Why don’t North American indirect tanks have larger heat exchangers?

Figure 1.


Over the last decade I’ve had several opportunities to attend European trade shows dealing with heating and plumbing. Dozens of tank manufacturers show their vessels at these events, many with full cut-aways of their products.

Figure 1 is one example seen at the recent 2011 ISH show in Germany. This tank includes a large bottom-coil heat exchanger - probably for heat input from a low-temperature source such as a heat pump or solar collector. It has a large stainless-steel upper coil, which could be used for heat input from a boiler or, in another application, for heating domestic water. This tank also includes stratification baffles designed to keep the hottest water separated from cooler water in the lower portion of the tank.

One readily apparent difference between the indirect water heaters offered in Europe (and I suspect other parts of the world) and those offered in North America is the size of the internal heat exchangers. Although there are exceptions, most North American indirect water heaters have substantially smaller internal heat exchangers compared to their European equivalents.

'It's what our machine makes'

That’s the response I got when I asked a certain North American tank manufacturer about how it determines the size of the internal coils in its indirect water heaters. Granted, it was honest. The size of the coil the company installed in its tanks is limited by what its coil-bending machine can produce.

Amidst the striving to improve the efficiency of all heating and domestic hot water systems, it seems that the indirect water heaters offered in North America haven’t changed much over the years.

I did a Web-based search of the product specifications offered by several manufacturers of indirect water heaters. The vast majority of them specify thermal performance ratings based on a coil (or other heat exchanger) inlet temperature of either 180 degrees F, 190 degrees F or 200 degrees F. Some reference a GAMA test method based on the domestic water being heated from 50 degrees to 140 degrees F, along with an associated heat exchanger inlet temperature of 180 degrees F. Some provide a thermal input rating (in Btu/hr.), but with no mention of the required heat exchanger supply temperature or flow rate.

I guess it’s up to the installer to keep cranking up the boiler temperature or flow rate until the unit delivers sufficient hot water recovery rates.

Here's my beef

A mod/con boiler operating at a water inlet temperature of 180 degrees F and a temperature rise of 20 degrees F has a thermal efficiency of around 86 percent. These high water temperatures are well above the dew point of the exhaust gases. Under such conditions, the mod/con boiler is not operating in condensing mode. Instead, it’s operating an efficiency comparable to a modern conventional boiler operating under the same water temperature conditions.

The same mod/con boiler operating with an inlet temperature of 110 degrees F would have a thermal efficiency of around 92.5 percent.

If one assumes a daily domestic water heating load of 100 gallons heated from 50 to 120 degrees F, the annual energy required for domestic water heating would be about 21.3 MMBtu (million Btu). At 86 percent efficiency, this requires an energy input of about 24.8 MMBtu/yr. At 92.5 percent efficiency, the input is about 23.0 MMBtu/yr. Thus, the potential fuel savings associated with heating water at the higher efficiency would be about 1.8 MMBtu. That’s about 18 therms of natural gas per year, not including energy used to make up for standby heat loss.

At a price of $1.40 per therm, the current annual savings would be about $25 per year. Some would argue this does not justify a higher-cost tank with larger heat exchanger and better insulation. Others would look at it from a life-cycle standpoint. Assuming 5 percent annual inflation on the cost of gas, the savings over 20 years would be at least $826, again not counting any energy for standby heat loss.

Other factors

Besides increasing boiler efficiency, there are other reasons to supply larger heat exchangers in indirect water heaters. They include:

1. Less fouling. Lower coil surface temperatures decrease precipitation of minerals on the domestic side of the coil. This helps retain the initial “clean coil” thermal performance ratings. It also reduces the length of boiler heating cycles based on maintaining better heat transfer, compared to a coil experiencing steady accumulation of scale.

2. PVC venting on mod/con boilers. As I’ve suggested in past columns, PVC venting of mod/con boilers operating at elevated water temperatures makes me nervous. At least one major manufacturer of PVC pipe does not warrant the use of their products for boiler venting, regardless of water temperature. You might get away with it in systems that always operate at low water temperatures, but repeated excursions to 180-200 degree F water temperature push boiler exhaust gas temperatures way past the thermal rating of PVC pipe. I’m not comfortable with pushing the limits that far.

3. Higher recovery rates for high-demand applications. Sometimes you get projects with peak DHW demands of 20 gallons per minute or more. Assuming a 70-degree F rise in water temperature, 20 gpm demand equates to 700,000 Btu/hr. boiler input. Indirect tanks with larger coils are better able to handle such loads, albeit with higher coil inlet temperatures.

4. Safety issues. Lower water temperatures in the boiler, piping, circulators and other components are inherently safer.

5. Longer component life. Any hydronic component with elastomeric parts is going to last longer when operated at lower temperatures. This includes circulators, expansion tanks, valves, vents and sensors.

6. Better compatibility with renewable-energy heat sources. Larger coils allow heat sources such as heat pumps and solar collectors to contribute energy while still operating at lower fluid temperatures. The larger the coil, the lower the approach temperature difference, and the lower the “thermodynamic penalty” associated with having any heat exchanger in the heat transfer process.

Debugging

The issue of Legionella also must be considered. Some suggest that the DHW tank should always be maintained at a minimum temperature of 140 degrees F, which kills Legionella bacteria within 32 minutes. Others suggest a daily “Pasteurization” period, usually at night, during which the tank’s temperature is increased to at least 140 degrees F for one hour. Either strategy is relatively easy to implement given current control technology.

From the standpoint of energy conservation, the latter strategy is appealing, especially in private homes where the risk of Legionella being spread is lower than in situations such as health-care facilities.

While you're at it

Another readily apparent difference between European and North American indirect tanks is insulation. The tank seen in Figure 1 has approximately 5 inches of a soft foam insulation material. Other European tanks have about 3 inches of rigid urethane foam insulation. Most North American tanks stop at 2 inches of foam insulation. Yes, it costs more to provide more insulation, but from a life-cycle standpoint I feel that cost increase is justified. Especially with tanks designed for lifetimes of more than 20 years.

So here’s what I suggest to some potential North American tank manufacturers: Produce a tank that can operate with coil inlet temperatures in the range of 130 degrees F to yield 120-degree F domestic water outlet temperatures, and recovery rates in the range of 75 to 100 percent of current recovery rates based on 180-degree F coil inlet temperatures. I contend that recovery rates could be reduced by up to 25 percent, given the fact that hot water delivery rates in many homes are lower than in the past, either through voluntary reduction or by mandated low-flow fixtures.

Provide 3 inches of closed-cell urethane insulation, or its equivalent thermal R-value, on all surfaces of the tank. Bring this tank to market at a price not more than 50 percent higher than current indirect tanks of the same volume. Offer the same tank as both a high-performance indirect water heater and a storage tank suitable for use with solar collectors and heat pumps.

A domestically produced high-performance tank would likely have a significant cost advantage over a tank of similar performance manufactured in Europe or Asia, which requires a long boat ride to North America. The thermal performance of such a product would stand out from the crowd. It would probably win product innovation awards at trade shows and help projects attain more “green points” from various energy-rating agencies. It would probably be embraced by enthusiasts in the growing heat pump and solar thermal markets.

Well, at least you know where I stand on this issue. If you agree, I would love to hear from you. Drop me an email, or even drop a letter to the editor of PM (editor@PMmag.com). If you think this is utopian thinking incapable of market acceptance, I also would like to hear from you. Finally, if you’re a manufacturer who decides this concept deserves a product, please keep me posted.

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