Homeowner's FAQ: HVAC

What is HVAC?

HVAC stands for Heating, Ventilation & Air Conditioning. Quite often the V part of HVAC gets overlooked, don't make this mistake.

HowStuffWorks provides a good starting point for understanding the basic facts about HVAC, it's actually quite fascinating.

What is a heat load?

This is a short name for "heat loss and gain calculation". This is something that every HVAC contractor should do at the very beginning of every job, but in reality there are very few that will even mention it to you, and if you even ask them about it, they will try to convince you that it is either very expensive or completely unnecessary because there's a "rule of thumb", or press you with their alleged experience and tell you how many thousands of people they have made happy.

This is all, forgive my French, bullshit.

It is neither complicated nor expensive. All it takes is the heat load software application that will cost you $49 if you get it from hvaccomputer.com, an hour of your time and the measuring tape, plus a little bit of information about your house that you are supposed to know before you start (like, what it's made of).

You'd probably be surprised - the calculated heat load value for your house may be well below your expectations, and quite often below the capacity of the unit you already have. If you don't feel comfortable with your current unit - the problem is somewhere else.

WARNING: Heat Load Calculation, a.k.a. Heat Loss and Heat Gain Calculation, is a critical component of any HVAC related project. Insist on having one done, and if the contractor refuses to do so or keeps talking about "rule of thumb" or "square feet per ton", you're talking to a wrong contractor.

What about "rule of thumb"?

There is no "rule of thumb". This is what the contractors use to save some money, on your behalf.
Every house is different. Even the same floor plan will have different head load if it is oriented in a different way, or insulated differently - for example, just replacing single pane windows and glass doors with tinted double pane may as well mean 1 ton difference in
the heat load.  That's several hundred dollars worth of equipment.

WARNING: There is NO rule of thumb.

Is bigger unit better?


Proper is better.

If the unit is too small, you may be able to get away with it if you have a good zoning system - for example, if parts of your house are not occupied at all times. This is not the only justification for zoning, but zoning is outside of scope of this question - we'll get back to it later.
On the other side, if the unit is too big, you have automatically
  • Paid unnecessary premium for something bigger than you need;
  • Committed yourself to paying higher electrical bills because the bigger unit will "short cycle" (switch on and off more often), and the maximum energy is consumed at startup (starting current may very well be ten times operating current). To add insult to injury, HVAC units (with exception of electric) get to optimal working conditions within no less than fifteen to thirty minutes for air conditioners and heat pumps and no less than five to fifteen minutes for furnaces;
  • Condemned yourself to repair it more often, because the equipment doesn't like to be jerked (compare: highway vs. city driving);
  • Created yourself a problem because the bigger unit will not be able to control the humidity properly. This will bite you in the back when you are paying your bills as well, because the way you feel is not just the temperature, it's humidity as well, and you will have to lower the thermostat to feel comfortable, which will cause the equipment to work longer;
  • Forgotten about the "V" in HVAC - bigger unit will not provide sufficient air circulation because of short cycling, and the air in the house will feel "stale".
Hope you are convinced now.

Most frequently, the contractors will try to sell you a bigger unit than you really need. The reasons usually are:
  • It is more expensive. They live on markup. The more expensive is the unit, the higher is the profit, even if the profit margin doesn't change;
  • This is what they have in stock (if they let you walk away, you will cool down and the sale will not happen);
  • They don't want to do the heat load calculation because this costs them money, however, they are afraid that the unit will not be big enough to satisfy the load requirements. The inadequacy of a unit that is too big is much more difficult to identify than inadequacy of the unit that is too small;
  • They fail or neglect to analyze the reason the customer requested a new unit in the first place. Quite often, the problems perceived to be related to a unit that is too small are in fact caused by inadequate ductwork, or improper balancing, or improper humidity control. Or such a simple thing as a dirty indoor coil.

What is a normal runtime for a HVAC unit?

Let's define runtime as time that passes from the moment the HVAC unit turns on to the moment it turns off during its normal cycle.

Short answer: 10-15 minutes and more.

Long answer: Generally, the longer the unit works, the better. Part of the reason is that at the beginning of the cycle the unit is just "warming up", and the temperature difference between the coil and indoor air is insufficient for efficient heat transfer. The coil gets to normal operating temperature in at least 5-10 minutes (FIXME: include graphs), and only then the unit approaches its design efficiency.

It is not unusual for the unit to run 30 to 60 minutes and longer - this fact alone does not mean the unit is undersized.

What about the filter?

WARNING: Changing, or cleaning, the air filter in your HVAC system is a critical maintenance procedure.

This is what happens when you don't do it often enough:
  • You are breathing dust now. Hope you don't have allergies.
  • Your unit is breathing dust now. You can cough to help your lungs to get rid of the dust, your unit can't.
  • The restricted airflow causes the unit to have a higher temperature drop across the indoor coil, which may cause the refrigerant not to be completely evaporated, which will cause the liquid refrigerant to reach the compressor, and the compressors are not designed to compress liquid, they are designed to compress gas.  There are some compressors that are able to withstand such an abuse, but not all of them.  The ones that are not, will die. You will pay dearly for that.
Even if it doesn't get this bad, the dirt will invariably accumulate on the indoor coil. Some units are notorious for a high price that you have to pay to clean the indoor coil.

For example, Goettl (the site seems to be down quite often) units are designed in such a way that it is not possible to get to the indoor coil without disassembling the ductwork. Some contractors just cut the holes in the ductwork, clean the coil, and patch the holes back. Disgusting design.

What is "multistage"?

The units referred to as "multistage" have two or more possible values for the compressor output. It may be a single compressor operating at different speeds, or it may be more than one compressor with any combination of them working at any given time.

The point of doing so is to provide heating or cooling capacity adequate to the demand and thus conserve the energy and optimize the unit operation.

What is a "variable speed fan"?

To be exact, there are two different kinds of fans: multi-speed and variable speed.

Multi-speed fan can be thought of as a single speed fan able to operate at different speeds and thus producing different airflow.

Variable speed fan is a more complicated animal: as a multi-speed fan, it will have more than one possible operating speed, but unlike multi-speed fan, the variable speed fan will take active measures to keep the airflow at the specified level. So, while multi-speed is just multi-speed, variable speed is multi-capacity.

Variable speed fans are known to be able to deliver much higher static pressure than other kinds, and a lot of HVAC contractors think they are a blessing. However, variable speed fans appear only in relatively expensive units.

Multistage & Variable Speed?

A combination of a multistage compressor and variable speed fan is the way to go to achieve the best comfort and energy savings.  Here's where they shine:
  • Harsh climates, where the periods of extreme cooling demand exist along with the periods when the cooling is barely required. This is true on both day-by-day and seasonal basis. Example: Southwestern US.
  • Humid climates, where the need for dehumidification is no less than need for cooling. Example: Southeastern US.
It should be noted, though, that they are expensive, but then again, you get what you paid for. Just make sure your contractor has experience in dealing with such systems.

Update: according to inside information, infinitely multistage and infinitely variable systems are quietly planning a loud comeback. Expected to happen around 2005.
Update: alas, it didn't happen at least until 2010.

What is "compressor slugging"?

Compressor slugging is a phenomenon when some amount of refrigerant doesn't evaporate and reaches the compressor in a liquid form. Liquids are not compressible, and if the compressor design doesn't take this into account, the compressor will invariably be damaged.

A particular design of a compressor called a "scroll" compressor is not affected by liquids entering its internals (which doesn't mean the efficiency doesn't suffer, though).