Integrated Residential Heating Controls

Mon, January 01, 2016

This article focuses specifically on how integrated controls make a hydronic HVAC system more efficient. One of the most basic and common systems is a boiler outdoor air reset control. This type of control looks at the outside air temperature and adjusts, or 'resets' the boiler water temperature accordingly. The idea here is that is the boiler needs 180'F water on a -10'F outside air day, the system could use a lower water temperature is the outside air is warmer than the -10'F the system was designed for. By running the boiler at a lower water temperature, there are less standby losses in the piping. Reducing the boiler supply water temperature will also help to keep the space from overheating due to getting much hotter water through the radiation device than needed. This type of control is set-up using a reset curve. The reset curve is the ratio of outside air temperature to boiler supply water temperature required. An integrated control system actually goes one step further by also incorporating indoor feedback. Indoor feedback is looking at the indoor zone air temperatures and using these to automatically adjust the reset curve. What happens a lot of times when not using indoor feedback, we find that it becomes difficult to set-up a perfect reset curve based on outside air temperature alone. When indoor air temperatures are incorporated, the reset curve can automatically be adjusted. So for example, if the control is calculating a boiler supply temperature based on outside air, but the zones are becoming satisfied quicker than expected, the control system can adjust the reset curve down to supply a lower water temperature. This will help to enhance energy savings as well as comfort levels. There is another aspect of integrated controls that will help to achieve better system efficiency; controlling the boiler cycling. When you have a complete control system that is looking at all of the individual zones, an integrated system can synchronize the zones. All of the zones will follow a common cycle schedule. Let's say that the control has been set-up to have 20 minute cycles. At the beginning of a cycle, the control system will look at all of the connected zone thermostats and use their indoor feedback to determine the hottest reset water temperature required. The control will then fire the boiler in an attempt to get the system water up to the desired water temperature. Individual zones will open up to allow water to flow if they currently need heat. Zones will shut down at different times based on if they need more or less heat. It is also possible that a zone that does not currently need heat will allow a small amount of water to flow in an attempt to prevent the zone from dropping below set-point. Why not, the boiler is already running for the other zones. The key here is that once the boiler turns off during the cycle, it cannot be turned back on until the next cycle begins. This may seem a bit odd, but preventing the boiler from short cycling every few minutes will help to improve the overall efficiency of the system. In a system without integrated controls, any thermostat can force the boiler on. So, one zone may have the turned on the boiler and as soon as it shuts off, another zone turns it back on. This would cause major short cycling of the boiler. The reason that short cycling of this nature is such a waste of energy is that every time the boiler turns on, there is a few seconds of running gas just to get the flame established. There is also time spent when the boiler turns off to purge hot flue gases from the venting system. These things combine to waste a lot of energy when boilers are turned on and off. It is much more advantageous to run a boiler for a longer time and then turn off for a longer time. Integrated controls help to do this by synchronizing zones cycles. This concept of integrated controls can be even more powerful on a system that uses a condensing boiler. Since condensing boilers also typically modulate, they allow for much lower reset water temperatures. Using an integrated control system with a modulating condensing boiler means that you will be able to run the boiler almost constantly by modulating the water temperature to exactly match the heat required by the building. Since the boiler will not be turning on and off as often, just modulating the firing rate, it will be able to achieve a much higher overall efficiency. As you can see, using integrated house controls will allow for much better cycle efficiency of a boiler system. Since the controls are looking at outside air as well as indoor air feedback, the control system can much more accurately create the water temperature that the system needs. More importantly though, the system can synchronize the cycling of the indoor zones to prevent the boiler from excessive on and off cycling. All of these things combined will help to improve the efficiency of your hydronic system.