Understanding Air to Water Heat Pump Systems: Part 2
By Mike MillerHPAC General
A high-level overview of setting up an air-to-water heat pump system in a Canadian climate.
In part 1 of this two-part article I provided a brief explanation on the growing adoption of air-to-water heat pump (ATWHP) systems in Europe and why that trend, supported by the move to decarbonization and electrification in North America, will lead to growth of these systems closer to home. I also provided a basic explanation of the technology, its strengths and limitations, and how more recent developments are expanding their capabilities in colder weather climates.
In this article I will go through a very high-level system design example including control logic. Nothing will, or should ever, take the place of a proper heat loss calculation for a project like this, so do not skip this step. The following is just an example with fictitious numbers for the purpose of this article.
To begin, we will be considering a home of about 2,000 sq. ft , at let’s make it 20 BTUh/sq.ft. to keep the math easy, which would yield about a 40,000 BTUh heat loss.
If the outdoor design temperature is -20C, then we know (based on heating capacities discussed in the previous article) that the particular ATWHP in this example could only produce about 6 KW or 20,000 BTUh on this outdoor design day, and we would require supplemental heat of an additional 6kw or 20,000 BTUh to heat the home. Keep that in mind when we look at the mechanical for this example. Supplementary heat could come from anything, but since we are fossil fuel free, let’s go with electric backup here.
Before we proceed to the mechanical and the control logic behind this system, I want to draw attention to the heat emitters we would choose for this home given the limited fluid temperatures we have available for heating and cooling.
High mass radiant floor heating would be an obvious choice for heating, but radiators and fan coils are not to be forgotten. Heat emitters can be upsized in surface area to work with lower fluid temperature for heating. Looking at a fan coil, as an example, as it would be usable for both heating and cooling season, one would need to ensure its output requirement can be achieved with the fluid temperatures available. See the example in figure 1 for a suitable fan coil example.
DOMESTIC HOT WATER
A similar consideration must be kept in mind when picking an indirect domestic hot water (DHW) tank for potable requirements. Given the lower fluid temperature available from a heat pump, the surface area of the coil inside of the tank must be much larger than that of a tank that would be fed by a high temperature boiler.
Depending on the manufacturer, some have specific models available for this kind of application and often feature surface areas between 29- to 35-sq.ft. (2.7- to 3.3-sq.m.) of heat exchanger, allowing for DHW production of 50C (122F) potable water with 60C (140F) fluid delivered from the ATWHP.
There are several ways to pipe an ATWHP into a system. One of such, but surely different approach than most would be familiar with, is shown in Figure 2. Please pay careful attention to flow direction and the strategically placed flow check valves in the piping.
Let’s review this piping diagram in a little more detail. Follow the red supply pipe leaving the ATWHP and follow it going into the return of a backup electric boiler. From that electric boiler, the piping leaves out of the supply and moves to a branch. From this branch, either P1 will create the flow in a heating or cooling requirement, or, P2 will create the flow through the indirect DHW tank.
It would be an either/or scenario, where the DHW tank would typically be the priority. In a heating or cooling requirement, P1 would create the flow through the buffer tank only and return the fluid back to the ATWHP. The two parallel horizontal check valves above P1 ensures flow can only go into its appropriate direction.
Up higher, P3 is what would be creating the flow for any heating and/or cooling system. Remember, those could be any types of heat emitters or a mixture thereof, as long as they are sized appropriately for the fluid temperatures available for the system.
Take a minute to review this in your mind by imagining only one of the two pumps would be running at any time. You will see how the check valves will fix the direction of flow as would be desired. Those check valves, as well as the bridge that you can see the S3 sensor on, create a means of hydraulic separation between the two. In practice, P1 and P3 would be able to run together if there is a call for heat or cool in the building (P3) and heated or cooled fluid needs to be added to it (p1).
Now let’s examine the high-level control logic that could be employed. The inverter driven ATWHP will always automatically regulate its output to maintain desirable delta-T across supply
and return in any call. Delta-T and flow rate through the heat pump can vary to best match the current need.
The DHW tank is typically set to be the priority. The ATWHP control system would have the option to provide priority only during certain hours of the day if so desired. The setpoint for the DHW Tank (50C or 122F) is set in the controller and a desired ON differential is set (5C or 9F).
When the tank temperature drops below setpoint by the ON differential, a call for DHW is generated and the heat pump is modulated to achieve 60C or 140F leaving fluid temperature until the tank is satisfied and back at setpoint.
If required, and only when it is colder than -5C or 22F outside (remember the temperature output limitations?), the electric boiler is enabled to supplement the DHW production requirement. The electric boiler, in this example, has its own onboard two stage controller to operate and stage its internal two 3KW elements. Its setpoint should be set at least 5C or 9F higher than the highest ever required fluid temperature for any call.
The system controller also allows for scheduled DHW Tank temperature boosts to accommodate for legionella prevention. The electric boiler is utilized to elevate the fluid temperature from the ATWHP sufficiently to achieve this safe temperature boost.
This system’s heating or cooling mode is adjusted using an outdoor temperature disable setpoint (IE: <18C or 64F) for heating and outdoor temperature enable for cooling (IE: >21C or 70F). These setpoints, along with adjustable time delays, must be met in order for one mode to switch to the other. A stand-alone thermostat, set-up for the same heating and cooling disable or enable setpoints and time delays as the system control, will then operate the fan coil and enable the P3 pump as necessary to maintain the desired comfort.
With P3 enabled, flow past the S3 sensor will trigger a demand with the system controller and it will then modulate the ATWHP to maintain a water temperature that is either reset based on outdoor temperature or to a fixed setpoint temperature. This system controller would have the ability to setup a reset or setpoint water temperature control for heating and/or for cooling.
So to summarize, S3 is used to gauge the level of demand from the system (larger delta-T = higher demand) and the heat pump is modulated using its supply S1 and return S2 sensors. As always, an operating differential will vary based on load/demand in the system.
A 30-gallon buffer tank is used to add mass to the system to ensure proper and lengthy run-times for the heat pump light loads throughout shoulder seasons. A general rule of thumb is that a heat pump should not run more than three times an hour to ensure its life cycle is not compromised. The fluid volume will help ensure those run times.
In the winter months, a heating cycle of the heat pump can and typically will ice up the refrigerant coil in the heat pump. The 30-gallon from the buffer tank is also used during the defrost cycle, where the volume of fluid is now taken advantage of to provide quick defrost and to minimize down time during peak demand.
SET UP OPTIONS
Some ATWHPs are available for purchase with just the outdoor heat pump by itself and a contractor/installer can build his own mechanical and control system to satisfy his application needs. Others offer complete systems that go together in plug-and-play fashion to provide an efficient installation time-wise. This option provides a complete system solution that has been tested with a
control logic and hydraulic piping arrangement to make it duplicable on the next job, while keeping the installation costs as close to being fixed as possible.
The example I provided in this article was geared towards smaller residential applications, and I am sure it was noted that this is just one of several examples for making a system work. More advanced and larger residential applications may need multiple ATWHPs staged together for heating and cooling needs. In some of these systems certain parts of a building may need heating while others are looking for cooling. A much more advanced control strategy would be required for those applications. As they typically increase in costs very quickly, some systems need the flexibility that those can provide. Perhaps there will be more on that topic in future articles.