Thursday, March 21, 2013

Understanding Expansion Tanks

One of the most overlooked and misunderstood components of a hydronic heating system is the expansion tank. When a heating system is designed the heat loss and boiler sizing are usually carefully calculated. The circulator is sized based on the length and heat carrying capacity of the piping. When it comes to the expansion tank, most guys just install the same size tank they have always used. The phrase, “We’ve always done it that way.” comes to mind. Oddly enough, most of the time it works. It’s when it doesn’t work that the problems arise. The most common symptom of an undersized expansion tank is the relief valve blowing off. To avoid this, the installer should understand the function of the tank and the importance of proper sizing.

Let’s start with the basics. We all know that things expand when they are heated and contract when they are cooled. Water is no different and this becomes a major factor in a closed loop hydronic heating system. Upon installation of a new system, you fill the pipes with 40 to 50 degree water and purge the air. Then when you fire the boiler and heat the water to 180 degrees, the water expands. As the water gets hotter the volume increases. Without compensating for this expansion of the water, the pressure in the system would go up and at 30 pounds, the relief valve would open. An expansion tank is designed to accommodate this increase in water volume in a closed loop system. Water is not compressible but air is and the expansion tank uses a cushion of compressed air to accept this increased volume of water as it is heated.

Old style expansion tanks mounted between the floor joists in the cellar ceiling were made out of steel and had tappings for water, air inlet, and a water level gauge. When the system was filled, the water entered the tank and compressed the air at the top. As the water heated, the level in the tank rose and the air compressed even more. Eventually, the air cushion would be absorbed into water and the tank would become waterlogged. This of course caused the pressure to rise which set off the relief valve and always led to a service call.

In the early 60’s the modern diaphragm tank was developed. It uses a rubber diaphragm to permanently separate the system water from the compressed air in the tank, thus eliminating the waterlogging problem. These tanks became popular because they were small and compact, inexpensive and needed much less plumbing. This type of tank, however, still can cause potential problems if not sized properly.

Let’s take a look at the five things we need to know to properly size a diaphragm expansion tank.

Fill Pressure ---In a closed loop system, static height of the piping is a factor only when filling the system. One pound per square inch (PSI) of water pressure is required for every 2.31 feet of static height. In the average 2-story house with the boiler in the basement, we must raise that water about 16 feet which would require about seven PSI. In order to ensure positive pressure and aid in air removal, we should add another five PSI to this. Pressure reducing valves (PRV) are factory set at 12 PSI and expansion tanks are pre-charged to the same 12 PSI. Most of the time the 12 PSI pre-charge is enough but if the system height is greater than 16 feet  the pressure reducing valve must be adjusted higher and the air charge in the tank must be increased accordingly. A tire gauge and a bicycle pump are all the tools you need to accomplish this. Always remember the tank must be isolated from the system when checking pressure. When checking the tank pressure, the system pressure must first be reduced to zero and the tank isolated by means of the isolation valve. If you don’t follow this procedure, you are not reading the tank pressure; you are reading the system pressure.

Relief Pressure---We need to know this in order to size the tank. Most residential relief valves are set at 30 PSI.

Fill Temperature---When you fill the system with tap water, the temperature is usually around 50 degrees.

Average Water Temperature--- This is the normal high limit temperature of the boiler. The more we heat water, the more it will expand.

Total Water Content--- This is critical to tank sizing. Old gravity systems that have been converted to forced circulation will have much more water than a new baseboard system. Three quarter inch copper pipe contains .025 gallons of water per linear foot while inch and a half steel pipe holds .106.  Measure the total footage and multiply by the appropriate amount. Then add the boiler water content and allow a couple of extra gallons for the fudge factor.

There are many factors in determining expansion tank size and any unusual variations in any one of these areas could dramatically affect the tank size. Manufacturers have plenty of experience in this regard and have developed guideline charts based on average systems. Looking at the chart, you can see that systems with higher water content like cast iron radiators and baseboards will require a larger tank than a copper baseboard system.

Monday, March 11, 2013

Sizing A Circulator Just Takes Some Math

The dictionary defines a system as a group of interacting, interrelated, or interdependent elements forming a complex whole. When designing and installing a hydronic heating system, we are in fact creating a group of elements that interact, that are interrelated and are interdependent, and they do form a very complex whole that is designed to keep people comfortable while minimizing energy usage. The system’s relative success or failure depends on how well these elements work together to perform the system’s stated function. The definition of hydronics is the science of transferring a definitive amount of BTUs from a source device to a heat transfer device and back via the movement of water or solution thereof. A key component of a modern hydronic system is the circulator and its main function is to move heated water (BTU/HR) through a distribution system (the radiators) and back.

