Heat Pump Sizing - Factor 3: Proper Installation
The effectiveness of a heat pump system can sometimes come down to how it is installed. You can insulate and air seal your house, have an accurate Manual J calculation, and still the system doesn’t seem to work as well as you hoped. There are many mistakes that can be made during an installation, but there are a few that are seen all of the time and are the first things to look for. These mistakes will be separated between ducted systems and ductless systems. By avoiding these mistakes, you will ensure that your system has been installed properly.
Ducted Systems
Duct systems are a great way to condition a house and are so common because you can provide the right amount of conditioned air to small and large rooms, and there is nearly no visible impact on the interior of the home. The problem is that duct systems can easily be missized, and it takes a lot of experience and knowledge to properly build a duct system.
There are two proper ways to size a duct system, a Manual D, and the written method of using the system size, CFM, a “ductulator,” a friction rate chart. If you pay attention while sizing the ductwork, then neither option is better than the other, but the written method has a lot of room for mistakes, especially if the installer in inexperienced. With that said, duct design should be done for every house. Just like with the Manual J, each house is its own system and requires its own calculations. To understand how someone can missize ductwork, we should start by explaining the duct system.
The duct system starts at the plenum. The plenum is the supply ductwork directly above the air handler, and the return ductwork either beneath or at the side of the air handler. We are going to focus on the supply side of the system for this duct system description. Conditioned air is pushed by the blower out of the air handler and into the plenum. The plenum is the first point pressure builds in the system and it sends air down the trunkline.
The trunkline is the long and large rectangular or circular duct that runs down the length of the house. This trunkline has supply ducts coming off of it that feed the rooms conditioned air. As you move further down the trunkline and further from the air handler, conditioned air flows into the smaller ducts, reducing the amount of air in the trunkline, slowing the air’s velocity. To prevent the slowing velocity, you install a trunkline reduction to keep the pressure and velocity up. The further down you go, the more you reduce the trunkline’s size.
The supply ducts that feed the rooms within conditioned space have a duct fitting that promotes air flow out of the trunk and into the supply duct. The air moves through the supply duct and up and out of the register that enters conditioned space. Supply ducts come in sizes as small as four inches (in most systems) and increase in size depending on the CFM and BTU requirement of the room it feeds.
These are the basics of a duct system. It begins to become more complicated when you think about the fitting types, boot types, the CFM of the mechanical equipment, the trunklength, etc. Each fitting and register has an equivalent length value assigned to it. The equivalent length is the measurement of friction and pressure loss of a fitting or boot, and how that fitting compares to straight pipe. A 90 degree elbow can have an equivalent length of 30 feet, so as the number of elbows add up, the friction and pressure loss do as well. There is nothing wrong with any fittings available to us as long as they are calculated into the duct design.
Building a duct system is a careful calculation, and when a ducted system isn’t working properly, it may not be the heat pump system, it may be the duct system.
Ductless Systems
Ductless systems are easier to install, but there are mistakes that can be made that will decrease efficiency, or cause the system to improperly function. The main issues we see are refrigerant leaks and low refrigerant due to the contractor not measuring the lineset lengths and following installation instructions. Proper installation requires attention to detail, and preventing these major issues.
When reading Reddit or forum posts, you will frequently see people say their system had low charge, and the contractor topped of the system. HVAC systems are not supposed to lose refrigerant, and contractors should always recommend looking for the leak, and not topping the system off. A leak in a ductless heat pump system will usually be at the flares (the end of the lineset that is shaped so it flares out and fits to the refrigerant line on the indoor or outdoor unit), in an area where the lineset needed to bend, or where two separate linesets are connected with fittings. Copper linesets are delicate and a kink will lead to a refrigerant leak that may not be noticed untill months later. This is preventable by following the installation manual, and pressure testing the lines. A system that does not hold pressure has a leak, and should be fixed before they system is started up.
Each outdoor unit comes prefilled with a certain quanitiy of refrigerant. The amount of refrigerant is determined by the system size, and whether it is a single port or multi port unit. When installing multi port outdoor units, the installer needs to verify the total lineset length connected to the outdoor unit, and look at the installation manual to know if refrigerant needs to be added. A system that does not have as much refrigerant as it should will perform poorly, and lead to an uncomfortable house.
Conclusions
Heat pump systems must be installed properly to work properly. With that said, there are tools and knowledge available to both new and experienced contractors to make the right choices during installation. If you plan on having a new system installed, make sure you ask the contractor how they do the installation, what the system start up looks like, and verify that the system they will install has been selected based on your home’s needs, and not rule of thumb.
