how I should bypass solar collector whenever this useful energy gain is negative?

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I'm going to supply a constant temperature using a solar field and a boiler as backup. I have fixed the set point temperature of boiler on 250 C. So whenever the boiler output temperature is lower than 250 C, it will use fuel to reach this temperature. But there is a problem with this model, whenever sun is not available, the solar collector will waste heat so I should bypass the solar field whenever this useful energy is 0 or negative. My thought is that I should apply a control method for comparing the solar collector outlet temperature versus it's inlet temperature and whenever it's lower than input, bypass the solar collector. Using controllers, how I should apply it in TRNSYS?