stack-effect drive of building with 12' ceilings, and 13-14' per floor rather than ~10', and those CAN be corrected.

   If the AC is cooling the space in 15 minutes after being left off all day on a sticky July you're probably looking at more than 5 tons of air conditioner, probably more than 6 tons, but go ahead tell us the model # or the nameplate BTU numbers for the cooling coil on the air handler.

   If I had to guess the furnace is probably more than 150 BTU/hr too, and operates at no more than a 20-30% duty cycle even when it hit's 0F outside. But read the nameplate BTU in/out, and compare that to the fuel use heat load calculation.

   Even if it's "only" 3x oversized (enough BTUs to keep the place 70F when it's -100F outside, which will happen sometime during the next ice age perhaps? :-) ) The low duty cycle alone is enough to create a substantial floor-to-floor temperature imbalance.

   ASHRAE recommends no more than a 1.4x oversize factor from the load at the 99% outside design temperature. So if the load comes in at say, 40,000 BTU/hr from the fuel use calculation, the largest furnace that you should be installing should be 1.4 x 40,000= 56,000 BTU/hr out. A condensing 60K-in/57K-out 95% efficiency furnace would be the "right" choice.

   If the place leaks air like a sieve or has only leaky single pane windows the load could be in the 50-55K range, but the right solution there would be air sealing and adding tigth low-E storm windows. But even a 55K load @ +15F only calls for no more than a 1.4 x 55K= 77K-out (say, an 80K-in condensing furnace) to provide the higher and more comfortable duty cycle.

   At a 1.4x oversize factor for the load at +15F the place would be fully covered for 70F indoors down to an outside temperature of -7F. Temperatures below -7F do happen in NYC, but not more than a handful of times over the lifecycle of a furnace, and when it does it doesn't STAY that cold for very many consecutive hours.

   So, start figuring out the load and the equipment capacities and report back. At some oversize factor (say, 5x) it could be that the right thing to do is to replace the equipment. But in the meantime moving the thermostat down stairs and throttling back flow to the upstairs (within limits) and opening up return flows paths near the floor level downstairs might get you to a livable compromise, if it appears gross oversizing is a primary or strong secondary factor.

4. 
   jcryder | Jan 17, 2018 11:47am | #4

   Thanks to both Dana and Martin for the comprehensive responses. I have a science background (doctorate) and I think I need a translator for much of this. I will read the other metrics off the furnace shortly, but you (collectively) guessed right --my furnace is under powered (56000btu) and the kitchen is at the end run of the system so this combined with ceiling registers (and ceiling fan) is grossly inadequate. Installing supply lines at floor level and a return or two in the first floor would be messy and expensive.

   Is there a ductless minisplit unit you could recommend size/brand wise? The downstairs is 900 ft2 but again, 12 ft. ceilings equate to over 10000 ft3. Of course, I don't need to heat the stratospheric portion of the floor. I had a quote for a unit that would be over 6K. Are these expensive to run? We are only in the unit early evening to early morning so I would not have to run it continuously.

   Interestingly, an insulation guy recommended as you did, that I run a couple of hydroponic wall based panels off the hot water heater and he said few HVAC guys would agree with this. He's right. I've asked four and they won't do it. They say that there is not enough heat coming from the HWH (140) to heat and the pumps/ and the mixing of potable water with heating water are issues. I don't even know if this is code in the 5th world city of NY.

   Thanks for your wisdom.

   JCRyder

5. 
   tommay | Jan 17, 2018 12:08pm | #5

   Got ceiling fans?

6. 
   jcryder | Jan 17, 2018 12:20pm | #6

   Yes, I have two ceiling fans in the downstairs at both ends of the floor and another at the top of the stairs to drive warm air down (from related posts this may not be effective). I did find the quote for a ductless mini split , actually less than 4 K (I was wrong). I've pasted it below and wonder if the specs are appropriate for one floor of 900 ft2.

   Fujitsu Duct, 9,000 BTU, 27.2 High Efficiency Seer
   Heating down to -15 degrees -- Advanced Comfort
   Includes 50’ line set piping, slim duct 1 pipe cover, condensate pump with drain line.
   $3,800
   Condensing Unit will be set on Steel Channel.
   Electric not included.
   7 year compressor, 5 year parts, and 1 year labor.

   Fujitsu Duct, 9,000BTU, 16 Seer Efficiency Seer
   Heating down to +14 degrees.
   Includes 50’ line set piping, slim duct pipe cover, condensate pump with drain line.
   $3,200
   Condensing Unit will be set on Steel Channel. Electric not included.
   7 year compressor, 5 year parts, and 1 year labor.