Choosing Heat Pumps
This seemingly magical technology has been with society since the adoption of refrigerators. We've used them for decades as reliable ways to heat and cool our homes and buildings. Their efficiency is being applied to water heaters and clothes dryers. It runs on renewables with no combustion needed.
Efficient for heat, only way to cool and dehumidify.
In heating and cooling homes we make the case that heat pumps are a natural fit for most climates. Cooling with dehumidification is a matter of comfort in the summertime and it is healthier for the building materials and furnishings.
Super efficient air-to-air heat pumps are changing traditional thinking on the need to include other fuels for heating like natural gas and propane. Better building envelopes and newer heat pumps without gas assist, are proving cost-effective and reliable in New England and Canada.
For one unit of electricity, we get 1.5-6 units of heating or cooling with dehumidification energy. With energy efficient building envelopes, proven airtight, energy requirements for heating and cooling gets small. Choosing heat pump efficiency is less important than building envelope efficiency.
Building envelopes are the most cost-effective way of reducing energy needs. Heat pumps serve these reduced needs well, without bringing combustion inside and are a good fit for renewables and batteries. Here are choices with rough costs for small to average homes.
Traditional Heat Pump
The picture is an outdoor unit of a traditional split system. The refrigerant lines run to the indoor unit's air-handler, where energy is exchanged and ducts distribute the air throughout the house. While this system tends to have a bad reputation, they are one of the most capable heating and cooling appliances, ever invented. Most bad experiences are better blamed on inefficient building envelopes, architecture, and ductwork, with room for major improvement. Variable or multi-stage equipment also helps.
These systems still represent one of the best heating and cooling values available. With professional design and installation, they are a great fit for efficient building envelopes. They are affordable, reliable and easily serviced. Costs from $7-20K.
Pass Through Air Conditioning PTAC Heat Pump
Most affordable option best described here as a built-in window unit. Commonly used in hotels, they are efficient but aesthetics and sound can be a challenge. A better fit for retrofits and isolated spaces. They can be a good choice for those on small budget or saving up to invest in a new system or technology.
My frugal father saves money with my mother's willingness to spend most time in the living-bedroom area served by a PTAC unit. The extra heat pump keeps the room cooler than the rest of the house, which uses a tradtional split system, set at a higher temperature. Of course dad says the extra heat pump costs him more. With installation, $500-2K.
Mini-Split Heat Pumps
Mini-splits are the fastest growing heat pump style for heating and cooling. Extremely efficient with thier use of electricity and capable of heating in outdoor temperatures as low as -10. Many net-zero and high performance projects are relying on this technology, even in the north. Alex Wilson has a good review of his on the GBA site.
Most systems are ductless. They usually have one, indoor head which is like the air handler of a traditional unit without the ductwork. This makes them similar to PTAC units in that they are point-source systems. Increasingly, multiple indoor heads are run to the outdoor unit. Costs $2-20K.
Problems with Mini-splits
While they are the perfect fit for many projects, they get as expensive upfront, as traditional systems when installed on average sized homes. Zoning is best handled with seperate outdoor units. To get things cheaper than a traditional system, there will be a certain loss of control. Homeowners need to keep interior doors open for best distribution. Dont attempt this strategy without international code level building envelope performance or better.
Are mini-splits cost effective?
Our primary HVAC contractor suggests mini-splits don't make it very far past their warranty dates (~7yrs) and because the electronics are so complicated, it usually makes sense to replace the whole unit. The cost effectiveness of this scenario is in question compared to the serviceability of a traditional system.
Mini split mold?
There were also hints of an industry cover up on the tendency of the indoor unit's fan blades to accumulate mold. This doesnt happen to all mini-splits but appears to be a growing problem if the right conditions are met. Check back for more soon.
No one wants to look at those ugly indoor heads. You can hide them to some degree with built-ins but keep in mind accessibility, especially if the thing is going to die in 7 years or needs to be sprayed with mold cleaner. Outdoor units are smaller than traditional units but stick out more, being painted white from the factory. Apparently their makers havent figured out what US makers have known for years, grey blends in more. Take that asian efficiency.
Dont let the contrarian views spoil the big picture on mini-splits. I plan on using one in my personal home's future renovation. Iam looking forward to the extra cooling benefits in the summer and eliminating the oil-fired furnace in winter. I expect that by switching to a mini-split, I will be more comfortable, have better indoor-air and have lower energy and environmental costs. It will be capable of being powered by solar PV and a tesla battery system (fingers crossed).
Mini-splits are especially well suited to small, open floorplans and renovations.
Ducted VRF (Variable Refrigerant Flow)
In the near future, all split systems will feature variable speed technology. DC pumps and circulators better match thermostat and humidity demands. It offers a blend between mini-split technology and traditional system control and accessibility for repair and service. Higher upfront costs have fast paybacks. Stay tuned for our upcoming project and check out Matt Risingers videos on ducted VRF.
A word on ducts
Ducts have negative implications to people but are usually required for removing pollution, right at the source; dryer vents, bath vents, kitchen vent hoods are best served with some amount of ducting.
Supply-air ducts are the bigger concern for dust accumulation. Good design and construction practices keeps concerns manageable and to a minimum. Filter placement and homeowner maintenance is critical.
As mentioned in radiant floor heating, negative experiences are usually based on leaky, inefficient building envelopes with outdated equipment, poorly installed, and under-insulated ductwork. Code minimum levels of building envelope performance result in very small air-flows to meet energy demands. Good equipment selection, and third-party verified ductwork results in systems whose operation is barely noticeable.
Outdoor-air ventilation usually requires ducts
Ductwork allows the most control for indoor air quality in multi-level homes, 3 bedrooms and up. There are newer outdoor-air ventilation, balanced-ductless-systems that can be used for smaller, more open floorplans.
Never put ducts in vented crawlspaces or vented attics.
New, vented crawlspaces should be illegal, and should never contain heating, cooling or outdoor-air ventilation ductwork. Vented attics are only acceptable when they have little to no HVAC ductwork.
Use Programmable thermostats with care
Ensure that programmable thermostats are not forcing the heat pump's emergency resistance heating to quickly respond to satisfy thermostat settings.
All the above systems are considered air-to-air heat pump technology. Geothermal, Ground Source Heat Pumps are a water-to-water technology and are increasingly popular but currently may not be the best investment for most projects. It's also possible to use air-to-water heat pumps on radiant floors.
Springtime Builders is an Asheville NC custom home builder specializing in building science craftsmanship and indoor-air quality.
Posted in Heating and Cooling