The main entrance of an upscale hotel in downtown Chicago was sweltering Thursday morning — and not just because of the rising sun and the week’s heat wave. As pickets marched under the Park Hyatt’s glass awning, the inn switched on 10 heat lamps installed to warm guests in the dead of winter.

The picketing began a daylong strike in which dozens of Park Hyatt employees walked off their jobs at 7 a.m.

Their union, UNITE HERE, is trying to put pressure on Hyatt Hotels Corp. during negotiations to replace Chicago-area contracts that expired almost two years ago. The union represents about 1,800 workers at the Park Hyatt, Hyatt Regency Chicago, Hyatt Regency McCormick Place and Hyatt Regency O’Hare.

The company has agreed to match wages and benefits spelled out in four-year contracts that UNITE HERE reached this year for Chicago employees of two other hotel chains, Hilton Worldwide and Starwood Hotels and Resorts Worldwide Inc.

UNITE HERE says the Hyatt negotiations are stuck on the company’s ability to outsource work to nonunion firms.

“If they replace me, it doesn’t matter how good the benefits are because I’ll be out of a job,” says Park Hyatt restaurant server Gabriel Carrasquillo, a picket captain. “I’m HIV-positive so I have a lot of medical expenses. Without these health benefits, I wouldn’t be able to have the care that I have today.”

UNITE HERE says another contentious point is Hyatt’s housekeeping workloads, which the union calls hazardous. The union funded a peer-reviewed study, published last year in the American Journal of Industrial Medicine, that placed Hyatt fourth among the top five U.S. hotel chains for workplace safety.

Hyatt responded by commissioning a critique from an occupational health expert who concluded that the study’s authors “have not disrupted the conventional wisdom that housekeeping tasks are not inherently hazardous.”

Hyatt accuses UNITE HERE of holding up the Chicago talks to pressure the company to recognize the union at hotels in four other cities. “Instead of acting in the best interests of its members, the union is using them to grow its membership,” says Farley Kern, Hyatt’s vice president of corporate communications.

The strike was the fourth work stoppage to hit a Chicago-area Hyatt since May 2010. Park Hyatt’s reservations desk said the hotel’s 198 rooms were booked solid Thursday morning. Kern insisted the strike would not affect guests because managers were filling in.

The company’s next bargaining session with UNITE HERE is Monday.