My Upstairs is Always Too Hot. Why?

Ducts too small
Ducts too large
Large stairwell
Too large a furnace

There are several reasons why the upstairs in a home is nearly always hotter and there are some things that can be done about it.

               
If the ducting is too small to the downstairs in your home or the return air register is in the upstairs you will not have even heating and cooling in your home. The problem you are having with your heating system is not that too much heat is going upstairs, but that you have too little of it going downstairs. If the return air register is in the ceiling of the first floor or in the ceiling of the second floor the homes heating will stratify. This means that it will not mix properly with the downstairs air. The cold air downstairs stays stagnant and all, or nearly all, of the heat goes upstairs. This can be fixed in a variety of ways including re-ducting, moving registers, adding zone controls and installation a variable speed furnace or splitting the home into 2 or more systems. Some homes require extensive work while most only require a little guidance and are very inexpensive to correct.

Ducts too large

Most heating and air conditioning system in our climate are designed to work best during the cooling season. The ducts, if sized for cooling properly, are way too big, by twice, for air conditioning. This means that during the heating mode the furnace will not have enough air to reach the farthest locations in the ducting system starving those areas for precious heat. The normal solution to this problem is to install dampers, normally manual dampeners, and adjust them twice a year. Wide open for the summer months and the upstairs choked down for the winter months.

Large stairwell

Those great grand opening in the entryway of newer homes are very taxing on the heating system. Warm air is lighter than cool air and will therefore rise. The stairwell acts like a large duct enable the heat to escape the downstairs and cook the upstairs. To solve this problem the systems must be zoned or must be two separate systems. Both systems need to get their return air from the downstairs for maximum effectiveness and both system should be variable speed motor equipped with the thermostats able to control the lower speed constant circulation mode.

Upstairs will get heat from the downstairs system so we can largely ignore that, but what about the air conditioning mode and no return air upstairs? It will work better. If the upstairs air handler is getting its air to cool down from the coolest part of the home the air coming out the upstairs supply registers will be cooler and promote more mixing of the air. This will be more comfortable in the summer.

When the upstairs unit is on circulation mode it will be removing the coldest air of the home and mixing it with the warmest air in the home upstairs. This reduces stratification by encouraging air movement using the naturally occurring thermal changes in the airflow.

Too large a furnace

This is constant problem that we run into. Most heating and air conditioning service technicians will change out a 120,000 btu older furnace that was 70% AFUE (annual fuel utilization efficiency) with a brand new 95% 120,000 btu energy saver model. Sounds great doesn’t it, but it is a horrible thing to do. That 120,000 btu 70% furnace put out 84,000 btus of heat into the home (120,000 x 0.70) and the 120,000 btu furnace at 95% puts out 114,000 btus (120,000 x 0.95). This means, if the old furnace was the correct size, that the new furnace is now oversized by 136% (114,000 / 84,000). Over the years you’ve probably had doors and windows replaced and the attic was insulated. The chances are very good that your home needs no more than 80,000 btu of heat at the very most.