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Outdoor reset has arrived

Oct. 1, 2011
The purpose of outdoor reset is to reduce energy use and cost without sacrificing comfort. The controller lowers boiler water temperature when the outdoor temperature is warmer and increases it when the outdoor temperature is colder.

"Outdoor reset" sounds a little like being able to change the weather if you don’t like it. If only that were true! But for your customers, it may be the next best thing — a way to pay for enough heat for the weather happening right now, but not always for the coldest night in January.

Estimates are that we need traditional boiler water temperatures only about 10 days a year. The other 80% of the heating season would be satisfied with lower water temperatures.

The purpose of outdoor reset is to reduce energy use and cost without sacrificing comfort. The controller lowers boiler water temperature when the outdoor temperature is warmer and increases it when the outdoor temperature is colder.

Outdoor reset makes sense to homeowners. Besides reducing energy bills, lower water temperatures can mean even warmth rather than spikes of too hot. And the clicking noise of hot baseboard is minimized. In radiant floor heating, hot and cold spots are reduced.

Yet many plumbers and heating technicians still resist outdoor reset. At first this glance this is puzzling. Outdoor reset comes with most new boilers. Minimally, it's a matter of wiring an outdoor sensor to the outdoor sensor terminals and putting it outdoors. And here's a big one: 90% plus boiler efficiencies cannot be obtained unless the water temperature is lowered, so that the boiler can condense.

Even conventional cast iron boilers have outdoor reset as an easy and inexpensive option. Included is protection against return water being too cool. For older boilers, a retrofit outdoor reset control is a matter of four hundred dollars. To the homeowner, it’s a no-brainer for projected energy savings of at least 15%.

So if there is no or low additional cost, little difficulty of installation, and an opportunity to make more money, what are the objections?

Objection No. 1: Fear of call backs. What if the homeowner notices that the boiler temperature is lower than it should be? What if the homeowner tries to test the boiler in August and the boiler won’t come on? If either of these happens, you can remind the homeowner that they have a feature that saves them money, called outdoor reset, and that it is performing normally.

Objection No. 2: The boiler guy wants the boiler to run. Outdoor reset makes it not run when heating is not required. Outdoor reset also takes away the certainty of what the water temperature is supposed to be. The perfect boiler temperature is not always the same because the control changes it based on outdoor temperature. The thing is the homeowner would rather save money.

Objection No 3: Outdoor reset is a new idea. It is not a new idea. The Sarcotherm Weather Control for Hot Water and Radiant Heating appeared in the 1930s. The Honeywell T475 has been successfully used in commercial applications for decades.

Objection No. 4: Outdoor reset is harmful to boilers. For high-efficiency condensing boilers, outdoor reset is essential for maximum energy efficiency. High efficiency depends upon low return water temperatures. Conventional boilers are protected by the minimum water temperature setting.

Objection No. 5: Outdoor reset is complicated. Setting up an outdoor reset control used to require figuring reset ratios and reset curves. You can still put yourself through that agony, but it’s not necessary anymore. The controller comes with default settings so no one has to enter any information at all for it to work minimally, which is better for the homeowner than not at all.

But doing outdoor reset right is very simple. After plugging in the outdoor sensor, a mere three pieces of information need to be entered: design temperature (which comes from ASHRAE data), minimum water temperature and maximum water temperature. Design temperature, by the way, is the normal low temperature for the last three to five years. It is not the lowest temperature that anyone can remember, and it doesn’t include wind chill.

It is helpful to answer these three questions. 1. What temperature should the water be on a normal cold day? 2. What is the hottest water that should go to the home? This is very important if there are hardwood floors over radiant since too hot can damage the wood. 3. At what outdoor temperature is heat no longer required?

Objection No. 6: Warm weather shut down (WWSD) turns off the heat when you need it. WWSD assumes that when it gets warm outside, no heat is needed inside. Even though there is a default temperature in the control, you can adjust it.

Objection No. 7: Domestic hot water (DHW) won't be hot enough. For DHW, select that option and the boiler will kick up to a hotter temperature to make domestic hot water. The boiler returns to the lower reset temperature once the domestic hot water is satisfied.

Objection No. 8: The terminology is confusing. In outdoor reset talk, both new and old terminologies are sometimes off-putting. Here are some translations.

For the reset ratio, we don't have to figure it out anymore, the control figures it out. But here's what the old-timers are talking about. Reset ratio is the degrees of boiler temperature change for each degree of outdoor temperature change. A ratio of 1:1 means that for every degree of drop outside, you get a degree of increase in the boiler water temperature. A ratio of 1-1/2:1 means that a 1-1/2 degree outdoor temperature fall results in a one degree water temperature rise. A ratio of 1:1-1/2 gets a 1-1/2 degree water temperature rise for each degree of outdoor temperature fall.

Reset curve doesn’t have to be figured out anymore either. The reset curve is the reset ratio plotted on a graph. It’s actually a straight line at an angle. Sixty degrees of boiler water temperature increase makes a steeper curve than 30 degrees. To select the correct curve, match outdoor design temperature on one side of the curve with the temperature water you need on the other side of the curve. Many code officials now require a heat loss calculation as part of the permit process. Design temperatures are normally shown on the heat loss calculation. Here are some terms to be familiar with:

  • Input, a new term, just means to enter, as in "input your desired temperature."
  • Parameter, another new term, is just a setting. For example, the maximum boiler temperature parameter is simply the maximum boiler temperature. "Input a parameter" translates to "set the temperature."
  • Algorithm is a mathematical formula that the manufacturer puts into an electronic control to make it do what it supposed to do. We can't change it or fix it and will never know what it is, so there’s no need to worry about it.

Outdoor reset has arrived! It comes with virtually every boiler. Homeowners like it because it reduces their energy bills. It also makes their heat more even and comfortable, and sometimes quieter. Boiler manufacturers like it because it increases the efficiency of high efficiency boilers, and it doesn't harm conventional boilers. Why should plumbers and heating technicians also like it? Because it’s an easy and inexpensive way to deliver to homeowner’s what they want, and they’re the ones paying the bills, both yours and theirs.

Carol Fey is a degreed technical trainer who has worked as a heating technician in Antarctica. She has published five books especially for the HVAC industry. Her website is:

About the Author

Carol Fey

Carol Fey is a technical trainer and writer, specializing in easy electricity, hydronics and troubleshooting books.  She also writes about HVAC work in Antarctica. 

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