Heating Systems 101 – Learning the Lingo

She Blinded Me With Science!

One of the toughest things about researching a new heating system is learning the tech talk. Your HVAC company will throw out all sorts of terminology assuming that you understand what they’re talking about. Some might even be happy that you *don’t* understand so they can confuse you and sound like experts. Well, no more!

This post covers the most common terms that you’re likely to run across. I’m sure I’ll miss some or confuse you, so please post questions if there’s anything you’d like clarified.

HVAC – Heating, Ventilation and Air Conditioning

This one’s easy. You’ll hear it pronounced “H-vac” or “H-V-A-C”. It’s just a catch-all term to cover anything related to your home’s heating and air conditioning systems.

BTU – British Thermal Unit and Ton

BTU is a measurement of heat. For the big systems that heat or cool your home, you might hear about Tons, which is 12,000 BTUs per hour. So a 3-ton system can produce 36,000 BTUs per hour.
So what is a BTU? It’s about the energy produced by a single wooden match. A 1,000 Watt hair dryer or space heater puts out about 3,400 BTUs/hour.

Domestic Hot Water (DHW)

This is the water that comes out your faucet that you drink or wash with.

Boiler vs. Furnace vs. Heat Pump

You’ll want to understand the difference between the various types of systems. Fortunately, this is easy and intuitive 🙂

  • Boiler – a heating system that uses hot water. Got it? Boil…water ==> boiler. Once you know this, you’ll always remember which is which. You might also hear these called “hydronic” heating systems – hydro => water. Again, easy to remember.
  • Furnace – a heating system that uses hot air. They might also call this a “forced air” or “scorched air” heating system.
  • Heat Pump – 99% of the time, a heat pump is a forced air system based on the same technology as an air conditioner, just run in reverse. Sometimes these are referred to as electric heating systems, but that term is broad and can cover different types of systems.

If you’re in a house and it’s got radiators or baseboard “convectors” (defined below), then the home uses a boiler of some sort. If it’s got air vents (registers) in the floor, wall or ceiling and doesn’t have radiators or baseboard heaters, then it uses a furnace or heat pump. There are rare exceptions, but that’s the general rule.

On rare occasions, you’ll hear about “electric boilers” or “electric furnaces.” Both of these use heating coils, just like an electric oven or toaster, to heat the water (boiler) or air (furnace). I won’t discuss them further since they’re rare and, at current utility costs, these systems cost 2+ times as much to operate as oil or gas. You’ll also see electric baseboard heaters. These operate and cost the same to run (expensive).

Radiant Heating and Underfloor Radiant Heating

Radiant heating refers to heating via a warm object. Think of “radiators.” Technically, it’s heating by “radiation” but that term has multiple meanings, so radiant it is. Stand out in the Sun and you can feel the warmth from radiant heat.

To confuse matters, most of the time when people talk about radiant heating, they’re referring to underfloor radiant heating. So if your HVAC person talks about radiant heating, assume that they’re talking about the type built into the floor.

Underfloor radiant heating uses either hot water in tubes or electric heating built into the floor to provide heat.

I could write reams about underfloor radiant heating systems and how most are installed incorrectly, but that will wait for another post.

Heat Exchanger

A device that transfers heat between things without letting the two things touch one another, almost always to prevent contamination. Most often, this means water. So you might have water from your boiler that you don’t want contaminating your drinking water supply. The boiler heated water goes through a heat exchanger to transfer the heat from that water to your drinking/bathing water.

Indirect Water Heater

An indirect water heater is a water tank connected to a boiler so that the boiler heats the water and pumps it over to the water tank. Usually the water tank has a heat exchanger so that the water from the boiler doesn’t actually touch the DHW (see above: domestic hot water).  This allows you to use a single hot water producer (the boiler) and use it for different things – heat the house and heat water for showers. It’s typically more efficient and reliable.

Outdoor Reset Control

You’ll rarely hear this term because most boilers don’t have this capability. Plus, I bet most HVAC technicians don’t even really understand how to do it optimally 😦

An outdoor reset control is a system that changes the temperature of your boiler based on the outdoor temperature.

The idea is that you want the boiler to run at just the right temperature for your heating requirements. So when it’s warmer outside, the boiler can run at a lower temperature which is more efficient. When it’s cold out, the temperature ramps up.

Air Handler

The air handler is the big box that’s part of the furnace or air conditioner or heat pump where the blower fan is located. It sucks air from the house through a “return” and supplies it to the house via a network of “registers” or “supply ducts.” They’ll say “this air handler looks too small for your system. You’ll have to replace it with a new variable speed unit that’s matched to your furnace BTU output.”


