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I had a brief scare just now. It is 7-degrees right now and getting colder so I definitely don't want a furnace failure.

I came home and house was at 62 with thermostat at 64 (my standard not-at-home setting) and I manually upped it to 72. Normally when I do this the furnace comes on in about 4-5 seconds. This time, however, nothing happened. The thermostat said "heat on", so I go down and every thing looks ok. Plenty of oil, water pressure is correct, furnace temperature is normal. I tried pressing the reset button but nothing happened.

So I go upstairs and get ready to call the emergency service when I hear the furnace come on, a good 3 or 4 minutes after I had originally turned the thermostat up.

What could have caused the delay?

  • low battery in thermostat? – jsotola Dec 29 '17 at 3:08
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    The boiler may have just cycled off. I believe there is an end of cycle time delay that may have delay initial start-up. – Paul Logan Dec 29 '17 at 10:35
  • A power outrage can cause a long delay as Paul is pointing out to prevent short cycling I was working on a small system that has a 3-1/2 minute delay for power up or end of last cycle. – Ed Beal Dec 29 '17 at 14:49
  • @PaulLogan Answers don't belong in comments – mmathis Dec 29 '17 at 15:01
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That is normal, many furnaces are designed to avoid a condition called a "short cycle". That's where they quit, and restart again shortly after.

It's not quite as big a problem with furnaces, but it's a huge problem for A/C units (and heat pumps!) and can damage the unit. If the furnace is designed to work with A/C, the anti-short cycle logic may be the same for both heat and A/C.

Unbeknownst to you, the furnace had just quit before you walked in the door. It waited the requisite timeout period to avoid a short-cycle, then started up normally.

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furnace temperature is normal

That is why. The furnace controls the water temperature. It doesn't matter how high you turn the thermostat. If the water temp is hot enough, it's not going to turn on until the water cools down after circulating the house.

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