It’s important to remember than when sizing a circulator, you do not need to take into account the height of the building.  The physical height of the building does NOT equal the feet of head. As part of defining a circulator as opposed to a pump is the fact that we are in a closed loop system versus an open system which has to over come static head as well as pressure drop. Examples of this would be a well or a sump pump system. The circulator does not need to lift the water to the top of the building due to the simple fact that what goes up must come down.  The circulator doesn’t have to lift the water to the upper floors – the weight of the water coming back down the return side is a counterbalance. Think of the circulator as the motor on a Ferris wheel.  The motor doesn’t have to lift the weight of the people up – there are people on the other side of the wheel coming back down. All it has to do is overcome the friction loss of the bearing assemblies in the wheel.  A circulator doesn’t have to lift the water – it only has to overcome the friction loss – or head loss – of the system. 

All piping systems impart friction loss on the fluid in the system, and understanding this is key to making sure your hydronic system functions properly. If you do the math, calculating the flow requirements for circulator is pretty simple - it's basic arithmetic. Calculating the "other" half - head pressure (or friction loss) - is a little tougher. Use the Universal Hydronics formula to determine how much flow the circulator has to be capable of.

GPM = BTUH ÷ ΔT x 500

GPM is gallons per minute. BTUH is the calculated system load. ΔT is the temperature difference across the system at design conditions and we use 20° F for our systems. 500 is a constant - it is the weight of a gallon of water (8.33 lbs.) times 60 minutes. When we have determined the load of the system all we need to do then is to divide by 10,000 (20 x 500) and we have our GPM requirement for the circulator. As an example, let’s say we are zoning with circulators and have a 30,000 BTU zone of baseboard or 50 feet of element. When we divide the 30,000 by 10,000 we determine a flow rate of 3 gallons per minute.

Choosing The Pipe Size
What size pipe should we use for this zone? Well, the guidelines for pipe sizing are as follows: 
  • 2 to 4 gallons per minute of flow, use ¾” M copper;
  • 4to 8 GPM, use 1 inch;
  • 8 to 14 GPM, use inch and a quarter;
  • 14 to 22 GPM, use inch and a half. 

These all fall within hydronics guidelines for pipe sizing and keeping flow velocities at no less than 2 feet per second and no more than 4 feet per second. At velocities greater than 4 feet per second, the system will produce velocity noise and customer complaints. At velocities lower than 2 feet per second, dissolved oxygen will tend to come out of solution and cause air problems within the system. 

To determine the head loss of a zone, start by measuring the total length of the zone, including the element. In this case we have 80 feet of  ¾” pipe connected to the 50 feet of element for a total of 130 feet.  Now multiply the total by 1.5 to allow for fittings, valves, etc. Fittings and valves produce pressure drop in a system that is the equivalent of a few feet of pipe each, so multiplying by 1.5 accounts for most basic fittings and valves.

If you have high head items in your system like flow checks or 3 way valves, you will have to add some more head later. You now have the total developed equivalent length of the circuit and you multiply that by .04. This number represents 4 feet of head per 100 feet of copper pipe. That head number applies as long as the pipe has been sized according to the velocity guidelines shown in the previous paragraph.  The end product is the head loss for the zone.  120 x 1.5 x .04 = 7.2 feet of head. We must now find a circulator that will deliver 3 gallons per minute against 7.2 feet of head.

If we take a look at the Taco “00” series performance curve chart – we can determine which circulator we should use for this zone. As long as the point at which the system operates is inside of or on the line that the pump operates, you are assured that the pump will deliver heat at the right temperature to the zone. If that point falls outside the pump curve, your pump will not be able to deliver the maximum amount of BTUs needed under design conditions. Simply put, in the coldest weather, the system can’t reach the required comfort level. In the case of an indirect water heater it will be slow in recovery.

First, on the bottom axis, we find the flow rate – in this example, it is 3 gallons per minute. On the vertical axis we have head loss – in this example it is 7.2 feet of head. We follow the two lines until they intersect to find our system operating point of 3 GPM at 7.2 feet of head. Next, we look at the performance curves to find out which circulator would make the best selection.  In this example, a 006, 005 or a 007 would be good choices – with the most likely choice being the 007, since it’s the most common and most readily available. 

Once you understand the dynamics that are going on in a system, you can size and select the right circulator for the job. Learn more about Taco products on our website

Friday, March 1, 2013

Taco "March To Efficiency"

During the entire month of March 2013, Taco will be running a promotion called the "March To Efficiency". Everyday they will be giving away a product to a lucky FloPro member. That means 31 days, 31 prizes and 31 chances to win! Take advantage of this great opportunity and register now! To be eligible you must be a Taco FloPro member. To register to become a FloPro member go here.

Already a Taco FloPro member? Then enter yourself for a chance to win today's featured product here. All you have to do is watch a short video to be entered to win that days prize. That's it!

And be sure to stop by the March To Efficiency website everyday in the month of March 2013 to enter to win that days prize.Watch a video for a chance to win a prize. It doesn't get much simpler than that! Good luck, everyone!

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