Coil is one of those terms that refers to a number of different things depending on context. Here’s a few:

  • Air conditioner or heat pump coil – the parts with all the aluminum fins inside the air handler or the outdoor unit. In both cases, the “coils” serve to transfer the heat/cooling from the system to the air. Often, the coils will develop leaks because they make the copper tubing thin to increase efficiency. Beware! Often, when your air conditioner or heat pump stops working properly, the HVAC technician will shake their head and say “this doesn’t look good – you’ve got a leak in your coil. I can add some Freon but it will just leak out and you’ll end up spending a couple hundred dollars every couple months. You should replace your unit since it will cost just as much to replace the entire thing as to replace the coil.” If you hear this, ask to see the leak and have them use their refrigerant leak detector to pinpoint the leak. If they don’t have one of these, don’t trust anything they say. If they tell you they don’t need one because they can “see the oil stain”, they’re a hack. This is an essential tool that’s required for quick and accurate location of a refrigerant leak.
  • Boiler coils – mostly in older systems. Also called “tankless coils.” These are copper heat exchanger coils that are surrounded by the hot water in the boiler. They’re used to heat domestic hot water. These often develop leaks over time and have to be replaced. This is such a prevalent problem that there’s a page dedicated to them: Inspectapedia – leaks in tankless coils

Condenser and Evaporator

Air conditioners and heat pumps use refrigerant and magic, I mean physics 🙂 to cool and heat your home. Two of the main parts of these systems are the condenser and the evaporator. The evaporator is the cold side and the condenser is the hot side. With an air conditioner, you want the heat outside your home, so the big unit with a fan sitting outside is the condenser. It also has the compressor. Here’s a little explanation of how this works.

With an air conditioner, or a heat pump in air conditioning mode, the evaporator is inside the air handler in your house.

To keep these straight, think of evaporation when you have water on you. Remember the chilly feeling you get? That’s due to the water evaporating. When something evaporates, it sucks heat from its surroundings. That’s how air conditioning works, through the miracle of evaporation.

The condenser is where gases condense. When gases condense, they add heat to their surroundings. That’s why the outside unit of an air conditioner blows hot air. The refrigerant is condensing. In a heat pump, when heating, the condensation occurs in the air handler in your house.

If you’re paying attention, you’ll see that a heat pump switches the operation of the indoor and outdoor units depending on whether it’s heating or cooling. When it’s cooling, the condenser is outside and the evaporator is inside. When it’s heating, the condenser is inside and the evaporator is outside. Tricky, eh?

Returns and Supplies

Return usually refers to the duct attached to your air conditioner or heat pump where the air “returns” to the circulation fan. In plumbing, the return is the pipe that carries water back to the boiler from your radiators and the supply lines run to the radiators.

Registers, Vents, Grills

All of these refer to the place where the ductwork from your air handler comes out your wall, ceiling or floor. They might say “your air registers look dirty” or “don’t block the return vent.” Often they’ll leave out “vent” and just refer to “supplies” and “returns.” Technically, they mean slightly different things but in practice, you’ll hear the terms used interchangeably.

Advanced Topics

Heat Gain and Loss calculations


Unfortunately, you rarely encounter these terms unless you research the correct way to size heating and cooling systems.

Heat gain and loss calculations refer to the quantitative methods for computing how much energy your home gains or loses under various conditions. For example, if you’re installing a heating system, you’d want to know how much heat the house loses at various typical winter conditions. In the summer, you’d want to know how much heat the home gains due to high temperatures, sunlight, and so on so you can get the right sized air conditioner.

The Manual-J calculation method is the industry agreed upon method for performing these calculations. There are entire books to read and spreadsheets to fill in including such things as the amount of insulation in each wall, the size and type of every window in the home as well as orientation of each wall and roof overhangs. It’s a process for the detail oriented person because the calculations are only as accurate as the numbers entered. Garbage-in, garbage-out as they say.

Because the process is labor intensive, most HVAC technicians take the (totally invalid) shortcut and use rule of thumb, like “one ton for every 500 square feet.” It doesn’t take a genius to comprehend that this is complete bullsh*t. They’re trying to say that a 50 year old home with poor insulation requires the exact same heating/cooling systems as a new home, built tight with low-e windows and spray foam insulation! Obviously, if they do this, they’re completely incompetent.

The only semi-valid shortcut you can take is if you’re replacing a system in a home where you’ve been living for a number of years. If your systems have provided stable, comfortable, reliable heating and cooling, then you can replace them with the same sized systems. This is making a number of assumptions, and isn’t perfect, but it’s better by far than using rules of thumb.

One word of warning – air conditioner sizing is particularly hard to get right. If you size it too large, you’ll be blasted with cold air for a few minutes, then the system will shut down. This has two negative consequences – the temperature will swing wildly and it won’t effectively reduce the humidity. Ideally, the air conditioner should be as small as possible to satisfy the cooling requirements on a hot and humid day but not the worst day of the year. The air conditioner should run long and low for best comfort.

