Information on adjusting Outdoor Reset Curve

Note to homeowners: A heating system operating with Outdoor Reset (ODR), parameters, the radiation will not be as hot as it was with your old low or mid-efficiency boiler until the weather outside gets colder.

Most controls allow you to adjust a minimum of 4 parameters. The best way to determine the required water temperatures is to conduct a room-by-room heat-loss analysis to properly size the boiler; hopefully, this was already done before the boiler was installed. Doing the heat loss allows us to determine the optimal water temperature for each zone. Outdoor reset adjustments will maximize boiler run times and affect boiler cycling. Properly sized boilers will allow lower turndown of boiler input and system water temperature for zones dependent on\ outside air temperatures.
1. Low outdoor air temperature
2. High outdoor air temperature
3. Low boiler water temperature
4. High boiler water temperature
5. Not all boilers offer a minimum water temperature parameter but if it is offered it is important to adjust with radiant heat or cast iron radiation.

All of these parameters are used to maximize operation, fuel savings, and home comfort. How do we know which one to adjust? It's fairly simple. What usually happens is that the ODR is disabled, mainly because ODR is misunderstood. DO NOT JUST DISABLE IT, that's unfair to the homeowner. Let's take a look at the reasons to adjust each parameter.

by most technitions.

1. Low Outdoor Air Temperature Scenario -
the outdoor temperature starts getting colder, say 7Â°F, and the boiler cycles on and off, but the thermostat does not satisfy. The water isn't hot enough to maximize the radiation's BTU output, so it can't supply enough heat to the house. If we raise the Low Outdoor Temperature from 0f to +10F the maximum water temperature will now be 180F at +10F instead of 0F.

The Low OD air temperature changed from 0F to +10F

The default setting is the Red line, and the Brown dashed line is the new curve after raising the Low Outdoor Air Temperature to +10F. At 50F OD temperature, the system water temperature only increased about 3Â°F, and at 30Â°F, it increased 6Â°F. At +10f the temperature went from 170F to 180F. Raising the water temperature by 2F to 3F is noticeable in the home.

2. High Outdoor Air Temperature Scenario - If the home doesn't heat well during the shoulder season (warmer days) but seems to heat well when it starts getting colder, change this parameter to make the water temperature higher to raise the system water temperature on the warmer days.

I show the OD air temperature changed from 70F to 80F

3. Low Boiler Water Temperature Scenario - There are times you may need to change the low boiler water temperature. It may need to be turned up if there isn't enough radiation in the home to offset heat loss at warmer outdoor temperatures. It may also have to be turned down if the home has radiant heat or cast-iron radiation. Yes, cast-iron radiation is considered low-temperature heating, with one exception: if the system was converted from a steam to a hot-water system. After the conversion, the radiation output could drop by up to 40%.

Changing Low Boiler Water Temperature

4. Lower High Boiler Water Temperature Scenario - If you have radiant heat that works at a much lower water temperature, you will need to lower the high boiler water temperature. Running the water temperature in the heating system too high may damage the tubing. If the tubing is in concrete or a lightweight pour, extreme temperatures may crack the concrete or the pour. If you have a cast-iron radiator system, you may want to lower the high boiler water temperature. Perform the calculation after determining the heat loss to determine the maximum water temperature needed. Making it go higher than needed is a waste of energy.

Lower High Boiler Water Temperature

5. Minimum Boiler Temperature Scenario - While not all boiler manufacturers have this option, it may or may not need to be adjusted, depending on the system the boiler is attached to. If you lower the low boiler water temperature, this setting will also need to be adjusted accordingly. The reason is that most manufacturers that offer this parameter set the default temperature to 130Â°F. If the maximum temperature your heating system requires due to a radiant system installed in a slab is 120F, the boiler will fire to 130F, as that is the default setting. Drop this to meet the low boiler water temperature for best results.
If you have a cast-iron radiator system that is not a steam conversion, reset the minimum water temperature to 100F and the low boiler water temperature to 90F.
If you have a hot-water baseboard system, set it to 120Â°F or leave it at 130Â°F. The boiler will send out no water, less than the minimum water temperature, even if the control says the water temperature should be less than the minimum water temperature parameter.

Changing Minimum Boiler Water Temperature

If you want to play around with your settings, you can plot them here.

You can print a Blank ODR Worksheet and play with changing outdoor air temps and water temps.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for any improper information, application, damage to property, or bodily injury arising from the application of information found on this website, as a professional should review it.