We have a Rinnai tankless water heater in the basement. Our housemate has a bathroom in the basement. About once a week, the water turns cold during her showers or when she uses the sink. We have no problems at all upstairs, not even filling the bathtub. She recently told us the low water pressure light comes on on the water heater whenever this happens. I don't understand- if the flow is too slow then it should just not come on at all.
I just changed her shower head from 2.5gpm to 1.5gpm and tightened the valve for hot water to the sink a bit. I'm hoping this takes care of it but I'm also hoping to understand better.