We had a new hot water heater installed 18 months ago. A few months after installation (3-6 months) we began having problems with hot water flow into the bath tub in the master bath - furthest point from hot water heater. Once or twice monthly the tub valve would only trickle hot water. I would solve the problem by opening the tub hot water valve full and then open the pressure relief valve on the hot water heater - water would immediately flow. Over the last 12 months the problem has gotten worse - more frequent, every hot water valve in house affected, etc. The only other 'plumbing' change during this time was replacing an old top loading washer with a new front loader. I mention this because the problem seems to have worsened since installing the washer, though that may be totally coincidental.

It seems to me that there must be a leak somewhere allowing air into the system. However, more than half of the plumbing system is viewable and I have checked it inch-by-inch multiple times to see if I can find a leak. Nothing. Further, the problem did not exist until this hot water heater was installed - have lived in the house for 30 years.

Is there a more effective way to 'bleed' air from the system other than the pressure relief valve on the hot water heater? Is it possible that the hot water heater installation is the problem? Any other thoughts?

Thank you in advance for any help / advice you may offer.

First, you do not have air in a domestic water system. If you had a leak water would come out, not air get in. Air being lighter than water, if you did have air in your pipes, would rise to the top and come out as soon as you opened a faucet. So you cannot have air in your domestic water system. You can in a sealed hot water heating system but not in your domestic water system.

Your problem is that something is blocking a valve somewhere. If you do not get water out the hot side of the bath faucet it is because something is blocking the bath faucet or something is blocking shut off valve on the cold water input side of your water heater. The way you describe it, it sounds like you have both. First you had something blocking the bath faucet, now in addition you have something blocking the cold water input valve to the water heater.

What could be blocking your valves? Assuming you have copper pipes that something is probably solder. When soldering copper pipes if you use an excessive amount of solder, the hot liquid solder runs inside the joint, drops and hits a cold pipe. The hot solder will not stick to a cold pipe. The hot solder hits the cold pipe, splatters into a thin flat disk. Later the flowing water moves it to the next available valve. The ports of the valve being smaller than the pipe, the solder blocks the valve.

Turn the water to the house off. If you have a single lever valve remove the cartridge. If you have a two handle valve, remove the hot water stem. Hold a cloth over the open valve, have some one turn the water on briefly to flush out the valve.

If the cold water input valve to the water heater is a compression type valve you may be able to remove the valve stem and flush out that valve as well. If you have a gate type valve or a ball valve you will have to cut out the valve and clean or replace it.

P.S.

Leave the T&P valve alone. Sooner or later you are going to get debris caught on the seat and then the valve will not seal and you will have to replace it also.