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Question DetailsAsked on 7/13/2014

How do I know whether the contractor's recommended furnace and AC are the right size for my condo?

My unit is 1200 sqft with 14ft ceilings. It's an old brick warehouse converted to condos in 1995. There is almost no insulation, just the 18" thick brick walls. Because it's an old brick building, we are not allowed to vent directly to the exterior, so we are limited to the 80% efficient furnace models. The ductwork is exposed.

I live in Chicago so it's freezing in the winter and boiling in the summer.

The first quote is for a Lennox SL280 UH 090v36b furnace and a Lennox XC13 036 AC unit. I need to request other quotes but I don't understand how to judge whether the size is right.

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4 Answers


you do not know he will sell you the brooklyn bridge if you let him. I copied what I use to determine what I would need to keep the customer comfortable.

(Moderator note: Removed extraneous HTML and included link to article referenced.)


Answered 6 years ago by Raymond Gonzalez


Makes no sense to me you are not allowed to vent through the wall - not sure where that came from, because it is done all the time. Also, high-efficiency furnaces can be vented through the roof - that also is done all the time, though of course has to be fan-driven venting.

If all three come up with pretty much the same sized system (BTU for furnace or Tons air conditioning), then they are probably right in the ballpark - provided you make sure to bring to their note the all-brick walls and high ceiling and probably poor roof insulation (if any), so your heating and A/C needs are both going to be greater than normal for the space, and the high ceilings of course means a lot more volume of air to be heated and cooled relative to the condo square footage.

One of the primary things they should be looking at is either a supplementary circulation system (similar to a whole-house system) to pull the air at the top of the ceilings and recirculate it in winter, or to put return vents up there to reclaim your heated air.

Answered 6 years ago by LCD


A competant HVAC contractor will perform a manual J heat /AC load calulation.

Your question is confusing- if you cannot vent to the exterior? then how is the existing heating system flue gasses vented? A 80-85% efficiency means the unit is non-condensing and generally uses an existing chimney, higher efficiency do not use a large chimney as the exhaust gasses are lower temp and use smaller diameter flue piping. and what is the size of your existing system?

Answered 6 years ago by hosey


I think Hosey misunderstood - you are venting to the roof, just are not allowed to vent directly through the wall, as I read it. Wall venting through the wall can cause issues, especially with the idiot designs some plumbers and manufacturers use where it is discharged right at the wall surface instead of 6 inches or more in front of the wall as used to be required in about pre-2000 era codes. There is an issue with surface venting on brick surfaces softening the mortar, but a foot or so flue extension should mitigate that risk.

Whether a furnace is "condensing" or "non-condensing" refers to whether the discharged flue gases keep the moisture (created by combustion, which produces primarily water vapor, carbon dioxide, some carbon monoxide and small amounts of nitrous gas compounds and a few stray huydrocarbon compounds) in gaseous form till it reaches the outside air or not. Units with efficiencies over about 95-97% are all condensing - basically discharging only luke warm "wet" gas right out of the furnace, so they typically use PVC flue pipe the shortest distance possible to an outside wall. Between about 85-95% efficiency roughly tend to be condensing in flue pipes over about 5-10 feet, so for practical purposes cannot discharge to the outside without condensing water vapor either. These condensing units almost always require a fan system to vent the gases because they are not hot enough to vent by themselves from natural convection even if vented vertically. Units below about 85% efficiency have hot enough exhaust air that they will retain the water vapor in gaseous form, without condensing, for some distance of pipe - typically long enough to make it to the outside of the house through the roof, so they typically vent through the roof though can go laterally, though that commonly requires a booster fan because most units below about 85% rely on convective flue gas evacuation.

Whether a furnace is condensing or not is sometimes rated by the manufacturer based on the exhaust temperature from the firebox and whether it condenses there or not, but sometimes based on an assumed height (from a first floor mounted furnace to the second story roof typically) - so actually in colder climates with an unheated attic a non-condensing furnace can be condensing in the flue - my 80% efficiency older furnace does so in winter below about ten below zero even with insulated ducting, and drips down into the furnace. One rated for not more than say 16' vertical flue can become condensing ifmounted in a three-story house or evenn if in the basement of a two story house at times. Likewise, a normally condensing rated furnace can actually be non-condensing if fitted with a smaller exhaut duct and an exhaust fan that evacuates the gases before they can condense, so you need to carefully review the manufacturers literature and contact them if your situation is potentially out of the normal operating range.

Pretty much any configuration can be used in any given location - just requires supplemental fan driven exhausting in some cases, which does burn up a bit more electricity and of course requires a unit designed for a duct fan, and of course there are more components and sensors to go bad.

One after-thought - which of course might be out of the question if this current furnace/AC selection is based on immediate need due to failure rather than a choice to upgrade. It may well be your situation is so bad thermally that you would be better off first investing in insulation and high-ceiling warm air recovery as that will save you both in the size of units you need, and in annual energy costs - if your current units work it might well turn out that the same or less money put into insulation will be your long-term better investment. It would take an energy audit and some computations to determine that.

One thing I spaced on that one of the other commenters touched on - there are a lot of websites with the Manual J and so forth computations available as web-based calculators. You might have to do a bit of web research on your lowest and highest expected temps and number of heating and cooling degree-days but that is easy to find onthe web in a few minutes. Then you could run the sizing calculations (read upon them at yourself using the simplified calculators to cross-check what the contractors are coming up with. Not tough or complex - will just require an hour or two of learning what they mean and running the calculations.

Answered 6 years ago by LCD

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