I have a 30 year old house with original faucets. I'm getting lukewarm water upstairs and hot water downstairs.

I thought it was the old hot water heater and had a new one installed last week, but the problem remains. I spent all my money on the water heater, so have no resources to hire a plumber.

The shower was nice and hot one day, then lukewarm the next, so it happened suddenly. The weather is very cold here and has been below zero.

Thanks for your help, I really appreciate it!

Diane

I would just about guarantee it's the cold and wind. Check your HW and see what its set at. Insulate pipes off heater to keep hot in pipe. And Extra insul wrap around HW would save some money if its not there already.
Do those things and see if it helps and monitor the temp and wind to get a good history on how it might affect the hot water. I assume the problem was before the new HW. Don't feel bad you upgraded the WH The only other could be the faucet. What style do you have?
Signed 21 boat

Sometimes an older faucet has washers that expand when they get hot. Is the pressure OK?

The faucet is single, pull to turn on the water, that fills the bath and a pull-up knob on top to start the shower.

I've lived here for eight years through all kinds of weather and this has never happened before. I don't regret the new water heater, the old one was nearly 20 years old and it was going to fail sooner or later. Yes, the problem started about a week before I got the new WH, and around the time the temps dipped to zero outside.

The pressure is the same as always.

Thanks so much, I am very grateful for your knowledges and expertise.

The faucet sounds like a moen château. Typically cold weather can cause incoming water to be cold enough to reduce the temperature half way during the shower but the first 5 minutes should be great. By code we are required to not adjust the temp above 125. On my home I adjust to 135 in the colder months. Be aware of possible scald dangers.
If the water temp is OK at other fixtures you may have a scald guard on your faucet that may need to be adjusted. Google moen château and see if that is your faucet and see how to adjust scald guard.