If you’ve followed these articles over the years, you’ve read about measuring and calculating the performance of forced air heating and cooling systems. Due to many recent requests from Hotmail readers, it seems like this is the right time to discuss the other heat transfer fluid…water. Since winter is on its way, let’s take a look at how to measure the Btu delivery of a residential hot water heating system.
This will be a brief introduction to water system Btu measurement, and if the response is good, we may continue to add more to the basics of water system performance measurement.
The Formula
Understanding math is the key to understanding how Btus move through a system. The simple formula is System Delivered Btu = 500 x GPM x System Temperature Change. Let’s take a look at the formula see what each piece means so we can understand it better.
The Btu constant in the formula is 500. Because Btus are measured per hour, the 500 comes from one gallon of water that weighs 8.33 pounds times 60 minutes in one hour (8.33 pounds times 60 minutes = 500).
The second piece of the formula and sometimes the hardest to identify is GPM or the system gallons per minute. We’ll talk more about this subject below.
Finally, we need the system temperature change. Notice we’re talking about the system temperature change, not the equipment temperature change. Temperature change is the effect of the Btus transferred from the system into the conditioned space. So if you measure the temperature of the water leaving the heat exchanger and subtract the temperature of the water returning from the system, you’ll find the system temperature change.
Calculate Pump Pressure and Plot GPM
For the purposes of this article and since we’re only considering the basics, let’s take a look at calculating pump pressure and plotting GPM in a residential hydronic heating system. We could discuss far more accurate methods, but this is just a starting point. It’s an initial performance test for beginners.
Since we don’t have to deal with duct leakage issues, we’ll assume the pump GPM is the system GPM. Two bits of information are needed to estimate pump GPM. The first item is the pump curve. When a pump is built, each manufacturer published the pump’s performance curve. You must have the exact manufacturer’s curve matching the installed pump with the right impeller size, RPM, and exact model number or your Btu test can be off by more than 50%. Simply Google the words, pump curve, with the model number and the Manufacturer’s name. Most current pump curves can be found on the Internet.
Like a fan curve, this table graphically represents the performance of the pump under certain field conditions.
Ideally, pump pressure is measured using pressure gauges or a circuit setter. For the introductory purposes, we’re going to calculate pump pressure using a time-tested formula.
Also, when performing the test, make sure that all the zone valves are open and calling for heat. The system performance test will be inaccurate if one or more of the zones are closed.
To calculate pump pressure on a simple residential system, use the following formula. Pump Pressure in Feet of Head = Feet of Pipe x 1.5 x .04.
First, to find the Feet of Pipe, measure the total linear feet of supply and return pipe to and from the farthest heating device in the house. The 1.5 in the formula is a factor to include the pipe resistance to flow (pressure) and the pressure drop of the system components (coils, baseboards, radiators, and excess fittings). The .04 represents the typical pipe friction rate per 100 feet of pipe.
Example: Say a home has 90 feet of pipe in a hydronic heating system. The formula would be 90 feet x 1.5 x .04 = 5.4 Feet of Head.
Once we have calculated the pump pressure, we can use the pump curve to plot pump GPM. First, mark the calculated pump pressure on the left side of the pump curve where the feet of head pressure is found. Second, plot a straight line horizontally to the right until the line intersects the rounded pump curve line. Third, plot straight down to the bottom of the table to find the GPM the pump is moving.
Now you have found the pump GPM and you are a step closer to finding the system delivered Btu.
System Temperature Measurement
To be completely accurate, a submersion thermometer should be inserted into the water. But I’ve assumed you probably don’t have Pete’s Plugs to get access to the water temperature or pressures. So we measure the temperatures on the surface of the pipe, wrapped with insulation or by using a clamp-on thermometer specially made for pipe temperature measurement.
Since we’re checking the system performance, not the equipment performance, measure the temperature of the water at least 10 pipe diameters downstream from the pump or heat exchanger where the water is leaving the equipment. Read and record the temperature to the closest 1/10th of a degree.
Measure the returning water temperature by measuring the temperature of the pipe at least 10 pipe diameters before the pipe returns to the equipment. Be sure not to measure directly over the boiler or too close to the flue so you don’t pick up the equipment heat in your water temperature measurement.
Subtract the supply water temperature from the return water temperature to find the system temperature change.
Calculate System Delivered Btu
To find the system delivered Btu, multiply the Btu constant of 500 x the estimated Pump GPM x the System Temperature Change.
Example: Say you calculate the pump pressure at 8.0 feet of head. Using the pump curve you plot and find the Taco 007 Pump is moving 8.0 GPM. Then you measure the system temperatures and find the discharge temperature to be 168.2F and the return pressure to be 152.4F. You subtract to find the system temperature change of 15.8F. Now that you have all the facts, apply the hydronic Btu formula: 500 x 8.0 Gallons X 15.8° = 63,200 Btu.
Is the system Btu delivery close to what to the equipment performance specifications, or is it a new 100,000 Btu output boiler hacked into a 40 year old mucked up piping system? Perhaps your customer would like you to prescribe some additional system improvements.
That’s all you need to complete the initial residential hydronic system Btu calculation. Remember this is just an initial test. There are far more definitive tests and procedures required to increase accuracy and calculate exact system delivered Btu. But this is a great start.
Unfortunately, it’s not uncommon to find the hydronic system performance well below 60% of equipment rated output. It’s probably not a good idea to promise your customers that their hydronic system is perfect until you measure its performance. Assuming a system is performing at the published equipment rated capacity is not a good idea.
So, how well was the last hydronic system you worked on performing? Or, how badly was it performing? If you can’t answer that question honestly, you might want to measure next time.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a procedure to measure the performance of a hydronic heating system, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.