This passive house building, located in Vancouver, was heated and cooled using water-to-water heat pumps with four-pipe coils for terminal units and six-way valves that modulate.No secrets here, the plot twist is in the headline. Heat Pumps are not boilers, and we can’t treat hydronic heat pumps as boilers. As we see the electrification Act build momentum across Canada, more and more manufacturers are bringing new heat pump technology to Canada. This means that we are going to see a whole new industry open up in front of us as hydronic heat pumps begin to offset the traditional natural gas and propane boiler business, just as these technologies did for coal and oil before them.
The principal purpose of this article is to talk about some of the differences and challenges we will face as we adopt hydronic heat pumps, specifically water-to-water and air-to-water equipment. We need to be fully aware of the ramifications of what will happen when we don’t take the time to get properly educated on this evolving technology. It is the old saying, “You do not know, what you do not know.” Which means, when you do go to hook up your first hydronic heat pump piped up like a standard boiler, you will not know at first glance that A) it is wrong and B) what is going to happen if it is wrong.
Moment in the spotlight
There are currently two common types of hydronic heat pumps in Ontario. They are water-to-water geothermal units and air-to-water heat pumps. Both of these units generally don’t like to see water temperatures above 120 F. A few will read that last sentence and say not true, and they are somewhat right. A vapour injected water-to-water, or air-to-water heat pump can, in theory, produce higher leaving water temperatures but at the sacrifice of efficiency and capacity.
On an air-to-water, the higher the outlet water temperature is from the air temperature, the lower the overall performance of the unit and in many cases the heating and cooling capacity. The same is true for a geothermal hydronic heat pump, only it is the ground loop temperature versus the air temperature that will determine overall performance and capacity. A geothermal hydronic heat pump has a higher efficiency, but it has some limitations in terms of where it can be installed due to the requirement for a ground loop.
To illustrate this point, the table adjacent is from the Advantage five-ton vapor injected variable speed air-to-water heat pump. As you can see, this unit can easily achieve 130 F water and can run down to -13 F. But should we run the equipment so hot? For those not familiar with COP, it is the coefficient of performance. For every unit of electricity you purchase, you will get X amount back for free.
This unit can easily achieve 130 F water and can run down to -13 FBoost in efficiency
So, on the table above, at 14 F for every unit of electricity, I buy my equipment has a COP of 2.4 or 240 per cent efficient at 131 F water. Do we actually need 130 F water? A properly designed radiant floor system will easily heat a home with 100 F water. If we look at the exact same conditions, 14 F outside temperature, and use 113 F water, we end up with a COP of 2.8 or 280 per cent efficient.
That is a hefty bump just by working from a design versus winging it. On a boiler, many do not even notice. The boiler still condenses, and you have no penalty. But on an air-to-water heat pump, some of our habits are significantly hampering the overall return on investment of the heat pump. Even more impressive if we were to use a low temperature convector, we can heat the space using 100 F water. For example, a Jaga low H20 product at 100 F water would produce 9,682 BTU/h. That is a staggering amount of energy for very low water temperatures. We have now taken the same piece of equipment and taken the COP up to 3.4.
All of the statements I just made also apply to a water-to-water unit only with even higher COP numbers because the ground loop temperature is stable all winter long, whereas the outdoor temperature is not.
Cautionary tale
With a heat pump, it is very important that we design around the correct water temperatures. If you don’t, it won’t immediately result in a callback, but it will definitely cause performance, comfort, and cost of operation issues. The other thing I caution you about is using this product on copper fin tube rads. I have seen in the past, where boilers are installed, and afterwards, when it can’t heat with 120 F water, the end user or the service technician cranks the water temperature up to 150 F or 160 F. This is a fix that eats into the cost of operation and generally goes unnoticed by the homeowner.
If you do this on a hydronic heat pump, you are putting the refrigerant circuit under significant stress. An R410 compressor circuit running at 140 F water can result in catastrophic equipment failure. Looking at the table below in heating mode, you can see that there is a solid red line, and we want you to stay inside of that envelope which is max of 130 F but again, understand what that does to your overall COP by running a system hotter than is needed. At 140 F, you are creeping outside of the safety buffer.
Determining capacity
Your design temperature will dictate the capacity of the equipment as will your leaving load temperature. With an air-to-water heat pump, your outdoor design temperature will dictate your overall capacity and with a water-to-water unit. Your ground loop temperature will dictate your capacity.
You want to stay inside of the envelope.A geothermal hydronic heat pump is superior because regardless of outdoor temperature, you know what your COP will be and what your capacity will be because with a properly sized earth-loop, the ground temperature is stable. In some places, geothermal makes more sense than air-to-water and vice versa. Can you install a ground loop being the big driving question.
You want to also be aware of when you pipe a hydronic heat pump; what are the actual required flow rates? On a boiler, we are used to a 20 F Delta T and one GPM. This will equate to 10,000 BTUs. On a hydronic heat pump, depending on the design, that may not be the situation. Take for example a water-to-water geothermal heat pump. Some of this equipment requires three GPM per ton, which means we are not operating on a 20 F Delta T but a 5 F to 8 F Delta T. This is why on a five-ton geothermal heat pump you will see the manufacturer calling for one and a 1/4-inch or one and a 1/2-inch pipe where you would be used to using much smaller piping if this were a boiler.
Know your requirements
Not only do we need to be aware of the need for properly sized piping based on the equipment type, but we also need to be very aware of the requirement for a buffer tank. On a geothermal water-to-water heat pump, we require that buffer tank to ensure the equipment gets a minimum run time of 10 to 15 minutes to maximize its performance. Some water-to-water equipment is fully modulating, and some utilize a two-speed compressor. These heat pumps need a buffer so that when the load does not perfectly match the equipment output, we do.
If you do not pipe the equipment with properly sized piping and a properly sized buffer tank, the end results can be quite catastrophic. The good news is that there are talented designers and suppliers with decades of experience that can help you navigate the exciting new world of air-to-water, and the not-so-new world of geothermal water-to-water equipment.