How Ductless Heat Pump Technology Works
Today, we are going to outline manifolded, ductless heat pumps. We first learned about these systems a few years ago, while working on a LEED hotel in Colorado. We want to give readers some background and thoughts on this technology.
Used in Europe and Asia for the last 2 decades, ductless heat pumps are still relatively new here in the US. How are these different from our more familiar heat pump systems? Heat pumps that we may be familiar with are known as Split Systems – compressor outside connected to the heat sink, and inside the energy is moved to a coil in the air handler. The air in the air handler blows air over the coil, changing the temperature. The heat exchange goes on in the air handler. The heat sink maybe their ambient air, a ground loop, a cooling tower, or a pond. Refrigerant is pumped through loops to transfer heat to the sink. In the winter heat can be draw in from the ground that is at 55°F year round below frost depth.
Ductless systems are also Split Systems, but are more complex than the normal arrangement. These systems have the outside compressor, heat sink, like the other system. However, inside the building, it gets much different. The refrigerant from the compressor is piped to a Manifold, and then from the Manifold it is exchanged to pipes leading to Terminal Units spread throughout the building.
How does this operate differently from the normal HVAC ducts? First in the duct, the system moves heated or cooled air to the zone(s) based on the thermostat setting for that zone. The familiar “click” we know and then the sound of the fan blowing air to the room. In these ductless systems, controls rely on a more complex system of moving existing heat around the building. Each Terminal Unit has a thermostat that is tied to the computer that runs the Manifold. The Terminal Unit senses it is above or below the set point and then tells the Manifold to send the correct temperature refrigerant. The Manifold checks the available heat or cold difference in other terminal units and adjusts the flows until the set points are met. If it needs to it will activate the heat pump outside to reach the set points. Terminal Units typically have a coil and a fan unit. They use very little power as compared to baseboard electric heat.
Here is an example of how it works; you have a building, the west side and south side rooms are gaining solar heat and will require more cooling. Now factor then also on the opposite sides of the building, those rooms are losing heat through walls and windows. What the Ductless system does is first try to move heat to the cooler parts of the building. It first absorbs the heat from the warm rooms, and moves it to the cool ones. Then based on the set point will activate the outside compressor system to toss out more heat from those other rooms.
Other advantages are on remodels, only the holes for the piping need to penetrate walls. Systems come with a variety of Terminal Units, from wall mounted, to ones that fit in suspended ceiling grids, even ones that look like picture frames. For the architect this opens up many possibilities.
On that LEED hotel project, our initial plans were to replace the typical PTAC (Packaged Through-wall A/C) with terminal units in each guest room. In effect, we would use the entire hotel as the heat sink, solar gain rooms would have heat dumped into shade side rooms, and the occupied rooms would dump heat into the unoccupied rooms. Most of this would go on without any compressors kicking in. That has huge implications for energy savings. In addition, these units have SEER ratings of 19 to 26, very efficient, compared to PTACs, with a SEER of 13 to 15. Combined with the other systems and considerations we are incorporating into the LEED plans it looked good to get that LEED Gold. Or does it?
Hotels are difficult typologies when it comes to LEED applications. Hotels are like large houses, with everyone getting up at the same time and wanting a hot shower, TV on and the AC blasting. The customers are purchasing the right to have it their way, LEED or nor LEED plaque on the front of the building. As such, the demands are greater at peak usage than for other types of buildings.
What is an energy and money saver in a house, bank, or office has the opposite effect sometimes in a hotel. We found that out the hard way with the electric instant-hot water taps. Likewise, for the ductless system all though it’s efficiency was in every way superior to the best PTACs, the install cost was the killer.
Here is what we found: Installation was several times more expensive, the overall system was more expensive to supply. Second, durability – commercial PTACs last about 5 years in the hotel environment, the ductless system can last at least 20 years. So installed in a house, office building, even condominium it would make sense because these are long-term investments for the owners. Hotels have to be profitable very quickly, typically in 3 to 5 years. Even though our client was intending to keep the project long term, part of the business plan is to be at a profit in 5 years. Part of achieving that was the choice to go back to high efficiency PTACs with advanced controls and vacancy sensors – even though PTAC replacement four times in the next 20 years will cost more than the ductless system installed.
Additionally Hotels have a higher level of fire compartmentalization, and we would need to run fresh air supply ducts in order to balance the typical bathroom exhaust from each room. A PTAC typically brings in enough fresh air to meet those requirements. In other building types, this is not as strict so undercutting doors, and other means of natural ventilation are used.
We did have the system used in the offices and public areas of the hotel, and that improved our overall building efficiency. Because as architects we had an open mind when the engineer suggested the system we explored its possible applications and implications to ventilation and energy design. We were fortunate that we had very good energy modeling services. They helped the client choose the best system that met LEED goals and the business plan.
We looked at Mitsubishi Electric and Sanyo (through Trane). Both systems function in the same way, but carefully consider how they communicate with the rest of the temperature controls in the building. We feel that these kinds of systems will find their way into more housing and office projects. We feel that this will be another useful tool in many applications. Architects should ask their engineers if it is a possible fit, but have sober ideas about the cost in larger applications.