Air to Water Heat Pump Applications
When you are asked to do an installation of an AWHP or have a chance to enlighten a homeowner about an AWHP, it will be one of three options. The first is new installations in new construction, the second is new hydronic installation in an existing home, and the third is a retrofit into an existing hydronic system. We will address this individually below.
The first thing you need to do is learn all you can about AWHPs. Learn how they work, how they are installed with pipe sizing, flow rates, and Delta-Ts. These are the main differences between boiler hydronics and AWHP hydronics. We offer most of this in pre-recorded seminars or live via online or in-person sessions, which carry a higher price tag.
The second thing you need to do is learn how to do a good heat/cool analysis. Gather the proper information to enter into your manual J calculation software. Learn more about creating zones. What rooms can be joined to create zones, especially when considering heating and cooling together? Learn the layout of the home, such as floor plans, either by creating a rough sketch or by using blueprints. For those of you not using a good heat loss Manual J calculation, don't be reluctant to believe the calculated heat losses. I was concerned about the first winter after doing heat loss calculations and using units half the size of existing equipment in the 90s.
We offer forms you can download and use. The forms were designed when I was working for the oil company back in the 1990s, but they should be closed. You can Download them and use them as a basis to change or design your own.
New home, New Construction
If you're going to do new house construction, try to find an upscale builder in your area and network with them. Present your AWHP ideas and possible solar ideas to them, setting a precedent for well-insulated upscale homes in the future, especially in mid-sized homes. Remind them of the country's push to transition away from fossil fuels and toward fully electric homes. Now is the time to decide whether to start offering solar PV. Looking into my crystal ball, I would suggest you start positioning yourselves for the next decade.
I would assume the homeowners are going to be interested in heating and cooling. The question becomes, do they want to maximize the savings, which may be two separate systems? You could offer radiant panels for heating and duct systems for cooling, or duct systems with heating and cooling combined using a water coil in the air handler or duct system.
The home should have a heat loss of approximately 15 BTUs per sq ft or less. This would be similar to the following R-values, along with extensive air sealing.
Ceiling = R49+
Walls = R25-R35+, includes Exterior sheathing to reduce the conduction through the framing
Floors = Slab = 1.5” to 2”+, Above unheated basement = R-19+
Windows = R4 – R10+
Ay can be used to heat DHW or maybe only as a preheater for an electric DHW tank.
After all this, you may have to add an HRV or ERV to improve indoor air quality. Consider solar PV with electric storage.
Remember, almost all purchased watts of electricity produce a BTU output. Electrical motor loads add BTUs of heat to the home. For instance, a 3-ton ECM blower motor can add just under 3,000 btu/h to the home. Think of these little heaters as refrigerator compressors, light bulbs, TVs, stoves, ovens, you get the point.
Retrofit AWHP into Existing Home
Here is where your skills need to be sharpened. When using a manual J heat loss, you can play with the what-if numbers. I have done this for years with standard heating equipment. What if they added more insulation to the attic or replaced windows? For instance, I worked at a quadruplex apartment house with a single boiler rated at 350,000 BTUs. The heat loss revealed that the proper boiler size should have been 280,000 BTUs. If they added 6” to the 3” they had, we could do a 160,000 BTU boiler. We ended up installing two 80,000 BTU boilers, staged firing. We also added Outdoor Reset, and the owner claimed he saved over 60% on his fuel bill.
When I worked as a contractor, we networked with an insulation company to insulate under our radiant floor jobs. They would do any insulation you needed at a reduced rate in most cases. If insulating an attic, homeowners often want to do it themselves. Include a clause in the proposal stating that they will undertake the work, ensuring it meets standard insulation application practices.
After the heat loss is performed, verify the water temperature required. If you are installing a new baseboard, look at the newer low-temp style baseboard line like HE2 by Smith Environmental Products supplies about 363 BTUs per ft @ 115°F water temperature. Since any AWHP Monobloc units can supply 120f and a split system can supply 130f. When installing AWHPs, you normally design a flow for a Delta-t of °F through the system. If a zone of heat requires 6000 BTUs for a few rooms, you would require 16 ft of baseboard. But what if you design for 100F water? You will need 26 ft of baseboard, divided by the heat loss for the three rooms. Sound impossible? This could be very doable in a home built in the 2000s of 2800 sq. ft. or less.
If using a ducted coil application using an air handler, it can be a bit tricky. Do not hesitate to incorporate the coiler manufacturer into this process. They will normally oversize the coil to accommodate the heating and cooling load. You will normally apply a variable-speed circulator for the air handler piping for proper flow.
You may or may not require a booster boiler for this application.
Retrofit AWHP Using Existing Hydronic System
You will connect the AWHP to the existing hydronic heating system and add an oversized buffer tank with a 3-pipe application. This will allow the heat pump to do more of the work to the balance point.
Most of the time, you will need a booster boiler for this application. If there is an existing boiler onsite you may leave it there and use it. If it is a gas or electric boiler, make sure it is a modulating boiler to maximize efficiency. If the homeowner wants to use a cast-iron boiler, they should be aware that it will incur higher operating costs. If it is a newer boiler, it may help keep the installation cost lower until it needs to be replaced.