On Sunday, February 17, 2019 at 5:07:32 AM UTC-5, Alan Holbrook wrote:
The water pressure in my domestic hot water supply has recently dropped to
very low levels. The water temperature is fine and there is no problem
with the cold water supply. When I open a hot water tap anywhere in the
house, the water dribbles out for a while and then suddenly gushes for a
couple of seconds and then reverts to the dribble. I have an oil fired
furnace and a tankless water heater, and when I had a serviceman out
recently for another problem, I described the low pressure problem and
asked him if the low pressure could be caused by something in the
furnace/tankless system. He told me that as long as I had hot water, the
pressure problem wouldn't be in the furnace/water heater, and I should call
a plumber. I'm willing to do that, but before I do I thought I'd ask if
anyone in the NG had any idea of what might be happening. Any ideas?

Are you sure that serviceman said that the problem couldn't be in the
tankless or the furnace? Or was he a furnace serviceman and he just said it's
not in the furnace? The latter would make sense, if it really is a
furnace and not a boiler that also heats your water. A furnace has nothing
to do with hot water. A tankless sure does. Since it's in all outlets,
then it's a problem with the flow into the tankless, the tankless itself,
or after the tankless before the first faucet. Sounds like it's time to
call the plumber.