Modern Radiator Heating
Radiators tend to incite strong reactions; those who grew up with them tend to either love or loathe them with equal passion (mostly the latter if you’ve ever had your wardrobe misted by a rusty steam valve). But today, with American home heating dominated by forced hot air registers and ductless mini split heads, you could be forgiven for thinking of the radiator as a technology of years past. And it was, until 2016.
Since 2007, Japan has been investing in a technology called EcoCute: an air source heat pump water heater that uses carbon dioxide (CO₂) as the refrigerant. Since the first commercial release of EcoCutes in 2001, the performance and ecological benefits have seen it gain traction in markets across the world, with recent emergence in North America. As a building science driven architect, it is absolutely the only heating system I will specify on small- to medium-sized residential projects, and here’s why:
- Heat pumps are 2–5 times more energy efficient than natural gas or electric resistance heating.
- As a split system, the heat pump is located outdoors and does not inadvertently cool the interior of the house.
- CO₂ refrigerant pollutes far less than other options.
- Domestic hot water heating is on the same system as space heating.
- Modern radiators are things of beauty.
In 2016, the Japanese equipment company Sanden unveiled their SanCO₂ unit to the North American market; it is one of several competing EcoCute heat pumps available and perhaps the most widely adopted in the US. While they can be used to produce domestic hot water alone, I am excited by their ability to simultaneously produce space heating with the same system.
Let’s break down how it all comes together in a case study house we are building in Roslyn, WA.
Stats
Uniform Energy Factor (UEF): 3.75
Coefficient of Performance (COP): 2.8–5.5
Operating Temperature: -20°F to 160°F
Nominal Heat Production: 4.5 kW per heat pump unit
Space Heating Capacity: 3.0kW per heat pump unit
Component Costs:*
- $2898 SanCO₂ heat pump
- $1250 SanCO₂ 83 gallon hot water tank
- $1783 Taco X-Pump Block
- $1632 Runtal RS2–84 radiator, 18" long
Refrigerant: 22oz CO₂ (R-744) precharged closed loop
Refrigerant Global Warming Potential (GWP): 1
Installation Difficulty: Easy
Use
How does this compare to other ways of heating a home?
In most low- to moderate-use cases, it is superior.
- Electric resistance has a maximum of only 100% efficiency of electrical energy input to heat energy output, whereas heat pumps have 200% to 500% efficiency. Electric resistance has low upfront costs, but very high operational costs. These systems need an enormous amount of power to heat a home, meaning high monthly bills on top of a higher amperage service. For these reasons, I would only use electric resistance as a backup source or to heat very low-demand spaces.
- Natural gas, oil, and propane are all fossil fuels that release CO₂ when ignited to heat your home. Despite marketing efforts, natural gas is not a ‘clean’ fuel or even a decent bridge fuel. In Eugene, OR, for example, oil pollutes 12x and natural gas pollutes 7.5x more than a heat pump. Strangely, even if your electricity is generated using 90% natural gas, a heat pump will still pollute less than a high performance natural gas furnace. Fossil fuel furnaces provide large amounts of heat very rapidly, and are most useful in very cold climates or for very large heating loads that cannot easily be supplied by other means. Some cities are beginning to outlaw these systems due to their polluting effects.
How does this compare to other methods of home heat distribution?
It tends to be less demanding spatially, environmentally, and economically.
- Forced Hot Air: The best case scenario is a forced hot air system tied to a heat recovery ventilator (HRV). There are some great examples of the SanCO₂ heat pump being used in combination with an HRV to provide moderate levels of space heating. If you are already ducting a home with an HRV, this can be an efficient heating solution. However, the recent emergence of ductless HRV units allows for homes with no ducts whatsoever. We like to minimize ducting for space savings, installation cost savings, and greater comfort. I haven’t yet met anyone who prefers dusty air registers over radiant heat.
- Ductless Mini Split: On our case study house, a ductless mini split heating system was quoted as $6700 more expensive than the modern radiator system. The modern radiator system is DIY-friendly with a heat pump that comes pre-charged with refrigerant, whereas ductless mini split systems must be professionally installed and manually charged with refrigerant. Mini split systems typically require multiple pounds of R410A refrigerant (a GWP of 2088 vs. CO₂’s GWP of 1), and can easily have a refrigerant global warming potential tens of thousands of times more than a SanCO₂ system. This becomes a critical issue if the refrigerant leaks or is not properly disposed of. Unfortunately, it is estimated that less than 2% of residential mini split refrigerants are recovered at end of life, and these units have a 10% leakage during operation. These units may be energy efficient, but they are extremely environmentally problematic.
- Hydronic Radiant Heating: I would argue that these systems have both increased costs and — in the case of radiant slabs — decreased performance in comparison to radiators. Most homeowners imagine toasty feet on a radiant slab, but that’s not how they work. When used to heat a home, they are a few degrees above the desired ambient temperature, which is well below body temperature, thus feeling cooler to your feet. A well-insulated slab without radiant will feel pretty much the same.
- Traditional Radiator Heating: Traditional radiators can be very noisy. They are usually heated by a boiler and fed by very hot water or steam under high pressure and high flow, whereas modern radiators use low temperature silent elements: less water moving through the system, less noise, and a greater change in temperature (ΔT) over the system.
- Passive Solar: Nothing beats passive solar. Glass facing the equator, trombe walls, and thermal mass are essential to every good design. But most homes can’t meet their full heating loads with passive solar alone.
Can I use this system in cold climates?
Yes. The SanCO₂ heat pump can produce hot water down to -20°F with full system capacity in ambient temperatures above 5°F. However, for space heating uses the minimum rated design temperature is 27°F. If your minimum design temperature is lower, ECO₂ recommends an ancillary heat source.
Won’t the water freeze?
No burst pipes. In cold climates, the water lines running from heat pump to the house envelope must be insulated and use trace heating. In case of a sustained power outage where the SanCO₂ heat pump approaches freezing, it is equipped with valves that automatically drain the water lines.
Can I use this system in hot climates?
Definitely. The heat pump performs well under high ambient temperatures and actually cools the space it is in. Locate in an attic or garage to passively cool during summer months.
What are the system limitations?
Limitations include:
- Space heating demands greater than 3.0kW can be provided by adding another exterior heat pump unit to double the space heating capacity. Multi-family installations can use a series of heat pumps and tanks to scale up to significantly higher heating loads. However, the system is simplest when used to heat high-performance homes with low heating loads.
- Because the SanCO₂ works most efficiently when heating cold water, the water returning to the tank must be cooler than 100°F. This means careful radiator selection and balancing to achieve high ΔT via low flow.
- The radiant water loop must be separate from the domestic water to avoid the health risks of drinking water that has been used for a heating system. A heat exchanger and pump will do the trick. In our case study, this is the Taco X-Pump Block.
How can I learn more?
The links below are a great place to start. We work with Small Planet Supply here in the Pacific NW to purchase components and for preliminary system design. They offer a great introductory class available for free.
References
Laboratory Assessment of Sanden GAU Heat Pump Water Heater Lab
Split-System Heat-Pump Water Heaters
What is Global Warming Potential?
The Cost of Comfort: Climate Change and Refrigerants
Annual Household Energy Cost & Carbon Footprint
Emission Reduction Potential from Electric Heat Pumps
What if Efficiency Goals Were Carbon Goals?
Translating the Noises My Radiator Makes
Photography by Runtal Design Studio
*Costs will vary. Indicated costs are in USD, reflect materials only, do not include shipping or taxes, and are based on manufacturer quotes provided at the time of writing in Seattle, WA.
