Onward & Downward
The future of hydronics is all about low water temperatures.
Other than the fact that they are all heat sources, what characteristic do mod/con boilers, solar collectors, and hydronic heat pumps have in common? Answer: They all perform best when coupled to distribution systems that operate at low supply water temperatures. Here is the proof.
Figure 1 shows how the thermal efficiency of a typical mod/con boiler varies as a function of its inlet water temperature. Although such boilers can operate at elevated supply water temperatures, (even up around 200F), doing so limits their thermal efficiency to only one or two per cent higher than a conventional boiler operating under the same conditions.
However, when the inlet water temperature drops below the dewpoint temperature of the exhaust gases (usually around 130F) thermal efficiency increases rapidly. With inlet temperatures under 100F, most mod/con boilers can deliver efficiencies in the mid to upper 90s.
Figure 2 shows how the coefficient of performance (COP) of a water-to-water heat pump is affected by its “entering load water temperature” (ELWT); the latter being the temperature of the water returning to the heat pump from the distribution system. When supplied with source water at say, 45F (a typical mid-winter fluid temperature from an earth loop), and an ELWT of 100F, the COP of the heat pump represented by this graph is about 4.7 – a very respectable number. However, if the distribution system forces the ELWT up to say 115F, the COP drops to about 3.7. That is a 21 per cent decline in thermal efficiency when operating at a load water temperature of only 15F higher.
Figure 3 shows how the instantaneous thermal efficiency of a flat plate solar collector is affected by inlet fluid temperature.
Assuming that the ambient air temperature and solar radiation intensity remain constant at the indicated values, which represent a sunny mid-winter day in a Northern U.S. climate, the thermal efficiency of the collector drops rapidly with increasing inlet fluid temperature.
Hydronic heat emitters determine the system’s operating temperature. The water temperature in any hydronic system only climbs high enough for that system to achieve thermal equilibrium – where the rate of heat dissipation from the heat emitters exactly balances the rate of heat input from the heat source. Once this condition is achieved, there is no need for the water temperature to climb higher.
As designers, we all want to maximize the thermal efficiency of the hydronic systems we design. Doing so means moving away from high water temperatures by specifying heat emitters with larger active surfaces, or other details that increase both convective and radiant heat transfer. This allows thermal equilibrium to occur at relatively low water temperatures, both at design load and partial load conditions.
This trend is not new. It has been taking place on a worldwide basis for over two decades. Those who work with hydronics in Europe accept this as common practice. North America is perhaps the last place where some new hydronic systems are still designed around higher water temperatures.
This is the case because most North American systems are designed around price.
Standard fin-tube baseboard is arguably the best example. Originally designed as an alternative to cast-iron radiators, most fin-tube baseboard has not changed much over the last several decades. When fin-tube baseboard first entered the market, fuel was cheap and nearly all boilers operated at water temperatures of 180F or higher. You could even find heat output ratings for residential fin-tube baseboard at water temperatures as high as 230F.
The economics is simple: the higher the water temperature, the greater the heat output. The greater the heat output, the shorter the required fin-tube length. The shorter the length, the lower the installed cost.
Please do not think that I am wagging my finger at our industry saying, “that’s a pretty stupid thing to do.” It made sense when fuel was relatively cheap. Even now, standard fin-tube baseboard would probably regain market share against more contemporary and higher cost alternatives if fuel prices reverted to where they were in the 50s and 60s. That is not going to happen, so the industry needs to move on.
There are some companies that sense opportunity as North America begins to grasp the necessity of low temperature
hydronics. For example, take a look at the fin-tube baseboard product in Figure 4. It is a product made in the U.K. that is now available in both the U.S. and Canada.
Assuming an average water temperature of 110F, this high performance baseboard releases about 290 Btuh/ft when the two pipes are configured for parallel flow, and the total flow rate through the element is one gpm (0.5 gpm through each tube). This increases to about 345 Btuh/ft with a total flow rate of four gpm (or two gpm per tube). Both ratings include the 15 per cent heating effect factor that is often added to the tested thermal performance of baseboard. Typical residential fin-tube would need an average water temperature of about 152F (at four gpm flow rate) to yield the same output.
Although it has twice the water volume of standard fin-tube (two tubes versus one), the thermal mass is still relatively low compared to other heat emitter options. This is important for fast thermal response in low energy buildings with significant internal heat gains.
This product finally acknowledges what is quickly becoming a “progress or perish” situation for traditional fin-tube baseboard. It moves fin-tube baseboard beyond a commodity that trades heat source efficiency for low price, into a contender within the arena of future hydronic systems.
Figure 5 shows another contender that combines a deep, multiple tube fin-tube element with a sturdy enclosure. Although its heat output at 100F average water temperature is about 24 per cent of the output at 160F water temperature, the optional rack of “microfans” seen in Figure 5 can boost low temperature heat output by as much as 250 per cent. These low voltage fans draw about 1.5 watts each at full speed.
Now that we have looked at both low temperature heat sources, and low temperature heat emitters, the next logical question is: How do I pipe these together? Figure 6 shows one possibility.
The anchor component in this system is a well-insulated heating appliance that offers several features including:
• A modulating gas burner and internal condensing heat exchanger.
• Plenty of thermal mass (water) to stabilize burner operation, even with extensive zoning.
• A drainback protected solar thermal subsystem that can contribute to both domestic water heating and space heating.
• An instantaneous DHW generating subsystem using a stainless steel heat exchanger.
• A self-contained captive air volume that serves as both an expansion tank and drainback reservoir.
The solar thermal subsystem adds heat to the lower portion of the storage tank whenever possible. It uses drainback freeze protection. When the collectors are a few degrees warmer than the tank, the collector circulator operates to create flow through the collector array. When the collector temperature drops close to the tank temperature, this circulator turns off and all water in the collectors and external piping flow back into the tank. A captive air volume at the top of the tank, under slight positive pressure, provides both drainback space and an expansion volume for the system.
On sunny days, the collectors may keep the tank well above the required temperature for either space heating or domestic hot water. Both situations are addressed through the use of mixing devices.
When there is a draw for domestic hot water, a flow switch, set for 0.5 gpm, turns on a low power variable speed circulator that moves h
ot water from the top of the storage tank, through the primary side of a stainless steel brazed plate heat exchanger. Cold domestic water flows through the other side of this heat exchanger and is instantly heated. The speed of this circulator is controlled based on the temperature of the leaving domestic hot water. If the leaving water temperature drops, the circulator speeds up to increase flow of hot water through the primary side of the heat exchanger and vice versa.
A single ECM-based pressure regulated circulator provides flow to a homerun distribution system for space heating. Each length of baseboard is supplied by its own ½” PEX or PEX-AL-PEX supply and return tube. With good piping design, this circulator could supply the entire distribution system under design load conditions using no more than 40 watts of electrical power.
Each baseboard also has an adjustable thermostatic radiator valve that monitors room temperature, and adjusts flow rate as needed to maintain that temperature. No wires, batteries, transformers or programming – just simple, effective and reliable room-by-room temperature control.
The three-way motorized mixing valve operates using outdoor reset control to provide the optimum supply water temperature to the baseboards.
THEN & NOW
Low supply water temperatures used to be more about getting conventional boilers to “play nice” with radiant floor heating systems. Now, it is the cost of fuel and related need for high efficiency that is driving the trend toward lower water temperatures. New heat emitters, as well as classic low temperature radiant panels, are the enabling technology. I urge everyone in the North American hydronics industry to embrace low temperature hydronics and tool up to deliver solutions that ensure its implementation. <>
John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.
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June 21, 2021