Letters: Plumbing Retrofit

May 01, 2006
May/June 2006
A version of this article appears in the May/June 2006 issue of Home Energy Magazine.
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Plumbing Retrofit

        I’ve recently completed the California Building Performance Contractors Association (CBPCA) training program and have become a subscriber to your magazine. One year ago I bought an old home on San Pablo Park in Berkeley with all original plumbing, heating, and electrical systems. I am in the process of retrofitting the entire house for energy efficiency and comfort.
        The Atlas Gravity Heater has been replaced by a 92% efficient, electronically commutated motor (ECM) forced-air system with all new (and properly installed) ducting. I have just fired up a new exterior-mounted direct-vent Rinnai 2532W tankless water heater. The heater itself is performing well, but I am concerned about convenience versus energy and water consumption.
        The system is plumbed in a traditional way. The trunk lines are all 1/2-inch copper to each fixture area, then 1/2 inch to the fixtures themselves. The heater is mounted on the outside wall of the bathroom and therefore favors the bathrooms in terms of quick delivery of heated water to the fixtures. The kitchen is on the other side of the house, which results in longer wait times for hot water. The aerator on the kitchen sink flows 2 gallons per minute (gpm). On a cold morning, it takes 60 seconds to get hot water at the tap. If that water is run down the drain just to get hot water, that’s a loss of 2 gallons of hot water. That’s 2 gallons down the drain, then a period of hot water use, then 2 gallons of heated water remaining in the pipes that will cool before the next usage. So my question in a nutshell: To loop or not to loop?
        My Rinnai heater requires an independently heated loop to maintain full warranty. All my plumbing is still exposed. Therefore, I can install the loop system as noted in the diagram that came with the water heater. At this time,my hot-side plumbing is still uninsulated. All hot-side pipes will be fully insulated prior to closing walls. I would install the system using a timer and an aquastat. I would use a 110V electric heater and a properly sized pump.
        Do you have any energy consumption data from real-world installations like this? Is there a good way for me to estimate the likely energy consumption?
        Another question: The diagram indicates that full 1/2-inch pipe is not required for the return portion of the hot loop. Smallerdiameter pipe is a little cheaper, but will restrict the flow slightly and may cause the circulation pump to draw more power. On the other hand, 1/2-inch return plumbing means more surface area of the heated loop system and a possibly higher rate of heat loss.
        I appreciate your magazine and look forward to learning of your expertise on the matter.

Technical Editor Steve Greenberg replies:
        There are better solutions than a loop kept continuously hot, or even kept hot many hours per day. Probably the best (lowest energy and water use, low first cost, medium convenience) is the ondemand pump: when you want hot water at a remote fixture, you push a button and the pump comes on, pumping water from the hot pipe into the cold pipe until the hot water reaches the pump, which then turns off. The water is available much sooner than it would be if you drew it from the tap, and almost no water is wasted. Such pump and control systems are available from, for example, Metlund (www.gothotwater.com); Taco (www.tacohvac.com/en/products.html? current_category=59); and Chilipepper (www.chilipepperapp.com).
        You should check to be sure, but there's no good reason why these would cause a warranty problem, since hot water isn't circulated through the tankless heater.

Runtime or Energy Use?

        I need information on a device to monitor the amount of time individual thermostats are on in our three apartment buildings. I know that some of these monitoring devices use runtime modules inside each unit that are connected to a zone valve, which will transmit a signal, and through another device send it to an office where data are collected.

Author Henry Gifford replies:

        I do not know of a product that fits your description. What is more common is something called a BTU meter, which is a fairly ordinary water meter, consisting of two temperature sensors and a black box that calculates water flow and temperature drop and gives a readout in terms of energy used. Numerous companies make and sell them, including Istec of New Jersey (www.istec-corp.com). Last I asked, they cost about $800 per apartment, including the central box. I think there are also companies that do the reading and billing automatically. Some of these companies are probably already in the water-metering business, which could be a good way to find them.
        I think individual metering is a good idea, and has a bit more credibility than simple runtime because of differences in water temperature, couches blocking heaters, and so on.

High-Velocity HVAC

        Can you provide me with information about high-velocity heating and air conditioning?

Steve Greenberg responds:

        I haven't seen a lot of information. A search on Google finds some discussion and an analysis by a consultant for the efficiency standards. The big manufacturers are Unico (www.unicosystem.com) and SpacePak (www.spacepak.com); the technical information on their sites is probably okay, but there's lots of promotional material there to take with a grain of salt.
        These systems have a natural handicap compared to normal systems in that the small-duct, high-velocity (SDHV) systems have a relatively high fan pressure and thus typically have higher fan energy use than the conventional systems. All other things being equal, the ducts have lower conduction losses and may not leak as much air, so the SDHV systems may have an edge there that may compensate at least partially for the fan power penalty.

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