We just installed a new water heater. Now all that comes out is luke-warm water. The pipes shake when I turn the warm water on, and I think it is air? Any suggestions would be appreciated!

I haven't been around in a while (being punished by AMHD), but due to your predicament I'll try to help.

Air is compressible, that's why is makes a lot of noise. The noise will eventually go away, but it's EASY when you do thing right the first time.

Now, I could ask you some questions to make it easier on me and you, but let's try. You probably have both cold and hot air at this point. I'm going to suggest "starting over" at this point,

So, turn off the hot and cold water and make sure the hat water heater can't start. Can you do that? Gas: set gas to Pilot; Electric: Power off

The pipeing layout of your house matters.
Locate the highest elevation fixture, and hopefully it's a bathtub.
Open this fixture as as far as your comfortable unattended.

If the water branches aren't too massive, then this may be the only one you need to open. If there are other tubs, do the same. Open so they will drain about 1/2 hot and 1/2 cold water.

Laundry sinks are good too.

Remember, layout is very important.

Find the lowest point that has a water outlet and open it. Air from the top fixture will displace the water removed. If you have a garden hose, it will be a good point for cold water. A laundry tub may be a good place for hot and cold water. Apriori, you should know how far you can open these faucets without flooding.

OK, so we have the water off, the highest faucet open and the lowest faucet open. So far so good. Open the cold water inlet on the water heater.full. (Generally there will only be one water valve on the water heater, Cold in; some might have a cold in and hot out. Be sure all are open full.

Now with spotters (kids) if you have them located at the taps SLOWLY turn on the cold water. You actually want to hear a smooth flow.

The water heater will fill and the air will be pushed to the highest elevation.
Close the lower elevation valves and open say a laundry sink or things you might have forget. Get a nice flow and shut them off.

If there is a second floor faucet open, just crack it open (hot and cold).

Now go back to the water shutoff and continue opening slowing, stopping a bit, until there is no noise. Open all the way.

Check all faucets starting at the water main. If any were open outside, time to make sure they are closed.

The basic idea is to
1: Empty all of the air chambers. Some are tougher than others. They protect against water hammer and should be filled with air.
2. When water enters the pipe it will either compress or replace the air. We want it to replace the air or keep pushing it out. It can't go out if the faucet valves are closed. Since we want to chase it all out, let's chase it out to the furthest tap and the widest taps.
3) lets use taps that we know won't make a mess. Laundry, shower, outside faucets.

Now go turn the water heater on.

Getting the air out works best if you gently push it out through big openings.

Getting the water out of a line, first make sure that when the valve is off and doesn't leak internally and have air to displace the water from a high point.

I'm glad your back kiss, I always enjoyed you answers.

Thanks. I'm not really back and I have no intention of cleaning out my mailbox. Don't know if you can email me with a full mailbox. Mailbox limits were severely CUT when I became an Uber member. It does create problems when certain experts can email you, but you can't email and PM you, but you can PM them back. A little silly, I'm afraid. I miss a lot of you, but this site and moderators/owners had a negative impact on my well-being starting with one event.

Try composing a reply when not logged in. You will be disapointed as to what happens.