GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Need a furnace/AC for an apartment

brennief | Posted in Mechanicals on

I’m a senior citizen on fixed income and just bought a 75 year old 2-family to help with expenses. I am living on first floor and have a 500 sq ft apartment upstairs. I added central AC to existing gas furnace, but at present I control the tenants heating and cooling. Most people love the apartment, but not me controlling their environment. My heating guy said there is no way to give tenant their own heating/AC unit using the existing ductwork. Does anyone have a suggestion, especially a “green” suggestion?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Unless you plan to do the installation work yourself, you should call a contractor.

    Options include one or more ductless minisplits; a ducted minisplit; a PTAC (Packaged Terminal Air Conditioning) or PTHP (Packaged Terminal Heat Pump) unit; a window-mounted air conditioner; a gas-fired space heater with a through-the-wall vent; or electric-resistance baseboard heaters.

  2. Dana1 | | #2

    Blocking off the existing ducts (both supplies & returns) would be necessary to avoid interactions with the separate system. If they had just brought the apartment up to 72F when your space was 65F, then five minutes you cranked your thermostat up to 70-72 they might get cooked or have to turn on an air conditioner. Blocking off ducts will slightly reduce the efficiency of your system, and may have air-conditioning coil icing issues if blocking those ducts reduces the total air flow through the coil too much. A good HVAC person could assess that risk ahead of time and make adjustments to some of the ducts if it's too close to the margin.

    Hydronic baseboards running off the hot water tank (with an isolating heat exchanger to keep the heating water separate from the potable hot water), is another option that may be a viable & reasonable heating solution to add to Martin's list, and in some instances may be about the same cost as installing electric baseboards. Code requires that every room be able to be heated to at least 68F at the 99% outside design temperature, which may be difficult to do with point source heating like mini-splits & PTHPs if the apartment has multiple doored-off rooms.

    Narrowing down to the right solution starts with a heat loss calculation based on the construction of the house, and the difference in temperature between 68F and the 99% temperature bin. An ACCA Manual-J calculation is the "gold standard", but using a simple I=B=R method spreadsheet would be good enough, and easy enough to calculate yourself. Surprisingly, many contractors still use crummy "X BTU/hr per square foot" rules of thumb estimation rather than a real calculation (why take an hour when you can take 15 seconds?) , which ends up reliably oversizing the equipment, usually to the detriment of both comfort & efficiency, and at a somewhat higher up-front cost.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |