Hot Water Questions Recirculate

The water temperature is described in the article as ranging between 120°F and 160°F, but 120°F is the highest reasonable temperature for hot water. In the study, water temperature in the buildings is controlled through mixing valves, which mix cold water with hot water from the boilers. Mixing valves of the $200 variety don't work well when new, and fail quickly, while mixing valves of the $1,000 variety control water temperature accurately when installed according to the manufacturer's instructions. 

The manufacturers recommend installing a return line pump that runs continuously. If you cycle the return line pump as recommended in the article, the mixing valve will fail and the water will become too hot. 

Figure 1. A qualified contractor should install a false loop and TACO 006 bronze self-lubricating pump with a minimum 30 ft of properly insulated 1.25-in to 2-in piping.

The focus of the study was the energy-saving potential of hot water recirculation, not of pipe insulation. Obviously, pipe insulation would be greatly beneficial wherever it is missing. However, in multifamily buildings like those that were part of this study, the pipes (which I believe are often uninsulated) usually run within brick walls--sometimes exterior walls that are also uninsulated--so it is impossible to know or control whether there is any pipe insulation. However, it is reasonable to expect a fair amount of heat loss through the pipes.

In New York City, hot water temperature is required to be a minimum of 120°F at the tap. In order to meet that requirement, it must be hotter when it leaves the basement--in some cases as much as 140°F, depending on the length of pipe it will travel through. This is true no matter how frequently the water is recirculated. 

In my experience, even the more expensive mixing valves do not meet long-term performance expectations, even when they run continuously. My impression is based on research work I've done for NYSERDA that focused on mixing valves. During this work, my colleague and I looked at several different makes and types of mixing valves, none of which functioned within acceptable parameters after two years. In other cases, when monitoring about 50 buildings for up to about a year each, as well as while consulting on individual buildings, I've observed daily high and low swings as well as hourly and five-minute temperature recordings, and all too often have observed swings of up to plus or minus 10°F -20°F allowed even by mixing valves that claim to hold their temperature within 1°F. A colleague of mine with equal if not more experience has observed this problem in hundreds of other buildings. 

If you are still concerned about meeting the manufacturer's conditions, you can install a false recirculation loop with a TACO 006 bronze self-lubricating pump and a minimum of 30 ft of properly insulated 1 1/4- to 2-inch piping (size should be equal to the size of the mixing valve installed). This will create continuous circulation around the mixing valve without circulating the 130°F-140°F water around the entire building, where it will unnecessarily lose heat and waste energy. (See Figure 1.) 

Tucson Takes Issue with Comfort

Representatives from TEP did meet with people from Comfort Home while conducting research for our program. However, TEP felt there were several additional areas that we wanted to include in our program; mainly the following:

• the pressure balancing in Guarantee homes necessary to address safety issues related to backdrafting fireplaces or combustion appliances; 
• returns in each room to alleviate comfort problems that may result when doors are closed in tight homes;
• the source of makeup air required to provide pressure balancing when exhaust fans are in operation; and
• a fresh air ventilation system that TEP requires in every Guarantee home to address indoor air quality issues. 

Comfort Home was an inspiration for us to take our program to the next level. We applaud the efforts of everyone, including Comfort Home, in taking on the challenge of building homes that use energy more wisely.

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