My New, Upside-down DHW System

IN MY DECEMBER 2001 column, I discussed the heating system that Id installed in my own live-in laboratory (Lessons Ive learned from Thistled House, pg. 24). The system used a tankless wall-hung boiler for providing domestic hot water. It performed as flawlessly as a tankless heater can be expected to perform, but due to major changes in my physical plant, it was time for it to leave. My DHW system

IN MY DECEMBER 2001 column, I discussed the heating system that I’d installed in my own “live-in laboratory” (“Lessons I’ve learned from ‘Thistled House,’” pg. 24).

The “old” system used a tankless wall-hung boiler for providing domestic hot water. It performed as flawlessly as a tankless heater can be expected to perform, but due to major changes in my physical plant, it was time for it to leave.

My “new” DHW system needed to be different in more ways than one. My good friend Greg Gibbs with Shamrock Sales in Denver hooked me up with a tank of a different design. The outward appearance does look a little different, but what has really changed is what’s going on inside. I’ve nicknamed this tank the Poseidon, and after I get done explaining its layout and design, you will understand why. If you’re not a big Shelley Winters fan, then you probably don’t recognize the name.

My goal, when specifying the new system, was to keep my condensing boiler operating in its “sweet” range of condensing. In other words, I didn’t want to ramp the boiler up to 180°F to satisfy my 120°F load requirements. That would cause a significant drop in energy efficiency by taking the boiler out of its condensing mode.

This, by the way, is what I’ve dubbed “the American way.” As I’ve pointed out in my previous columns, Americans in their infinite wisdom and with cheap energy, run the water temperature of the boiler up to 180°F so that they can use a smaller heat exchanger to heat the potable water in the storage tank and maintain the tank temperature at about 140°F or lower.

Onward and upward. I wanted to use the tank at a substantially lower temperature because my actual end-use water temperatures are really more like 120°F instead of the typical 140°F. I also wanted to keep my condensing boiler well loaded and not allow it to deviate much from the DHW temperature. I was originally thinking of using one of the European-style tanks that are merely a can that is filled with a massive copper heat exchanger, but then Greg volunteered this new tank that seemed like a good match to my design criteria. It also had another feature I’d not considered, that being a removable buffer tank.

The final tank design looks and works like this: It’s an Amtrol 80-gal. stainless-steel tank with the company’s standard copper internal heat exchanger, except that the heat exchanger is in the top of the tank instead of the usual bottom of the tank installation. Also, I’ve switched the water routing. Instead of operating the boiler water through the copper coil, I’m running the boiler water through the tank, and I’m running the potable water through the coil that’s immersed in the top of the tank. You still with me? Can you see why I dubbed it the Poseidon? I was going to dub it The Upsidedown but thought that might be too obvious.

In any case, I can maintain my storage tank temperatures at about 140°F. The sensor for controlling the DHW call is located in the very bottom of the tank directly below the immersed coil. During my testing of the tank, the response time of the sensor was amazingly quick and accurate. I have a 10°F (split) differential built in to the DHW controller so that the water is drawn down to 135°F before the boiler is enabled, and it runs until the tank temperature reaches 145°F.

During peak loading tests, the tank was capable of delivering copious amounts of adequately hot water while keeping the boiler in a relatively low temperature range and in the condensing mode. Typically, I see an entering water temperature of 120°F to 140°F and a co-relating DHW temperature of 120°F to 130°F. This keeps my heat source nearer the “sweet” spot of condensing and more than satisfies my needs for DHW.

It does have its limitations, but they’re nothing more than what we’d gotten used to with the tankless system. Basically, only one person or machine can use hot water at any given time. I don’t consider this a major problem.

The “buffer” feature kicks in at anytime my space heating system calls for heat when the outside air temperature is less than half of my design temperature, which for Denver is 35°F. When that condition is present (65°F to 35°F outside temperatures) the tank acts as a buffer tank to keep the boiler from short cycling. Once the outside air temperature falls below 35°F, the boiler pumps its energy directly into the one-pipe distribution system. As the outside air temperature increases, the tank comes back on line to help buffer the operation of the system, increasing efficiency and avoiding short cycling.

Tune in next month as we continue looking at the latest in “out of the box” hot water heating system designs.

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.

TAGS: Hydronics