AFUE and Efficiency

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AFUE and Efficiency

Jeff Brideau: Benchmarks. We need to know where to start.

Jeff Brideau: So I’m Jeff Brideau

Mike Breckon: I’m Mike Breckon.

Jeff Brideau: And today we’re going to continue our series by talking about how we measure a heating systems or cooling systems against each other. Because when we’re shopping for new equipment, we need some way to say which one’s better or worse, for the money. And one of the few tools that we have to sort-of level everything out, is a general term, what we call efficiency. What is efficiency? Because you don’t get the same numbers from every piece of equipment. You know, we also get different perspectives on what “efficiency” means. So for example, one of the common questions I get when we talk about heat pumps in the market is a customer will say: “Well, but is it efficient?”. And what I think they’re really trying to ask me is, “is it dollar cost efficient to use it? “. That is, you know, “does it cost a lot of money to run?” is really what they’re asking. What is it going to cost?

Mike Breckon: That’s right.

Jeff Brideau: Because I usually respond with, “well, they’re fairly efficient but they’re not nearly as efficient as electric baseboard”, “if that’s what you’re asking, then electric baseboard is 99.9% efficient”.

Mike Breckon: Correct.

Jeff Brideau: But it costs a lot to run it.

Mike Breckon: Yes.

Jeff Brideau: So, let’s get a grip of what we are talking about when we say “efficient” and then takw the next step. Well, once you get past efficiency, then you’ve got to talk about what cost of operation is and we’ll get into that in another episode where we talk about different fuel sources and how to compare what a machine costs to run on different types of fuel. So I think first thing to start with is just a brief discussion of what efficiency means. So why don’t you give us a brief rundown,

Mike Breckon: Well, for different pieces of equipment, there are yard sticks to compare the four fossil fueled equipment. There’s what they call an Annual Fuel Utilization Efficiency (AFUE). AFUE so you can compare natural gas furnaces for example, one-to-the-other from about 78% up to, I think now they get to close to 98%. And if your doing an air source heat pump, there’s what they call HSPF or Heating Seasonal Performance Factor.

Jeff Brideau: So what do all these numbers mean though? When we talk about the efficiency and percentages versus you know, HSPF, what is it that these numbers actually mean?

Mike Breckon: The higher the number of HSPF the less electricity you will use to do the same job.

Jeff Brideau: Right, so if we look at, or start with efficiency because most people are more familiar with that number, it’s a little easier for people to grasp, get their mind wrapped around. Efficiency is the amount of heat that you get out of an appliance for the amount of energy that you put into it.

Mike Breckon: Correct.

Jeff Brideau: So, and that’s a percentage. So we can all imagine that if you have a gallon of water or let’s go even further, say it’s heating fuel, right? If we pour that gallon of heating fuel into this oil boiler behind us, how much of that gallon actually gets turned into useful heat in a house? And that’s what efficiency is. So this thing here probably runs in the 80 to 85% range. So that means 15% of the oil that you’re putting into the machine actually goes up the chimney pipe or leaks into the basement.

Mike Breckon: The heat from that from that oil goes up the flue.

Jeff Brideau: The oil itself actually also goes up, it gets turned into other things…

Mike Breckon: Right.

Jeff Brideau: …You know, carbon dioxide and other chemicals after its been combusted. But the other part, we’re assuming, it goes into the building and that’s the 85% part. When we get into heat pumps, we get a whole other set of numbers. And why is that? We get Coefficient of Performance (COP). So why do we have to have that different numbering?

Mike Breckon: Because we’re using different fuels, we’re using electricity.

Jeff Brideau: Wwll, it gets a little more interesting on that. Webster’s dictionary says efficiency, and if you look in your physics books, can never be greater than one. So when we say, okay, uh, let’s take an air source heat pump, and in simple terms, let’s sa they publish COP numbers for them sometimes. So at 50 degrees, the COP may be 2.5. What does that number mean? Well, it turns out it’s actually on the same exact scale as efficiency. So if we say a boiler might be at 85%, that means that less than one of the gallon that you put in actually gets turned into heat.

Mike Breckon: Right.

Jeff Brideau: Well a heat pump doesn’t suffer that problem. A heat pump can actually make more heat than the energy that you put into it because it’s not burning fuel. So it actually performs such that it produces a greater than 100% efficiency (in practical terms). And since the dictionary says you can’t call it efficiency if it’s over one, we had to make up a new term and that’s what we call a Coefficient of Performance (COP). So it’s actually on the same scale. And we’ll draw a little picture here.


AFUE Efficiency Chart

Mike Breckon: Which is the ratio of the heat output of the device versus the electrical energy you put in.

Jeff Brideau: That’s correct.

Mike Breckon: Converted to what they call BTUs.

Jeff Brideau: That’s right.

Mike Breckon: British thermal units.

Jeff Brideau: Yup, which is the same thing when you’re down below one. You could still call it COP, It would just be a fraction. This would be a cop of 0.85. So that’s where we get into why we have these silly terms that we have to deal with that confuse everybody. So we get into get into COP and then you brought up a great one, HSPF. So why do we have two numbers for heat pumps? Well the big thing is when you try and do a calculation just based on COP, the problem is COP changes with the weather.

Mike Breckon: Yes.

Jeff Brideau: So then an air source heat pump, and we’ll get into details of what that means that later, but that means you have a piece of equipment outside that’s in the weather, that’s trying to capture heat and bring it into the building. Well, it’s performance changes with the outside temperature. It’s, much more productive at higher temperatures than it is at lower temperatures. And that means it’s efficiency changes. It’s less efficient when it’s cold out than it is when it’s warm out. So how do you..

