Outdoor Reset

Anyone with a military background will recall the U.S. Government’s penchant for acronyms. MREs, BDUs, MRAPs, the list goes on. Unfortunately, this dynamic is also present when discussing the efficiency enhancing feature known as outdoor reset. ODT, SWT, LWT, etc. It’s enough to cause a BM in your BVDs! Well, U.S. Boiler Company has once again developed a solution to make all this NBD.

First a few simple facts.

1. Flue gas will condense when temps are at or around 130° or less.

2. When vapor contained in flue gas condenses it releases a significant amount of heat.

3. Without question, optimal efficiency is achieved by heating a home using the lowest possible water temperature that will maintain the desired indoor target temperature, usually the thermostat set-point.

4. Using a Mod-Con, or high efficiency, gas boiler like the ALTA can take full advantage of the condensing action allowed by low water/flue temps.

Outdoor reset sensors do a wonderful thing. They enhance efficiency by matching water temperature to varying conditions.  For example, one would want to maintain indoor design temp, normally 70°, during outdoor design temps (in CT normally 7-10 degrees.) Assume a 7° ODDT, and required SWT of 180. This means the boiler must deliver 180° supply water consistently at a 7° outdoor design temperature to maintain the desired indoor temp (IDT). This all simply means we need the hottest water when it’s coldest outside. Good so far?

As Paul Harvey used to say, “And now-The rest of the Story!” We now all understand (hopefully) what happens in our heating systems on the coldest day of the year. What about the rest of the time? Sure, that 180° water will heat the house just fine on a 50 degree day. But what about all the energy used to unnecessarily heat the water way past what would be required to keep the home comfortable? This is a net negative on many fronts, but most significant is the increased wear and tear on the heating unit.

So, your now asking “Tom, how low can my water temp be?” The answer will likely surprise you. Standard fin tube radiation works ok down to 130° water temps in most applications. Same with cast iron radiators. For the fin tube baseboard system in my house the minimum boiler water temp on my ALTAC-136 is 125 degrees.  Hydro-air (hydronic coils added to or contained within hot air furnaces or air handlers) can often be run around 130° as well, depending on the occupant’s expectations. Remember, these SWT’s are typically targeted when the ODT is ~70°. We don’t need to transfer much heat to maintain indoor setpoints at these conditions. How does all this magic happen? Outdoor reset. Using a thermistor mounted outside, typically out of the sun on the north facing wall we monitor ODT and adjust SWT temp accordingly. So my personal curve used to be 180° SWT @ 7° ODT, and 125°SWT@70° ODT, but no more.  My ALTA figures this out automatically without an outdoor reset sensor.

While outdoor reset simply monitors outdoor temperatures, rate-based reset works by looking at boiler run time, firing rate, and other parameters and adjusting accordingly. Here’s a few examples. Say it’s a beautiful spring day. Nice and sunny with a gentle breeze and 50° outdoor temperatures. The boiler run times are likely short, and the firing rate is likely low. The boiler knows from these conditions the heat loss from the house is minimal, and adjusts, incrementally dropping the boiler water temp until the minimum setpoint is reached, or our algorithm (the “secret sauce”) calculates that based on run time and firing rate the water temp will deliver enough heat to match the rate of heat loss and maintain indoor temp setpoints at the lowest possible SWT. As night rolls in and the temperature drops, average run times and firing rates increase, and once again, rate-based reset will adjust (increase) SWT’s to maintain indoor comfort and optimal efficiency. “Brilliant!” you say. I agree. But wait-there’s more!

Conventional ODR has limitations. Say the calculated water temp for a 45 degree day is 142°. It’s a moderately windy day. Consider two different houses, both 2,500 ft. The first has a well-sealed envelope, double pane windows, and effective insulation. The second has old single pane windows, minimal (or no) insulation, and is as drafty as old fence. The boiler doesn’t know what the indoor conditions are, just that it must target 142 degrees. That 142° SWT might be fine for the first, but far too low for the second. Now to be fair, all U.S. Boiler products with outdoor reset offer a “boost” feature that will automatically bump the temperature up every “X” (boost time value) minutes, default being 20. This means every 20 minutes with an uninterrupted call, the SWT will rise 10 degrees until the max setpoint is reached. The benefit of rate-based reset is we are always looking at indoor conditions and adjusting accordingly. Rate-based reset always uses the lowest possible water temp that will maintain the desired indoor temp, and then has the boost feature if an increase above the calculated target temp is required. Like a samurai sword: simple, elegant, and effective!

There are other benefits as well. Remember the test I mentioned? Here it is. What factors showcase the benefits of rate-based reset? See the answers below. Correct your test answers accordingly…

1. No sensor to install. Rate-based wins!

A) No sensor for landscape professionals to damage.

B) No sensor for contractor to install (or throw away!)

C) No wire required from boiler to sensor.

D) No worries about where the contractor will mount the sensor, if it will be impacted by the sun, or heat migration from the interior affecting the sensor calibration.

E) Enabled from the factory. No worries for homeowner regarding whether the contractor enabled ODR. The Alta boiler is set up for maximum comfort and efficiency right out of the box!

2. No Lemon Faces! Rate-based wins!

A) Always monitoring indoor temp. Onboard algorithms are constantly at work to meet heating demands while maintaining maximum efficiency. Tens of thousands of units installed with zero complaints about indoor temperature. When not writing informative and engaging articles such as this, I am a member of the renowned U.S. Boiler tech support team. I’ve honestly never received one complaint regarding rate-based reset. If you have, please contact me.

B) Not reliant on a technician with advanced, comprehensive knowledge of ODR curves. Works fine out of the box with no programming in most applications, and, where required, only minimal adjustments for almost all applications, including low temp radiant.

C) If the homeowner is made aware that during mild temperatures the radiators will not be the same temperature as the skin of the space shuttle during re-entry, and that these cooler temperatures mean more money in their pocket for candy/wine/shuffleboard, they will instead have smiles every time they touch the radiator. Or better yet, they will abandon the practice entirely…

3. Takes full advantage of condensing aspect of mod con boilers. Again, rate-based wins!

Regardless of ODT, rate-based reset always targets the lowest possible temp, thereby maximizing the potential for the boiler to run cool enough to operate in a range where flue gas condensation occurs.

I can almost feel grizzled old contractors firing up HP 386’s and waiting for the comforting squawk of their dial up modems connecting. I imagine most of the responses will begin with:

“Dear factory stooge/shill. Your article may fool the unwashed masses, but not me. My mom’s uncle’s grandmother has a hunting cabin with 1 radiator and 600 feet of ¾” pipe to connect the boiler to the house (it’s in the barn where we milk the squirrels). How does your new-fangled boiler work in my application? Hah!!! Gotcha!!!”

It will be this, or something substantially similar. There’s always at least one. For the rest of the world (and fortunately, most of the squirrels) these anomalies are rare. There is no magic solution for a poorly designed or installed system. But when a competent, capable, and progressive contractor is installing a new or replacement system, the advantage of rate-based reset over conventional sensor-based reset is clear. Reduced (or no!) programming required, no wires to pull, and no exposed exterior sensor susceptible to damage. Save time, money and aggravation.  Go ahead and skip a step with ALTA.

For more information on the Alta boiler, visit our Alta Product Page.

GUEST BLOGGER: Tom Secondino is a Technical Sales Support Specialist for U.S. Boiler Company, a manufacturer of residential and commercial heating products, including condensing, non-condensing and electric boilers.