Diving Deep Into BTU Meters
By Curtis BennettCooling Heating HPAC General HVAC Systems devices energy meters hydronics temperature
At its core a thermal energy meter is a device that measures the amount of thermal energy used in a fluid system over a given amount of time.
I’ve titled this article ‘Btu Meters’ because I think that name is more well-known than the “proper” term of thermal energy meters. I can tell you this subject is pretty near to my heart, as I have spent last few years working on them. So if I seem like I’m getting way too deep, it’s because I think some of the information is necessary to really understand what they do and how they work. So let’s dig in.
What is a BTU meter?
Btu meters are also known as heat meters, but I prefer the term thermal energy because the word heat excludes the possibility of calculating cooling thermal energy which these devices can also do, well at least some of them.
I’m sure you have started to hear a little more about these devices online or in print. The reason is that thermal energy metering is kind of the final frontier of metering devices. It was a “hole” in the metering industry, at least in North America, for companies or people to bill based on the amount of thermal energy used.
At its core a thermal energy meter is a device that measures the amount of thermal energy used in a fluid system over a given amount of time. To put this into perspective, a water meter measures the amount of water used, an electrical meter measures electricity consumed and a gas meter, you guessed it, measures the amount of natural gas consumed at a location. There are “metering devices” for all these, and these devices are certified for billing purposes. This is a very important aspect,as this makes them very accurate and as such there are stipulations to using them.
How Does it Work?
As you all probably know, the calculation for a Btu is: Flow (gpm) x DeltaT x 500. So, to calculate the thermal energy used in a hydronic system we need some flow and we need the temperature at the supply and the return.
Then there is this elusive number “500”. This number is the heat coefficient, and the heat coefficient of water is 500 (well of pure water anyways, but let’s not muddy the waters, LOL). This number represents how much energy the fluid can hold. There is a very complex calculation that goes into this number taking into account the density of the water, which depends on the temperature and pressure of the system, and if there is any glycol or other antifreeze in the fluid. And there is another variable called enthalpy, which basically means how much energy can actually be put into a fluid. This number will vary anywhere from around 300 all the way up to 500 or even a bit higher.
Once we have all of our values we can achieve a very accurate Btu calculation. This calculation has to be done to five decimal points of precision. Alright, that’s enough boring stuff for one article!
So now let’s calculate the “energy” of an imaginary “apartment”: it’s Btu = 5.6(gpm) x 23 (DeltaT) x 457, which equals 58,861.6. But hang on, that’s an instantaneous number of 58,861.6 Btu’s/hour.
The accuracy of these devices has to be very high. So we actually slice this number every second and do the accumulation that way.
So that would be, in this case, 58,861.6/3,600 (seconds/hr) which equals 16.3504 Btu’s per second. We add this number to our accumulated number.
This Btu value can actually move around all the time, so to get a perfect value over a given amount of time, you need to take very small amounts and add them together. If this is not done the actual value will be out of range and customers will be billed incorrectly. So the calculating portion of the device is very important.
Now let’s move on to the “guts” of the thermal energy meter. Let’s start with the flow portion. There are three main types of flow sensors: vortex shedding technology, ultrasonic and turbine. Each has advantages and disadvantages. Sensing challenges like debris, fluid concentration, or orientation of the device can affect the performance of the energy meter. Proper mounting of the flow sensor also very important.
This comes right down to turbulent items in the flow steam as well. These could consist of valves too close, pumps too close or even a change in size of pipe too close. Size can also be a deciding factor when it comes to using an energy meter. Mechanical design may make it so some units just don’t fit in a space.
All energy meters, until recently, used the same basic technology. They have a flow meter and two PT-100 or 1000 (thermistors) temperature sensors. This technology has its limitations. With a resistive temperature sensor you are not allowed to modify the sensor length. This is part of the reason in North America the adoption has been slow to take off.
These types of meters cannot be installed into many current heating environments, at least not very easily. The typical energy meters come with around three meters of length on the sensor. You can get longer, I think up to 15m, but the issue then is that you can’t modify that wire. So you have to find a clear path to put the temperature sensor at the other end of the system.
The temperature differential is super important in the calculation. The differential of the supply and the return temperatures can change the outcome a lot, hence why the sensors have to be so accurate and calibrated to be less than 0.185C different from the factory.
This leads us to the different classes of energy meters: Class 1 is an accuracy of +/- 3.5%, Class 2 (where most meters fall) is +/- 5% accuracy and Class 3 is also +/- 5%, but there are some other small differences, these are called the maximum permissible error. These are the total of all the errors that can happen with the device. This includes the calculator errors, the error from flow, and the error on the DeltaT. When the certification is done they total up all the deviations to find the total. We don’t need to get into the error calculations, just know, it’s not easy to get a device fully certified.
Why do we need them?
The energy meter, as with most of our hydronics overlying technology, has its roots in Europe. The European market is laser focused on using the least amount of gas or electricity to get the most output. Metering how much energy you use in a space helps to achieve this goal. In Europe they have been using energy meters for years. They have a whole industry to install, maintain and calibrate these meters.
They need to be certified and calibrated at regular intervals. Remember, these devices are used to bill customers, so they need to start out accurate and stay accurate for many years. In North America we are just getting started in this new frontier. I think it’s a really good thing when it comes to the push for more energy efficiency.
I used the example of an apartment because it’s the natural place to use these devices. Tenants who use small amounts of energy are rewarded, and the large users have to actually pay for what they are using. It will bring the efficiency of the whole building up. When you know you are paying for what you are really using it is a natural tendency to use less.
So there you have it, a mind-boggling entry into the field of thermal energy meters. This article got a little long so we did not cover communications or IoT aspects of these devices, for which there are many variants. Maybe next time.