Heating, Air Conditioning, Fridge, HVAC/Improving Return Duct in an old 3 family house
QUESTION: Hi Fred,
I live in a condo of an old 3 family house where there is a basement, and three floors where each floor is a condo; I have the top floor.
Currently, I have an oil forced hot air furnace, that I'm looking to switch to gas. My return duct is pulling air from a common area stairwell of the house and not my unit. Plumbers and HVAC guys have been giving mixed opinions on this; some say it does not matter too much, others say it will take large sums of money to fix (one was around $15,000) and thus, saying it isn't worth it, another said to extend the duct from the common stairwell into my unit, and someone else said to just get flexible round ducting and pull one of these in my unit through one of the stairwells.
There really is no consensus among the people who I met and thus, not sure what an overall adequate solution would be in this case, in terms of time/money, efficiency, and cosmetics.
I don't think I can extend the existing return duct in the stairwell as one had suggested, but there is another stairwell in the back which potentially we could use 2ft square of space.
I think there may be some closet space and some walls where some duct work can be run but I don't think the duct can be much bigger than 1ft square here, but possibly two chases can be run.
First, should I care too much about the return? And if so, what do you think a solution or two would be in this case of living in the third floor and trying to get adequate return in my unit?
Also, does ductwork need to be enclosed and protected rather than simply putting it into walls or in the stairwell? If so, there would be much less space to work with for actual ducting.
Think of a stairwell as a chimney in the event of fire. The last thing I'd want would be a return in that chimney. You have a very unsafe condition that must be corrected ASAP. The cost to do so should not be your primary concern.
Likewise, the back stairwell is not an option.
I should note using an exit stairway as part of a return system is specifically prohibited by NFPA 90A (this a three family dwelling, so you're excluded from the less stringent 90B), and the International Mechanical Code.
The return grille(s) must be inside the condo, and every room separated from the return by a door must have provisions to transfer air from the room to the area where the return is.
1 SF of duct is good for 600-odd CFM, 1-1/2 tons of cooling (Return ducts are limited to 700 FPM velocity).
The ducts would have to be enclosed if they pass through the lower floors. Note that the condos cannot share a common return duct; that too is prohibited.
You didn't say where the furnace is located. Also, you didn't give the AC size, so I don't know how much air the return must handle. I'll also need your city and state. Please send a follow-up.
---------- FOLLOW-UP ----------
I'm in Boston, MA. The furnace is in the basement, and I currently do not have an AC and am not considering getting one with the install.
I read some info on NFPA 90A to get a better understanding, but need some help on understanding it. So ducting into the stairwell is not correct, but is running actual metal ductwork from the furnace and through the stairwell and terminating inside the condo acceptable? I'm trying to figure out if the stairwell altogether cannot be used at all to house ductwork or if it was just because actual air is being exchanged through the stairwell and not directly from the furnace and condo.
Also, is metal ductwork required to be framed and drywalled around in some areas of the house?
The duct must be framed in where it runs in the stairway. I believe you'll need a 1-1/2 hour rating, and you may need fire dampers. You should check that with your AHJ ("Authority Having Jurisdiction" - Fire Marshal, Building Department, etc.).
The size of the duct will be a function of furnace size, so be sure someone measures windows, etc. and performs a proper heat loss calculation: The bigger the furnace, the bigger the duct; make sure you get the proper size, but no larger (I imagine 40MBH would be all you need, but do have someone do the math). Follow Manual D for selecting duct size, of course.