Outdoor Reset Control
What’s outdoor reset control? It sounds like maybe it’s a weather control to “reset” the temperature outside when we don’t like it. That could be a handy device, but alas, that’s not what it is.
Outdoor reset is a weather-responsive control rather than a weather controller. Based on changes in outdoor temperature, it automatically adjusts boiler water temperature. Wow, what a concept. If the temperature is colder outside, it takes more heat to overcome its effect inside the building than if the weather is warmer! Outdoor reset is the same as turning down the boiler water aquastat in the spring and fall - automatically.
We set up heating systems as if it’s always the middle of the coldest, darkest night, no matter what the weather is doing. That results in wasted energy (and money), and in the discomfort of being too hot.
While it may sound like a new concept, outdoor reset has been around for a long time. The “Sarcotherm Weather Control for Hot Water and Radiant Heating” may have been the first. It’s from the 1930s. (Great name - Sarcotherm Weather Control. It does sound like it can control the weather, doesn’t it?)
I think outdoor reset has remained fairly obscure until now because of our heating industry’s commitment to delivering too much heat - the “just in case” nature of making sure everyone stays warm. However, with outdoor reset, we still can size systems for “just in case,” but reduce the amount of heat produced when it’s not needed.
What has made outdoor reset a topic of conversation now is energy conservation concerns and the advent of the condensing boiler. The advantage of a condensing boiler is its energy efficiency. But your customer gets nothing from that boiler except added purchase price unless you can get the return water cool enough to condense in the boiler.
Having outdoor reset sends out cooler water when the weather is warmer. Cooler return water ensures that the boiler will condense when the weather is mild, 60-70 percent of the time. That can raise the boiler’s efficiency from 89 percent to 92-94 percent.
How It WorksAn outdoor reset controller measures temperature at two places at once. It has two sensors; one outside the building and one in the boiler water. As the temperature gets warmer outdoors, the controller decreases the boiler water temperature accordingly.
Installing outdoor reset can be very straight-forward. If you choose one of the simple controllers, you just dial in the desired maximum and minimum boiler temperatures and the minimum expected outdoor temperature. This outdoor temperature is what’s known as “design temperature.” It’s the normal low temperature for the last three to five years. It’s not the lowest temperature that anyone can remember ever hearing about, and it doesn’t include wind chill.
You also can get simple outdoor reset control that allows for domestic hot water. And you can use a night setback thermostat with it, provided you allow a couple hours for the boiler to recover in the morning.
What's Reset Ratio?Understanding how to select reset ratio has been one of the barriers to outdoor reset being widely used. Hardly anyone understands what it is. The fact is, you probably don’t need to do anything with it. The simpler controls don’t even allow for anyone to adjust it.
But the advantage of using an adjustable reset ratio is that you can tell the controller how much you want the boiler water temperature to increase in relation to the outdoor temperature drop. A ratio of 1:1 means that for every degree of drop outside, you get a degree of increase in the boiler water temperature.
The first number represents outdoor temperature. The second number is boiler water temperature. A ratio of 1 1/2:1 means that a 1 1/2-degree outdoor temperature fall results in a 1-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.
Each of these reset ratios can be plotted on a graph. The result of each will be a straight line at an angle. The angle represents how much boiler water temperature increase you get for a degree of outside temperature drop. In a ratio with the same amount of outdoor temperature change, 60 degrees of boiler water temperature increase will make a steeper “curve” (called that even though it’s a straight line) than will 30 degrees of boiler water temperature change.
What To UseThe reset ratio you choose depends upon the temperature range required by your heating system. For example, if your job is radiant floor heating in concrete, your temperature may be 90-120 degrees F. That 30-degree range would indicate a flat curve. For a baseboard temperature of 120-180 degrees F, the curve would be steeper. A staple-up floor job might have a range of 110-140 degrees F. That’s still a 30-degree flat curve. So the reset ratio would be the same as in concrete, but the boiler design temperature is higher.
A beautiful thing about outdoor reset is that you don’t have to get it exactly right the first time. It’s adjustable. It’s tweekable. In fact, to get the best performance, you will probably be doing some fine adjustments. So if you set your range for 90-120 degrees F and there’s not enough heat, no problem. All you need to do is adjust the range up a few degrees to 95-125 degrees F and see how that works.
At the same time, you can count on newer controls being able to self-adjust. For example, we can expect controls that learn which zone needs the most heat and adjust the temperature range and pitch to that zone.
Outdoor Reset MythsOutdoor reset is a way to help your customers who aren’t ready to replace their old boiler save on energy bills. However, some contractors believe they can’t use outdoor reset with noncondensing boilers. In fact, the only limitation is that the return water temperature must be hot enough not to condense in the heat exchanger. You can control that with the minimum boiler water temperature that you choose.
Similarly, there’s a belief that outdoor reset can’t be used with baseboard. The only limitation, again, is the minimum return water temperature required by the boiler. Otherwise, outdoor reset might add to comfort by preventing overheating in the spring and fall.
There is also an objection that outdoor reset can’t be used with domestic hot water. This one is easy to overcome, too. Select a control with domestic hot water override. When there’s a call for hot water, the boiler temperature bumps up to satisfy the hot water demand.
Outdoor reset is an essential part of advanced boilers. It makes it possible to have only as much stand-by heat as the weather demands. This increases energy efficiency and comfort. And outdoor reset makes it more likely to have the cool return water temperatures required for condensing boilers to perform up to efficiency expectations.