Can i use my old vent pipe i had for my old 40 gallon water heater

1) I believe you can immediately elbow the PVC pipe.
There is a limit for how many elbows can be used between blower and termination, and limit is determined by diameter of PVC pipe.

2) Copy following link for venting resources and typical Rheem and Richmond power vent manuals, and diameter-elbow pdf from Bradford White.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#venting

3) Richmond is Rheem. Product manual will show specifications for venting.
Product warranty is voided if installation does not follow specifications shown in manual.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) Copy following link for troubleshoot resources by type of gas control valve, Venting resources, and pro water heater forum:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/

2) This odor can indicate incomplete combustion due to lack of air supply. Get more fresh air to unit. Check that flame is clear blue color. John Wood is American/AO Smith product so air intake screen is located on bottom of unit. Use mirror and flashlight to inspect. Clean with compressed air ad or feather duster.

3) Jw840nvh is 40 gallon natural gas. Natural gas is lighter than air and rises upward, unlike propane gas that is heavier than air. Install gas detector at height correct for type of gas.

4) For safety, install CO carbon monoxide detector

5) Review product manual for correct vent lengths and diameters.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I believe it takes 4" vent pipe up to 100,000 BTU
BTUs are shown on label side of tank.
http://waterheatertimer.org/pdf/Gas-venting-specs.pdf

Does new water heater have power vent? What does manual say?
Are you experiencing vent problem now? When you light a match and put it under vent stack, does smoke immediately get pulled upward? Are furnace and waster heater connected to same venting. Is there dark exhaust stain around vent hood? Does exhaust vent back out of vent hood when furnace kicks on? Are flue pipes stained with rust and white coating? Are vent pipes vertical, or are there level runs? Are the pipes made of galvanized and screwed together with 3 screws at each joint?

Frankly I would double-triple-check any internet answer dealing with possible CO poisoning at your home.
1) Ask local plumber for specific code in your area.
2) Call gas company
3) Post question on pro water heater forum:
http://www.thetankatwaterheaterrescue.com/forums/forum3/
4) Read whole manual for new water heater before buying.
5) Contact manufacturer.

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

You have 75 gallon gas water heater.
I install water heater insulation 3 ways:
1) Easiest: buy ready-made blanket from Lowes and use duct tape
2) Harder: wrap heater in ordinary fiberglass insulation and use tie wire
3) Hardest: Build a box out of blue styrafoam >> leaving room for air intake for gas water heater, and space around vent pipe

Instructions for gas water heater:
Do not cover TP valve.
No insulation is put over gas valve or air intake.
No insulation touches vent pipe.
Remember the gas water heater has to breathe air into the burner, and aftter combustion takes blace, hot gases escape so extra space is left around hot vent pipe to avoid fire.

Find age of Rheem: look at 3rd and 4th digit of serial number, for example 06 was manufactured in 2006.

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Also take advantage of fixya phone service.
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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

If you'r really lucky you may find a leaking fitting on top of the water heater thats draining down inside the shell and out the bottom, otherwise its time to replace a rusted out water heater.
Sorry

i don't fully understand your question. which model heater are you using? what do you mean by "concentric pipe-heater"

If water is leaking from a pipe. then replace or repair the pipe or fitting. If the leak is coming from the unit itself, then replace the unit. Hope this helps.

Shut the water off to it (should have a valve very near it on the cold side) and kill the breaker at the electrical panel. This needs replacing if it's the pipes I am thinking (the two 3/4 inch nipples coming directly from the top?).

Dear Hank McNeal; I have successfully used a "boiler plug" to seal a water heater tank leak in 2007. It was a 100 gallon electric commercial hotel unit. To this day it's still in use. It depends where the leak is located....Predrilling the leak is necessary to be sure the plug will ernguage it's threads into good material... Start as small as possible, then use the best suited plug. they come in an assortment of sizes.