A nanny cam caught the powerful blast and destruction on video.
Footage shows the uninhabited room containing deflated kid-size blow-up house, various toys and balls.
After a few seconds, the wall can be seen exploding and sending shrapnel, nails and other debris flying into the room. A cloud of smoke is then seen filling the room, and it’s all you can see for a few seconds until it clears and the destroyed playroom comes back into view.
"We are all lucky we are alive," homeowner Chris Renoud told ABC News affiliate KOMO News, who reported his 3-year-old daughter Lily was napping just above the room with her nanny and that he, his wife Anne-Marie Faiola and their 4-year-old son Jamison were not in the house.
The nanny called the kids’ mother saying she heard a ticking and then a bang -- with smoke coming out of the vents, Faiola wrote on her blog.
“I immediately logged into our nanny cams and saw the entire house was filled with smoke and every detector was going off,” Faiola said, adding she rushed home, where she saw seven firetrucks and her family outside the house.
The boiler was ripped from its foundation 15 feet across the room, and the blast raised all the flooring on the first floor and put cracks in every wall, KOMO reported.
Fire investigators determined the boiler malfunctioned, KOMO added.
Renoud, Faiola and the Bellingham Fire Department did not immediately respond to ABC News’ calls requesting additional comment.