Have 2 electric water heaters connected in series. Both 60 gal Rheem. The hot water pressure is low on all the faucets. When I turn on first, it is OK and then it reduces. If I turn on 2 faucets, it goes exactly half. Cold water pressure is fine. Water heaters are about 10 years old and I switched off the first one ( I needed only one) long time back. So the first one is staying as a reservoir.

Can it be a clog in the heater? I drained both of them and got some little black sediment. Also the pressure relief valve, also works fine. Removed the connector from the second heater which is close to him where it feeds the home. Put a small screw driver in it and found no deposits in it.

When I turn on 2 faucets and then turn off 1, the pressure comes back as if like it was released from a block. Sudden.

How to diagnosis this problem properly?

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I drained both of them and got some little black sediment.

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You should flush not drain both water heaters. Flushing is not the same as draining. When you flush you attach a hose to the drain valve and open the drain valve leaving the cold water input stop valve open. Using the force of the water pressure to flush any sediment out the bottom of the tank.

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Can it be a clog in the heater?

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No, but obviously there is a restriction somewhere. Most likely that restriction would be in the heat trap nipples if equipped.
Replace heat trap nipples, both heaters.

So to flush, I keep the input open? So basically I just connect a garden hose to drain valve and open it. The water input is open (normal). So this way it pushes the sediments out? Draining is same, but for a short time? This one, do it for long time?

How to remove the nipple? Basically 4 of them right? Input/output for both heaters? Is that with a pipe wrench? It is not hex as for an adjustable wrench?

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Draining is same, but for a short time?

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Draining not the same. When you drain you turn off the cold water stop valve, open a hot water faucet somewhere to let air in, and open the drain valve. Water drains from the tank but only under there force of the water in the tank.
When you flush the water is under the pressure of your water system.

Yes there would be four, two cold and two hot. Cold should be marked blue and hot red.
When replacing make sure you get them right, and yes it is a pipe wrench job. Use Teflon tape of pipe joint compound on threads.

Thanks. I will I will replace the heat trap nipples one at a time. I did the drain so many times and the water was clear.

This was a slow process. I mean, the water pressure reducing little by little. Started from last winter.

I have one heater off. Will it affect anything. Basically it is sitting there as a reservoir. Also the ball valve which controls the water input (replaced by a plumber 1-2 years back), is not stopping the water completely. Even if I turn it off, I am getting little water of faucets continuously.