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This is our first house with a boiler and I have a question about the most efficient way to use it. We have multiple zones and it seems like heating one zone while not heating others might degrade the efficiency. For instance, if the bedrooms are being heated but the living room/kitchen isn't (but they're still colder than we would want if we were up) are we losing efficiency by not letting both zones heat at the same time while the boiler is running to heat the bedrooms?
Folks I have known with boilers do just as you proposed. They run rooms they are in at a normal temp, and the unoccupied rooms around 10 or so degrees cooler. The claim of cost savings with split duct A/C is based on this premise. Condition the room you are in, and spend less energy on the others. I wish I had a boiler instead of forced air. A co-worker just replaced his 40 year old boiler with a modern one and said he cut his gas use by 25-30 percent.
Radiant heat has a longer lag and the air temp can be a little lower than with a forced air system. The layout of the radiators or heated surfaces has an effect. Even with high efficiency boilers, you don't want to turn zones off or down more than maybe 5 degrees or you will lose some efficiency. Radiant systems are not well understood by some owners and that is essential to getting the best comfort and efficiency. I've worked on numerous lawsuits of claimed deficiencies in radiant heat systems where the problem was improper operation of the system due to the owners runing it like a forced air system. Your specific system may have nuances as well.
If you are so inclined, try running it a few different ways for one month corresponding to your billing cycle. With that data and the heating degree days (HDD) an analysis can be run to tell you what mode is the most efficient.
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