shower problem - hot water is not working well in winter

Hi, I recently moved into a ten-year old house. The hot water of the shower worked fine in summer. However, when the outside temperature begins to drop down around 30, the hot water in the shower does not seem to work. The faucet water is hot, but the shower water can only reach warm level at the best. The heating system is in the garage and it is a new system with only 5 months on it. Two bedrooms have shower, it seems both showers have the same problem. The shower has one bolt - turning to the right is cold water and turning the same bolt to the left is the hot water. Would anyone please give me any hint on what might be wrong with the hot water in the shower? Thank you very much. - Lily
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