Mike Breckon: Plus, when it’s colder out, even though there’s not a lot of moisture in the air, there’s some, so you have to have that coil, outdoor coil, cooler than the outdoor temperature for the heat to flow from the outside air to the coil. And, if you get below the dewpoint, you will form frost

Jeff Brideau: that’s right

Mike Breckon: So the unit, then we’ll have to reverse itself and melt that frost and that has to be factored in

Jeff Brideau: And that’s non-productive time.

Mike Breckon: It’s non productive time.

Jeff Brideau: So that, and that also impacts the total efficiency. So if you’re trying to compare an appliance that runs without an impact of weather, like the boiler here, it doesn’t much care what the outside temperature is, unless you tell it. How do you compare that to a machine that has to deal with the outside weather? And that’s why the government came up with these two different classes of measuring sticks. One is HSPF, which is Heating Seasonal Performance Factor, and that tries to estimate these weather changes throughout the seasons and things like the defrost capability. And the sister factor of that is on the air conditioning side. They use a different term, unfortunately. So that is Seasonal Energy Efficiency Ratio and it’s trying to do the same thing. The hotter it gets outside the harder an air conditioner has to work. So, we’ve got a term that tries to incorporate the seasonal impacts so that you can have a better measuring stick to compare the performance of one machine versus another. So an example of how you would do that, because there aren’t as many choices in air conditioning, right? You’ve basically got packaged or unpackaged air conditioners that sit outside, or what else? You have water source and not too many of those around. It’s getting more popular. But geothermal is a great example of a water source air conditioning system. It’s not impacted by whether. It’s using the ground temperature that’s fairly consistent all year round. So you typically don’t see, SEER ratings for geothermal equipment, that doesn’t really work that way.

Mike Breckon: Right? You see Energy Efficiency Ratio…

Jeff Brideau: Correct

Mike Breckon: ..numbers for that. And that changes a little bit based on the water temperature coming to the unit.

Jeff Brideau: Right? And, and there can be some variations with geothermal equipment throughout the seasons as well. As it works more and more, it might warm up the ground or…

Mike Breckon: Right

Jeff Brideau: …or cool it down in the wintertime.

Mike Breckon: And there is a fancy formula that you can multiply your HSPF by a number that a ASHRAE came up with as some sort of average and it will spit out a SEER rating.

Jeff Brideau: Yeah, So these are all the little nuances that you get into and then as you might need your comfort technician to help you walk through. Because even we sometimes need to reference our paperwork too. Certainly we can’t remember all the numbers all the time.

Mike Breckon: Right. You can go on different websites like Efficiency Maine has a website and they have pellets and they have a cord wood and they have natural gas and they have lines that you plug in your cost of fuel.

Jeff Brideau: And compare fuels.

Mike Breckon: But, what you can’t figure out on their site is what the building needs for either heating… Well it’s just the heating.

Jeff Brideau: I don’t think they let you put in the efficiency either they, they just give you a category.

Mike Breckon: No, no if it’s energy star, Its just that.

Jeff Brideau: Energy Star is a big window. You might have a really good Energy Star or you might have a less than good, it’s not that they’re bad because they’re all Energy Star. But, so what we were trying to get to is that when we’re looking at appliances and we see all these numbers, how do we compare them to each other? And My company website, at Intelligent Heat and Power, used to have a nice little calculator, but we’ve just taken it down in our re-do to make it mobile friendly. But hopefully that will be going back up and you’ll be able to plug in, you know, what your fuel costs is and plug in what your appliance efficiency is and it will spit out a dollar per btu number, which is handy. And we’ll talk about that in detail when we get into the dollar-per-BTU fuel episode. But, the long and short it is we’re looking for a way for us to make decisions when we’re buying a new piece of equipment, a new system. Do I go with the Hydro Air System? Do I pick a an air source heat pump or the geothermal ground source heat pump? You know, which way do I go? Which one’s going to cost me more in the long run versus what its going to cost me to install? And these are the measuring sticks we have: we have a efficiency, and that comes in kind-of two varieties, your instantaneous efficiency. The thermal efficiency is just what it’s doing at any given instant. But, the AFUE number, much like HSPF and the SEER numbers are trying to take into account some o f the practical impacts. So we’ve got the two efficiency numbers, AFUE and thermal efficiency. When you’re shopping, AFUE is really going to be the only one that you see. And then we get into to air source heat pumps. We’ve got, well, sometimes you see the COP number, but is more easy to compare with the HSPF number. And then on the cooling side, we’ve got our last couple of big brackets and that’s going to be the EER and the SEER. The Energy Efficiency Ratio and the Seasonal Energy Efficiency Ratio. And those are our tools to be able to measure things back and forth. Now, that’s not the end-all be-all. We’ll probably do another episode in not too long discussing the other impacts that you might want to consider when you’re purchasing a heating system. Because efficiency is just one piece of that number, the other that come in is comfort. And one of the big topics that we talked about that, is, particularly with air based equipment, is when you have modulating output and what that does for how you feel.

Mike Breckon: Right.

Jeff Brideau: And we’ll give you a little teaser if anybody’s familiar with having an old fashion forced air system. A lot of, I hear a lot of complaints about hot, cold and, and drafts,

Mike Breckon: Right.

Jeff Brideau: And when we get into modulating equipment, a lot of that goes away. So efficiency alone isn’t the only answer. So, let’s wrap this episode up for now and if you like what you see, please hit subscribe and take a look out for our next episodes coming up and we’ll see you then. Thanks.


Mike Breckon: Thank you.