When Water-To-Water Heat Pumps Don’t Work Well

By Barbara A. Checket-Hanks

“Dear ASHRAE:
“A building I am working on has an existing chiller-boiler arrangement. The owner wants to install a water-to-water geothermal system, but wants to use as much of the existing system as possible (ventilators and fancoils). Is this possible? Will the geothermal system work as intended?
— Signed, Nervous Engineer”

This isn’t a letter from a syndicated advice column. It’s from a forum on “Water-to-Water Heat Pump Operating Problems,” held at the 2002 Annual Meeting of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) in Honolulu, HI.

One of the problems with water-to-water heat pumps, it was acknowledged, is that they need a volume of water flowing at all times. Their biggest advantage is that the heat pump never “sees” the building’s load; it sees the water loop. The water loop is what sees the building. Therefore, water-to-water heat pumps “can work on a retrofit and you don’t care how people use the building,” commented an engineer.

However, “We’re always running into water-flow problems,” stated another mechanical engineer. The system “has to have adequate evaporator and condenser flow; otherwise, units lock out on high head pressure.”

When adding a water-water heat pump system to an existing building and replacing its HVAC system, the problems that arise can be attributed to “the whole issue of commissioning,” an engineer stated. “It’s the lack of understanding by people who have always dealt with a boiler and a chiller.”

CONSIDERATIONS

“One job I was on had seven ‘cooks,’ and it looked like we wouldn’t get any broth,” recalled a forum participant. “One ‘cook’ demanded an economizer cycle. That was OK, and the controls guys designed it that way, but at that time, no water was circulated [and] no water was available for the zones that needed it.”

System integration and zoning are possible, but system limitations need to be spelled out clearly to decision makers. For example, it may be possible to design for a north zone and a south zone, but not in adjacent rooms. And “Nothing says we can’t mix and match water-to-water and water-to-air systems,” said an engineer.

However, “We have uncovered design flaws from trying to reutilize the existing system,” specifically the low-side pumps, stated an engineer. “We didn’t take into consideration the pressure change.”

In another case, a 40- to 50-year-old system wasn’t flushed out properly before the combined system was started up. New components were damaged downstream.

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Finally, problems can result if the existing system itself was oversized — not an uncommon occurrence. “We have yet to find an old radiant system that didn’t have the terminals matched to the boilers, and the boilers were oversized,” stated an engineer.

Most importantly, though, “In water-to-water systems, water flow is a key issue. We’re seeing things with three-way valves involved, or trying to design geothermal when they don’t use antifreeze; then we see cracked heat exchangers from the freeze-up.

“A lot of things can be resolved if you really look at your application.”