Low Hot Water Pressure after Hot Water Tank Flushing

October 17, 2020 5:17 PM

We flushed the hot water tank today - we live in a hard water area, and do this annually. Now we have low water hot water pressure from all faucets - what should we check?

We've quadruple checked the valves to turn the water back on to the tank (there are two on the top of the tank for some reason, but both were shut off for draining and cleaning the tank). To me, it seems like the tank is getting inadequate pressure, because when I first turn on the hot water, the pressure is the same as the cold, but then it gradually drops.

Based on the pictures at this Home Depot link one of the shut offs is a gate valve and the other a ball valve.

Is it possible that either valve has failed and not opened all the way, even though they feel all the way open? What else might cause this issue?

posted by hilaryjade to Home & Garden (4 answers total)

If you have a whole house tempering valve disturbed sediment may have plugged something up.
posted by Mitheral at 6:53 PM on October 17, 2020

Best answer: Of the valves on top of the tank, I think one must be the cold water intake which admits cold water into the tank and forces it to go down a long tube (the fill tube) to the bottom of the tank, so that when cold water comes into the tank it doesn't mix with the hot and cool it off, but instead goes to the bottom of the the tank and lifts the hot water up at a boundary layer across which there isn't much mixing, and which rises as you use up the hot water. And that hot water has to leave the tank at another valve on top for that system to work. There should also be a third valve on top of the tank, the safety valve which comes into play if the pressure in the tank gets too high.

I would try a quick and dirty fix which doesn't involve taking anything apart or completely emptying the tank again. First, turn the breaker which supplies the electricity to the tank off (if it's electric of course). Wait a minute or two for the heating elements in the tank to cool, and then turn off the cold water intake on top of the tank. Then go to a sink which has a single faucet that all the water comes out of and position the lever in the middle the way you would if you wanted a 50-50 mixture (or turn on both taps). Only cold water will be coming out at this point, of course.

Then go back to the tank and open the drainage valve at the bottom a third or half way, and then go back to the sink where the water is running and hold your hand over the faucet so that no water can come out.

This will have the effect of forcing cold water backward along the pathway that hot water follows as it comes out of the tank and to that sink, and that cold water will probably blow out the obstruction which is blocking the hot water. I'd say hold it for 3-5 minutes in that configuration, and then turn off the drainage valve at the tank, shut off the faucet at the sink, turn the cold water intake valve on top of the tank back on, and see how your hot water pressure is at that point. If it's OK, you can turn the power back on at the breaker after you run your hot water for a few minutes to fill the tank back up completely.
posted by jamjam at 10:23 PM on October 17, 2020 [1 favorite]

Also maybe try cleaning the aerators on the faucets. Unscrew them and clean out any gunk that has accumulated. The changing pressure in the pipes from flushing a water heater usually stirs up rust, etc that collects at the end.
posted by jtexman1 at 6:11 AM on October 18, 2020 [1 favorite]

Response by poster: Thanks all - I didn't describe the valve situation well - for some reason there is a gate valve THEN a ball valve both on the same cold water in line. It turns out the gate valve was not all the way open. jamjam - your answer helped. I started by turning off the breaker again, then opened up the hot tab in the laundry sink (closest to the water heater). I went to the valves on the cold water in line and turned them on and off, listening to the water flow as I worked. With the ball valve open, I turned the gate valve closed, then open again, and this time could hear the water into the tank increasing.

So - we have "replace that stupid valve" on the to do list instead of "get a new hot water heater" :-)
posted by hilaryjade at 9:44 AM on October 18, 2020 [1 favorite]