The diminutive cost of operating a well-designed hydronic distribution system
August 28, 2018 by John Siegenthaler
Many hydronic heating professionals like to promote that the modern boilers they install are capable of operating with thermal efficiencies in the range of 95 +/- per cent. Professionals who install geothermal heat pumps often boast that their equipment is capable of coefficients of performance (COPs) in the range of three to perhaps four. Both claims are possible given the right equipment, the right application, and the specific definition for thermal efficiency and COP used when evaluating the equipment.
However, if past practice is any indication, those same professionals may not bother to mention that a well designed and properly installed hydronic heating system has a distribution efficiency much higher than that of either a forced air system or a system that moves heat through a building using refrigerant.
BEYOND THE HEAT SOURCE
For the system to be considered best in class it needs a high efficiency heat source, as well as the ability to move heat (or cooling effect) through a building using minimal amounts of electrical energy. The latter criteria can be quantified using an index called distribution efficiency. That index is defined by Formula 1.
Distribution efficiency has nothing to do with converting any type of fuel into heat. Instead, it describes how many Btu/hr of heat the distribution system can move from the heat source to the heat emitters for each watt of electrical power supplied to operate that distribution system. The more heat the distribution system can deliver, per watt of electrical power input, the higher its distribution efficiency.
To show how wide the gap is between two contemporary systems I will compare a relatively simply hydronic heating distribution system, using currently available off-the-shelf hardware to a forced air system on a state-of-the-art geothermal heat pump.
The hydronic system assumes a building with a design heating load of 30,000 Btu/hr. The building will be heated using eight identical panel radiators measuring 24 in. high by 72 in. long by 4 in. thick. These relatively large panels were chosen so that the building’s design heating load could be met using a supply water temperature to the radiators of 120F, along with a 20F temperature drop. That temperature makes the system compatible with mod/con boilers, heat pumps, and solar thermal collectors.
The piping for the system is shown in Figure 1.
The system is equipped with wireless thermostatic radiator valves, allowing each panel radiator to function as an independent zone.
This allows each room to individually respond to internal heat gains, temperature setbacks, and personal comfort preferences.
Each radiator is connected to a manifold station by 120 feet of ½ in. PEX tubing (60 feet on supply plus 60 feet on return). The result is a simple homerun distribution system. A buffer tank is shown between the heat source and distribution system. Its purpose is to prevent heat source short cycling given the relatively low design load combined with a highly zoned distribution system.
From the standpoint of distribution efficiency the heat source that warms the buffer tank is irrelevant. It’s the hydraulics of the distribution system and those of the variable speed pressure-regulated circulator that count.
The total system flow rate required to deliver 30,000 Btu/hr with a 20F temperature drop is only three gallons per minute. This divides up equally among the panels so each circuit only requires 0.38 gpm.
The combined head load of the 120 feet of PEX tubing and the panel radiator at this small flow rate is only about 4.2 feet of head. I added 10 per cent to this to allow for some head loss in the short 1-in. piping connecting the manifold station to the buffer tank.
The circulator duty point in this system is therefore three gpm at 4.6 feet of head.
The flow / head requirement can be used to estimate the electrical power input to a circulator. To be conservative, I assumed an ECM circulator with a wire-to-water efficiency of 30 per cent at the calculated duty point.
The estimated electrical input to the circulator can be calculated using Formula 2.
If a circulator operated continuously at this power input, and at a location where electricity costs 20 cents per kWh, the total daily operating cost would be a paltry 4.1 cents. If the circulator operated continuously throughout a heat season lasting 3500 hours, the total cost would be about six dollars. I suspect that most people could work that into their budget.
A few years ago, you would not be able to find a circulator that could operate at such low power. Today there are several circulators on the North American market that can operate in the nominal eight to 10 watt input power range. The European market has even more selections available in this range. One such circulator, with a listed input power range of three to 40 watts is shown in Figure 2.
The distribution efficiency of the example hydronic distribution system can be calculated using Formula 1.
So how does a distribution efficiency of 3,581 Btu/hr/watt rank relative to that of other systems?
To put this number in perspective, let’s compare it to the distribution efficiency of a modern geothermal water-to-air heat pump with a variable speed ECM blower motor.
One manufacturer lists the wattage of an ECM blower on a nominal four-ton (48,000 Btu/hr) rated water to air heat pump, at full speed, as 746 watts. Assuming that the heat pump delivers 48,000 Btu/hr using 746 watts input to the blower, its distribution efficiency would be:
The two calculated distribution efficiencies can be compared by forming their ratio: 3581/64.3= 55.7. This means that the hydronic system is delivering almost 56 times as much heat to the distribution system, per watt of electrical input energy, compared to the heat pump using forced air distribution.
The reciprocal of that ratio, 64.3/3581 = 0.018, or 1.8 per cent, implies that the hydronic system delivers heat to the building using only about 1.8 per cent of the electrical input power required by the forced air system.
Either way you look at it, the hydronic system in this comparison is far more efficient when it comes to delivering heat.
Unfortunately, this profound performance difference goes largely ignored by heating professionals, utilities, energy auditors, and government agencies, all of which tend to focus on high thermal efficiency (that is high AFUE, or high COP) of the heat source.
THIS NEEDS TO CHANGE
The concept of high distribution efficiency should be a starting point (rather than an oft-forgotten afterthought) when discussing the advantages of modern hydronic systems. Our industry needs to get this message to trade allies such as architects and engineers involved with low energy or net-zero buildings, energy auditors, professionals involved in thermally-based renewable energy heat sources, and regulators who determine which HVAC technologies are incentivized through government programs. The superior distribution efficiency of hydronic systems could (and should) play an important role in helping provinces achieve their carbon reduction goals.
At a time when energy efficiency enjoys support from a wide spectrum of society, the hydronics industry should not be leaving its best cards on the table. There are lots of decision makers with eager minds who have never heard about distribution efficiency. Some of them would embrace the concept based on real comparisons. It is time we tell them what is possible.
John Siegenthaler, P.E., is a mechanical engineering graduate of Rensselaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing hydronic heating systems. Siegenthaler’s latest book is Heating with Renewable Energy (for more information see www.hydronicpros.com).