Inspiring and Innovating our way.


80, 93, 95, Up, Down and Horizontal – A Guide to Choosing Your New Replacement Furnace

When homeowners plan to purchase a furnace replacement, there are a few simple questions you need to be prepared to answer in order to get the right furnace for your application.

1. What efficiency do I want or need? (80%, 93% or 95%)
2. How will I vent the unit (vent pipe or PVC)?
3. Airflow direction (up, down or horizontal)


  • The three basic options available in the market today are 80%, 93% and 95%. It seems obvious to choose the unit with the highest efficiency. It would seem so, but the reality is that you have to make some decisions here.
  • Conventional 80% gas ovens are vented with standardized metal tube vents and often require an outlet through the ceiling. The 80% oven is the lowest price point in the oven market, but with that price comes tradeoffs. First and foremost is efficiency. An 80% efficient oven is just that, 80%. That means 20% of every dollar of fuel you use to heat your home is lost through burned exhaust.
  • The 93% gas heater has become the heater of choice for most homeowners. One of the most attractive features of the entry-level high-efficiency heater is its ventilation flexibility. 93% furnaces are called “Condensing Gas Furnaces”. Most have primary and secondary heat exchangers, which add to the efficiency and allow these units to be vented with standard plumbing-type PVC plastic tubing, and can be vented in a variety of configurations, including the ability to come out of a basement sidewall or basement and vent directly out on a horizontal run. This flexibility of not having to penetrate roofing materials is a great advance. The other notable advantage of the 93% efficient gas oven is the greater savings achieved. A 93% oven is about 14% more efficient than the standard 80% version. This can add up to significant savings and although the 93% high efficiency furnace is on average 40% more expensive than the 80% models, the savings achieved can easily make up the difference in a short period of time.
  • The best option is the 95% and above model gas ovens which, by description, appear to be only slightly more efficient than their 93% counterpart, but on closer look you will find that this group of ovens is one of a kind. class. . Ultra-high efficiency 95% gas ovens are often paired with variable speed blower units, two-stage gas valves, and a host of other additions that not only make these ovens more efficient, but also more comfortable. , besides being almost silent in its operation. . Along with the 2% increase in base gas efficiency, two-stage gas operations allow the unit to run and fire at significantly lower speeds during marginal outside temperatures. This adds up to savings on energy bills. Variable speed fan drives not only increase comfort and efficiency of operation, but since they are programmed to “raise” and “raise” during operating cycles, the normal airflow you hear with systems conventional ventilation is greatly reduced. It is not uncommon for owners to never hear this unit run. These long duty cycles and variable airflow also add to the efficiency of air conditioning systems that work in conjunction with these heaters. The price of these 95% Optimum heaters may be 30% to 40% higher than the 93% single stage versions, but you need to consider the added benefits of comfort and efficiency.


  • As mentioned above, there are multiple options for venting your gas oven and knowing what you want or need is an integral part of choosing your oven.
  • Metal or PVC? This choice is determined by the efficiency of the oven you choose or the limitations you have with the structure of your building. As noted above, 80% gas furnaces use standardized sheet metal vent pipe and are typically vented through roof penetrations. In some cases, you may already have a vent that you want to use and your choice of furnace could be dictated by that vent material. Furnaces that have rated and rated at 90% or higher typically use PVC pipe to exhaust flue gases. Vented PVC heaters offer greater flexibility when venting, as in addition to roof penetration venting, these heaters can also be vented in horizontal directions, through sidewalls and basement foundations.

Airflow direction:

  • This last piece of the oven selection puzzle is critical when choosing your oven. Here are some guidelines to help you decide.
  • Upflow – This type of setup is often found in basement locations. The return or intake air of the upflow heater will enter at the bottom and the supply air will exit at the top of the heater and then be directed to the upper ducts and vents. This configuration can also be found in the same lava as the living space and is then vented up and into the ceiling or floor registers in the rooms above.
  • Downflow or Counterflow – This type of oven is exactly what it sounds like when compared to upflow models. Return or intake air enters through the top of the heater and exits through the bottom. This type of furnace is often found in main living areas and feeds floor vents as well as ceiling vents on lower levels. They can also be used in attics where a downflow scenario is needed.
  • Horizontal – Often the upflow and downflow ovens mentioned above can also be set up in a horizontal position. To visualize this, think of the oven lying on its side. Instead of up or down, this configuration allows them to draw at one end and run out at the other horizontally. This can be useful in attics that are too low to accommodate a pedestal oven. They are also used in basements or cellars that have lower than average ceiling heights that cannot use the vertical oven.

If the above options seem confusing, talk to your heating and cooling expert before you buy your next system. Most online HVAC stores have phone numbers to call or live support agents to help you make the right decisions for your home.


Your email address will not be published. Required fields are marked *