Heating is more forgiving and combustion systems (gas, oil, propane) tend to be oversized because nobody wants to be cold on a cold winter day. Also, the heating system isn’t controlling humidity, so it’s easier to predict. Heat pumps are tougher because they lose heating capacity as it gets colder out. So they’re designed to be able to supply heat until it gets to a certain temperature, then they switch on backup heating. For heat pumps, it’s especially important to know the heat loss characteristics of your home or you’ll end up very uncomfortable and spend a fortune on electricity.



4 thoughts on “Heating Systems 101 – Learning the Lingo

  1. Hi Again Ted 🙂

    Read with great interest, like if I was reading a thriller :), Jessa’s story (about her house in Boston with the attic’s water problem) and both your communications last Wednesday (Dec 2). I hope I’ll get to read the end of this story.

    Now it’s one AM and I am still absorbing all this precious information one story after the other. Just read everything you wrote about heating system 101 + other articles on heat pump.

    I retained from everything I read this that applies to any problem in our lives:

    ” sleuthing skills

    In order solve mold problems, you have to know the science of moisture movement and be methodical in finding the source of moisture.

    One of the biggest mistakes people make is ignoring the big insulation picture and how it all connects together something doesn’t make sense to you, don’t let the contractor do it. Sometimes they’ll be offended and say something like “I’ve been doing this 30 years, I know what I’m doing.” Well, maybe they’ve been doing it wrong for 30 years! If you have any doubts, get another opinion or two. Or post a question on my Q&A section. There’s a science to this and there’s a good chance that your contractor doesn’t know the science. I’m not saying that they’re not a good contractor, but they may simply not be knowledgeable about the current best practices. Regardless, if they’re not willing to listen or back up what they say with facts, you should find another contractor.”

    I just can’t imagine Ted a different way of looking into things. The problem is that none of the guys (3 company until now) who came to submit even measured my house before recommending options for the electric heating furnace and the heat pump. And they didn’t seem to take in account the fact that it was an old house. Or the other hand, as you pointed out, doing otherwise take time. And time is money.

    That said I have a energy report that point out what would be, if I isolate my house better, the heating loss of my house (10202 Watts) and the cooling charge (1.5 tone) . The report recommend to give this last information to the person who will replace my old heating system. To tell you the truth I didn’t have the impression they were really understanding what the report was talking about.

    Neither do I !

    I was heating my house with oil and the system needed to be replace because it was getting very old (more than 25 years). Took a while to decide what to do because we have ducts (air pulsed) and I didn’t know exactly what to do because some were telling us to take everything out (all the ducts) and migrate to electric heating and other (majority) were telling us to keep the ducts and buy a new electric furnace and a heat pump. I opted for this last solution even so the first solution seems more appealing because of the space I would have gained. Also a more costly solution (i think).

    Anyway all this to tell you that it’s obvious for me more than ever that finding a really professional that knows about the science is next to impossible because they work without taking in account the dynamic of the whole house and I see it which makes it very very hard for me to make a choice.

    I have checked the name and the credibility of all the companies who came and the reference are all excellent. I have 3 other companies who are coming this week and then I’ll have to decide because meanwhile we’re heating with temporary heating and hard winter is arriving with it’s snow and cold.

    Other people around me , most of them actually, ask for 3 submissions when they have work to be done and then they choose the cheapest one and that’s all. Not me. I need to understand at least the basic. And reading everything you wrote has now a double effect on me: I feel encourage and discourage at the same time because after all I don’t have any scientific background when it comes to your expertise.

    Thanks !

    Michel 🙂

    • Good on you for doing your homework! I admit it’s daunting. Many of the lessons are hard learned after years of seeing mistakes.
      One note – if you are going to have air sealing and insulation work done, that will very much affect how much heating system you’ll need. I can’t tell from your message if you’ve already had a new system installed or you’re still researching it.
      Over the years, I’ve found the most effective way of determining how big a system you need is to analyze your previous heating usage. With oil, it’s pretty easy because there’s a conversion between gallons burned and BTUs generated. It’s not 100% accurate because efficiency can vary a lot, but it’s better (in my finding) than most of the abstract calculations.
      Based on everything you’ve written, I would personally air seal and insulate, then see how your existing heating system works through this winter and compare your oil bills to see how much of an effect the work has had. From this, you can make a more informed decision about how to proceed with a new heating system.
      If you need help with calculations, I can write up a sample with your numbers to show you how I would do the analysis. To do this, I’d need to know where you live, the type of existing heating systems (models and details if possible) and your oil bills (gallons and date) for as long as you have records. Usually I need a few years of bills since usage varies so much and doesn’t fall on easy dates, like gas bills which show monthly consumption. This would be a really useful exercise for this site, so I’m happy to run through it for you and the rest of my readers.

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