Ask the Engineer: A Tale of Two Stories

A Tale of Two Stories

Why to install two HVAC units on a two story home.        taleoftwostories1


If you want to see me hit the roof, sell me something I don’t need; couple that, with me being a firm believer in treating others as you would want to be treated.  A dear couple asked that I come take a look at the home they had just purchased.  You could tell as you pulled up that it had “good bones.”  Solid construction –  one of those timeless looks to it, a modern colonial style.

When inspecting the HVAC, I could tell that the previous contractor didn’t really know what they were doing.  They put in a 3.5 ton packaged unit with gas heat (a “gas-pack” for short) that served both the downstairs and upstairs.  The duct work wasn’t hung properly, collapsing in several places, and the insulation wasn’t wrapped appropriately.

The solution they received from a second contractor, that performed the inspection at the closing of the home, was to install a damper that would sertaleoftwostories2ve the upstairs and open/close via a signal from a second thermostat.  This is what we in the business call a horrible idea.  The unit is a few years older and single-stage compression; basically it is either on or off (imagine your gas pedal in your vehicle, and you have only two options, foot off, and foot all the way down on the pedal).  You can’t just add a damper, because you have to completely resize the ductwork to take all the air in one direction or the other depending on how the damper(s) are installed.  If a damper is closed and the unit is still running, you have to send all that air through a small piece of duct (imagine fitting a baseball through a straw- doesn’t work so well).

If you will allow me to back up, the reason the one unit serving two floors was not a good idea is because in Columbia, South Carolina, there is a reason we are known as being “famously hot.”  So if a thermostat is downstairs, and we know hot air rises, when the thermostat says it is 72 degrees downstairs, is that the case upstairs?  No.  I have seen it range from 4 degrees up to 12 degrees, depending on the building envelop and taleoftwostories3the associated R-value of the home (R-value = how well it holds in cold/heat).

The solution we provided was to install a 1.5-ton heat pump upstairs and a 2-ton gas pack downstairs.  The reason why is simple.  What is the most efficient machine in the world?  Answer: the one that is turned off.  In this particular home, the only thing upstairs were bedrooms.  Do they need to be cold in the day?  Nope.  Does the downstairs need to be holding taleoftwostories472 degrees at night time?  Nope.  Then the last question – if your downstairs unit goes out and your HVAC service provider can’t get there until tomorrow, can you just hang out upstairs until they show up?  Yes.  Same scenario, but now the upstairs unit, can you just make a pallet downstairs to muscle through the night? Yes.taleoftwostories5

So you’ve got efficiency, redundancy, and two different types of heat source if something goes awry in the dead of winter (gas heat and electric heat via the reversing valve and heat strips in the heat pump unit).  Lastly, we put Wi-Fi programmable thermostats in their home so that they care able to set up schedules and see their power consumption (yes, I own one and it is installed in my home; it’s great and I will tell you about it in the near future).  As is customary, I follow up with them from time-to-time, and they love the system’s comfort and efficiency.  Later, I will discuss units with the capability to unload and will discuss the ductless split option.

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