Hot Water Furnace Temperature Q&A

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Dear NH,

I have a problem with the temperature of the water in my home. I have a hot water heating system, and my water is heated by the same furnace. I do not have a hot water tank. Instead, the water comes directly from the furnace. According to my oil burner company, I must keep the temperature of the water in the furnace above 140 degrees or my heating system will not work properly. This seems to be extremely hot and I am concerned that my little one (when he is old enough to reach the faucet) may burn himself. Should I insist he lower the furnace temperature?

PB from Scranton, PA


You are without-a-doubt correct to be concerned about your water temperature. Back in the good old days, it was not uncommon for water heaters to be set at 160 degrees, more than hot enough to cause instant severe burns! Nowadays, the standard setting for hot water is between 110 and 120 degrees.

However, I would advise against changing the water temperature of your furnace. Your furnace guy (or gal) is absolutely correct. Lowering the temperature will radically change the built-in efficiency of your furnace and of your heating system as a whole.

Think about it… if you lower the temperature of the furnace, the temperature of the water circulating through your radiators will likewise be lowered. This will in turn increase the amount of time it will take for your home to be heated. All things being equal, it takes the same amount of oil to keep your home at a certain temperature regardless of how hot the water is. Therefore, your oil burner will have to cycle on and off more often to maintain this lower temperature because it will take longer for the temperature to rise. This will cause increased wear and tear on the furnace without any gain (or even a loss) in efficiency. The most inefficient moment in your furnace's operation is when it first starts up!

So instead of focusing on the furnace as the culprit, you can take measures to lower the faucet hot water temperature AFTER it leaves the furnace. This is done through the installation of a "mixing valve". A mixing valve is a simple thermostatically-controlled mechanism that mixes a little cold water with the hot water to lower the temperature. Mixing valves are adjustable to produce the desired water temperature, but it is wise to use a thermometer to verify the temperature at the tap. Installation does require some plumbing skills such as pipe cutting and soldering, but the end result is worth it!