Two-Pipe Systems

A common complaint that building maintenance departments hear is that the occupants are either too hot or too cool.

In some instances, both complaints may come from the same room. One of the systems with the most comfort complaints is a Two Pipe System. As the name implies, the system uses two pipes to the building; a supply and a return. In the heating season, the water in the pipes is heated with a boiler and in the cooling season, it is cooled with a chiller. During the peak of each season, this strategy works great. The comfort complaints with this type of system come during the shoulder times of the year, like Fall or Spring. During that shoulder season, the building may require heat in the morning and cooling in the afternoon. The complaints  occur due to the dead band of the temperature difference between the lowest hot water and highest chilled water temperature. Standard efficiency boilers are designed to operate above 140º F. Operation below that temperature could cause the flue gases to condense which can destroy the boiler and chimney. Conversely, most chillers cannot operate with temperatures above 90º F (Please check with chiller manufacturer as to their temperature limits) . So, we have a temperature difference of 50º F between the heating and cooling set point.  You have heard that saying about how long it takes for a watched pot to boil, that is nothing compared to the time it takes for the temperature inside a two pipe system to drop  from the minimum heating temperature to the maximum cooling temperature on a mild Spring or Fall day.  It may be anywhere from several hours to a full day and by that time, we may need heat again.

When a local school with a two pipe system had to replace their boilers, they had several choices. They could have used a condensing boiler that could handle lower water temperatures. The economics of converting the heating plant to a condensing system were more than the owner had budgeted.  

Another option was to use standard boilers with a three way valve that allows lower temperatures in the system. I am not a fan of three way valves because they could lead to thermal shock of the boiler if  not installed correctly. Thermal shock can cause cracked sections or tube leaks. It is caused by a wide temperature swing from the return to the supply  piping. Most boilers were designed to handle a 20-25º F  rise through the boiler.  A three way valve allows the water to bypass the boiler and be reset to a much lower temperature. In many boiler  control strategies, a One to One ratio is used. That means that for every rise in outside temperature of one degree, the system supply temperature will lower by one degree. The typical reset schedule will range from 180º F supply water at 0º F outside temperature to 120º F supply water at 60º F outside. At the low end of that reset schedule, the supply temperature will be 120º F and the return temperature will be 100º F. When that  cool 100º F water is returned back to the hot boiler filled with 160º water, rapid expansion and contraction happens and the boiler literally shakes itself to death. Remember the advice from Dr. Egon Spengler from Ghostbusters, "Don't cross the streams." Another drawback to three way valves is that many boiler manufacturers require that you install  an additional  pump that will pull water from the supply and inject it back to the return to avoid shocking the boiler. To me, this is like driving with a parking brake engaged.

If your state adheres to the International Energy Conservation Code, there are a couple of regulations that they require when controlling a two pipe system.  According to the 2009 International Energy Code, the following must be done:

Section 503.4.3.2 Two-pipe changeover system.

"...systems that use a common distribution system to supply both heated and chilled water shall be designed to allow  a dead band between changeover from one mode to  the other of at least 15⁰ F(8.3⁰ C) outside air temperatures."   A common set point arrangement is for the heat to be enabled below 50º and air conditioning enabled above 65º F outside temperature.  In between those two set points, most buildings use a combination of economizer operation for cooling and return air for heating.

"...provided with controls that will allow operation in one mode for at least 4 hours before changing over to the other mode."  This may require some planning for the building owner to avoid complaints.

"...provided with controls that allow heating and cooling supply temperatures at the changeover point to be no more than 30⁰ F (16.7⁰ C) apart." In other words, the heating loop shall be no warmer than 120º F if the chiller is designed for 90º F entering water temperature at the changeover point.  This is below the typical lowest operating temperature of most  standard-efficiency boilers.

With all these things to consider, the engineer and I put our heads together and came up with another system that works perfect. One of the standard efficiency boilers we used was a combination boiler with an internal coil that was traditionally used for low temperature operation such as domestic hot water. This internal coil could handle temperatures from 60º F to 130º F without risk of Thermal Shock. This was our first stage of the heating system. It would handle the water heating during the shoulder times of the year. This allowed  the loop temperature to drop to the 90º F that the chiller could handle. As the reset schedule required warmer temperatures, the space heating  side of the combination boiler and the other boilers took over and supplied water up to 180º F. The benefit to the client is that  this system could change from heating to cooling in less than an hour. The result is that comfort complaints are greatly reduced.

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