Lyman Hall High School in Wallingford had a three-hour delay on Thursday because of a heating issue, according to school officials.
They said they have a temporary boiler to work and school started at 10:30 a.m.
The high school was closed on Tuesday and Wednesday because of a problem with the heat.
School and town officials said they are working to fix this issue and Wallingford's mayor said the town has authorized money to replace the boiler.
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No other Wallingford schools are impacted.