Low Hot Water Pressure
Turning on the cold water will pressurize the hot water in your hot water piping. The hot water will come out of the shower head with greater pressure for a while, until the cold water starts to mix with the hot water, causing the temperature of your shower to drop.
Look on your water heater for two "thick pipes" above the water heater. If it is a high efficiency water heater, those thick pipes are check valves. The cold water inlet has a check valve with a ball in it made of polypropylene (which floats). That valve prevents hot water from rising up the cold water inlet line and cold water from spilling into the water heater. The hot water outlet line has a nylon ball in it (which sinks) and prevents the hot water in the water heater from rising up into the distribution piping, and the cooled down water in that piping from spilling back into the water heater. Maybe try giving them both a good shake to loosen up any dirt that's causing those plastic balls from moving.
It could be that one of your check valves is stuck closed cuz of dirt in it.
It could also be that one of the ball valves, either inlet or outlet to your water heater is partially closed.
Buy a small pressure gauge and connect it to your water heater's drain valve. Measure the pressure. Now connect it to the spout on your laundry room sink, and turn the hot water on. You should record the same pressure. Now, start your washing machine on "whites" so that the washer starts filling with hot water and see if the pressure gauge on the faucet spout drops. If so, then the blockage is somewhere upstream of the laundry room tub faucet.
Crude as it may sound, solving problems like this is just a matter of doggedly tracking down where the pressure drop is occuring. It's just as simple and as hard as that.
Also remember that a restriction to flow is only going to cause a pressure drop when there is flow. When there isn't any flow, the pressure throughout the hot water distribution piping will be what it should be everywhere. So, you need to measure the pressure in the hot water pipe WHILE there's flow through that pipe. It doesn't matter WHERE you measure the pressure; as long as it's downstream of the flow restriction, you'll see a drop in pressure when water is flowing.
Last edited by Nestor_Kelebay; 12-06-2008 at 03:18 AM.