furnace keeps going on even tho thermostat temp reached

hello to all you heating gurus:
we have just moved into a new house, and the furnace was working fine. for some reason, this afternoon when i went upstairs to put my son down for a nap, the thermostat said it was 74. it was hot. it was set at 62. i turned it down all the way, and i still heard the furnace kick in.
is there any simple explanation to this before i call the plumber??
any help would be appreciated. the furnace is quite old, about 25 years, oil burning baseboard heat.
jae
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Don't call a Plumber! See if there is a sticker on the boiler of the company that regularly serviced it. If not, ask your new neighbors whom they would recommend. You should have it looked at right away by a Heating/Boiler company. Do you know how to shut the boiler off should there be an emergency situation with the boiler/burner? Ask all the questions you need when your boiler is being serviced. Ask the service person to please instuct you on all the safty issues with regard to your boiler. Your symptom describes a number of things besides just the termostat. Spend the extra time reserching who is the best local Boiler/Oil Burner service company. THAT is the real job you have (but time very well spent. $$$).
-zero

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In some systems the boiler runs without the thermostat. It has its own internal thermostat that controls the temperature of the water in the boiler always keeping it hot when the boiler is turned on. The wall thermostat will turn on a pump sending hot water to the radiators when needed and turning the pump off when not needed. But the boiler always keeps itself hot. Other systems work differently.

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Yes, that would be a maintained temperature boiler as opposed to a cold start boiler, usually found on boilers that have a coil for domestic hot water. An aquastat in the water jacket controls the temperature, connected to a triple aquastat relay. Of course these days with hi end digital controls, that stuff will soon be a thing of the past

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The two things come to mind are bad thermostat, or a bad contactor (relay) in the furnace. Couple years back, some friends of mine had a furnace that would heat the house up into the 80s, and would not turn off. They had replaced the thermostat four times, no joke. The problem turned out to be a bad relay
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Christopher A. Young
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Most of the answers so far assume that you have a separate hot water heater. Is that true or does your boiler also supply domestic hot water?
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Peace,
BobJ



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