I installed a 30 gallon gas hot water tank and sometimes i get strong water pressure but my hot water doesn't last long enough for 2 showers, but if i have low water pressure we can get in 4 showers. Any idea what could cause this?

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

large water heater, turn p the themo stat to a higher setting ( watch for burns with young childern and older folks)

or the best install a tempering valve. this valve mixs hot water with cold water before it goes into the building.

Again if you have young ones or old people they can burn faster than any other group.the best is a larger heater for the DIY person.

Thanks Tom

Have you tried it with both (hot rod and Gas ) turned on at same time? Is tank well insulated? (looking from behind tank.)
Does temperature drop off quickly, or gradually? Did it supply hot water better before, or this is first time you really measured it out? Need to keep in mind that as hot goes out the top fitting to hot lines, cold is coming in the bottom fitting and beginning to mix with hot, which obvioulsy will cause hot to gradually become warm as it enters. 2 gallons out hot tap, 2 gals., at bottom of tank, then mixing as it flows in usually equals, about 2 gal or less of warm, then cold again. Get many complaints of only enough for a short shower when they have 6 gal tank. You can try adjusting temperature up a little to see if it helps. Keep an eye on pressure relief valve dripping excessively as well as shut pump off for 20min-30 min, then turn it back on before touching any taps, to see if it cycles at all, indicating a leak in the system. If pump doesn't cycle when turned back on, no leaks.

Your tank is rusted out and needs to be replaced.
Link below is a troubleshoot for leaking water heater.
http://waterheatertimer.org/Leaking-water-heater.html

Protect yourself first:
If your tank is rusted, there is danger of rupture.
My friend welded his leaking tank, but he assumes the risk.
Ruptured tank can cause massive damage and water runs until it is shut off.

Hot water tank is under pressure because hot water expands and pushes against tank wall.
Turn off water to tank.
Turn off power supply:
Electric: turn off circuit breaker.
Gas: rotate knob to pilot so pilot stays lit but burner will not fire. To turn back on, rotate knob to ON.
Open hot side bathtub valve and relieve pressure on tank.

You can limp by with rusted tank by turning water heater on when hot water is needed.
Shower uses 5-9 gallons hot water
Bath uses 12-15 gallon hot water
Ordinary gas heater makes 41 gallons hot water per hour
Ordinary electric heater makes 20 gallons hot water per hour.

I buy water heaters at Lowes, Home Depot, Ace.
Look for scratch and dent models.
Buy drip pan for water heater.
Wrap electric heater totally with insulation except TP valve.
Buy new flexible connectors for water lines.
Offer 15% less than marked price on tank, because the store makes money on additional items.

> Most people can install their own electric heater since you just connect water pipes and electric wire, and it's done. Read link below since there are tips that help you buy and install electric.
> Generally people are nervous about installing gas water heater, so step-by-step instructions are shown on link below. Remember gas line is under less pressure than water line, so if you can install a water pipe, you can install gas line to water heater with crescent wrench, and soapy water to check for leaks.
How to install gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater, or any do-it-yourself project.

A leaking water heater cannot be repaired, assuming it's the tank leaking and not nearby water pipe.
http://waterheatertimer.org/Water-heater-leaking.html

First thing is to protect your property from catastrophic tank failure where tank ruptures and water runs until it is shut off:
Tanks are under pressure when they are hot.
Close cold water shut-off located above tank.
Turn control knob to 'pilot' so pilot is lit but burner will not turn on.
Open hot side on tub for few moments to relieve tank pressure.

Now your property is safe.
You need a new water heater.
In mean time, gas water heater makes 41 gallons hot water per hour.
Shower used 5-9 gallons hot water.
Bath uses 10-15 gallons hot water.
You can turn on water heater for short stretches and limp by until you get a new heater.

Steps for installing gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

1) Your water heater needs to be replaced.
It cannot be repaired.
Tanks rust out after anode rod is depleted unless anode is inspected and replaced 1-3 years.
Water softener causes anode to deplete quicker, so inspect every 2 years.
Also stray household current can cause tank to rust out. Put bare copper ground wire between hot and cold pipes on new heater.

2) First thing is protect yourself from tank rupture where water heater bursts open and water runs until it's shut off. Ruptured tanks are big topic in homeowner insurance industry today because they cause major damage inside house.

- Turn off heat source so tank is not under pressure:
Gas: Rotate dial to pilot so pilot light stays lit, but burner does not turn on
Electric: turn off circuit breaker

- Turn off water going into tank using shut off above tank.
- Open hot side to bathtub to relieve pressure from tank.
- Check label on side of tank to get tank specifications for replacement.

3) What to do until replacement.
You can get by for a while on leaking tank by turning it ON only when needed.
Make sure tank is full of water before turning heat source back on.
This will buy time until replacement.
Electric tank heats 21 gallons water per hour.
Gas water heater 41 gallons per hour
Shower uses 6-9 gallons hot water, bath uses 12-15 gallon hot water.

4) Additional reading so next tank lasts longer
http://waterheatertimer.org/Replace-anode-rod.html

http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

I wrote a page on leaking water heaters with resources and information:
Page lists things to check.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

Also run hand over tank to find warm spot that says where leak is coming from
A rusted tank cannot be repaired.

If your tank is rusted out, it might burst open and flood the area.
Hot water inside tank is under pressure.
Turn off heat source to water heater.
Turn off water valve located above tank.
Open tub faucet on hot side and relieve pressure from tank.

You can limp along with a leaking water heater by turning it on when hot water is needed.
For example a shower uses 5-9 gallons hot water.
Turn gas water heater on for 15-20 minutes.
Turn electric on for 20-30 minutes.

To conserve resources with next heater:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

this is usually a problem with all tankless water heaters.
the solution is to install a small buffer tank (5 or 10 gallon electric water heater) after the Rinnai. this will give the water something to mix in. it will solve your problem.

did you make sure to use a 3/4" gas line to the unit. not just 3/4" at the unit itself. sometimes when there is not enough gas, the unit will act funny and cycle from hot to warm or even cold. Make sure you check the gas line size, its very important.

I would use the instant unit for a secondary unit to a large conventional heater, 40 gal min, gas or electric. They are both about the same now, but when the power goes out, you might not have the pump running, but you will have hot water! And whether it is a secondary or a primary water source, you will have a hot supply feeding it just in case. We have 2 people in the house, and a 30 gallon unit, and it is not big enough. The 40 gallon tank worked great. Hope this helps.